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Old JFET in mic goes noisy?
Old JFET in mic goes noisy?
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Old 25th January 2004, 04:55 AM   #11
marz.tv is offline marz.tv  Sweden
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Thanks John and Eric for the insight. No, the legs weren't all blackened, still I tried rubbing a little with a q-tip and isopropyl alcohol where I could reach, but there wasn't much immediate difference.

I got a really helpful mail from Bernt Malmqvist at Pearl (not many companys care to support 35 yr old products!).
He said that 2SK879 is what they use in their mikes nowadays, but that it is surface mount. He also recommended J201 as a good TO-92 fet in this application. Does anyone have experience with these parts?
He also said that the old tantalum caps can be a noise problem, so maybe I'll swap these parts first before going transistor shopping.

I noticed the noise suddenly dropped a little in one of the mikes after half an hour or so of warmup time. Would this be some old cap reforming, or perhaps breakdown of an oxide trace somewhere? (ok, this might be deep water I know
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Old 25th January 2004, 09:53 AM   #12
In my experience old electrolytic capacitors often become noisy, especially if they are in the path of very small signals (i.e. phantom power blocking caps with mic signals).
I found this in noisy channels of older mixing consoles.

I think replacing the old tantals would be a good start.
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