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Author Topic: Crossover, L-Pad, Impedence, Resistance question  (Read 10571 times)

Ivan Beaver

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Re: Crossover, L-Pad, Impedence, Resistance question
« Reply #50 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.
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Ivan Beaver
Danley Sound Labs

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Ivan Beaver

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Re: Crossover, L-Pad, Impedence, Resistance question
« Reply #51 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.
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A complex question is easily answered by a simple-easy to understand WRONG answer!

Ivan Beaver
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Jeff Schoonover1

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Re: Crossover, L-Pad, Impedence, Resistance question
« Reply #52 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)
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Jeff Schoonover1

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Re: Crossover, L-Pad, Impedence, Resistance question
« Reply #53 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...
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Tim McCulloch

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Re: Crossover, L-Pad, Impedence, Resistance question
« Reply #54 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.
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Jeff Schoonover1

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Re: Crossover, L-Pad, Impedence, Resistance question
« Reply #55 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. 
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Chris Grimshaw

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Re: Crossover, L-Pad, Impedence, Resistance question
« Reply #56 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
« Last Edit: November 22, 2017, 02:14:23 pm by Chris Grimshaw »
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Ivan Beaver

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Re: Crossover, L-Pad, Impedence, Resistance question
« Reply #57 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
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Ivan Beaver
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Ivan Beaver

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Re: Crossover, L-Pad, Impedence, Resistance question
« Reply #58 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
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A complex question is easily answered by a simple-easy to understand WRONG answer!

Ivan Beaver
Danley Sound Labs

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Ivan Beaver

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Re: Crossover, L-Pad, Impedence, Resistance question
« Reply #59 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.
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A complex question is easily answered by a simple-easy to understand WRONG answer!

Ivan Beaver
Danley Sound Labs

PHYSICS- NOT FADS!

ProSoundWeb Community

Re: Crossover, L-Pad, Impedence, Resistance question
« Reply #59 on: November 22, 2017, 02:07:31 pm »


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