July 10, 2016 at 11:20 am #16048
Hmmm…. The indication is that there’s a ground path that is not quite optimum, and it could be that it is there on all that version. Certainly at that time we did not have the analytical equipment in the factory that we have now.
I suspect that there is a ground track that is carrying grounds from several places; the very tiniest of ground current will create hum or buzz. The stage that is being affected must be the compression stage and it is clearly only on one side.
I’m sure that if I had the unit here I could isolate the problem and re-route the ground, but the advice is all I can achieve at the moment.
This could be an example of why circuit board layout is so critical in audio circuits; there are so many variables that it’s very much an art, and we don’t get it right all the time! But it’s interesting that I have not come across this before…. and I have looked closely at the compressors I have here and such an effect is not apparent.July 11, 2016 at 11:27 am #16049jtvrdyParticipant
looking at the output stage,
the A&B resistances are 10k but the C&D are 20K, should not be of the same value?
also the pins 2&5 goes through A&B to ground but the pins 10&13 goes to C&D but not to ground,shoould be connected to ground as A&B?July 14, 2016 at 9:11 am #16050
The design of that output stage is complex…. it is not symmetrical as it performs the matrix function of re-combining M+S/M-S into the L and R.
If, as you say, the noise remains the same when the output gain control is turned down, the the problem is associated with the output stage.
The difference in noise behaviour has to be to do with the way the ground paths are arranged. If I had the unit here and wanted to achieve the very lowest noise levels, I would experiment with some very heavy copper wire to try to eliminate the track paths that carry grounds from more than one place.July 14, 2016 at 9:18 am #16051
I have just re-read the messages and see that you actually say that the hum/noise changes with the output gain control.
If that is so then the output stage is OK. But if the problem is in an earlier stage then it should affect L and R outputs (as all earlier stages are in M/S mode). So apart from being certain that this hum/noise is to do with ground paths, I can’t suggest anything else.July 14, 2016 at 9:57 pm #16052jtvrdyParticipant
ok thanks Ted, I will send the unit to you, shipping address?
I checked another unit and it has de same noise/hum problem..also some resistors have different values.
if you found an easy solution let me know so I can replicate it on this second unit.
thanks,July 17, 2016 at 8:08 am #16053
I will send you a separate email.
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