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Title: Crossover, L-Pad, Impedence, Resistance question
Post by: Jeff Schoonover1 on November 14, 2017, 06:34:27 pm
Can I use an 8-ohm L-Pad after a crossover for a 16-Ohm tweeter?  I've read one can only use an 8-ohm L-Pad with an 8-Ohm driver.  It seems to me that resistance is resistance and I'd just have to factor the different resistances (tweeter and L-Pad) when designing the X-Over.  What am I missing?

I don't suppose there's a nifty calculator for all of this somewhere?  I've found some web-based calcs, but none include the ability to add an L-Pad to the equation.
Title: Re: Crossover, L-Pad, Impedence, Resistance question
Post by: Paul G. OBrien on November 14, 2017, 06:49:36 pm
Are you talking about one of those continuously variable potentiometer type L-pads? If so then what power levels are we talking about here? These rotary devices are really only good for very small power levels.. like 50w or less regardless what power rating they have, if you need more than that you should be building a fixed L-pad from power resistors. It is OK to use what you have to determine the required resistance for each leg of the L-pad.
Title: Re: Crossover, L-Pad, Impedence, Resistance question
Post by: Jeff Schoonover1 on November 14, 2017, 07:12:33 pm
Are you talking about one of those continuously variable potentiometer type L-pads?
Yes, an 8-ohm model from Eminence.
If so then what power levels are we talking about here?
The amp driving this unit (a mid-high box) will be 2-3KW, but I expect only a tiny fraction of that will be in use most of the time.  I have run the same speakers before with a crossover built with components made for less than 200VDC.  The tweeter itself is rated for 100 Watts RMS.  I used the same 100W L-Pad for a short time, which didn't blow up, but I took it out as I was paranoid.  I did this with slightly less power, but again, I don't think I will ever come close to the maximum available.
Someone else designed that X-over, but I think they did it wrong - it sounded poor, and the pad didn't fix it.  This time, I want to do it from scratch and be sure.

These rotary devices are really only good for very small power levels.. like 50w or less regardless what power rating they have, if you need more than that you should be building a fixed L-pad from power resistors. It is OK to use what you have to determine the required resistance for each leg of the L-pad.
  So use the L-pad, to find a good level, and then swap it out with resistors in series and parallel?  What wattage should they be able to withstand?
Title: Re: Crossover, L-Pad, Impedence, Resistance question
Post by: Ivan Beaver on November 14, 2017, 07:51:29 pm
Yes, an 8-ohm model from Eminence. The amp driving this unit (a mid-high box) will be 2-3KW, but I expect only a tiny fraction of that will be in use most of the time.  I have run the same speakers before with a crossover built with components made for less than 200VDC.  The tweeter itself is rated for 100 Watts RMS.  I used the same 100W L-Pad for a short time, which didn't blow up, but I took it out as I was paranoid.  I did this with slightly less power, but again, I don't think I will ever come close to the maximum available.
Someone else designed that X-over, but I think they did it wrong - it sounded poor, and the pad didn't fix it.  This time, I want to do it from scratch and be sure.
  So use the L-pad, to find a good level, and then swap it out with resistors in series and parallel?  What wattage should they be able to withstand?
The idea behind a L pad is to provide a constant impedance load to the xover as the levels are adjusted.

The "simple answer" is yes, you can use a 8 ohm lpad with a 16 ohm driver.

HOWEVER, you MUST take this mismatch into account in the crossover design.

It depends on how you are doing the "Design".

If you are "designing" using simple xover calculators, the results will be off.

If you take into account the load impedance by measuring the actual impedance and amplitude response of the lpad and the driver, then enter that information into a program that will calculate the response, it can work.

It might be easier to simply measure the response/impedance of the woofer and add that to the HF response, and adjust/design the xover for the total response the way you need it, and don't even worry about the lpad.

As usual, it depends, on how the "design" is being done.
Title: Re: Crossover, L-Pad, Impedence, Resistance question
Post by: Jeff Schoonover1 on November 14, 2017, 08:54:51 pm
I knew this could get complex, LOL.  Maybe I should just say what I have, and what I'm thinking, and ask for suggestions? 
I have a JBL 2426 horn, with which I can use either an 8ohm or 16ohm diaphragm.  Both on paper (110-117db, depending on horn) and in real life, it's easily 10-15 (or more) dB more efficient than the woofers, which are -
Two Eminence Delta 10A 10" speakers at 8 ohms each (so, 4 or 16 ohms load) with a sensitivity of 98.8dB each, (about 102dB or so with both.)

 The woofer's response is flat-ish until it begins to rise at about 900Hz until their peak at 2.4K, where they drop off sharply.
The tweeter's response is dependent on the horn used. But basically, its curve begins at about 900Hz and rises gradually over about a 10dB spread to about 12.5K where it drops off.
I was looking at different alignments (BW, L-R, 2nd, 3rd, 4th order, etc.). Then I saw that I already have the components to make a simple low pass with a 1st order BW at 850Hz.  For the high pass, a 1st order BW at 2K.  Considering the way the components behave, I'm thinking the large gap will be compensated by the much louder tweeter, the efficiency bump of the woofers over 900Hz, and the very gradual 6dB slopes of 1st order filters. 

I could even just use equalization on the higher tweeter frequencies if need be, and not even bother with an L-pad... 

With what I had before using these speakers, there was a jump around about 1.5-2.5K anyway which was unpleasant.

Thoughts?
Title: Re: Crossover, L-Pad, Impedence, Resistance question
Post by: Ivan Beaver on November 14, 2017, 09:25:47 pm
I knew this could get complex, LOL.  Maybe I should just say what I have, and what I'm thinking, and ask for suggestions? 
I have a JBL 2426 horn, with which I can use either an 8ohm or 16ohm diaphragm.  Both on paper (110-117db, depending on horn) and in real life, it's easily 10-15 (or more) dB more efficient than the woofers, which are -
Two Eminence Delta 10A 10" speakers at 8 ohms each (so, 4 or 16 ohms load) with a sensitivity of 98.8dB each, (about 102dB or so with both.)

 The woofer's response is flat-ish until it begins to rise at about 900Hz until their peak at 2.4K, where they drop off sharply.
The tweeter's response is dependent on the horn used. But basically, its curve begins at about 900Hz and rises gradually over about a 10dB spread to about 12.5K where it drops off.
I was looking at different alignments (BW, L-R, 2nd, 3rd, 4th order, etc.). Then I saw that I already have the components to make a simple low pass with a 1st order BW at 850Hz.  For the high pass, a 1st order BW at 2K.  Considering the way the components behave, I'm thinking the large gap will be compensated by the much louder tweeter, the efficiency bump of the woofers over 900Hz, and the very gradual 6dB slopes of 1st order filters. 

I could even just use equalization on the higher tweeter frequencies if need be, and not even bother with an L-pad... 

With what I had before using these speakers, there was a jump around about 1.5-2.5K anyway which was unpleasant.

Thoughts?
Here is the basic problem with using "off the shelf" passive crossovers.

They DO NOT take into account the ACTUAL impedance curve, or the freq response.

They assume both are flat and constant.

It would be VERY VERY rare that any designed (based on actual impedance and amplitude) would meet any standard slope or type (butterworth etc) curve.

I am not sure how you would design a passive 1st order xover of any particular type, as you say BW.  A 1st order rolloff via a capacitor is what it is, it has no particular shape.  If it did, what would be different if you choose to make it a 1st order LR or bessel?  With one part, the only thing you can do is change freq.

But be ready for the math not to work, unless the impedance and amplitude are flat.  In both cases, that would be rare around xover freq for a HF driver.

Often the math is actually a octave or two off to make it work properly

Title: Re: Crossover, L-Pad, Impedence, Resistance question
Post by: Paul G. OBrien on November 14, 2017, 09:29:23 pm
Thoughts?

Biamp it.

1st order crossovers are not an option in Pro audio IMO, one of the worst things you can do is cross a CD too low or not steep enough, they just get ugly sounding and you risk blowing diaphrams.

So back to the biamping thing. Do you have an active crossover and a couple amp channels you could use to at least find out what minimum crossover points and slopes you need to use to keep everything alive and then sounding the best it can? With that info you could then build a passive crossover to match if  it is financially fesable... as in not more expensive than buying a second hand analog crossover and a small amp to drive the horns directly.

Title: Re: Crossover, L-Pad, Impedence, Resistance question
Post by: Stephen Kirby on November 15, 2017, 12:27:51 am
As folks have said, the L-Pad itself is not the problem.  Most crossovers use fixed attenuation on the HF drivers although there are a few with switchable HF levels and things like bass guitar cabinets and very old "hi-fi" speakers that use variable attenuation.

Passive crossover design is a very complex art.  As Ivan said, the reactance (not resistance because it isn't static with different frequencies) of a speaker system (note I said system as the cabinet and other things hooked to the crossover play a part) is a complex curve.  Creating filters that balance things out is complex.  Then if you really want it to be good, you have to balance out the phase shift of those filters (or create additional phase compensation with other out of band filters) against the reactance of the drivers in the cabinet to try and get reasonable phase response.

