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Bridge mode and damping factor

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Sonic the hedge View Drop Down
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    Posted: 15 May 2020 at 4:47am
I keep reading on this forum that using amps in bridge mode halves damping factor, while this is partly true I think it is only half the picture.

In bridge mode the two channels are effectively in series across the speaker load, so it is indeed correct that the source impedance is doubled, and therefore the amplifier's available damping capability is halved.

What's missing is that at the same time (assuming in all cases the amp is fully loaded) when in bridge mode the number of drivers is reduced by a factor of four (vs dual channel mode)

e.g.

4 ohm dual channel = 4x 8 ohm drivers
8 ohm bridge mode = 1x 8 ohm drivers

2 ohm dual channel = 8x 8 ohm drivers
4 ohm bridge mode = 2x 8 ohm drivers

Since DF is essentially the ability of the amplifier to sink the back EMF generated by the drivers, reducing the number of drivers will obviously reduce the total back EMF the amp has to sink by a corresponding amount. 

So while the source impedance is doubled (and therefore DF halved) the back EMF the amp has to sink is quartered. So effectively the actual available DF is doubled. Another way to think about it is dividing the DF (bridge or dual) by the total number of drivers, same result. 

e.g

Dual channel DF 1000 / 4 drivers = 250
Bridge mode DF 500 / 1driver = 500

Dual channel DF 1000 / 8 drivers = 125
Bridge mode DF 500 / 2  drivers= 250

So IMO running an amp in bridge mode doubles the effective damping on each driver, not halves it.

For this reason (and a couple of others) I have always run bass amps in bridge mode, and I believe the results speak for themselves.

Discuss...


Edited by Sonic the hedge - 15 May 2020 at 5:31am
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Jo bg View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Jo bg Quote  Post ReplyReply Direct Link To This Post Posted: 15 May 2020 at 10:15am
Well this assumes damping factor is more noticeable (i doubt it) than the bigger distortion you get by lowering impedance, and that your amp and supply can handle the increased current draw and heat produced by loading it to 2 ohm nominal (it means ity could probably go lower, impedance varies with frequency), or it will clip / limit, and this will surely be way more noticeable.
I prefer running a single 4 ohm nominal speaker per channel on 2 ohm capable amplifier, not stressing the amp too much, less distortion, plenty current reserve for peak to preserve dynamics, less troubles if I lose one channel or a cable goes bad.
And more efficiency means also less waste and pollution.
Not the cheapest way but if you talk about quality it usually is not.
Hanging 8 subs to an amp seems more a matter of economy than quality.
Now a k20 may be more suited to you use, but not the majority of amps in my book.


Edited by Jo bg - 15 May 2020 at 10:18am
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Post Options Post Options   Thanks (0) Thanks(0)   Quote levyte357- Quote  Post ReplyReply Direct Link To This Post Posted: 15 May 2020 at 11:20am
No one here has mentioned, impedance curve of cabinets.

So we can actually forget talking about, 2 ohm loads, 4 ohm/8 ohm cabs.

So if we assume, we are talking about very capable, Crest, QSC, Crown transformer amps, placing 2x cabs across the amp in bridge mode, is not the same as running the amp in 2 ohm stereo.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Jo bg Quote  Post ReplyReply Direct Link To This Post Posted: 15 May 2020 at 12:00pm
Actuallly i did
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Post Options Post Options   Thanks (0) Thanks(0)   Quote levyte357- Quote  Post ReplyReply Direct Link To This Post Posted: 15 May 2020 at 12:16pm
Originally posted by Jo bg Jo bg wrote:

Actuallly i did


Well done, my bad.

My point being, so many amps, just do not accomplish their 1khz specified rating, 40hz-100hz.

Back when I used to bridge amps, (Dec 2019 LOL), my weapon of choice was the QSC RMX 5050.

Many, many times bridged these across many modern 5" VC, 1kw+ drivers, and obtained some of the smoothest, fullest bass tones, with monumental SPL, control, sound quality, never ever going anywhere near clip.

Not even the illustrious Powersoft K20, driving 2x cabs per channel, could match the sound obtained, from 2x bridged 5050s.

However no one wants to walk with 4x 5050s, to drive 8x cabs.

Now on a different path..
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Jo bg Quote  Post ReplyReply Direct Link To This Post Posted: 15 May 2020 at 12:28pm
Can you explain your statement, and what consquences you imply?
I don't get your point, or how this could be helpful.
I agree it will not be a perfect 2 ohm load, no speaker is, like no perfect 8 or 4 ohm.
But the impedance seen by the amp with 2 speakers in parallel on brideged channels will indeed be, per channel, the same as stereo operation with 4 of the same speakers per channel.
Say minimum impedance for such speaker is 6 ohm, if you hang four on one channel it will see 1.5 ohm load.
Now if I bridge the amp and connect two of these speakers it is a 3 ohm min load, 1.5 ohms per channel same as before.
So what is the difference, except the op is using nominal impedance, which is a universally agreed semplfication?
And how amplifier brand can have any influence on the load ? It only depends on the speakers.
Different amps will deal with the load differently, but will see the same load.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Jo bg Quote  Post ReplyReply Direct Link To This Post Posted: 15 May 2020 at 12:35pm
Maybe I got it
Still, if an amp can't deliver th3 goods down low I believe it is mainly a current problem, so lowering impedance  asks more current and should deliver even less?

Trying to put some science behind this, not to put a fight.

