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Author Topic: Amps - Bridged or bi-amped, which mode sounds better and WHY?  (Read 6620 times)

Jeff Schoonover1

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Amps - Bridged or bi-amped, which mode sounds better and WHY?
« on: November 24, 2017, 01:45:13 pm »

Was talking about this a bit in the other thread I posted, figured it best to start its own thread.

Seems there are different opinions on whether it's best to bridge or bi-amp a box containing two or more drivers. The differing impedance capabilities are a factor as well.
 
I have a 2x18" woofer box.  Speakers are 18" Znom 8-ohms with an AES rating of 1.8KW each.  I have a crown XTi 6K. In bridged mode, I can opt for 6KW into 4-ohms, or roughly 3K into 16ohms. Or, I can separate the drivers, and run bi-amp for 1.2K from each channel into 8-ohms Znom.

I have several issues/questions. 
Is the greater efficiency of a bridged rig at 6KW into 4 ohms worth it over bi-amping 1.2KW per channel?
I believe Bi-amping offers higher impedance, lower THD, better damping control, and reduces the workload on the amp, correct?  Will this be an *audible* improvement over a bridged setup? 
Bi-amping also potentially may flow less current, keeping things cooler, right?  Further, it seems that I'd lose less than 3dB (big deal) by using a bi-amp setup. So the "extra power" (made by the 6dB boost combining channel inputs) isn't all it's cracked up to be on the sales datasheets.  I'd also be drawing less current from AC mains, which is a big benefit in clubs and small stage environments, where I likely don't have the full and sustained 20 Amps the amplifier's specifications call for.

Or, would it be better to bridge the amp, but run the drivers in series, rather than parallel for 16 ohms instead of 4?
In short, will my rig be AUDIBLY better bi-amped? I don't care nearly as much about wringing every dB out, as I do about the best sound quality I can achieve.

Opinions?
« Last Edit: November 24, 2017, 01:50:13 pm by Jeff Schoonover1 »
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Ivan Beaver

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Re: Amps - Bridged or bi-amped, which mode sounds better and WHY?
« Reply #1 on: November 24, 2017, 01:56:39 pm »

I would not call it "biamped" but rather dual amp.

There are other factors to consider when looking for the "best sounding".

Things like your speaker cable size and length come into play.

With the same cable, a lower impedance will result in more signal loss over the cable, lower damping factor etc

Does it matter? is it audible?  It depends.

Consider that bridged means the damping factor is already half of a single channel (because the 2 channels are in series).

Damping factor is basically the ratio of load impedance to the output impedance of the amplifier.

The higher impedance of the load, the higher the damping factor

There are plenty of debates on how much damping factor is actually important-so I won't go into that here.

I am just throwing out the numbers.

Having extra headroom is a good thing-if you like a clean signal.  But lots of people actually PREFER a slightly clipped signal.  That is a personal opinion.

And then there is the whole "how much can I realistically send to my speaker" debate.

So-as usual-lots of  "It depends".
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John Roberts {JR}

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Re: Amps - Bridged or bi-amped, which mode sounds better and WHY?
« Reply #2 on: November 24, 2017, 02:20:02 pm »

Was talking about this a bit in the other thread I posted, figured it best to start its own thread.

Seems there are different opinions on whether it's best to bridge or bi-amp a box containing two or more drivers. The differing impedance capabilities are a factor as well.
 
I have a 2x18" woofer box.  Speakers are 18" Znom 8-ohms with an AES rating of 1.8KW each.  I have a crown XTi 6K. In bridged mode, I can opt for 6KW into 4-ohms, or roughly 3K into 16ohms. Or, I can separate the drivers, and run bi-amp for 1.2K from each channel into 8-ohms Znom.

I have several issues/questions. 
Is the greater efficiency of a bridged rig at 6KW into 4 ohms worth it over bi-amping 1.2KW per channel?
Perhaps more cost effective in W/$ but not more efficient.
Quote

I believe Bi-amping offers higher impedance, lower THD, better damping control, and reduces the workload on the amp, correct?  Will this be an *audible* improvement over a bridged setup? 
Probably not audible, but amps will be happier, run cooler, and perhaps longer between failures.
Quote

Bi-amping also potentially may flow less current, keeping things cooler, right?  Further, it seems that I'd lose less than 3dB (big deal) by using a bi-amp setup.
biamps vs passive crossover is not simple analysis.
Quote
So the "extra power" (made by the 6dB boost combining channel inputs) isn't all it's cracked up to be on the sales datasheets.  I'd also be drawing less current from AC mains, which is a big benefit in clubs and small stage environments, where I likely don't have the full and sustained 20 Amps the amplifier's specifications call for.
I'm shocked...  :o
Quote
Or, would it be better to bridge the amp, but run the drivers in series, rather than parallel for 16 ohms instead of 4?
In short, will my rig be AUDIBLY better bi-amped? I don't care nearly as much about wringing every dB out, as I do about the best sound quality I can achieve.

Opinions?
All else equal running speakers in series means less control. In bridge mode amps are working harder.

