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Author Topic: Amps - Bridged or bi-amped, which mode sounds better and WHY?  (Read 6291 times)

Paul G. OBrien

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Re: Amps - Bridged or bi-amped, which mode sounds better and WHY?
« Reply #10 on: November 24, 2017, 05:38:58 pm »

I have a 2x18" woofer box. Speakers are 18" Znom 8-ohms with an AES rating of 1.8KW each
What drivers are they exactly? The AES spec(the old one) is with a 6dB crest factor which is double rms.

I have a crown XTi 6K. In bridged mode, I can opt for 6KW into 4-ohms. Or, I can separate the drivers, and run bi-amp for 1.2K from each channel into 8-ohms Znom.
That amp won't really generate that much output at sub bass frequencies but lets ignore that for a moment. Because the load is shared by the two channels in bridged mode each channel is effectively running at 2ohm versus running at 8 ohms if you discretely power each driver. 2 ohms operation really stresses everything, the power supply and amp outputs are asked to supply maximum current which generates lots of waste heat and shortens component life. We generally don't like talking about damping factor around here because it's a non issue with modern amps driving 8ohm loads, but you will hear and see an obvious difference in how much apparent control the amp has over the drivers at 4ohm bridged vs 8 ohm dual mono, dynamics will be less defined sustained tones less clean. So yes there is a sound quality penalty to pay IMO and best way to find out how much is to try it out yourself.

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Art Welter

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Re: Amps - Bridged or bi-amped, which mode sounds better and WHY?
« Reply #11 on: November 24, 2017, 06:37:53 pm »

What drivers are they exactly? The AES spec(the old one) is with a 6dB crest factor which is double rms.
I mistakenly had spread the misinformation that the crest factor of the AES signal being 6dB rather than a sine wave being 3dB crest factor was the reason to "downgrade" by 3 dB, but that is incorrect- watts are watts regardless of crest factor or waveform.

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Helge A Bentsen

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Re: Amps - Bridged or bi-amped, which mode sounds better and WHY?
« Reply #12 on: November 24, 2017, 06:59:27 pm »

Keep in mind that some half bridged class D amplifiers can start bus pumping if you give them an uneven load on the outputs and run them hard.

For that reason, sometimes you have to bridge a amp to get stable performance.
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Jeff Schoonover1

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Re: Amps - Bridged or bi-amped, which mode sounds better and WHY?
« Reply #13 on: November 24, 2017, 07:02:33 pm »

What drivers are they exactly?
18-Sound TLW-3000
http://www.eighteensound.it/Products/Articles/Detail/catid/4051/eid/3470/15tlw3000
That amp won't really generate that much output at sub bass frequencies
Agreed, I don't think it can even draw enough off of a 20 amp circuit to do it.  Even if it could, I'm not sure the power supply could; based on the little I've read about what makes some amps more expensive than others.
Because the load is shared by the two channels in bridged mode each channel is effectively running at 2ohm versus running at 8 ohms if you discretely power each driver. 2 ohms operation really stresses everything, the power supply and amp outputs are asked to supply maximum current which generates lots of waste heat and shortens component life.
Yes, and I can't imagine pushing that much current and generating that much stress on components won't make an audible difference.  I know in testing this box thus far, it doesn't take much at all to make fans come on at maximum, and for the chassis to get pretty warm.  It's gotta shorten longevity.
We generally don't like talking about damping factor around here because it's a non issue with modern amps driving 8ohm loads, but you will hear and see an obvious difference in how much apparent control the amp has over the drivers at 4ohm bridged vs 8 ohm dual mono, dynamics will be less defined sustained tones less clean. So yes there is a sound quality penalty to pay IMO and best way to find out how much is to try it out yourself.
I have done so already, and have witnessed all you describe in person - hence this thread.  I kept telling myself that I really ought to try running each driver on a separate channel and see if it sounds better and stresses the amp less.  To use the race car analogy, I'm running down the whole track in first gear.  I really want to see what it sounds like using each channel independently, and I'd like to understand more about the WHY before I do.  I want to understand the wisdom as I go against the conventional folklore of "more watts equals mo' betta." Again, the volume is secondary to sound quality.  A few dB will make no practical difference.
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Paul G. OBrien

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Re: Amps - Bridged or bi-amped, which mode sounds better and WHY?
« Reply #14 on: November 24, 2017, 07:39:50 pm »

I want to understand the wisdom as I go against the conventional folklore of "more watts equals mo' betta."
There have been more than a few threads here over the years about swapping out a lesser quality amp running bridged mono at 4 ohms for a more powerful amp running at 8 or 4 ohms dual mono, and in all cases the authors reported improved sound quality and even more apparent output in some cases even when the total power available per box(on paper) went down.

