Disharmonic distortions

[txstrat]

My double channel amp has a master volume in the second channel, right before the main master volume (pre PI) that regulates both channels. (2nd channels MV is just there to adjust the level of the 2nd to the 1st channel).
When the 2nd channel MV is all open, the amp sounds great, even when I turn down the main MV.
When I turn down the 2nd channels MV the sound begins to distort in a disharmonic way. Don't know what's wrong.
Any help is highly appreciated.

[txstrat]

I'm planning to take the main MV out of the circuit, to see if it's the 2nd channels pot or the circuit itself (cascading master volumes).
Any ideas?

[Robert M. Martinelli]

Hi Matt,
I'm unsure I got the picture right, can you post a schematic ?
Regards
Bob

[txstrat]

Hi Bob,

here's a block diagram.
When the relay is switched ON the 2nd channel MV pot is right before the main MV. Both pots are 1meg audio.

cheers

Matt

[Robert M. Martinelli]

Matt,
the block diagram looks OK ( IMHO ), do you have an o-scope to take a look at the waveforms before and after each stage? Have you checked the coupling caps for leakage? ( if there's some DC around the master could act as a DC voltage divider as well and bring the PI out of balance.....just a wild guess....)
HTH
Best regards
Bob

[txstrat]

Bob,
unfortunately I don't have a scope. Would have to change parts for a check.
I'm thinking about a bad solder joint as well, since all sounds great, when the 1st pot is all open. Bad idea?

[Robert M. Martinelli]

Redoing the solder joints is not a bad idea, for sure it won't do no harm.....
The lack of an o-scope will make things harder, but I' m sure you'll sort it out.
Are the ground connections ( especially the pots' ones ) and the relay connections OK?
Regards
Bob

[txstrat]

The connections on the relay should be ok since there was no issue with them before I redid the circuit and I haven't touched 'em. I'd suspect the ground of the 1st MV since that's the pot which wasn't there before.

[Robert M. Martinelli]

Me too, that's why I advised to check the GND connections, with so high an impedance, having the thing "floating" could cause weird things to happen.
Good luck!
Bob

[Jag]

Just curious, why you didn't wire it like this and have an independent master for each channel?



I don't think you need the .0005 before the PI either. That's a pretty small value as well and it probably killing alot of bottom end.

[txstrat]

Yes, I thought about that but actually I have two main master volumes (also switched by relay) to have two independant volumes switchable by footswitch. Very comfortable.
The first channel has a MV too, but thats to be switched in by a DPDT switch with a third gain stage. (I think this doesn't belong to this thread as it's not part of my problem, so I left it out)

I adopted the .0005 from the 6G16 schematic and the amp has a good protion of bottom end, anyway. I might try .002 to .005 or so.

Thanks for your thoughts.

[txstrat]

I checked the solder joints and moved the ground lead to the main masters ground to no avail. Still distortions. But I found out, that it distorts only between 7 and 9. Under 7 and full open there are no distortions and all sounds great. I could live with that (not easily) but just for the sake of the matter, does anybody have an idea what to do?

I don't think you need the .0005 before the PI either. That's a pretty small value as well and it probably killing alot of bottom end.

I changed the value to .005 and the sound became, well, I'd say - overall punchier. Even better than before.