Fet for Neumann mic

[Ricren]

Hi Folks,

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?

Thanks for and clue on this.

[BrianL]

Contact David Josephson at Josephson Engineering.

Try david@josephson.com or info@josephson.com

[Zappa]

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.

[Ricren]

Thanks a lot for the data.
Have you found wild differences in gain on the 170? Do you find usefull to buy several just to select two ?

cheers

Ric

[Zappa]

"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...

[Zappa]

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...

[Ricren]

Zappa, tks. for the data.

The noise issue is from the mic, no dubt. I have several high end mics that do not "hiss" like this. Even a $200 U87 chinese clone is quieter!

Another disturbing thing: I noticed that the Neumann have some tantalum capacitors, une of them is in the signal path. Would I consider to upgrade to a film? Unfortunelly there's not much room inside.

Cheers,

RIC

[Zappa]

A dryed cap may be a source of serious noise too...

The lack of space is probably the primary reason why they put a tanlalum there. Any film type of the same value will apparently be significantly larger...

Maybe you could try an high quality low ESR litic here. (Black Gates FK, N, NX etc...)

Depending on the design of this pre and the role of this cap, chances are you to change the signature of the mic but, isn't it better than keeping it unused? Anyway, you will learn!

Keep us informed!

[Ricren]

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.

cheers


Ric

[BrianL]

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.