Bridged amplifier output?

[Brotaco]

Hey all, noob on the site, but been tinkering with car audio since the 90's.

I have a question about gain setting on bridged amplifier channels. It seems like most manufacturers rate their bridged output in watts to be 2X single channel output. But I've noticed with several amps that when I bridge channels I typically get 2X the voltage output before clipping. Using the standard calc of power = voltage^2 / resistance... am I not in reality getting 4X the watts?

I have an amp now that will output 20V clean signal per channel, which equates to 100 watts @ 4 ohms. When bridged, this amp shows a clean signal of 40V, which comes out to 400 watts @ 4 ohms. Or is there some other factor affecting the true output in bridged mode? The amp manufacturer rates the bridged output for this amp as 200W @ 4 ohm.

Sorry if this is a dumb Q. I tried searching for answers in the past at various sites including BCAE but if the explanation is there it went over my head.

[dejo]

I would say that depending on the amp specifics, that is a possibility. a well designed amp should double output power going from 4 to 2 ohms. when an amp is bridged each channel see half the impedance per channel. so power should be the summed power of 2ohm rating. IE- if rated at 100x2 at 4 ohms and also rated at 200x2 at 2ohms would equate to 400x1 at 4ohms. most amps these days dont fully double ouput power though

[captainobvious]

This should help you out

This excerpt is taken from Stephen Mantz's 2012 Zed Audio manual. Link here: http://www.zedaudiocorp.com/pdfs/ZedManual-2012.pdf
Pages 11-12. The whole thing is a good read though and explains quite a bit.

Bridging two channels of an amplifier is not a magical thing. Most are mystified by the
power figures quoted under the “bridge” column. It is actually very simple. When two
channels are driving a common load, one channel is out of phase with the other by 180
degrees. So when one channel swings positive the other swings negative. There is a
catch however. Each channel “sees” fifty percent of the common load and that means that
each channel of the bridged pair must be capable of delivering current to this lower load
impedance. Thus a 4 ohm bridged load presents a 2 ohm load to each of the bridged
channels. The power into a 4 ohm load in bridged mode is twice the rated 2 ohm power
per channel, the reason is that the power supply voltage is effectively doubled when two
channels are bridged.

[Brotaco]

I had to read that a few times to comprehend it, but now I get it. Thanks!

Interesting reading at that link. My first good amps back in the day were Zed designed Hifonics Vulcan and Odin amps. Still wish I had them.

[chad]

People have a hard time understanding why an amplifier "sees" half the impedance of the load and honestly is LITERALLY as easy as this.

Each channel sees 1/2 of the voice coil, really, no joke and this is why.

Normally one side of the coil is un-driven (it is improper to call it ground, we COULD call it a common but let's say un-driven to be correct.) The other side of the coil is driven by an AC signal. If you were to be able to magically hold a meter at the midpoint of the coil you would see 1/2 of what you would see at the driven input.

In a bridged configuration, as explained above, there is no undriven input, both inputs are driven by an equal and opposite signal... Because of this, if you were to reference the exact electrical center to the amplifier's un-driven outputs you would find zero volts because that is the point where the two inputs that are equal and opposite null out. The coil at that point is basically acting like the center-tap of a transformer.

And that is why. It's not that the amp is seeing half the impedance magically, it is because, in a physics sense, each channel is actually electrically seeing 50% of the coil itself.

[dejo]

and chad to the rescue for some impedance tutorial

[Brotaco]

Cool, thanks all. I understand it now. On to my next question:

Should you set amp gains with a load connected? I've read a few threads over at DIY mobile audio on the subject but there's conflicting opinions for sure. Even some of the people suggest the best method is to set gains by ear but I don't trust my ears.

[chad]

If you don't do it by ear then how are you going to know if it is right? I mean, technically, you could measure voltage gain and do the math knowing driver sensitivity in the environment.