Bridging amps

[e_c_mccollum]

Power amp configuration can be a hot topic because everyone has their own way. Another tech told me when an amp is bridged, messing with channel 2 gain does'nt damage anything. Is this true?

I know bridging an amp connects the output's feedback circuit of channel one to channel two's post variable gain stage preamp so it can be controlled by channel one and have the proper signal phase.

[Mark Black]

Depends on the amp.

Bridge mode uses the "hot" of one channel (usually Chan-1) to produce an in-phase output, and the "hot" of the other channel to produce a 180 degree out-of-phase output. With the load connected between the two hots you get effectively double the one-channel voltage at any given time, since the other channel is driving an equal but opposing voltage.

Some amps (usually older designs) rely on the Chan-2 input attenuator to be set exactly the same as Chan-1, because they do the phase-flipping before the Chan-2 control. Most newer amps do the flip after the Chan-2 control so it has no effect (unless you happen to also be driving the Chan-2 input jack with a signal, which would be an incorrect usage of the amp in Bridge mode).

I'm not a big fan of Bridge Mode except for very special occasions such as driving a high-power 8 ohm subwoofer with an amp that is too small to do the job on one channel alone, or to create a "70V" driving capability for a bunch of ceiling speakers.

Lots of folks run bridged into 4 ohm subwoofer loads all the time and get away with it, but it can be awful hard on the amp (like running stereo 2 ohm). Perhaps the inherent impedance rise most speakers have at low frequencies helps to mitigate the effects.

All of this is of course not to be confused with "Parallel" mode which is pretty much like sticking a Y-cable in the inputs, hooking speakers up normally, and using both input attenuators. Except, again, on some amps. Crown in particular have traditionally had a different idea of what "Parallel Mode" means and how it should be implemented.

So to sum up: what the tech told you is certainly true for some (maybe even most) amps, but I have a little trouble with it as a blanket statement.

[e_c_mccollum]

As a blanket statement it would get others into trouble. I does depend on design. Qsc's have dip switchs on the back which requires you to switch them into parallel input mode for bridging. Looking at the schematic, I can see where this blanket statement would be proven wrong on these qsc models since the same input signal/phase appears on each channels preamp before each variable gain stage.

I don't care much for brigding amps. I see too many burned out because of it. One customer briged a Peavey pv600 amp into 2 ohms. Only lasted for a while. If he only listened.

Crown parallel mode is unique. To me, paralleling the outputs is hard on the amp. Crown is certainly different.

[g1]

Like the others said, there is no "rule" that covers all manufacturers amps. Some even require a special xlr patch cord wired to flip the phase on the channel 2 input.
The big problem with amps getting blown running bridged is that the load impedance must be double the minimum of single channel operation (ex. if amp can drive minimum 2 ohms per side, it can only go down to 4 ohms bridged).
If the manufacturers instructions for running bridged are adhered to, it shouldn't be any more likely to blow than in stereo mode.

[Jazz P Bass]

That's the key.
Read the specific manufacturers manual.
Alot of the power amps are the same.
Some are not.