Sound quality is likely to be dominated by other things if amps are well designed (like most modern amps are).

JR
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Jeff Schoonover1

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Re: Amps - Bridged or bi-amped, which mode sounds better and WHY?
« Reply #3 on: November 24, 2017, 02:37:38 pm »


 
biamps vs passive crossover is not simple analysis.
For this conversation, let's assume the crossover is active and done by the amp's processor either way.  Passive vs active = audible difference, Active is best, no contest.
running speakers in series means less control.
Good point.
In bridge mode amps are working harder.
This alone might be a reason to consider separate channels.
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Bob Leonard

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Re: Amps - Bridged or bi-amped, which mode sounds better and WHY?
« Reply #4 on: November 24, 2017, 03:07:35 pm »

Buy an amp that has an rms output, per channel, equal to 1.5 or 2 times the rms rating (continuous rating) of the speaker being powered. I.E. speaker rated at 1000 watts continuous then you should power the speaker with an amp that can generate 1500 watts continuous. Then run them in stereo/separate channels, one cabinet per channel. More control, and best sound quality.
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Jeff Schoonover1

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Re: Amps - Bridged or bi-amped, which mode sounds better and WHY?
« Reply #5 on: November 24, 2017, 03:31:37 pm »

Buy an amp that has an rms output, per channel, equal to 1.5 or 2 times the rms rating (continuous rating) of the speaker being powered.
I have heard this rule of thumb, but I don't think it's nearly so simple.  The duty the amp has to perform has much more to do with it.  For example, mine are rated at 1,800KW RMS each.  But if you really were to throw heavily compressed EDM at them, I would bet they won't hold up at nearly that continuous power for very long.  Similarly, if you have an amp (like mine at 1.2KW per channel) with much less than the speaker's rating and throw some serious transients toward them, it could be more than enough to blow the cones right out of the frame.
one cabinet per channel. More control, and best sound quality.
I'm more interested in what goes on within a single box, as to differences in running each driver on it's own circuit or the same circuit for all.
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Mac Kerr

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Re: Amps - Bridged or bi-amped, which mode sounds better and WHY?
« Reply #6 on: November 24, 2017, 03:55:13 pm »

I have heard this rule of thumb, but I don't think it's nearly so simple.
That's why it's a "rule of thumb" not the law of the land.
Quote
  The duty the amp has to perform has much more to do with it.  For example, mine are rated at 1,800KW RMS each.  But if you really were to throw heavily compressed EDM at them, I would bet they won't hold up at nearly that continuous power for very long.  Similarly, if you have an amp (like mine at 1.2KW per channel) with much less than the speaker's rating and throw some serious transients toward them, it could be more than enough to blow the cones right out of the frame.
1200 watts is a barely discernible difference from 1800 watts (it's 1.7dB). You need to stop thinking in terms of watts and start thinking in terms of dB of difference.
Quote
I'm more interested in what goes on within a single box, as to differences in running each driver on it's own circuit or the same circuit for all.
Running each passband on its own amplifier, with a capable system processor, is almost always the way to go for better speaker performance. The downside is you need more amp channels and the processor. Like everything in life you have to decide where your priorities lie.

Mac
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Mark Wilkinson

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Re: Amps - Bridged or bi-amped, which mode sounds better and WHY?
« Reply #7 on: November 24, 2017, 04:38:05 pm »

Are you able to take an impedance curve...of the drivers in the box, wired in parallel?

It's a necessary step in the 'smell test' for using bridge mode IMO....
If the curve hangs out too far below, or even too long mildly below 4 Ohms, I say no go.

Curves can be made with DATS (parts express), or even REW with a little more effort...
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Chris Grimshaw

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Re: Amps - Bridged or bi-amped, which mode sounds better and WHY?
« Reply #8 on: November 24, 2017, 05:27:09 pm »

Hmmm...

Bridged with parallel drivers:
- More power
- 2-core cable (with the implications on damping factor and cable losses)
- More work for the amp

Stereo:
- Less power
- 4-core cable
- Easier life for the amp

Neither wins on "efficiency" as such (unless you count cable losses). The amount of power required to hit a given output from a given cabinet is fixed. Setting power compression aside for a moment, 1000w into a 96dB@1w cabinet gives you 126dB. Doesn't matter if you put the two drivers on one channel of the amp, bridge it, drive them stereo, whatever.

IMHO, consider this - fire up some simulation software, and see what happens when you put 1200w into each driver. Check out the excursion charts. Now try 3000w.

If 1200w per driver is enough to get the cones to go past their linear travel, there's not much to gain by going to 3000w. You're already using everything the drivers have to offer, and all you're doing with the extra power is pushing them further outside their linear region, resulting in more distortion and more heat.
If, however, the drivers are sitting pretty at 1200w and have more mechanical travel to give, then I'd probably go ahead and bridge the amp to really give them some stick.

Speakers have mechanical limits as well as thermal ones.