For an amateur it's probably easier to get a simple DSP and start learning about this stuff when there are less levers to push or at least the DSP can compensate for some things while you push some levers.
Title: Re: Crossover, L-Pad, Impedence, Resistance question
Post by: Chris Grimshaw on November 15, 2017, 03:26:43 am
Biamp it.

1st order crossovers are not an option in Pro audio IMO

+1.

For what it's worth, a second-order highpass only gives constant excursion as frequency decreases. For high-power PA use, you want excursion to decrease quickly as frequency goes down, so compression drivers should get a 4th order crossover IMO. 3rd order minimum.

Chris
Title: Re: Crossover, L-Pad, Impedence, Resistance question
Post by: Jeff Schoonover1 on November 15, 2017, 09:49:39 am
Here is the basic problem...
be ready for the math not to work...
I think you're absolutely right.  Regarding crossovers, and for whatever reason, real-world doesn't work out like the math says it will in my experience.
Biamp it. 
Do you have an active crossover and a couple amp channels you could use to at least find out what minimum crossover points and slopes you need to use to keep everything alive and then sounding the best it can? With that info you could then build a passive crossover to match...
I do, and thought of just that when I woke up this morning.  I have a Crown with active processing which I can set up on the box and use it's active X-over.  Figure out what sounds best, and then try to build a passive once I find a good set-up?
As folks have said, the L-Pad itself is not the problem.  ...Passive crossover design is a very complex art.  As Ivan said, the reactance (not resistance because it isn't static with different frequencies) of a speaker system (note I said system as the cabinet and other things hooked to the crossover play a part) is a complex curve.  Creating filters that balance things out is complex.  Then if you really want it to be good, you have to balance out the phase shift of those filters (or create additional phase compensation with other out of band filters) against the reactance of the drivers in the cabinet to try and get reasonable phase response.
This is another worry of mine. That the active X-over I use from my amp will not match the passive set-up I try to create based upon it.  If I understand correctly, shifting impedance, reactance, and sensitivity will have much more of an effect when using a passive unit than with an active one?
[/quote]
Also, yes, I can use an amp in two channel with active X-over, but there will be times where I want to use a more compact rig and use just one side of an amp to drive this entire cabinet.
Title: Re: Crossover, L-Pad, Impedence, Resistance question
Post by: Jeff Schoonover1 on November 15, 2017, 09:51:40 am
Has anyone messed around with Windows Passive Crossover Designer (WinPCD)?  On first look, it seems to have a solution for about everything we've talked about, as well as off-axis issues, phasing, etc.
Title: Re: Crossover, L-Pad, Impedence, Resistance question
Post by: Ivan Beaver on November 15, 2017, 01:13:23 pm
I think you're absolutely right.  Regarding crossovers, and for whatever reason, real-world doesn't work out like the math says it will in my experience.
Actually the math works totally fine.

The problem is that people don't use the right numbers in the right places.

For example, they "assume" the impedance is 8 ohms.  But what if it is actually 30 ohms at the particular xover freq? 

You MUST consider the curve-NOT a single simple number.

Then you come up with different numbers.  OH BTW, that 30 ohms is not constant, a half octave lower it is actually 7 ohms, so now the "math" is different.

I am working on a cabinet now that is "rated" at 8 ohms, but around xover the actual impedance is just over 100 ohms.

And what if the amplitude has a big hump in it around xover (like most do).  No you have to electrically cross it over higher, in order to get a good acoustical xover.

All the real crossover calculators (we use LSPCAD), simply use the math.  HOWEVER, they have all the numbers to put into the right places, so it works very well.

It is yet another, trying to put a simple single number on a complex problem.
Title: Re: Crossover, L-Pad, Impedence, Resistance question
Post by: Jeff Schoonover1 on November 15, 2017, 06:40:52 pm
... people don't use the right numbers in the right places.
Or omit needed values.  As in your example, not factoring impedance and frequency response of drivers, etc. could be a mistake.  All I ever did was use a simple calculator or chart, without regard to these other important elements.
Title: Re: Crossover, L-Pad, Impedence, Resistance question
Post by: Marc Sibilia on November 16, 2017, 12:17:12 pm
Has anyone messed around with Windows Passive Crossover Designer (WinPCD)?  On first look, it seems to have a solution for about everything we've talked about, as well as off-axis issues, phasing, etc.

I have used WinPCD to design a three way for my living room and find it very useful and pretty easy to use.  I prototype my speaker by building the cabinet, and installing the drivers.

Then, I adjust DSP crossovers until I get the measurements looking good and do final voicing by ear in my listening room.  Then I measure each driver individually in the cabinet with the DSP crossover active and each driver individually with no crossover (at low level on the tweeter) without moving the microphone or the speaker.  Then I measure the total response with the crossovers and all drivers active, again without moving anything.  Finally, I do impedance measurements of each driver in the cabinet.

With these measurements, I then proceed to design a crossover that matches the predicted response of the speaker in room with passive crossover to the actual measured response in room of the speaker with DSP crossover.

The key is that the measurement and the predicted response may not look perfect because of room interactions, but it will match the sound of the DSP speaker that you have spent a week playing with to get the sound right.  It is so much easier to A/B different crossovers using DSP than passives that I wouldn't do it any other way.

It works very well.  I set up the two speakers adjacent and touching (one active with DSP and one passive) and listen.  The passive design sounds very nearly identical to the DSP speaker.  There was a bigger difference in voicing by exchanging the speakers left and right than between the passive and the active crossover.

If I were doing a PA speaker with a passive crossover (I'll be building some coax wedge monitors soon), I would do my measurements and voicing outdoors rather than in my listening room.

Marc
Title: Re: Crossover, L-Pad, Impedence, Resistance question
Post by: Jeff Schoonover1 on November 19, 2017, 10:39:09 am
The key is that the measurement and the predicted response may not look perfect because of room interactions ...
It is so much easier to A/B different crossovers using DSP than passives that I wouldn't do it any other way.
I have been using the box with a Crown XTi 4K. Using that active X-over at LR24 1.3K, it's sounding better than the passive one ever did. I also am able to run the woofers at 4-ohms, to help overcome the efficiency differences with the tweeter. Additionally, I'm able to use EQ settings via my analyzer software to flatten out the peaks quite a bit. I had wanted to have the flexibility of using the other channel for lows on a compact rig, which is why I was wanting to go passive.  But this is sounding too good to ignore.
I would do my measurements and voicing outdoors rather than in my listening room.
I have found outdoors and away from reflections to be hugely valuable as well.
Title: Re: Crossover, L-Pad, Impedence, Resistance question
Post by: Jeff Bankston on November 19, 2017, 02:27:37 pm
Biamp it.
YES !
Title: Re: Crossover, L-Pad, Impedence, Resistance question
Post by: Jeff Schoonover1 on November 19, 2017, 03:55:38 pm
YES !
That's what I'm doing, and it sounds noticeably better than the passive one ever did.

So then, I had a thought.  What if I put TWO Speakon inputs on the back of my box instead of one.  Then somehow make a switch that would make one as a two-pole input which would go to the crossover.  Throwing the switch and connecting to the other would be a 4-pole which would disconnect the crossover and switch in wiring straight to the speakers.  Best of both worlds. 
Not sure they make a switch with that many poles on it though.
Title: Re: Crossover, L-Pad, Impedence, Resistance question
Post by: Paul G. OBrien on November 19, 2017, 04:18:44 pm
You don't need 2 speakon connectors to do this but they can be useful other ways. This is what you need electrically.. 1 DPDT switch for the LF section and another for the HF.

(http://)
Title: Re: Crossover, L-Pad, Impedence, Resistance question
Post by: Jeff Schoonover1 on November 19, 2017, 08:53:41 pm
You don't need 2 speakon connectors to do this but they can be useful other ways. This is what you need electrically.. 1 DPDT switch for the LF section and another for the HF.
Nicely done!

I think I see what you mean - this could be done with one 4-pole connector (?) And the same standard 4-conductor cable.   Position 'A' could use 1+ and 2+ only for when it's mono, lifting the negatives to prevent a short.  Position 'B' would switch in 1-/2- and re-route directly to drivers, bypassing the crossover.
Then I wouldn't even need to use a different cable (or adapter at the amp-side) for mono vs when it's bi-amped.

I wonder if it could be done with one switch though.  Seems like it might get confusing if someone other than me sets up the rig? Or stupid me would forget to switch both and think I'd blown a driver, LOL

Title: Re: Crossover, L-Pad, Impedence, Resistance question
Post by: Paul G. OBrien on November 19, 2017, 09:39:53 pm

I wonder if it could be done with one switch though.

Yes, technically you really only need to switch the positive connections to take the crossover out of the circuit.
Title: Re: Crossover, L-Pad, Impedence, Resistance question
Post by: Ivan Beaver on November 20, 2017, 07:26:23 am
Yes, technically you really only need to switch the positive connections to take the crossover out of the circuit.
Assuming the + of the HF is not hooked to the common of the xover.