Discretely powering sub drivers isn't unusual in pro audio and it's probably going to get more common when the latest generation of high output drivers are more widely used. Coincidentally I also power the drivers in my subs independantly.. mainly because they are loaded with 4ohm drivers, but that does make it easier to fully power them with something less than a top shelf amp.
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Jeff Schoonover1

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Re: Amps - Bridged or bi-amped, which mode sounds better and WHY?
« Reply #15 on: November 24, 2017, 07:55:28 pm »

There have been more than a few threads here over the years about swapping out a lesser quality amp running bridged mono at 4 ohms for a more powerful amp running at 8 or 4 ohms dual mono, and in all cases the authors reported improved sound quality and even more apparent output in some cases even when the total power available per box(on paper) went down.

Discretely powering sub drivers isn't unusual in pro audio and it's probably going to get more common when the latest generation of high output drivers are more widely used. Coincidentally I also power the drivers in my subs independantly.. mainly because they are loaded with 4ohm drivers, but that does make it easier to fully power them with something less than a top shelf amp.
Cool, I will report back.  Eventually... 
Makes me wish I'd opted for the 4-ohm version of the drivers, but again, I don't think the dB or headroom loss will be anything to worry about. Sound quality is the primary goal - at an adequate level to keep up.  As Ivan said, a little soft clipping might even be more pleasing to the ear.
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Stephen Kirby

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Re: Amps - Bridged or bi-amped, which mode sounds better and WHY?
« Reply #16 on: November 24, 2017, 08:43:10 pm »

It has also been pointed out that a given amp can only produce so much power.  Tricks like halving the load makes it seem louder, but the amp just runs out of gas sooner.  The power supply and output devices have finite limits.  There's no free lunch.

IME, bridging or running amps at low loads has always resulted in less clarity.  I've not done this with things the likes of Lab Gruppen or Powersoft.  But with various mid-range QSC, Crown, etc, it's noticeable.
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Luke Geis

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Re: Amps - Bridged or bi-amped, which mode sounds better and WHY?
« Reply #17 on: November 24, 2017, 10:05:27 pm »

You are only going to be about 3-4db shy of the best performance you can get anyway. I wouldn't worry about the last few db. You are essentially talking about making something more complicated to extrapolate a meaningless amount of extra energy.

If you only have XTI money, then you can't be worrying about how to get that last 3db out of the units. In my experience teh XTI 6k did not do very well with sub service in bridge mono mode. It provided the same performance with less of a light show in 2 channel mode. The QSC PL3602 is a very serious sub amp if you need to run bridge mono and performed for me, just as well as the XTI 6k with a couple thousand watts less in the bank.

As mentioned you have to look at what your system can provide in db not what the watts are. The difference between 3k watts and 6k watts is 3db, so not very much. No show has ever been ruined because the PA couldn't get 3db louder. If a show was ruined because the PA was destroyed trying to get 3db more out of it, then the problem was the driver behind the mixer / system provider who didn't spec a big enough system.

Going back to just the speaker cable alone as a factor in performance, you should consider what lengths and gauges you currently run. A 50' cable ( rather common to use ) with 12ga. wire connected to a 2100 watt amp with a 4 ohm load will reduce the wattage seen at the speaker by 157 watts!!! This is roughly a 7.5% loss in power! Or put another way a loss of .34db in potential output. 1/3 of a db for a 50' cable. It is so negligible right? That is the same kind of outlook you need when looking at peak potential output, vs actual probable output in db. 6db between peak and probable isn't bad. If you really need that 6db, then you DO NOT have enough PA to play with. I you really need even 3db more, then you again still DO NOT have enough PA to play with.

