Hafler P500 - One channel quieter than the other

[Pippi Lindenstroem]

Hi there, I am happy that I've found such a great oasis of knowledge here. I am ready to learn, so your help would be very welcome : )

I have a little problem with my Hafler P500. The amp has stepped volume pots (on the PC-20 board). With both pots in equal position, the left channel is always a bit quieter. What could be the reason for this? How can I adjust it?

Many thanks in advance

Pipp


There is a part of the schematic attached

P101 & P101L are the Volume Pots

P102 & P102L Trim pots to null out any common mode signals -Btw. How would I be able to measure common mode signals (Just to check up if it's all still healthy adjusted)?

From the manual:
Clipping Detector circuit - P103 adjust the amps output to the same level as the input. RI 18, R19, C106 & C107 shape the frequency of the imput signal to match the amplifier. The two signals are compared by IC 105, and any difference is amplifierd and triggers the one shot multivibrator IC 106 which lights the LED D202

 

[Pippi Lindenstroem]

Better Schematic

Her is the input schematic in better resolution

Maybe the installing of a small trim pot near to one of the volume potentiometer would help to adapt the volume to the other channel

 

[EchoWars]

If both channels sound OK, and the only problem is the difference in volume (and that difference is small), then the most likely culprit is the volume control itself.

See if you can bypass it as a test.

 

[Pippi Lindenstroem]

If both channels sound OK, and the only problem is the difference in volume (and that difference is small), then the most likely culprit is the volume control itself.

See if you can bypass it as a test.

Many thanks - The amp sound very nice at all. The soundstage is a bit shifted with the different channel volumes. I can solve this with an extern 2 channel level controller in the signal path. But I don't want to have an extra device in my chain needlessly
If I don't find an approach to solving the problem -like different bias settings (just as an example, because I don't know if this would express in different loudness)- I'll install a set of 50k conductive potentiometers. So I will not have the fixed stepped settings from the original ones.

Any other ideas what the cause could be ?

 

[EchoWars]

You need to eliminate the simple possibilities before you go digging for the difficult ones.

 

[Pippi Lindenstroem]

You need to eliminate the simple possibilities before you go digging for the difficult ones.

I've changed the volume pots. It's not ideal now (they are linear) but it works pretty well as long no one coughs. The new vishay conductive pots give the amp a tad more shine/brightness, that's ok. I installed two 68uf caps across the psu filter cans and it seems the amp plays a bit tighter.

I never had the opinion that the NE5532 is an old worse op-amp like many do. I lot of classic mixing desks and other quality devices have them. Anyway, the amp was already open and a played a bit around with some different types of op-amps in the buffer stage. I have nothing improved in the circuit like additional decoupling etc. The NE5532 played pretty alright in the top middle of the comparision field. It emphasis the treble freq. a bit what gives a nice 'stereo' impression. The soundstage was better than with many newer amps. Finally, I've found a combination of National Semiconductor op-amps which are pretty great for the Hafler.