And also assuming the crossover is a normal parallel type, and not a series type.

Yeah, the details matter.
Title: Re: Crossover, L-Pad, Impedence, Resistance question
Post by: Jeff Schoonover1 on November 20, 2017, 11:21:01 am
Yes, technically you really only need to switch the positive connections to take the crossover out of the circuit.
I think the X-over + terminals must be taken completely off all drivers, too.  Otherwise, the X-Over will still affect the drivers.  I think I can picture doing this with a 4-pole, double throw switch.  I'll have to draw it out. 
Title: Re: Crossover, L-Pad, Impedence, Resistance question
Post by: Tim McCulloch on November 20, 2017, 11:20:19 pm
I think the X-over + terminals must be taken completely off all drivers, too.  Otherwise, the X-Over will still affect the drivers.  I think I can picture doing this with a 4-pole, double throw switch.  I'll have to draw it out.

Yes.  Completely disconnecting the transducers from the passive crossover is the best practice.
Title: Re: Crossover, L-Pad, Impedence, Resistance question
Post by: Ivan Beaver on November 21, 2017, 07:28:24 am
I think the X-over + terminals must be taken completely off all drivers, too.  Otherwise, the X-Over will still affect the drivers.  I think I can picture doing this with a 4-pole, double throw switch.  I'll have to draw it out.
But if you look at the diagram shown, there is no path for the other parts to affect the signal.

Yes they are still hooked up, but they aren't doing anything, so it is fine.

It is only when there is a path for the passive parts that it becomes an issue
Title: Re: Crossover, L-Pad, Impedence, Resistance question
Post by: Jeff Schoonover1 on November 21, 2017, 12:14:40 pm
Well, here's a clean drawing, but without X-over removed from circuit.  I think I'd need an 8-pole to simultaneously lift the crossover connections when in Bi-Amp mode...  I'd really like to do this with one, not two switches.
Title: Re: Crossover, L-Pad, Impedence, Resistance question
Post by: David Sturzenbecher on November 21, 2017, 01:11:54 pm
Well, here's a clean drawing, but without X-over removed from circuit.  I think I'd need an 8-pole to simultaneously lift the crossover connections when in Bi-Amp mode...  I'd really like to do this with one, not two switches.


Could always do something like this....
(https://uploads.tapatalk-cdn.com/20171121/559ddb8bb8fa0960bded40724aba0907.jpeg)


Sent from my iPad using Tapatalk Pro
Title: Re: Crossover, L-Pad, Impedence, Resistance question
Post by: Tim McCulloch on November 21, 2017, 03:18:55 pm
Well, here's a clean drawing, but without X-over removed from circuit.  I think I'd need an 8-pole to simultaneously lift the crossover connections when in Bi-Amp mode...  I'd really like to do this with one, not two switches.

You're overthinking this, the 4PDT you proposed will work just fine.

The common terminals of the switch go to the transducer; one side of the DT goes to the crossover pass band outputs, the other side of the DT goes to your HI/LOW input jacks.
Title: Re: Crossover, L-Pad, Impedence, Resistance question
Post by: Chris Grimshaw on November 21, 2017, 03:52:10 pm
If you happen to keep the crossover parts connected to the driver(s), interesting things can happen to the impedance curve - you could end up with a near-short at some frequencies. A capacitor across a midbass driver, for instance, would tend towards a short at HF. That's gonna make a lot of amps unhappy.

Chris
Title: Re: Crossover, L-Pad, Impedence, Resistance question
Post by: Mac Kerr on November 21, 2017, 04:16:22 pm
You're overthinking this, the 4PDT you proposed will work just fine.

The common terminals of the switch go to the transducer; one side of the DT goes to the crossover pass band outputs, the other side of the DT goes to your HI/LOW input jacks.

That will switch the transducers between their 2 sources, but it leaves the input to the crossover in circuit as an additional load on the amplifier when in biamp mode. If this is not a problem a single 4PDT is the way to go. Be sure it has a sufficient AC current rating.

Mac
Title: Re: Crossover, L-Pad, Impedence, Resistance question
Post by: Mac Kerr on November 21, 2017, 04:18:04 pm
Well, here's a clean drawing, but without X-over removed from circuit.  I think I'd need an 8-pole to simultaneously lift the crossover connections when in Bi-Amp mode...  I'd really like to do this with one, not two switches.

Shouldn't the minus side of the input to the crossover come from the #1 minus contact on the speakon?

Mac
Title: Re: Crossover, L-Pad, Impedence, Resistance question
Post by: Ivan Beaver on November 21, 2017, 04:18:40 pm
If you happen to keep the crossover parts connected to the driver(s), interesting things can happen to the impedance curve - you could end up with a near-short at some frequencies. A capacitor across a midbass driver, for instance, would tend towards a short at HF. That's gonna make a lot of amps unhappy.

Chris
Totally agreed.

But with only 1 leg (common) hooked to the crossover, there is no path for any "weirdness" happen.
Title: Re: Crossover, L-Pad, Impedence, Resistance question
Post by: Tim McCulloch on November 21, 2017, 04:35:24 pm
That will switch the transducers between their 2 sources, but it leaves the input to the crossover in circuit as an additional load on the amplifier when in biamp mode. If this is not a problem a single 4PDT is the way to go. Be sure it has a sufficient AC current rating.

Mac

We added separate NL4 jacks for biamp operation.  While this was for a legacy product I'd do the same thing again.
Title: Re: Crossover, L-Pad, Impedence, Resistance question
Post by: Jeff Schoonover1 on November 21, 2017, 06:36:29 pm
Could always do something like this....
Yes, I suppose ganged switches are made.  I'd need another 4-pole double throw with the drawing above.
You're overthinking this, the 4PDT you proposed will work just fine.
The common terminals of the switch go to the transducer; one side of the DT goes to the crossover pass band outputs, the other side of the DT goes to your HI/LOW input jacks.
I *think* I kinda get what you're proposing, but what connects to the inputs of the crossover?  Also, having anything connected to the positive outs of the X-over will put it into the circuit as the others have said?
Shouldn't the minus side of the input to the crossover come from the #1 minus contact on the speakon?
No, when used in Mono mode, a crown amp uses 1+ and 2+ as positive and negative, respectively. The negatives aren't used.
We added separate NL4 jacks for biamp operation.  While this was for a legacy product I'd do the same thing again.
That might be something but in that case, we still need a switch.  I think one 4-pole input and one switch is do-able, I'm just having trouble visualizing.

I think I'll have to add two more poles in order to switch the driver positives out of the circuit in my drawing...
Title: Re: Crossover, L-Pad, Impedence, Resistance question
Post by: Mac Kerr on November 21, 2017, 08:46:32 pm
No, when used in Mono mode, a crown amp uses 1+ and 2+ as positive and negative, respectively. The negatives aren't used.

I'm aware of this, but I am totally opposed to running amps in bridged mono mode so I ignored that possibility. The reason most people are impressed by how much louder their system sounds in bridged mode is because with few exceptions bridging the amp gives you 6dB of input gain relative to stereo mode. This does not impact maximum output, it is only input gain. It is the same as turning up the volume. The loss in flexibility in the loads you can drive and the loss of an amp channel for other uses are too high a price to pay.

Mac
Title: Re: Crossover, L-Pad, Impedence, Resistance question
Post by: Chris Grimshaw on November 22, 2017, 03:34:06 am
I'm aware of this, but I am totally opposed to running amps in bridged mono mode so I ignored that possibility. The reason most people are impressed by how much louder their system sounds in bridged mode is because with few exceptions bridging the amp gives you 6dB of input gain relative to stereo mode. This does not impact maximum output, it is only input gain. It is the same as turning up the volume. The loss in flexibility in the loads you can drive and the loss of an amp channel for other uses are too high a price to pay.

Mac

Mac,
I've no idea where you got that from, but it ain't right.

Bridging an amplifier gives you 6dB more output from the amplifier.
You're effectively putting the two channels of the amplifier in series, doubling the voltage output.

Since P=V*V/R, when you double V (voltage), P (power) quadruples. That's your +6dB.

Sure, there are downsides. Each side of the amplifier effectively sees half the load impedance, so the amp is working harder. Pretty much anything will run bridged into 8ohm, though - each side is seeing 4ohm, which is fine. If you want to bridge into 4ohm, each side is seeing 2ohm, which is a tough life. A lot of amps will do it for "easy" program material, but if it was EDM at the limiters all night I'd have to have something really durable to start the gig bridged into 4ohm (or on 2ohm/ch). MA5k as a minimum.

I'd also argue there's an increase in system flexibility. Bridging amps means you've got spare channels that are currently in use. You can free them up by switching to normal output. You'll lose some output, sure, but you've got your emergency amplifier channels right there.

Chris
Title: Re: Crossover, L-Pad, Impedence, Resistance question
Post by: Ivan Beaver on November 22, 2017, 08:51:32 am
Mac,
I've no idea where you got that from, but it ain't right.

