# Bridging an amp?

Discussion in 'Amps and Cabs [BG]' started by GreyBeard, Feb 24, 2004.

1. ### GreyBeard

Sep 5, 2002
BARRACKVILLE WV
Just what happens inside a power amp when you push the Bridge switch. Is it possible to bridge 2 mono power amps? Can you bridge 2 mono tube amps together?

2. ### BillyB_from_LZSupporting Member

Sep 7, 2000
Chicago
With a solid state amp, I believe that it places a differential amplifier in the signal path so that one channel amplifies the positive wave and the other channel amplifies the negative wave.

I seem to recall looking at a DIY article for an autosound power amp long ago and that's what they did...

You ought to PM Bob Lee to be sure...

IIRC that you can parallel the outputs of tube amps without any electronic gynmastics at the input. Perhaps psychobassguy would be a good one to ask on this. Or ask anyone that makes them...Aguilar for instance...or maybe even Lord Valve or the guys at Triode Electronics in Chicago...

3. ### Bob Lee (QSC)In case you missed it, I work for QSC Audio!Commercial User

Jul 3, 2001
Costa Mesa, Calif.
Technical Communications Developer, QSC Audio
In bridged mono, both amp channels put out exactly the same signal voltage, except the one on channel 2 (or B) is inverted in polarity, so it is a mirror image of channel 1.

One side of each output is ground, so the complementary voltages on the "hot" sides means that the two channel outputs are effectively in series, so you get double the output signal voltage--similar to if you put two 1.5-volt D cells in series, positive on one to negative on the other, you get 3 volts as a result.

To bridge two mono amps, they would have to both share a circuit ground and have unbalanced outputs. You would have to put a straight signal into one of them and an exactly inverted signal into the other, and they would need to have identical gains. (In fact, they really should be identical amps.) I think I remember a "strapping" circuit for doing this that was published in Audio magazine back in the late 70s or early 80s.

4. ### GreyBeard

Sep 5, 2002
BARRACKVILLE WV
Thanks Bob.

5. ### BillyB_from_LZSupporting Member

Sep 7, 2000
Chicago
Yeah, that's what I meant