Chris
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Luke Geis

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Re: Amps - Bridged or bi-amped, which mode sounds better and WHY?
« Reply #9 on: November 24, 2017, 05:28:23 pm »

Let's break it down a little more.

When you run an amp in normal 2 channel operation, the output terminals have an output resistance that is usually between .5 and 1ohm. When you run a speaker load across both channels for bridge mono, the effective output resistance of the amp is cut in half. This lowers the amps damping factor, which is an amps ability to control the speakers movement. This is also why most amps will not run down to 2ohms in bridge mono. There is not enough resistance  to reduce and or mitigate back EMF from the speaker. I.E. the speakers overshoot will create enough current to actually send power back into the amp. The amp see's this as basically a dead short and gets mad at you, shows red lights and says I am not going to work anymore for you.

Damping factor is moot if you run any regular speaker cable gauge over 50' in length. Where things can change is if you use bigger wire, or increase the load resistance. Wait what? If you run your amps at 16 ohms in 2 channel mode, with larger gauge cable, you can run longer cable lengths and still maintain useful damping factor in an amp!!!! Or all things being equal, in theory, a higher ohm speaker should sound better than one of a lower ohm rating. By better I should say that it will be more accurate to the incoming power amp signal. Now that does not mean go out and convert all your speakers to have each driver independently powered. If you run shorter cable lengths typically less than 50', you will be fine down to about 4 ohms. I would refrain from trying to run 2ohm loads even in 2 channel operation.

Another thing to keep in mind is that running an amp in bridge mono increases the THD of the amps output too. This is of course at any given load relative to 2 channel operation. So with that extra power comes extra distortion too. Possibly not an issue if you are not running things that hard.

If we take your subs for instance and power each speaker with its own amp channel we will reduce the amp channel load down to 8 ohms. This means the amp will produce, as you mention, 1.2k watts into 8 ohms!!!! This is really freaking good. The speakers are rated for 1.8k watts AES which is another standard of RMS / continuous. What can be safely assumed is that the speakers can dissipate 1.8k watts all day every day. So you have an amp that will get to within 600 watts of that. This last 600 watts is worth only 10ths of a db in actual output, so not a huge deal. If you were to double up in amp power to 2.4k watts, you will see a 3db increase in output. If you were to power the speaker with 4.8k watts you would see yet another 3db in output, but also be really close to the speakers peak wattage rating, so no real use in doing that. Not to mention finding an amp that will produce 4.8k watts at 8 ohms is is also like finding a unicorn.

The downside to high power speakers at mid range resistance ( like 8 ohms ) is that powering them with typical amps to their program or 1/2 their peak rating, is very expensive or impossible to find. So most of the time they are ran in parallel with another like speaker to get a load that allows for more power dissipation and makes it easier to find an amp that can power them reasonably well.

If you factor that that last 3db of potential output of a speaker is off limits ( you can't utilize it for long anyway ) and that the the 3db difference between program and RMS power ratings is not enough to make or break any show, you quickly see that having the subs running at 8 ohms in 2 channel mode is not such a bad thing. It just doesn't make sense to run all the extra cable. You quickly start getting to a point of diminishing returns. In the case of your amp you get 2,100 watts at 4 ohms or 1,200 watts at 8 ohms. A 900 watt difference! That is almost doubling the wattage. There is a 2.4db difference in potential SPL in favor of 4 ohm 2 channel operation. This is free SPL in your case!!!

 I would recommend running the sub in its normal 4 ohms operation. 3k watts at 16 ohms bridge mono is nearly identical as running at 2,100 watts in 4 ohm stereo. That is assuming the amp actually pushes 3k watts at 16ohm's? It does 4,200 at 8 ohm's which is 1,800 less than its peak 4 ohm rating. Carrying that math over, I don't think the amp will produce 3k watts at 16 ohms. I think it will be something closer to 2,800 - 2,600 watts at 16 ohms bridge mono. Either case not a big deal. The difference in potential output between 3k watts and 2,100 watts is 1.6db....... Going in bridge mono at 4 ohms will give you more output at the cost of distortion.

What does lack of damping factor sound like? Well it has been said that if damping factor is really an issue, then you can't get better than sticking the amps output directly to the leads of the speaker. The truth is that cable lengths longer than 50' typically make damping factor in an amp useless. So chances are you don't really have a reference as to what any one speaker sounds like with optimal damping factor? I always advise that you should keep the cable length between the amp and speaker as short as you can. The shorter, the better. Next I suggest not using any cable less than 12ga in wire size, 10ga if you can afford it. 14ga has a noticeable loss in output when longer lengths of 100' and more are used. The cable itself has a large part to play in the speakers / amp circuit. Since the cable has a resistance and is part of the closed loop, its size and length have a HUGE impact on the systems performance. For instance a 14ga cable that is 100' long will reduce the amps power seen at the speaker by 11%!!!!! The same length of cable in 12ga will only reduce power by 7%.
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Re: Amps - Bridged or bi-amped, which mode sounds better and WHY?
« Reply #9 on: November 24, 2017, 05:28:23 pm »


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