Bridging an amplifier gives you 6dB more output from the amplifier.
You're effectively putting the two channels of the amplifier in series, doubling the voltage output.

Since P=V*V/R, when you double V (voltage), P (power) quadruples. That's your +6dB.




Chris
Not exactly.

The total output capability of the amp DOES NOT CHANGE when you go bridged.

ONLY the ability to deliver greater voltage into certain impedance loads.

It is totally load dependent.

Another good case of "it depends".

Over the years I have seen all kinds of people tearing up gear because they don't understand bridging, how it works, how to hook it up etc.

The most common is when people hook up speakers normally, and then simply flip the "bridge" switch, thinking they are getting more power.

But in reality it is a trip to the recone shop, because they overdrive the speakers due to the cancellation.
Title: Re: Crossover, L-Pad, Impedence, Resistance question
Post by: Jeff Schoonover1 on November 22, 2017, 09:35:52 am
I'm aware of this, but I am totally opposed to running amps in bridged mono mode so I ignored that possibility. The reason most people are impressed by how much louder their system sounds in bridged mode is because with few exceptions bridging the amp gives you 6dB of input gain relative to stereo mode. This does not impact maximum output, it is only input gain. It is the same as turning up the volume. The loss in flexibility in the loads you can drive and the loss of an amp channel for other uses are too high a price to pay.
*from a curious, not condescending perspective :) * I'd like to see the math on that.  AFAIK, with the input signal being equal, if you quad the power you will attain a 6dB gain boost. (most don't quad the power either, it's more like X2 or X3 for 3-4 dB)  This is hardly a reason to use bridging for a volume increase.   But (aside from extra distortion) it is absolutely valuable for driving loads with higher power handling to have more headroom.  I am aware of the input summing feature, and if used, it provides a 6dB boost in the input to the channel.  It won't give more output, it just reduces input headroom, so I understand what you're saying there, but it can be useful in attaining more gain on a lower level signal to get the most out of the amp.

I also agree with you that, in this instance, it is far better to Bi-amp, (using an active X-over) rather than using a mono input with passive.  Very noticeably, actually, as I just did both.  BUT - you may have missed that I'm trying to gain flexibility out of my system.  When I can use my whole rig, there is one amp for lows (2x18, bridged into 4-ohms) and this amp for highs, which will be bi-amped at 16-ohms into the highs, and 4-ohms (maybe 16, haven't totally decided) into the lows, as you say.  Other times though, I will only use one amp.  One side will drive a single 15", and the other will drive this box, hence the passive x-over.  Sacrificing volume and cutting the physical size of the rig by 2/3. No, it won't sound as good, but in those instances, a drop in clarity is acceptable if I'm working on a small club stage or in low volume.
The total output capability of the amp DOES NOT CHANGE when you go bridged.  ONLY the ability to deliver greater voltage into certain impedance loads.
Over one channel - but if you're adding another channel, don't you also increase the *available* voltage for the load?  I get that the overall power availability from the whole amp doesn't change, but each channel divides to half of the input potential from the wall in stereo mode, doesn't it?
I have seen all kinds of people tearing up gear because they don't understand bridging ... ... when people hook up speakers normally, and then simply flip the "bridge" switch, But in reality it is a trip to the recone shop, because they overdrive the speakers due to the cancellation.
I have never once done this.  I don't even know what you're talking about [wink]
Title: Re: Crossover, L-Pad, Impedence, Resistance question
Post by: Mac Kerr on November 22, 2017, 09:36:28 am
I'd also argue there's an increase in system flexibility. Bridging amps means you've got spare channels that are currently in use. You can free them up by switching to normal output. You'll lose some output, sure, but you've got your emergency amplifier channels right there.

Chris

Except your racks and speakers are wired in a way that means you can't use those channels.

Mac
Title: Re: Crossover, L-Pad, Impedence, Resistance question
Post by: Jeff Schoonover1 on November 22, 2017, 10:37:25 am
The new drawing.  The only 6-pole double-throw switches I can find are on Mouser for some obscene amount, like $500 or something.
It looks like I'm stuck using two switches - a 4-pole, and a 2-pole unless someone knows a better way while still using one 4-pole Speakon input...
The good news is that it can be wired so that if some goofball (me) forgets to flip BOTH switches, nothing bad will happen, there just won't be sound.
Title: Re: Crossover, L-Pad, Impedence, Resistance question
Post by: Tim McCulloch on November 22, 2017, 11:00:43 am
I'm aware of this, but I am totally opposed to running amps in bridged mono mode so I ignored that possibility. The reason most people are impressed by how much louder their system sounds in bridged mode is because with few exceptions bridging the amp gives you 6dB of input gain relative to stereo mode. This does not impact maximum output, it is only input gain. It is the same as turning up the volume. The loss in flexibility in the loads you can drive and the loss of an amp channel for other uses are too high a price to pay.

Mac

^THIS ^^RIGHT ^^^HERE

Staying in 2 channel mode also makes for fewer "hey, why isn't this working" interruptions to the work day.
Title: Re: Crossover, L-Pad, Impedence, Resistance question
Post by: Tim McCulloch on November 22, 2017, 11:02:26 am
Mac,
I've no idea where you got that from, but it ain't right.

Bridging an amplifier gives you 6dB more output from the amplifier.
You're effectively putting the two channels of the amplifier in series, doubling the voltage output.

Since P=V*V/R, when you double V (voltage), P (power) quadruples. That's your +6dB.

Sure, there are downsides. Each side of the amplifier effectively sees half the load impedance, so the amp is working harder. Pretty much anything will run bridged into 8ohm, though - each side is seeing 4ohm, which is fine. If you want to bridge into 4ohm, each side is seeing 2ohm, which is a tough life. A lot of amps will do it for "easy" program material, but if it was EDM at the limiters all night I'd have to have something really durable to start the gig bridged into 4ohm (or on 2ohm/ch). MA5k as a minimum.

I'd also argue there's an increase in system flexibility. Bridging amps means you've got spare channels that are currently in use. You can free them up by switching to normal output. You'll lose some output, sure, but you've got your emergency amplifier channels right there.

Chris

Bullshit.

Where does the extra power come from, the Amplifier Fairie?  Does she swoop in and leave a bigger PSU and extra output transistors under the pillow?
Title: Re: Crossover, L-Pad, Impedence, Resistance question
Post by: Chris Grimshaw on November 22, 2017, 11:13:26 am
Bullshit.

Where does the extra power come from, the Amplifier Fairie?  Does she swoop in and leave a bigger PSU and extra output transistors under the pillow?

Are you for real?

You double the voltage swing by bridging. If the amplifier can source the current, you'll get 4x the power.

When bridging into 8ohm, that's the case for any amp worth having in the rack. Bridging into 4ohm, as I've noted, becomes more difficult as you're driving down at 2ohm/ch which some amps aren't always happy about.

ProSoundWeb, remember?
Even if you're gonna disagree with me, lets keep it vaguely professional and polite.

Chris
Title: Re: Crossover, L-Pad, Impedence, Resistance question
Post by: Jeff Schoonover1 on November 22, 2017, 11:20:23 am
^THIS ^^RIGHT ^^^HERE
Staying in 2 channel mode also makes for fewer "hey, why isn't this working" interruptions to the work day.
Agreed - to a point.  Always willing to learn though, convince me further?  My low box is 2x18" with each driver at 3,600W program power, 1,800 AES RMS.  Right now I'm set up with a dedicated Crown XTi-6K, set at 6KW bridged into 4-ohms. I can also stereo at 1.2KW into 8-ohms.  The amp is dedicated to this box, so either or is fine.  Would Bi-Amp-ing outweigh the benefits of "more power" in bridge-mode? Will it be audibly better?
Links are fine if we don't want to get off the original topic too much.  :)
Title: Re: Crossover, L-Pad, Impedence, Resistance question
Post by: Tim McCulloch on November 22, 2017, 11:24:26 am
Agreed - to a point.  Always willing to learn though, convince me further?  My low box is 2x18" with each driver at 3,600W program power, 1,800 AES RMS.  Right now I'm set up with a dedicated Crown XTi-6K, set at 6KW bridged into 4-ohms. I can also stereo at 1.2KW into 8-ohms.  The amp is dedicated to this box, so either or is fine.  Would Bi-Amp-ing outweigh the benefits of "more power" in bridge-mode? Will it be audibly better?
Links are fine if we don't want to get off the original topic too much.  :)

Think about this:  how does the amplifier suddenly have MORE power?  POWER is not voltage.
Title: Re: Crossover, L-Pad, Impedence, Resistance question
Post by: Jeff Schoonover1 on November 22, 2017, 11:27:52 am
... you're driving down at 2ohm/ch
  Not sure about this?  One, two, or fifty speakers doesn't matter, a 4-ohm load is a 4-ohm load, right?
Title: Re: Crossover, L-Pad, Impedence, Resistance question
Post by: Jeff Schoonover1 on November 22, 2017, 11:29:40 am
Think about this:  how does the amplifier suddenly have MORE power?  POWER is not voltage.
Agreed, and understood.  But, doesn't an amp only using one channel of two possible channels, only use half of the power available to both channels?
Also, isn't a 4-ohm load twice as efficient as an 8-ohm load?  Meaning, if I use each channel independently, and only drive one driver, my impedance is 8-ohms per channel.  Parallel-ing them halves the impedance, making the overall load of the box more efficiently driven?
Title: Re: Crossover, L-Pad, Impedence, Resistance question
Post by: Tim McCulloch on November 22, 2017, 11:34:07 am
Agreed, and understood.  But, doesn't an amp only using one channel of two possible channels, only use half of the power available to both channels?

