Bottlehead Forum
Bottlehead Kits => Crack => Topic started by: sbelyo on May 02, 2011, 07:46:55 AM
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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?
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Shawn is checking a stock Crack as I type this, and we'll post our results.
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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?
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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.
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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
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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.
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I thought about that in the back of my mind, I'll have to measure with phones plugged in
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Yup, that's the thing to do. Electrolytics tend to wander around a bit if they have no load path, even in power supplies.