I have heard this 2 ohm sounds better many times, but noone gave me any explanation beyond subjective opinions.


Edited by Jo bg - 15 May 2020 at 12:36pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Mikkel Quote  Post ReplyReply Direct Link To This Post Posted: 15 May 2020 at 12:44pm
This is quite interesting, never thought of it that way. I may be wrong but my understanding is the size of the speakers also comes into the equation, i.e. less moving mass, more effectively the amp can control the cone.

So what if you have say 2x 18" bridged and 4x 12" stereo (all 8ohm), you would have circa same cone surface area but is the amp working less hard in stereo plus having better/equal cone control no?

Edit: no i see where ive cocked up. You would need 8x 12" driver to load the amp the same as 2x 18" in bridge. But again the moving mass is still a factor i think.


Edited by Mikkel - 15 May 2020 at 12:53pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Sonic the hedge Quote  Post ReplyReply Direct Link To This Post Posted: 15 May 2020 at 12:44pm
Originally posted by Jo bg Jo bg wrote:

Well this assumes damping factor is more noticeable (i doubt it) than the bigger distortion you get by lowering impedance, and that your amp and supply can handle the increased current draw and heat produced by loading it to 2 ohm nominal (it means ity could probably go lower, impedance varies with frequency), or it will clip / limit, and this will surely be way more noticeable.
I prefer running a single 4 ohm nominal speaker per channel on 2 ohm capable amplifier, not stressing the amp too much, less distortion, plenty current reserve for peak to preserve dynamics, less troubles if I lose one channel or a cable goes bad.
And more efficiency means also less waste and pollution.
Not the cheapest way but if you talk about quality it usually is not.
Hanging 8 subs to an amp seems more a matter of economy than quality.
Now a k20 may be more suited to you use, but not the majority of amps in my book.

Agreed, all valid points

Running one driver per channel on 2 ohm capable amp will have better damping than running two, three or four drivers per channel. And lower THD.

You could equally run two smaller 2:ohm capable amps in bridge mode, each with one driver, for same result. Either way you would not be able to use the full output of the amp though, but this is a choice, to allow for headroom and redundancy.

My point is, for the same amount of available watts output, all other things being equal, bridge mode doubles the effective damping available on each driver. 

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Post Options Post Options   Thanks (0) Thanks(0)   Quote levyte357- Quote  Post ReplyReply Direct Link To This Post Posted: 15 May 2020 at 12:52pm
Originally posted by Jo bg Jo bg wrote:

Maybe I got it
Still, if an amp can't deliver th3 goods down low I believe it is mainly a current problem, so lowering impedance  asks more current and should deliver even less?

So that a real fight is not started, I'll refer to only transformer amps.

Some transformer amps are endowed with big enough transformer, output devices, smoothing, power regulation, to at minimum, extract enough current from the wall (assuming no voltage drop), to meet output specification, down to 40hz.

Most are not. So is usually a problem with implementation, not just supply available.

Void Infinite 8Mk2 is a good example, of Transformer amp, built to achieve, and actually exceed specifications, down to sub notes, at 4 ohms.

Originally posted by Jo bg Jo bg wrote:

Trying to put some science behind this, not to put a fight.

Is more a case of money. Quality manufacturers, used to build amps that were fit for purpose down to sub notes. These were the types of amps that would get Dolby certification for use in cinemas, because they did what it said on the tin.

Originally posted by Jo bg Jo bg wrote:

I have heard this 2 ohm sounds better many times, but noone gave me any explanation beyond subjective opinions.


The simple answer is more power, more headroom, with sound quality on sub notes.

As i said previously, great amps as they are, Powersoft K20, or Void Infinite8 Mk2, could not provide same sub notes, quality, warmth, headroom, across 2x cabs per channel, as 2x bridged QSC 5050s, driving pair of cabs each.




Edited by levyte357- - 15 May 2020 at 12:53pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Jo bg Quote  Post ReplyReply Direct Link To This Post Posted: 15 May 2020 at 12:55pm
To MikkeI, I was comparing same driver in different configurations, with different drivers many variables will come into play, surface alone is not enough, you need to account for the whole speaker parameters.
Still I see bridge operation as a compromise to get more sound for the buck, not a quality choice.
Also damping factor seems more an hifi concern than a modern PA issue to me.




Edited by Jo bg - 15 May 2020 at 12:56pm
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Post Options Post Options   Thanks (1) Thanks(1)   Quote Sonic the hedge Quote  Post ReplyReply Direct Link To This Post Posted: 15 May 2020 at 12:55pm
Originally posted by levyte357- levyte357- wrote:

Back when I used to bridge amps, (Dec 2019 LOL), my weapon of choice was the QSC RMX 5050.

Many, many times bridged these across many modern 5" VC, 1kw+ drivers, and obtained some of the smoothest, fullest bass tones, with monumental SPL, control, sound quality, never ever going anywhere near clip

This highlights another advantage, for those on a budget using old iron amps with modern drivers.

Bridge mode allows to well match the output of ~3-4kw old iron amps, of which there are plenty available (8001, 5050, MX1200 etc) with the modern 1Kw drivers. 

However, I learnt this trick from others who were doing the exact same thing in the early 90s, with complete disregard for the 'rated' power handling of the drivers. I'm not convinced that the modern 1kw drivers actually have much  more powerhandling than old school 600w - 800w ones




Edited by Sonic the hedge - 15 May 2020 at 1:25pm
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