You either use the other channel, or not.  But the amplifier has only so much current it can deliver into a particular load regardless of voltage swing.  This is EXACTLY what Mac is saying - you get what appears to be a +6dB increase, but it's in the gain (or input sensitivity, if you will) when bridged.  The amplifier isn't automagically capable of additional current and that is exactly the notion that we've spent 20 years dispelling on the LAB and its subforums.
Title: Re: Crossover, L-Pad, Impedence, Resistance question
Post by: Tim McCulloch on November 22, 2017, 11:37:00 am
Are you for real?

You double the voltage swing by bridging. If the amplifier can source the current, you'll get 4x the power.

When bridging into 8ohm, that's the case for any amp worth having in the rack. Bridging into 4ohm, as I've noted, becomes more difficult as you're driving down at 2ohm/ch which some amps aren't always happy about.

ProSoundWeb, remember?
Even if you're gonna disagree with me, lets keep it vaguely professional and polite.

Chris

If you can't take the heat, disconnect the load...

Dispelling bullshit has been a major part of these forums from the very beginning.  Don't stub your toe on the Dave Stevens (LAB founder) Anvil of Reality.
Title: Re: Crossover, L-Pad, Impedence, Resistance question
Post by: Jeff Schoonover1 on November 22, 2017, 11:39:34 am
You either use the other channel, or not.  But the amplifier has only so much current it can deliver into a particular load regardless of voltage swing.  This is EXACTLY what Mac is saying - you get what appears to be a +6dB increase, but it's in the gain (or input sensitivity, if you will) when bridged.  The amplifier isn't automagically capable of additional current and that is exactly the notion that we've spent 20 years dispelling on the LAB and its subforums.
Do you have a link? I understand that TOTAL amount of current an amp can deliver remains the same.  But think of it this way, If you're only using half of the amp, aren't you leaving the other half on the table?  My understanding is that the potential current it can deliver is divided into half - one half for each channel?  Doesn't the channel limit its consumption before drawing more than it's been alotted?  Otherwise, there wouldn't be any 'clipping,' the channel would just keep drawing more until it blew up?  Not being stubborn, just learning.
Title: Re: Crossover, L-Pad, Impedence, Resistance question
Post by: Tim McCulloch on November 22, 2017, 12:06:36 pm
Do you have a link? I understand that TOTAL amount of current an amp can deliver remains the same.  But think of it this way, If you're only using half of the amp, aren't you leaving the other half on the table?  My understanding is that the potential current it can deliver is divided into half - one half for each channel?  Doesn't the channel limit its consumption before drawing more than it's been alotted?  Not being stubborn, just learning.

This requires logic, not a link...

As I said, if you don't use the other channel, that's up to you.  If you don't *need* the power there is no harm in "leaving power on the table" and you have a spare channel if something else takes a dive...

There are a 2 or 3 ways to power a dual transducer box, I'll use an example of subs - you can wire the loudspeakers in parallel, wire them in series, or separately power each loudspeaker (which is commonly done in Big Dog dual driver subs - more copper means less resistance and therefore less loss; the higher the load impedance also means maintaining a higher damping factor).

We've had amp racks of XTI6002 on sidefill duty (KF850 over SRX728).  I didn't plan or build these racks and the guy who did chose to bridge the sub amps, blissfully unaware that the subs go into power compression with a sustained input of around 500 Watts or so per cone.  Yesterday we started putting the sub amps back into 2 channel mode and pulling an amp out of each rack.  This also means I can now send out VerTec 4880 subs instead of SRX728 with these racks.

Perhaps a water analogy will work:  think about a fire truck that carries 2000 gallons of water.  The pressure out of the hose is very high (voltage) but when the water tank is empty, it's empty regardless of the pressure.
Title: Re: Crossover, L-Pad, Impedence, Resistance question
Post by: Ivan Beaver on November 22, 2017, 12:08:41 pm
Do you have a link? I understand that TOTAL amount of current an amp can deliver remains the same.  But think of it this way, If you're only using half of the amp, aren't you leaving the other half on the table?  My understanding is that the potential current it can deliver is divided into half - one half for each channel?  Doesn't the channel limit its consumption before drawing more than it's been alotted?  Otherwise, there wouldn't be any 'clipping,' the channel would just keep drawing more until it blew up?  Not being stubborn, just learning.
It is not simply divided up.

There is the limit each channel can deliver, current wise.  And then there is the total current available from the power supply.

Either one is the limit of a particular channel.

The voltage swing is another limit.

This voltage swing limit is the reason the "power" is not the same at all impedances.

You must describe the maximum output by using several different factors, NOT a single factor.
Title: Re: Crossover, L-Pad, Impedence, Resistance question
Post by: Ivan Beaver on November 22, 2017, 12:10:39 pm
If you can't take the heat, disconnect the load...

Dispelling bullshit has been a major part of these forums from the very beginning.  Don't stub your toe on the Dave Stevens (LAB founder) Anvil of Reality.
And that is EXACTLY why I like this forum. 

If you are wrong, you WILL be corrected, like it or not.

Yes, I have been on the receiving end of that, but I still come here.

It is a GREAT place to learn about the TRUTH, and not simply "wishes" or beliefs.
Title: Re: Crossover, L-Pad, Impedence, Resistance question
Post by: Jeff Schoonover1 on November 22, 2017, 12:24:19 pm
If you don't *need* the power there is no harm in "leaving power on the table" and you have a spare channel if something else takes a dive...

There are a 2 or 3 ways to power a dual transducer box, I'll use an example of subs - you can wire the loudspeakers in parallel, wire them in series, or separately power each loudspeaker (which is commonly done in Big Dog dual driver subs - more copper means less resistance and therefore less loss; the higher the load impedance also means maintaining a higher damping factor).

We've had amp racks of XTI6002 on sidefill duty (KF850 over SRX728).  I didn't plan or build these racks and the guy who did chose to bridge the sub amps, blissfully unaware that the subs go into power compression with a sustained input of around 500 Watts or so per cone.  Yesterday we started putting the sub amps back into 2 channel mode and pulling an amp out of each rack.  This also means I can now send out VerTec 4880 subs instead of SRX728 with these racks.

Perhaps a water analogy will work:  think about a fire truck that carries 2000 gallons of water.  The pressure out of the hose is very high (voltage) but when the water tank is empty, it's empty regardless of the pressure.
Understood, and nice analogy.  Two hoses or one hose, you will only ever get the same potential pressure (voltage) but, you can add another hose and draw that water out twice as fast (current) - assuming there are no restrictions on the other end of the hose (impedance). 
A simple analogy but effective.  It doesn't consider impedance though, the second part of my question, so this is my understanding of that.  Connect the hose to a fixed diameter pipe.  Whether you have one hose from the engine connected to a pipe or twenty hoses from the engine and connected to that same pipe, the water delivered will only be as much over time as the size of the pipe allows it to be (impedance) - given the same pressure (voltage).  Into the same size pipe (impedance,) it wouldn't matter how many hoses you connect to it.  But double the diameter of the pipe, by halving the impedance, (two speakers in parallel, instead of one) can you not then also draw more current at the same given pressure?  The Voltage never changes, and assuming the water doesn't run out, impedance regulates the current, but you can deliver roughly twice as much of it at a time....  I think(?)
P.S., I know the "pipe" changes diameter (resistance) based on frequency, but as a simple analogy)
Title: Re: Crossover, L-Pad, Impedence, Resistance question
Post by: Jeff Schoonover1 on November 22, 2017, 12:30:24 pm
It is not simply divided up.
There is the limit each channel can deliver, current wise.  And then there is the total current available from the power supply.
Either one is the limit of a particular channel.
The voltage swing is another limit.
This voltage swing limit is the reason the "power" is not the same at all impedances.
You must describe the maximum output by using several different factors, NOT a single factor.
Pretty sure I already understand this.   I'm just not good at describing my thinking...
So, anyhoo, regarding my latest drawing, Anybody know where I can get a 6-pole double throw for less than the $500 Mouser wants for one?

