I stumbled upon this today (explanation of the Schoeps mike pre-pre, if I can call it that).
https://www.youtube.com/watch?v=e5xenXTwAzo&t=8m4s
-Alex
Yes, he makes the common mistake of saying they use a big FET with high Cin which is wrong. Some well meaning modders do this all the time but the noise is dominated by the G Ohm bias resistor and the capsule capacitance.
Thanks Scott. Indeed I have seen schematics with the 2sk170 in that position. There is no noise advantage, and you lose a few dB output because the high capacitance loads the capsule? What would you use instead?
Here´s the first schoeps schematic that came up on a google search. It indicates BC264. And here´s the relevant data from a datasheet in german. Much lower capacitance than the K170. Comparable Rauschfaktor at 0,5dB, but the Equivalent Rauschspannung is listed as 40 nV/sqrt(Hz) versus 1dB typical for the K170@1mA.
-Alex
Here´s the first schoeps schematic that came up on a google search. It indicates BC264. And here´s the relevant data from a datasheet in german. Much lower capacitance than the K170. Comparable Rauschfaktor at 0,5dB, but the Equivalent Rauschspannung is listed as 40 nV/sqrt(Hz) versus 1dB typical for the K170@1mA.
-Alex
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These numbers don't really reflect the application. Noise figure re: 1M and spot noise at 10Hz are not indicators of the noise in the microphone application. If the FET's have a reasonably low noise corner the noise of an equivalent resistance of about .7/gm would be the number to use. Selected J305's do remarkably well, my favorite 2SK222 is unfortunately obsolete with rare stashes of NOS parts here and there.
I was told Rode uses SMT versions of the J305 in their mic that achieves 5-7 dBA noise which is not audible in any normal recording environment.
EDIT - Forgot to mention the back plate hole patterns used to tune many capsules have a real acoustical resistance component which often dominates the noise in the important presence region anyway.
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Very nice! I see you put in the gate stoppers, fingers crossed that it's all you need.
Well, I will officially pledge myself as the first interested party.
Lord knows I've pledged myself to be the first guinea pig in much worse.
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