Help on Bridging

I built 2 ESP 3a power amps. When i tried bridging the amps together i get a huge hum from the speakers. I tested the amps individually and it was allright. I tried using 10 ohm resistor to ground to isolate the loop and even disconnecting main's earth leaving the amp floating but still hum is there. Anyone can show some light on this area please do write in.
 

paulb

Member
2001-06-01 4:53 pm
Calgary
You can try some experiments.
First, disconnect the bridging circuit and ground both inputs, then measure the voltage between the two amp outputs (where in a bridge configuration you would connect the speaker). It should be very close to 0, because you said they work properly when not bridged.
With the bridging resistor still removed, apply the same input signal to both amps. Do you see exactly the same output on both channels (voltage between them is 0)? A scope in differential mode would be useful here.
Then, reconnect the bridging resistor and ground the input, then measure the voltage again. You should be measuring the "hum" that you can hear if the speakers were connected. Then measure each output relative to ground. Which one is the hum coming from? Or are you seeing signal on both?
If you can create a differential source (see Rod's Project 14, for instance), you can remove the bridging circuit and just drive both amps from opposite polarity input signals. I think this is a better approach anyway.
A diagram showing exactly how you hooked things up would be useful.
 
The solution

I didn't really fixed the problem. There was no problem at all. What happen was i left my preamp on and tested out the power amp. Normally everything should be fine, but sometimes when i turn on the preamp first and later switch on the power amps there would be hum present. Well on testing day, i left my preamp on and switch on the bridge-amp and found huge hum. I first thought that the hum was a problem. Then later when tried again hum was gone. So there was no problem at all the amp work fine except my stupidty got the better of me. Sorry for starting this thread but thanks for the help