Bridging 4 channel loose SQ fact or myth

[Thnking 10+ year member]

Techically your are "losing SQ", because distortion is doubled. However, it goes from 100x lower than what you can hear only 50x... Odds are if your music was harsher, you were simply pushing the components harder than you were before.

Distortion isn't necessarily doubled, more than likely THD remains the same or decreases due to current limiting, as it's relative to power/load.

Boulderguy:

I agree with Squeak. You misrepresented the example.

By bridging the amplifier channels the available "headroom" doubles while maintaining the stereo per channel load. What you're referencing as the "margin" would decrease as load is decreased, say from 4ohm to a 2ohm. That isn't happening when channels are bridged.

 

[Daishi 10+ year member]

I was told by several of the dealers here that THD doubles when you bridge an amp.

 

[thylantyr 10+ year member]

My vote:

* If you bridge a good amplifier, no audible SQ loss.

* When you bridge an amplifier you gain 2x more clipping headroom

Hypothetical;

Ordinary amp;

100w x 2 @ 4 ohms

150w x 2 @ 2 ohms

Clipping headroom {not counting losses} = Power supply rail voltage in

reference to ground.

Bridged

200w x 1 @ 8 ohms

300w x 1 @ 4 ohms

Clipping headroom {not counting losses} = +Vrail + (absoluate value of [-Vrail])

... amps that use +Vrail and -Vrail, 99.99999% of them will have the same absolute value.

 

[Boulderguy 10+ year member]

Boulderguy:I agree with Squeak. You misrepresented the example.

By bridging the amplifier channels the available "headroom" doubles while maintaining the stereo per channel load. What you're referencing as the "margin" would decrease as load is decreased, say from 4ohm to a 2ohm. That isn't happening when channels are bridged.

I don't have any idea what you're saying there but it illustrates that we've lost the meaning in explaining our examples.

So switch gears to real-world listening. Let's say you want more volume from your fr spks so you bridge your 4 ch amp. Set the gains & now you get about 3db more output or so unclipped. Great, now it's louder.

But the sound noticeably less dynamic. I have done this on numerous amps & it happens every time. It really boils down to "no such thing as something for nothing."

But don't take my word for it, do the A/B tests. Someone had to prove it to me that way before I'd agree. Then I hit him over the head with the amp for bursting my bubble! //content.invisioncic.com/y282845/emoticons/laugh.gif.48439b2acf2cfca21620f01e7f77d1e4.gif:laugh:

 

[wesl56 10+ year member]

My vote:
* If you bridge a good amplifier, no audible SQ loss.

* When you bridge an amplifier you gain 2x more clipping headroom

Hypothetical;

Ordinary amp;

100w x 2 @ 4 ohms

150w x 2 @ 2 ohms

Clipping headroom {not counting losses} = Power supply rail voltage in

reference to ground.

Bridged

200w x 1 @ 8 ohms

300w x 1 @ 4 ohms

Clipping headroom {not counting losses} = +Vrail + (absoluate value of [-Vrail])

... amps that use +Vrail and -Vrail, 99.99999% of them will have the same absolute value.

can you please explain this a little more?

 

[thylantyr 10+ year member]

I was told by several of the dealers here that THD doubles when you bridge an amp.

True.

You have look at the data to see if it's audible. If THD is already low

in a non bridged amp, then doubling it may still be in the inaudible range,

so who cares.

 

[Boulderguy 10+ year member]

True.
You have look at the data to see if it's audible. If THD is already low

in a non bridged amp, then doubling it may still be in the inaudible range,

so who cares.

I've never been able to hear THD anyway, so it's kinda a non-issue anyway.

 

[thylantyr 10+ year member]

can you please explain this a little more?

Take a typical good amplifier with switch mode powersupply and class AB

output stage, *not factoring in losses *

Non bridged;

Channel #1

Amp + output -> speaker +

Amp - output -> speaker -

Channel #2

Amp + output -> speaker +

Amp - output -> speaker -

The 'amp - output' = ground reference

Lets say your amp power supply is +50 volts, -50 volts, ground.

+Vrail = +50v

-Vrail = -50V

ground = 0v

Clipping is 50v across the speaker because the speaker negative terminal

is connected to ground.

Bridged; {one way to bridge an amp, also assuming you configured it properly to make this work}

Amp channel 1 + output -> speaker +

Amp channel 2 + output -> speaker -

The speaker negative terminal is not connected to the ground reference, it's

new reference is 50V below ground, ie -50V

The speaker 'sees' +50v plus the absolute value of -50V which is 50v,

+50v + 50v = 100v headroom. Double.

