How much DC offset is normal for this amp?

[sbelyo]

Hi All...

I completed my crack last night and I must say it is a fantastic kit for the money.  I am very impressed with the sound even after only 1 hour of break in.

Now, here's the question.  I am getting around 20 - 30 mV DC on the left and right output.  They will both float anywhere between -12 to 40 mV dc if you sit there and watch with a meter.  Is this an acceptable value and what would you normally expect to see with this amp after it has warmed up?

 

[Doc B]

Shawn is checking a stock Crack as I type this, and we'll post our results.

 

[sbelyo]

Thanks Doc...

It sounds real good by the way.

I did find a wiring error during my voltage checks where I had accidently connected B6 to U10 instead of U9.  This sent 90 Vdc to the left output.  When I corrected it everything fell back in line.

To the best of my knowledge this shouldn't have damaged the left output cap, am I correct?

 

[Doc B]

That cap should be fine. We just looked at a stock Crack with Speedball and a couple of different tubes. The behavior is consistent, but not the offset. It's not really a steady DC voltage but a randomly (slowly) shifting voltage that wanders from around 0 to 30mV, most of the time below 10mV of shift. My hunch is that it is something the 6080 is doing and as far as I can tell it is benign - too small to hurt anything and too slow to be audible.

 

[sbelyo]

Thanks doc... 

That's exactly what mine does.  I won't worry about it.

I'm used to my other solid state circuits where I trim the offset down to 0.5 mV

 

[Paul Joppa]

I expect that leakage in the electrolytic output capacitor plays a part. Electrolytics are always a bit leaky. The voltage should drop a lot when the cans are plugged in.

 

[sbelyo]

I thought about that in the back of my mind, I'll have to measure with phones plugged in

 

[JC]

Yup, that's the thing to do.  Electrolytics tend to wander around a bit if they have no load path, even in power supplies.