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Large offset and large differential input to input pairs... but amp works..?
Large offset and large differential input to input pairs... but amp works..?
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Old 4th July 2015, 06:11 AM   #1
Fusion916 is offline Fusion916  United States
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Default Large offset and large differential input to input pairs... but amp works..?

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?
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Old 4th July 2015, 06:26 AM   #2
Alan0354 is offline Alan0354  United States
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90mV at the output is not the end of the world!!! If you have complementary LTP IPS, it's not that surprising.
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Old 4th July 2015, 06:29 AM   #3
Fusion916 is offline Fusion916  United States
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Quote:
Originally Posted by Alan0354 View Post
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.
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Old 4th July 2015, 08:32 AM   #4
JonSnell Electronic is offline JonSnell Electronic  United Kingdom
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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.
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Old 4th July 2015, 09:01 AM   #5
Fusion916 is offline Fusion916  United States
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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.
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Old 4th July 2015, 09:05 AM   #6
currentflow is offline currentflow  United Kingdom
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LTP = Long-tailed pair and refers to a differential amplifier.
See Figure 2 here: https://en.wikipedia.org/wiki/Differential_amplifier
IPS = InPut Stage.
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Old 4th July 2015, 10:00 AM   #7
JonSnell Electronic is offline JonSnell Electronic  United Kingdom
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Quote:
Originally Posted by currentflow View Post
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!)
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Old 4th July 2015, 09:02 PM   #8
bwaslo is offline bwaslo  United States
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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.
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