What is your PA capable of? A little math and you can get a potential outcome. Then you have to figure what you need. If you need more than you have, then no amount of tweaking the system will save you. 3db is not a deal breaker, 6db might be? From a practicality standpoint a speaker can only get to around -3db of its peak rating on the best of days and with a great system designer. He may employ the tricks you are looking to try, but a good system designer will try and NOT have a system he has to play god with. If you need 130db @ 100' you can't do it with most MI, pro-sumer systems of standard deployment sizes. You are better off looking at what you have without having to get wild and making things complicated so you can do a cost benefit analysis on other potential outcomes. I presented one that gets you within about 1.6db of the best potential outcome you can potentially acquire and is 2.4db better than another way of running it, utilizing no special tricks and using what you currently have.

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Jeff Schoonover1

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Re: Amps - Bridged or bi-amped, which mode sounds better and WHY?
« Reply #18 on: November 25, 2017, 11:06:36 am »

Going back to just the speaker cable alone as a factor in performance, you should consider what lengths and gauges you currently run. A 50' cable ( rather common to use ) with 12ga. wire connected to a 2100 watt amp with a 4 ohm load will reduce the wattage seen at the speaker by 157 watts!!! This is roughly a 7.5% loss in power!
Another 'plus' in the dual channel column. Cable considerations.
 I'm using one 12AWG x 4 cable, 10' long.  Bridging means that I double up the 4 conductors to two, but still - much lower load/higher current.  With dual-channel, two conductors for positive - one to each channel/driver, and two for negative, one to each channel/driver.  Higher load/lower current, more conductor.  Win/Win/Win
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Luke Geis

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Re: Amps - Bridged or bi-amped, which mode sounds better and WHY?
« Reply #19 on: November 25, 2017, 01:10:16 pm »

If using 12 ga. wire that is 10' long, then the cable is no longer the weak link. If you were to double the connection inside the speakon up ( 2 wires used for +1 and 2 wires used for -1 ) then you will reduce the cable resistance in 1/2.

I am not sure what you mean by dual channel though? One channel for each speaker, or one channel for each whole sub?

I as other are suggesting that you simply run one whole sub per amp channel. This will present a 4 ohm load to the amp and you should have roughly 2,100 watts of power. This will drive the sub to roughly its AES / RMS power rating.

I DO NOT suggest running a single channel per speaker in the sub ( re configuring the sub for bi-amp operation ). This doesn't improve the outcome and as I presented, you actually loose 2.4db in potential output. The amp would run each speaker individually with a lower potential.

The third option you presented was running the sub so it has a 16ohm load ( running the speakers in series ) with the amp in bridge mono. We are not sure what the amp will provide at 16ohm bridge mono and I am pretty sure it will be less than 3,000 watts. Done this way, you could potentially gain 1.6db ( MAYBE ), but I still feel this is not ideal. If you blow one of the two speakers, neither will work.

The last option is running the sub in its normal configuration at 4ohms with the amp in bridge mono. This is the one that most people will go for. It provides the most power, but has the added cost of making the amp work harder and with a little more distortion. If you know that you won't be running close to the amps limits, then this is not a bad way to go. If you will be using all the watts available, it could end badly if you abuse that power.

6,000 watts vs. 2,100 watts is only a 4.6db difference in potential output. What that means is that running the amp in stereo, 2 channel mode with one channel going to each sub configured in its normal 4ohm operation will only produce 4.6db less output. This will be easier on the amp and in theory should sound better since it will have less THD than running in bridge mono.

The cable you are using is not of detriment to your use. Ready for some fun numbers! At 2,100 watts 4 ohm stereo operation, the cable you are using will reduce the power to the speaker down to 2,067.12 watts. You will have an effective damping factor of 100.8!!!!! This is amazing. Same setup with 6000 watts bridge mono would be reduced down to 5,906.07 watts and an effective damping factor of 83.89. You only loose about 35 watts in stereo operation and you loose almost 100 watts in bridge mono. To boot you have an effective damping factor that is higher by about 15 by going stereo mode. The difference in potential volume is still the same as I mentioned before with 4.6db, but that is really not a make or break amount.
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Re: Amps - Bridged or bi-amped, which mode sounds better and WHY?
« Reply #19 on: November 25, 2017, 01:10:16 pm »


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