Large offset and large differential input to input pairs... but amp works..?

[Fusion916]

Have a strange problem I have no encountered before. Here are the symptoms.

Amp works fine, I get a clean signal output, under load, is stable (clean square wave no overshoot). But there is something going on with the LTP offset and amp offset.

Speaker out (vout) offset is ~90mV.
Feedback input is ~90mV
Signal Input is +50mV (???????)

Closed loop gain is fine (25V/V vs 31V/V from feedback ratio), so I don't think the open loop gain is as bad as the differenital input to the LTP suggests. Somehow it seems there is some kind of offset going. I took a look at some the bias voltages and everything seems to look ok EXCEPT each input to the LTP.

Where to debug next?

 

[Alan0354]

90mV at the output is not the end of the world!!! If you have complementary LTP IPS, it's not that surprising.

 

[Fusion916]

90mV at the output is not the end of the world!!! If you have complementary LTP IPS, it's not that surprising.

Yes, it doesn't kill any audio quality or anything, but I still want to understand it. I've built many amps with LTP input stage and I usually get an offset of 1-10mV. Ive never seen 90mV and I have never seen a LTP offset this large before, which leads me to believe there is some misconnection, broken part, or broken trace on the PCB, but I can't find it so far.

 

[JonSnell Electronic]

A schematic would help but anything up to 150mV is OK. This is usually caused by a misbalance on the differetial matched pair input transistors hfe. If you are worried about this minor issue, swop them over and read the difference.
No idea what "LTP" or "IPS" stands for. Don't do text speak.

 

[Fusion916]

I was just dicking around in spice playing with mismatched emitter resistor values and yes mismatching values does create the exact effect I am seeing, which would translate into mismatched bjt re values...

Maybe ive just lucked out and never built and amp with LTP transistors that never mismatched this much.

 

[currentflow]

LTP = Long-tailed pair and refers to a differential amplifier.
See Figure 2 here: https://en.wikipedia.org/wiki/Differential_amplifier
IPS = InPut Stage.

 

[JonSnell Electronic]

LTP = Long-tailed pair and refers to a differential amplifier.
See Figure 2 here: https://en.wikipedia.org/wiki/Differential_amplifier
IPS = InPut Stage.

Thanks for the deciphering.
(I still send texts in English!)

 

[bwaslo]

Is it by chance a late 70s vintage Hafler DH200? I ran into and fixed a number of those, with those same symptoms, back in the day. The high voltage diff pair transistors went leaky. Fixes were to replace those (all 4 in each channel) or (more difficult) change the stage to a cascode.