Also, still off topic, but still wondering which is better - two channels into two speakers, 8 ohms each, (from an XTi-6k this would be 1,200W each) or bridged into both paralleled, at 4 ohms (Resulting in a theoretical 6,000W from the amp)?  The Speakers are rated at 3.6KW program power each, and 1.8KW AES.  Power compression is not an issue...
Title: Re: Crossover, L-Pad, Impedence, Resistance question
Post by: Tim McCulloch on November 22, 2017, 12:40:02 pm
Pretty sure I already understand this.   I'm just not good at describing my thinking...
So, anyhoo, regarding my latest drawing, Anybody know where I can get a 6-pole double throw for less than the $500 Mouser wants for one?

Also, still off topic, but still wondering which is better - two channels into two speakers, 8 ohms each, (from an XTi-6k this would be 1,200W each) or bridged into both paralleled, at 4 ohms (Resulting in a theoretical 6,000W from the amp)?  The Speakers are rated at 3.6KW program power each, and 1.8KW AES.  Power compression is not an issue...

Power compression IS an issue in the real world and is why bridging a 6002 and running it in stereo with the mythical lower output results in the same long term SPL.

Consistently hit your woofers with sustained 1.8kW and tell me how long they last... but we've already had this discussion.
Title: Re: Crossover, L-Pad, Impedence, Resistance question
Post by: Jeff Schoonover1 on November 22, 2017, 12:53:20 pm
Consistently hit your woofers with sustained 1.8kW and tell me how long they last... but we've already had this discussion.
Apparently, if I read what I have about A.E.S. correctly, they will sustain that rating indefinitely. 
Title: Re: Crossover, L-Pad, Impedence, Resistance question
Post by: Chris Grimshaw on November 22, 2017, 01:16:04 pm
If you can't take the heat, disconnect the load...

Dispelling bullshit has been a major part of these forums from the very beginning.  Don't stub your toe on the Dave Stevens (LAB founder) Anvil of Reality.

Very true. It's a reason I like these fora.

However, what I'm not too keen on is you saying that my post is "bullshit" without any presentation of evidence to the contrary.

So, lets try again.

Bridging an amplifier will double the voltage across the terminals. It does this by driving one side positive, and the other side negative. When that happens, each channel "sees" half the load impedance. Imagine a centre-tap on the driver and this becomes obvious.

So, the bridged power output of the amplifier is equal to the total power output from each channel if it were driving a load of half of the impedance of the load connected.

We can see this from a variety of spec sheets. The Crown MA5000VZ, for example, is as follows:

Stereo:
8ohm - 1300w/ch
4ohm - 2000w/ch
2ohm - 2500w/ch

Bridge mono:
16ohm - 2600w
8ohm - 4000w
4ohm - 5000w

We can see that there's a little voltage sag going from 8ohm to 4ohm (stereo), so the amp won't quite manage 4x the power going from 8ohm on one channel to 8ohm bridged. It will come pretty close, though.

I could go through a thousand spec sheets and consistently draw the same conclusion. So, Tim, which bit is bullshit?

Chris
Title: Re: Crossover, L-Pad, Impedence, Resistance question
Post by: Ivan Beaver on November 22, 2017, 01:59:22 pm
Pretty sure I already understand this.   I'm just not good at describing my thinking...
So, anyhoo, regarding my latest drawing, Anybody know where I can get a 6-pole double throw for less than the $500 Mouser wants for one?


Why don't you simply use the 4 pole drawing presented earlier?  It will work perfectly fine, be less complicated to wire, a cheaper switch etc.

There is no downside
Title: Re: Crossover, L-Pad, Impedence, Resistance question
Post by: Ivan Beaver on November 22, 2017, 02:04:46 pm
Apparently, if I read what I have about A.E.S. correctly, they will sustain that rating indefinitely.
You need to reread it then.

The power ratings are for a SPECIFIC waveform, for a SPECIFIC time period, in a specific mounting condition.

Music has a crest factor several times that of the test waveform, Music can last longer than the time period, Modern music has a different waveform (specifically on the lower freq) than the test signals.

 Loudspeaker are usually mounted inside cabinets, which restricts their air flow
Title: Re: Crossover, L-Pad, Impedence, Resistance question
Post by: Ivan Beaver on November 22, 2017, 02:07:31 pm
Power compression is not an issue...
How do you know it is not an issue?  Have you measured it? 

You might be surprised at how quickly some speakers get into power compression.
Title: Re: Crossover, L-Pad, Impedence, Resistance question
Post by: Tim McCulloch on November 22, 2017, 02:23:37 pm
Very true. It's a reason I like these fora.

However, what I'm not too keen on is you saying that my post is "bullshit" without any presentation of evidence to the contrary.

So, lets try again.

Bridging an amplifier will double the voltage across the terminals. It does this by driving one side positive, and the other side negative. When that happens, each channel "sees" half the load impedance. Imagine a centre-tap on the driver and this becomes obvious.

So, the bridged power output of the amplifier is equal to the total power output from each channel if it were driving a load of half of the impedance of the load connected.

We can see this from a variety of spec sheets. The Crown MA5000VZ, for example, is as follows:

Stereo:
8ohm - 1300w/ch
4ohm - 2000w/ch
2ohm - 2500w/ch

Bridge mono:
16ohm - 2600w
8ohm - 4000w
4ohm - 5000w

We can see that there's a little voltage sag going from 8ohm to 4ohm (stereo), so the amp won't quite manage 4x the power going from 8ohm on one channel to 8ohm bridged. It will come pretty close, though.

I could go through a thousand spec sheets and consistently draw the same conclusion. So, Tim, which bit is bullshit?

Chris
The part you're not looking at correctly....

Flip this the other way - in 2 channel mode, where did the power disappear to?

Have we forgotten the law of conservation of energy?
Title: Re: Crossover, L-Pad, Impedence, Resistance question
Post by: Chris Grimshaw on November 22, 2017, 02:29:17 pm
The part you're not looking at correctly....

Flip this the other way - in 2 channel mode, where did the power disappear to?

Have we forgotten the law of conservation of energy?

When using a 2-channel amplifier in stereo mode to drive a single load, one of the channels will be disconnected.

Conservation of energy works just fine. I happen to have a Physics degree from a decent university, so CoE and the like is still just about second-nature.

Chris
Title: Re: Crossover, L-Pad, Impedence, Resistance question
Post by: Art Welter on November 22, 2017, 03:13:29 pm
So, anyhoo, regarding my latest drawing, Anybody know where I can get a 6-pole double throw for less than the $500 Mouser wants for one?

Also, still off topic, but still wondering which is better - two channels into two speakers, 8 ohms each, (from an XTi-6k this would be 1,200W each) or bridged into both paralleled, at 4 ohms (Resulting in a theoretical 6,000W from the amp)?  The Speakers are rated at 3.6KW program power each, and 1.8KW AES.  Power compression is not an issue...
Jeff,
1) Just wire your passive crossover in to a separate box and use separate high and low outputs. This will make it much easier to change components when you realize that you don't like the passive crossover ;^).
2) More headroom is "better", 3000 watts is a bit more than 3dB more than 1200 watts. With your bass guitar as a source, power compression should not be an issue unless you use a lot of compression in your signal chain.

To elaborate on Ivan's explanation of AES power, the difference in "real world" compared to the AES rating comes down to the impedance. An "8 ohm" speaker in a band limited open air test will be much higher impedance over most of the pass band than it's nominal rating, so only "draws" a fraction of it's rated power, as the voltage used would develop the rated power at 8 ohms. When in a bass reflex cabinet, at Fb, the impedance is usually right around the driver's DC resistance, consequently the driver can generally only handle about half the AES rating.

Art

Title: Re: Crossover, L-Pad, Impedence, Resistance question
Post by: Stephen Swaffer on November 22, 2017, 04:28:45 pm
Very true. It's a reason I like these fora.

However, what I'm not too keen on is you saying that my post is "bullshit" without any presentation of evidence to the contrary.

So, lets try again.

Bridging an amplifier will double the voltage across the terminals. It does this by driving one side positive, and the other side negative. When that happens, each channel "sees" half the load impedance. Imagine a centre-tap on the driver and this becomes obvious.

So, the bridged power output of the amplifier is equal to the total power output from each channel if it were driving a load of half of the impedance of the load connected.

We can see this from a variety of spec sheets. The Crown MA5000VZ, for example, is as follows:

Stereo:
8ohm - 1300w/ch
4ohm - 2000w/ch
2ohm - 2500w/ch

Bridge mono:
16ohm - 2600w
8ohm - 4000w
4ohm - 5000w

We can see that there's a little voltage sag going from 8ohm to 4ohm (stereo), so the amp won't quite manage 4x the power going from 8ohm on one channel to 8ohm bridged. It will come pretty close, though.

I could go through a thousand spec sheets and consistently draw the same conclusion. So, Tim, which bit is bullshit?

Chris

Ohms law and power equation always apply-it's simply physics.  If you double the voltage with the same current, you double the power.  If it is a pure resistive load, doubling the voltage will cause the current to double, and in that case the power will be four times.  However, this assumes an unlimited power supply with zero internal impedance.

