What causes DC offset in mono Hafler?

[reiver]

My Hafler question for the week:

My Pro1200 amp bridges to mono by taking the signal from the feedback loop (see attached diagram), inverting it and running it into the input of the other channel. IN stereo mode I get 13mv and 20mv DC on the outputs which I can live with. There's no DC trimpot to adjust it.

When in bridged mode though, I get 90mv DC on the mono output.

My question is: when I run it in mono I was expecting it would sum the DC from the channels and I would get maybe 35mv. What in the circuit could be pumping it up to 90mv DC?

- Do I just need to replace and match all the input transistors? Or is low DC simply asking too much for a this old mono circuit?

[john65b]

I would try this.

Jumper both signal grounds to both output grounds together to power supply ground with with alligator clips.

Check if your offset drops.

[reiver]

The hitch is that when in mono mode you use both red/+ speaker terminals to drive the one speaker. They warn against using the grounds. I'll do some careful checking with my voltmeter.

.....Here's from the manual:Conversion to Mono Mode is accomplished by driving the negative input (feedback point) of the right channel with the output of the left channel. This causes the right channel to mirror exactly the output of the left channel, but 180 degrees out of phase. This creates twice the voltage swing to be available across the two red output binding posts. This simple method of bridging the amplifier (involving only one resistor) eliminates the costly and sonically degrading inverter circuitry used in other amplifiers.

[djk]

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