I have an old Neuman U87 condenser microphone that is a bit noisy. I was wondering if changing the fet inside ( an SK 107 type) would improve things.
Would someone care to recommend a better fet? Or is the SK part good enought?
Pass labs seems to like the 2sk170 and also the 2sk389 (<-twin) for low level signal. To my tests, these are two good choices, very low noise, very transparent. Maybe the 170 have a little more of these two qualities and comes in green, blue and violet flavours to my knowledge. Each colour describing the Idss range.
"Gain" is easily 2 or 3 times to 1 between the least and the most. If you need a pair, you HAVE to match them at least for Vgs (I would go 0.1% if possible). Idss is probably an issue of lesser order but it can't hurt to match on this one too. No need to say you have a great AND fine piece of engineering in your hands, don't fiddle with random parts in a magnificent vintage Neumann...
So the answer would be yes, and to get them matched as close, you would probably need 15 or 20 of them, as a minimum . Don't worry, they are cheap (60-75 cents).
By the way, are you sure that it's not your console (or outboard) preamp that makes this noise? The Neumann probably don't output gobs of voltage and maybe you have to crank up the gain there. High quality preamplification is prescribed here...
I went to my frendly electronic supply store and bought some caps. I got some 1 mf polyester caps. Would you say a polyester is better than a tantalum?
The one to change is between the fet and the output transformer.ALL the signal is passing thru it, so I guess is critical.
Be careful which FET you use with the microphone. Many
of the FETs loved for audio purposes have rather high
input capacitance. This is NOT what you want to connect
to a microphone capsule which is a variable capacitor of
very small capacitance.
But, search through the Toshiba FET offerings and you'll
find something that will work. Then again, you could
just replace with the same part as Neumann used, selected
for low noise from several.
As someone said, there are other noise sources, including
moisture in the capsule. One shouldn't just assume that
the noise is from the FET.
But, please, a Neumann is too valuable an item for you
to be dinking with if you don't know what you're doing.
I repeat my advice: contact David Josephson, or some other
person competent to fix your problem.