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In Reply to: RE: Preamp with high pitch whine from 1 channel posted by Cpwill on November 02, 2015 at 05:02:49
A little more detail. Is this a DIY or a commercial product? If DIY can you post a schematic or link to one.
Dan Santoni
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It is an early Emotive Sira will a few mods over time by me. I have no schematic. It has a ton of tubes for rectification and voltage and current regulation, but signal path is very simple. 5687 twin triode for gain followed by one 7-pin CF per channel. As I said, DC voltages are OK and match side to side, so I am assuming the other couple hundred tubes are not the problem. Present on all sources, so I suppose it is not the selector switch and at all volumes so I assume it is not the stepped attenuator.
Once you've gone through the basics - solder joints, sockets, cleaning etc.
Then you have to look to the circuit. It is actually a very complex power supply with current sources and regulators feeding each channel.
The oscillation is probably caused by one of the other tubes or the associated components. So don't rule out the other tubes. Pay particular attention to the 5651 tube. It is prone to oscillation if it's shunt capacitance is too high. There should be a .02ufd cap on that tube. Any bigger and the tube will oscillate.
Dan Santoni
OK, I will expand my search for the culprit. Anyway to measure oscillation with a DMM?
DMM's don't have the frequency response to track the oscillation. Try just swapping the other tubes first, you might find the culprit.
Dan Santoni
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