Double clipping headroom in bridged mode vs. non bridged.

You can factor in losses to get the absolute values of clipping but nobody cares

because you will have losses in non bridged mode and in bridged mode that scales

accordingly. Lets say you have only 35v volts of headroom instead of the +50v,

you will still have +70v in bridged mode vs. theoretical +100v, but it's still double

of the +35v.

 

[Thnking 10+ year member]

I don't have any idea what you're saying there but it illustrates that we've lost the meaning in explaining our examples.
So switch gears to real-world listening. Let's say you want more volume from your fr spks so you bridge your 4 ch amp. Set the gains & now you get about 3db more output or so unclipped. Great, now it's louder.

But the sound noticeably less dynamic. I have done this on numerous amps & it happens every time. It really boils down to "no such thing as something for nothing."

But don't take my word for it, do the A/B tests. Someone had to prove it to me that way before I'd agree. Then I hit him over the head with the amp for bursting my bubble! //content.invisioncic.com/y282845/emoticons/laugh.gif.48439b2acf2cfca21620f01e7f77d1e4.gif:laugh:

What I said is well represented in the above post. The "headroom" is doubled, not constant like you're referring to. For headroom to decrease, load would have to drop from X ohm to Y ohm, or the gains would have to be changed per input to output ratio. As far as dynamics come into play, I'm sure it has to do with psychoacoustics. I'm pretty confident in saying between building amplifiers and owning many amplifiers, I've done enough A/B testing to have formed a "real world" opinion of my own.

Daishi:

THD is a function of Voltage*Current through the amplifier stages. If an amplifier is bridged, Voltage remains the same (rails), while current is now decreased to relative power/load connected at the output. So if anything, THD is decreased per V*A output.

 

[Lizardking 10+ year member]

I don't think I would overpowering the Rainbows....they seem to love my XXK-2500 with 270 to each side..

Anyway, I heard the specs get cut in half when bridging..damping factor, slew rate, THD and so on.....I'm know expert but I've read enough that figure damping factor and slew rate mean nothing with todays amps.

But I will say when I bridged the amp it seem to loose that warm full sound. Is there a way to measure to find out if you really would loose any SQ?

Thanks for all the replies so far....by the way.

 

[helotaxi 5,000+ posts]

You could measure the THD, etc... but unless the amp has issues, you will find the level still in the inaudible range.

You want an A/B comparision. I have one. When I first installed my Eurosports, they were powered by a 225 HCCA Orion in 4 ohm stereo. Honestly they sounded fine but they lacked headroom. Dynamic passages just fell flat because there wasn't enough power. It took all the amp had to power the speakers to an output level that was able to overcome the noise floor of my car (unmuffled exhaust, turbo whine etc...). I bridged that 225 and added a second one. One on each front channel. The difference was night and day. Very dynamic, tons of headroom (because I didn't have to use the full power of the amp just to overcome road noise) and uttery transparent. Now this is with a high quality amp. YMMV based on the amp.

BTW you would have 2x the voltage headroom but 4x the power headroom. Theoretically (ignoring losses) there would be a 6dB gain by bridging the amp.

 

[Team Hammer 10+ year member]

Voltage drops as Current Rises and Current Drops as Voltage Rises that's the trade off!

If you use a cheap unit it just shuts down or burns up. As energy increases so does heat, That is your trade off!

I never tested a class A/B amp for it, But I would like to know when you bridge a amp when it switchs from A to B compared to not bridging.

 

[helotaxi 5,000+ posts]

Voltage drops as Current Rises and Current Drops as Voltage Rises that's the trade off!

If the power supply is overbuilt the trade-off is very small. True, it won't double output due to resistive losses, but in the case of high freq amps, you aren't pushing the kind of power that you do to subs and the losses are less.

If you use a cheap unit it just shuts down or burns up. As energy increases so does heat, That is your trade off!

Point? If an amp isn't stable into a load placed on it that's one thing, but that is not the thing being discussed. I can basically guarantee that the only difference that you would hear between my front stage running off a single stereo amp or off a bridged pair is that the setup with more power has better dynamics afforded by more headroom and that it can play louder with less distortion because I don't have to even approach clipping to get the volume that I want. In fact I can turn the gain down to the point that clipping from the amp is impossible and still get enough output. Don't tax the amp and distortion never rears its ugly head.

I never tested a class A/B amp for it, But I would like to know when you bridge a amp when it switchs from A to B compared to not bridging.

Should be at the same place. I don't go re-biasing my output devices when I run it bridged compared to stereo, do you?