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?"

 

[thor.zmt]

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

Q115 Q103 (PNP) is the constant current source, there is implicit looped feedback from the follower (Sziklai circuit) emitter via the zener diode.

R115 (8.2k) with C110 acts as the "flying supply" for the J-Fet, with Q102 acting as the same follower as in the Audio Technica circuit.

The second follower is also switchable highpass and perhaps an overengineered part.

For a simple modern version with 21st century parts, use 2 X E102 CRD from pin 2/3 of XLR (2mA constant current), J-Fet and source resistor (say 10k for 500uA Idss) and BSS84 as follower and to bootstrap Cgd, coupling cap. Capsule to ground, J-Fet gate sits at ~5V.

As pin 2/3 with P48 will be at 41.2V, we can easily derive 36V bias Voltage for LDC Capsules. If using P68 (my "personal" standard) we get 56V Bias.

Marginally more components that the simplest possible circuit, but better performance all around, including noise.

Thor

 

[k brown]

Q115 (PNP) is the constant current source, there is implicit looped feedback from the follower (Sziklai circuit) emitter via the zener diode.

R115 (8.2k) with C110 acts as the "flying supply" for the J-Fet, with Q102 acting as the same follower as in the Audio Technica circuit.

The second follower is also switchable highpass and perhaps an overengineered part.

For a simple modern version with 21st century parts, use 2 X E102 CRD from pin 2/3 of XLR (2mA constant current), J-Fet and source resistor (say 10k for 500uA Idss) and BSS84 as follower and to bootstrap Cgd, coupling cap. Capsule to ground, J-Fet gate sits at ~5V.

As pin 2/3 with P48 will be at 41.2V, we can easily derive 36V bias Voltage for LDC Capsules. If using P68 (my "personal" standard) we get 56V Bias.

Marginally more components that the simplest possible circuit, but better performance all around, including noise.

Thor

Did you mean Q105 (CCS)?

 

[jp8]

The bootstrapped JFET principle was discussed on that other microphone forum and inspired me to build something similar, with such a circuit on both the inverting and non-inverting outputs of the microphone. The circuit would have zero electrolytics, lending itself very well to miniaturization. Gain close to 0dB, and according to simulations very low distortion, extremely high SPL handling and low noise. Everything except the low noise came out as predicted. It could handle 7V RMS on the input at 0.1% THD. Just noisy as hell and I still don't understand why. Maybe I'll publish it here later on GroupDIY.

Jan

 

[micolas]

Q115 (PNP) is the constant current source

Thor, please excuse me,
There is no BJT Q115 in the schematic

 

[thor.zmt]

Did you mean Q105 (CCS)?

Thor, please excuse me,
There is no BJT Q115 in the schematic

Q103... It was by my first coffee.

My brain and memory ain't what they used to be. Then again, they probably never were.

Thor

 

[thor.zmt]

The bootstrapped JFET principle was discussed on that other microphone forum and inspired me to build something similar, with such a circuit on both the inverting and non-inverting outputs of the microphone.

Theoretically good idea.

Everything except the low noise came out as predicted.

Excess noise from poor resistor choices? I had an SMD thick film accidentally in the tail of a folded cascode circuit where the input side was capable of < 0.5nV|/Hz and had 40mA transconductance.

Replacing with a generic (Yageo) thin-film resistor dropped noise a lot, going MELF got close to predicted results.

Thor