Nelson Pass B1 Buffer as mic FET circuit.

[k brown]

With appropriate value subs (gate R up to 1-2 gig and input cap to a typical value), this 2SK170BL circuit is extremely clean as a mic FET. I replaced the circuit in a couple of Oktava 012s, and the mics are much more transparent.
https://www.passdiy.com/project/preamplifiers/b1-buffer-preamp

 

[k brown]

This circuit uses a constant current source in place of the usual bias resistor to gnd on the FET source, and a voltage divider to feed half the supply to the gate.

Thoughts on why this is seen in few if any microphones? Using well-matched FET pairs it eliminates the usual tedious biasing procedure - or am I missing something?

 

[bremusound]

Constant current sources were common in the 80‘s. AKG (C460b, C414B) and Beyerdynamic used J500 (…J503) current regulators a lot.

 

[k brown]

As a way to bias the capsule's FET, or elsewhere in the circuits?

Some reason they fell out of favor?

 

[bremusound]

C460B
No idea when or why they disappeared…

 

[k brown]

Yeah, I looked that up after you mentioned it; here the CCS appears to be on the drain, with the source conventionally biased with a resistor to gnd. And the op amp is doing . . . what?

 

[bremusound]

 

[k brown]

Thanks for that - from the service manual, I presume.

 

[bremusound]

Yes, it is available as a pdf file:
http://recordinghacks.com/pdf/akg/C460B-service.pdf
The Nelson Pass circuit looks interesting. Did you make a pcb for The Oktavas?

 

[Khron]

What exactly does "transparent" mean, though?

 

[k brown]

Yes, it is available as a pdf file:
http://recordinghacks.com/pdf/akg/C460B-service.pdf
The Nelson Pass circuit looks interesting. Did you make a pcb for The Oktavas?

No, I just stripped and adapted the '012 board. I wouldn't do it again, though - the traces lift too easily. Did another pair point-to-point.