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I built a 2 channel preamp, not my first, and noticed for the first time that the volume wouldn't go down all the way. Ch.1 is gain stage, volume, resistive divider, cathode follower. Ch. 2 is gain stage, volume, gain stage, resistive divider, cathode follower. The stages are coupling through the plate resistors of ch.1 and the 2nd gain stage of ch. 2 as they share the same power supply node. I separated the 2 channels to different nodes but it didn't seem to help much. Is this just the nature of the beast?
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If separating the B+ nodes didn't change the crosstalk, then it seems like they were not coupling there after all.
Crosstalk can come from many things. Apply a strong signal to one input, then scope the other channel stage by stage to see where the crosstalk does and does not appear. If you have reason to suspect B+ coupling, scope the B+ for signal riding on it.Education is what you're left with after you have forgotten what you have learned. -
Improper grounding can also be the culprit.Comment
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Amen.Education is what you're left with after you have forgotten what you have learned.Comment
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Looking back at your design abstract, it occurs to me that the channels are out-of-phase, and if the crosstalk is in the first stage, most of it should be canceled when the channels are summed. If it DOESN'T, then chances are it's farther down the line.
However, I maintain that you likely have a grounding issue. The clue is that you cannot turn the volume all the way down.Comment
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Is the tone of the unwanted signal thin and trebly, or bassy?
Do the channels share a tube? If so try a different tube, as some have excessive coupling between sections.
Has the physical implementation kept good seperation between the signal wiring of the channels (photos would be helpful)?My band:- http://www.youtube.com/user/RedwingBandComment
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I have found that changing the 1st preamp tube can also solve the problem.MandopickerComment
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The signal is very quiet, just audible through the speaker over the acoustic sound of the guitar, not tonally different. V1 and V2 are separate tubes, V2 is cascaded with volume in between, the two meet at a resistive divider into the cathode follower. Pulling V2 doesn't stop the crosstalk. V1 and V2 share the same power supply node and cap. Each stage is grounded to it's own cap and then onto the main reservoir cap. The PI and CF share an F&T can cap and the common ground. I have pulled all of the grounds on the first node and changed their order with clip leads to no avail. Grounding the signal at the grid of V1 stops it, grounding the output from the
downstream end of the coupling cap doesn't.Comment
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