Comparison of JFETs for mic applications

[ccaudle]

this method of biasing the 2SK660

I'm having trouble following, is this correct for the connections?


What is indicated with the connection (capacitor?) between source and case, and drain and case?

 

[k brown]

I'm having trouble following, is this correct for the connections?
View attachment 143471

What is indicated with the connection (capacitor?) between source and case, and drain and case?

Those are RF caps at the TB3M input jack of the pwr module (could have been drawn more clearly).

 

[thor.zmt]

I've always been curious about this method of biasing the 2SK660; it's from the Audio Technica AT8533 power module; so different from the way FETs are usually biased in microphones.

The PNP transistor acts in effect as a Zener Diode (or Vbe multiplier), which produces a fairly constant emitter voltage.

The emitter is supplied from a current source (CRD) from the centertap of the output transformer.

The 1k load for the Mic capsule FET comes from the PNP emitter. It seems to be a follower, with the Diode in the acting as "floating" powersupply of ~ 0.6V for the J-FET, that's DEEP in the triode region.

As the signal out of the J-Fet goes via a cap to the base of the PNP the PNP transistor in effect bootstrap's the 1k Resistor.

Overall capacitances are well cancelled, it's a fairly linear follower into a transformer.

It looks complex, but fundamentally it's simple.

I'm not surprised it sounds good, if we measure this circuit, I expect even order HD dominant with very low order of HD, plus likely a fair bit of pink noise.

Thor

 

[k brown]

The PNP transistor acts in effect as a Zener Diode (or Vbe multiplier), which produces a fairly constant emitter voltage.

The emitter is supplied from a current source (CRD) from the centertap of the output transformer.

The 1k load for the Mic capsule FET comes from the PNP emitter. It seems to be a follower, with the Diode in the acting as "floating" powersupply of ~ 0.6V for the J-FET, that's DEEP in the triode region.

As the signal out of the J-Fet goes via a cap to the base of the PNP the PNP transistor in effect bootstrap's the 1k Resistor.

Overall capacitances are well cancelled, it's a fairly linear follower into a transformer.

It looks complex, but fundamentally it's simple.

I'm not surprised it sounds good, if we measure this circuit, I expect even order HD dominant with very low order of HD, plus likely a fair bit of pink noise.

Thor

Thnaks

Did look like a bit of a bootstrap to me. Wonder why this type of bias isn't seen more often.

This is the Shure version (don't know for what FET):

 

[thor.zmt]

Did look like a bit of a bootstrap to me. Wonder why this type of bias isn't seen more often.

Most people get a headache looking at this.

The J-Fet operated at extremely low voltage.

There seem to be many thermally dependent voltages.

A lot goes against "received wisdom".

Peeps just "don't get it"!

Thor

"fast and bulbous

(Hey, do it again) I love that, I love those words

Fast and bulbous, that's right, the mascara snake

Fast and bulbous

Bulbous, also tapered (yeah but you gotta wait until I say it)

Also, a tin teardrop (oh, christ)

(Again, beginning)

Fast and bulbous, that's right, the mascara snake

Fast and bulbous, also a tin teardrop

Bulbous, also tapered, that's right

A squid eating dough in a Polyethylene bag

Is fast and bulbous, got me?"