Power amps have an internal impedance-how well this internal impedance matches the load determines how much power is transferred.  Hence the need for spec sheets.  You could test everything and do the math yourself, but doesn't it make more sense to simply match your loads to the amps impedance for each scenario?

Intersting pro's and con's for each method-might be easier if there weren't so many "right ways" to do things!
Title: Re: Crossover, L-Pad, Impedence, Resistance question
Post by: Ivan Beaver on November 22, 2017, 05:12:32 pm
Ohms law and power equation always apply-it's simply physics.  If you double the voltage with the same current, you double the power.  If it is a pure resistive load, doubling the voltage will cause the current to double, and in that case the power will be four times.  However, this assumes an unlimited power supply with zero internal impedance.

Power amps have an internal impedance-how well this internal impedance matches the load determines how much power is transferred.  Hence the need for spec sheets.  You could test everything and do the math yourself, but doesn't it make more sense to simply match your loads to the amps impedance for each scenario?

Intersting pro's and con's for each method-might be easier if there weren't so many "right ways" to do things!
There have been a couple of "perfect" amplifiers. 

By that, I mean one that doubles the power when the load is halved.

And then there are others that can drive any load, but the power is not what you would expect.

The Peavey CS800 is a good example.  It can drive any load-in fact the test after a repair is to drive a dead short into full clip until it thermals out.

The specs are 8 ohms 300 watts, 4 ohms 400 watts, 2 ohms 250 watts.

So once you get below 4 ohms, the power capacity goes down, but it will keep on working.

Even some of the modern big amps drop their power at 2 ohms.  Some are optimum at 2.67 ohms (3x8 ohm loads)

Reading and understanding specs IS an important aspect of understanding performance
Title: Re: Crossover, L-Pad, Impedence, Resistance question
Post by: Jeff Schoonover1 on November 22, 2017, 09:04:22 pm
1) Just wire your passive crossover in to a separate box and use separate high and low outputs. This will make it much easier to change components when you realize that you don't like the passive crossover ;^).
I like this idea.  Simple, except there's another thing to remember/carry.  But not like it's huge.
Why don't you simply use the 4 pole drawing presented earlier?  It will work perfectly fine, be less complicated to wire, a cheaper switch etc.
I thought we had decided that drawing won't work, as it leaves the positives connected to the X-Over outputs.  The last drawing I put up has two switches to lift those.  It's really just a 4 pole and a 2 pole switch.  I just need to be sure to throw both up, or down.  What I don't like is having two inputs.  More likely it insert the wrong cable or have the amp set incorrectly...
2) More headroom is "better", 3000 watts is a bit more than 3dB more than 1200 watts. With your bass guitar as a source, power compression should not be an issue unless you use a lot of compression in your signal chain.
More headroom = better was always my thinking too.  But reading the other guys reasons for using Bi-Amp, made a lot of sense too and got me thinking.  there's damping factor, and THD to think of too.  Also, it's easier on the amplifier, right?  I won't ever be running my low box at near its full-out capabilities, and you're right with bass guitar also - the continuous load will fluctuate considerably.  It's mostly to get low - cleanly.  I suppose the only way to really know is to try both.  I feel like 3dB is trivial when compared to less distortion, better damping, and easier loading.  Hmm...
To elaborate on Ivan's explanation of AES power, the difference in "real world" compared to the AES rating comes down to the impedance. An "8 ohm" speaker in a band limited open air test will be much higher impedance over most of the pass band than it's nominal rating, so only "draws" a fraction of it's rated power, as the voltage used would develop the rated power at 8 ohms. When in a bass reflex cabinet, at Fb, the impedance is usually right around the driver's DC resistance, consequently the driver can generally only handle about half the AES rating.
  Interesting.  So, would this also be more support for Bi-Amping?
Title: Re: Crossover, L-Pad, Impedence, Resistance question
Post by: Ivan Beaver on November 22, 2017, 09:24:39 pm
  But not like it's huge. I thought we had decided that drawing won't work, as it leaves the positives connected to the X-Over outputs.
PLEASE look at the drawing in question again.  I don't know where "we" decided that it would not work-because the Hots are still connected.

Connected to what?  Nothing that I can see.  I still say it will work just fine. If it won't PLEASE show me where or why it won't work.

The crossover is completely disconnected from the drivers and there is no path for any signal on the common.  So therefore no path for any signal.

That is the same thing I have said on SEVERAL posts in this thread.  I still stand by my comments.

BTW, you have quotes associated with the wrong people who said them.
Title: Re: Crossover, L-Pad, Impedence, Resistance question
Post by: Jeff Schoonover1 on November 22, 2017, 11:35:21 pm
PLEASE look at the drawing in question again.  I don't know where "we" decided that it would not work-because the Hots are still connected.  Connected to what?  Nothing that I can see.  I still say it will work just fine. If it won't PLEASE show me where or why it won't work.  The crossover is completely disconnected from the drivers and there is no path for any signal on the common.  So therefore no path for any signal.  That is the same thing I have said on SEVERAL posts in this thread.  I still stand by my comments.
Let's figure it out... Let's start with my original drawing in post 24.  Can you see that the positive of the woofer is always connected to the positive low output of the crossover?  The same is true with the positive on the tweeter.  It is always connected to the positive of the high output on the x-over.
BTW, you have quotes associated with the wrong people who said them.
Apologies to all concerned.
Title: Re: Crossover, L-Pad, Impedence, Resistance question
Post by: Paul G. OBrien on November 22, 2017, 11:52:53 pm
Let's figure it out... Let's start with my original drawing in post 24.  Can you see that the positive of the woofer is always connected to the positive low output of the crossover?
The same is true with the positive on the tweeter.  It is always connected to the positive of the high output on the x-over.

No they aren't. The diagram represents fullrange operation using the crossover, when the switches are thrown to bi-amp mode the only thing left connected to the crossover is the woofer common(-) which is fine because electrically it just goes straight through anyway.
Title: Re: Crossover, L-Pad, Impedence, Resistance question
Post by: Jeff Schoonover1 on November 23, 2017, 12:03:48 am
The diagram represents fullrange operation using the crossover
No, as pictured, the input to the crossover is switched out.  It shown in Biamp mode. 

The problem is that even though the x-over input is switched out, the positive poles of the drivers remain connected to the positive poles on the output of the x-over.
Title: Re: Crossover, L-Pad, Impedence, Resistance question
Post by: Paul G. OBrien on November 23, 2017, 12:27:44 am
No, as pictured, the input to the crossover is switched out.  It shown in Biamp mode. 

No it isn't!!!!!!

Here is what Bi-amp mode looks like.
Title: Re: Crossover, L-Pad, Impedence, Resistance question
Post by: Jeff Schoonover1 on November 23, 2017, 09:12:41 am
No it isn't!!!!!!
Okay, now I see where the miscommunication lies. 
Ivan Beaver and Paul G. Obrien are referring to Paul G. Obrien's drawing in post 17 (re-posted in post 70). 
I have always been referring to MY drawing in post 24.

I see what Paul did in post 17.  and what both Ivan and Paul are saying.  In post 18 though, several of us began the conversation regarding the positive leads to/from the X-Over.  This led to my drawing and the beginning of all this confusion.
I get Paul's drawing now.  I can use just one 4-pole Speakon and it will work.  I also see where I went wrong with the common connections.  There is no need to lift the common in Biamp mode, as I did with mine.
Sorry for all the confusion.
Title: Re: Crossover, L-Pad, Impedence, Resistance question
Post by: Paul G. OBrien on November 23, 2017, 11:03:39 am
Okay, now I see where the miscommunication lies. 
Ivan Beaver and Paul G. Obrien are referring to Paul G. Obrien's drawing in post 17 (re-posted in post 70). 
I have always been referring to MY drawing in post 24.

OH sorry if I was mistaken on that. Your schematic has a couple issues including the fact that the crossover inputs need to be on 1+/1- of the speakon. This allows one to use use a standard straight through 4 core speakon cable for both modes.. which is always a good idea. If you want to use a bridged amp when in fullrange mode that is fine but you need to make the wiring adjustment at the amplifier end and ideally with a patch panel in the amp rack if your amp doesn't have a dedicated bridged output connector.
Title: Re: Crossover, L-Pad, Impedence, Resistance question
Post by: Jeff Schoonover1 on November 23, 2017, 12:11:20 pm
OH sorry if I was mistaken on that. Your schematic has a couple issues including the fact that the crossover inputs need to be on 1+/1- of the speakon. This allows one to use use a standard straight through 4 core speakon cable for both modes.. which is always a good idea. If you want to use a bridged amp when in fullrange mode that is fine but you need to make the wiring adjustment at the amplifier end and ideally with a patch panel in the amp rack if your amp doesn't have a dedicated bridged output connector.
LOL, No worries.  It's funny now :)
Yes, yes, yes, and yes!  I understand completely now.
I already have made the two cables, one for bridged, and one for 4-way, I was hoping to figure out how to use the same Speakon input with either cable and here we are, thanks! 
Patch panel - A good idea, I even have one I can use for this.
Just to be sure, here's your idea using one input.
Oh - What was the 20uF cap for in your drawing? Can I not connect 1- and 2-, run the tweeter's common back through the X-over like the woofers?  Thereby eliminating that switch as well?
Title: Re: Crossover, L-Pad, Impedence, Resistance question
Post by: Ivan Beaver on November 23, 2017, 12:57:41 pm
No it isn't!!!!!!

Here is what Bi-amp mode looks like.
I'm sorry, but the input to the xover IS disconnected.

YES, the common is hooked up to the xover-but SO WHAT?  There is no path for it to go, therefore none of the components in the crossover do anything-no matter how they are wired.

The common is hooked to the neg of the woofer and the biamp input-but it doesn't matter.

It will take a lot more convincing (which I don't see how it can be done) to convince me it would nto work.

Hooking a single wire to something does nothing, ONLY if there is a path to somewhere-which there isn't in this case
Title: Re: Crossover, L-Pad, Impedence, Resistance question
Post by: Jeff Schoonover1 on November 23, 2017, 01:19:18 pm
I'm sorry, but the input to the xover IS disconnected.
I see what you're saying.  No matter what, this configuration will take two different cables to work.  A standard 4-conductor with 4-pole Speakons, and a special adapter cable for bridge mode with 1+ and 2+ at the Amp end (Negatives are not used for Bridge Mode) connected to 1POSITIVE and 1NEGATIVE on this diagram. 
The Speakon in the diagram isn't drawn the way a Speakon is in real life.  But it should work if connected by the diagram, not the number labels...  I'll fix the drawing tonight.
Title: Re: Crossover, L-Pad, Impedence, Resistance question
Post by: Ivan Beaver on November 23, 2017, 02:51:00 pm
I see what you're saying.  No matter what, this configuration will take two different cables to work.  A standard 4-conductor with 4-pole Speakons, and a special adapter cable for bridge mode with 1+ and 2+ at the Amp end (Negatives are not used for Bridge Mode) connected to 1POSITIVE and 1NEGATIVE on this diagram. 
The Speakon in the diagram isn't drawn the way a Speakon is in real life.  But it should work if connected by the diagram, not the number labels...  I'll fix the drawing tonight.
Whether or not the amp is bridged has nothing to do with switching the cabinet from passive to biamp.

They are not related to each other.
Title: Re: Crossover, L-Pad, Impedence, Resistance question
Post by: Paul G. OBrien on November 23, 2017, 05:22:03 pm
I was hoping to figure out how to use the same Speakon input with either cable and here we are, thanks! 
Patch panel - A good idea, I even have one I can use for this.

Here is a way to use a patch panel and the same standard speakon cable for both modes, no special cable or adapters necessary. NL4 panel mount speakons fan out to banana plugs that can go to separate channels or be quickly moved across the red terminals for bridged operation. If your amps don't use that config for bridged then you just have to mount another connector on the patch panel wired specifically for bridged mode And all of these can stay connected to the amp all the time, you just plug into whatever one matches the speaker config in use.


Oh - What was the 20uF cap for in your drawing?
I was wondering when we were going to get to that. It's an insurance policy protecting the CD from accidents like somebody using the wrong cable or a mixup with processing where the low frequencies or a fullrange signal is sent to the CD. It is sized to filter ULF only with a corner frequency well below the active crossover so as to minimize unwanted phase shift.


Can I not connect 1- and 2-, run the tweeter's common back through the X-over like the woofers?  Thereby eliminating that switch as well?
Depends if you are always using the same 2 channel amp for biamp operation or not, if using two separate amp chassis then you may have a problem with that common neutral.
Title: Re: Crossover, L-Pad, Impedence, Resistance question
Post by: Jeff Schoonover1 on November 24, 2017, 11:09:37 am
Ivan Beaver is correct, I believe.  The problem is that 2+ becomes negative and completes the circuit in bridge mode, and therefore has to be connected to the X-Over negative input at that time.  There will have to be a fifth switch. so that in Bi-Amp mode, 2+ can connect to tweeter +.
 
Since 2+ becomes negative, 1- negative may also need to be lifted in bridge mode, or it may short Channel 1's contribution to the bridge back through channel 1(?)  I don't know whether or not 1- and 2- are lifted at the amp when in bridge mode.

I think my original drawing stands, more or less.  Here's an update.



Title: Re: Crossover, L-Pad, Impedence, Resistance question
Post by: Jeff Schoonover1 on November 24, 2017, 11:30:42 am
Here is a way to use a patch panel and the same standard speakon cable for both modes, no special cable or adapters necessary.
I have a panel with one speakon mounted that will work just for this.  Like yours (I think, the pic is blurry),  my amp also has channel 1 as a 4-pole, able to output both channels independently, and can also be used for bridge mode with just the positive poles.  I can just use the Channel 1 output and put a piece of tape over Channel 2.  I love the idea of one cable, one connection, no matter what.  I understand the added safety value of what you suggest, as best practice.  Although I'm sure that when cycling through modes, my amp won't engage the circuit if it's shorted.  Not that I've EVER tried this accidentally ;) 
I was wondering when we were going to get to that. It's an insurance policy protecting the CD from accidents like somebody using the wrong cable or a mixup with processing where the low frequencies or a fullrange signal is sent to the CD. It is sized to filter ULF only with a corner frequency well below the active crossover so as to minimize unwanted phase shift.
I figured it was a filter, I didn't think of it for safety.  My tweeter is 16ohms, So something closer to 10uF for a ~1KHz roll-off might be better. 
Depends if you are always using the same 2 channel amp for bi-amp operation or not, if using two separate amp chassis then you may have a problem with that common neutral.
Actually, looking at it, tweeter common to the X-Over can't be done. 2+ becomes negative in bridge mode, so 2- has to be lifted, or it shorts to 2+.
 2- has to have it's own switched route to the tweeter, as it's the only route Channel 2 can complete its circuit in bi-amp mode.  This is what Ivan Beaver was talking about.
Title: Re: Crossover, L-Pad, Impedence, Resistance question
Post by: Paul G. OBrien on November 24, 2017, 06:01:42 pm
The problem is that 2+ becomes negative and completes the circuit in bridge mode, and therefore has to be connected to the X-Over negative input at that time. 
Yes and no. The 2+ output of the amp has to be connected to the negative input of the crossover for Bridged output but that is best done at the amplifier end, and the crossover inputs should remain at 1+ and 1-. You could make a special speakon cable for bridged or install yet another switch in the cab but this is an accident waiting to happen or at the very least time wasted troubleshooting a no sound problem at setup for an event when you unknowingly reverse that special cable. Don't under estimate the value of keeping it simple.

Title: Re: Crossover, L-Pad, Impedence, Resistance question
Post by: Jeff Schoonover1 on November 24, 2017, 07:11:52 pm
Yes and no. The 2+ output of the amp has to be connected to the negative input of the crossover for Bridged output but that is best done at the amplifier end, and the crossover inputs should remain at 1+ and 1-.
  So leaving 1- connected to the x-over at all times will not short channel 1 in bridge mode?
You could make a special Speakon cable for bridged
It seems I'd have to make a special cable for this anyway - I'd have to route the 2+ from the amp to the 1- on the input.(?) Right now, I have a 4-pole Speakon cable with only the plus pins wired.  This is my specially labeled "bridge cable."  The most ideal thing as you say would be to use one standard 4-pole cable, whether it's bridged or not, where the ends don't matter which component they're hooked up to, and just be sure the switch is in the correct position.
Title: Re: Crossover, L-Pad, Impedence, Resistance question
Post by: Jeff Schoonover1 on November 24, 2017, 07:15:37 pm
  So leaving 1- connected to the x-over at all times will not short channel 1 in bridge mode?
If this is true, I can leave out the 1- switch in my drawing, and leave 1- connected to the common of the X-over all the time, only switching 2+ to the common when I'm using bridge mode.  This then allows me to go back to just one, 4-pole double-throw switch.
Title: Re: Crossover, L-Pad, Impedence, Resistance question
Post by: Paul G. OBrien on November 24, 2017, 07:20:55 pm
  So leaving 1- connected to the x-over at all times will not short channel 1 in bridge mode? It seems I'd have to make a special cable for this anyway
No and no if you use a patch panel. You install a special Bridged speakon connector on it with 1+/1- at the speakon wired to 1+/2+ at the amplifier.
Title: Re: Crossover, L-Pad, Impedence, Resistance question
Post by: Jeff Schoonover1 on November 24, 2017, 07:42:35 pm
No and no if you use a patch panel. You install a special Bridged speakon connector on it with 1+/1- at the speakon wired to 1+/2+ at the amplifier.
So, like this drawing.  Excellent!  I will convert the 'special' cable back to standard, and use my patch panel.  Then if I lose my cable or it breaks, no biggie to swap it out.  One cable connected to one of two clearly labeled outputs on the patch-panel, one single switch, and one single input jack. Simple = better.  Thanks!