Built a JFET Source follower with JFET current sink load microphone

[hifizen]

2SK246 is roughly equivalent to 2SK30A / 2SK208 / 2SK879.

You could probably build the whole circuit using just the one type, and save those rare K170's for lower-impedance applications which need the high transconductance and quiescent current.

 

[k brown]

Reminds me a little of the Audio-Technica mic pre front end, except they add a 3rd JFET, "Erno Borbely" style, as a self-bootstrapped cascode on the follower FET to neutralize it's gate-drain capacitance...

I don't know if Mr. Borbely came up with the arrangement (probably not), he's just stuck in my mind as one of the most prolific advocates of it.

From Mr. Borbely's famous article "JFETs The New Frontiers Part 2" (a balanced pair version of this cascode arrangement) ... 2SK389 is basically a dual packaged 2SK170. Slice this circuit in half and put that B1 buffer's JFET current source in place of the J511 CRD shown, and you have something pretty close to the A-T front end:

" 'the' Audio Technica mic pre"? - didn't know AT made any mic preamps. If they did, what exact device are you referring to?

 

[hifizen]

Oh, I meant the circuit inside the mic itself, directly connected to the capsule. Impedance converter circuit I guess might be a better way to say it? ... Not the thing you use at the other end of your XLR cable to add signal gain into the recording console / audio interface.

 

[Wavelength]

Reminds me a little of the Audio-Technica mic pre front end, except they add a 3rd JFET, "Erno Borbely" style, as a self-bootstrapped cascode on the follower FET to neutralize it's gate-drain capacitance...

I don't know if Mr. Borbely came up with the arrangement (probably not), he's just stuck in my mind as one of the most prolific advocates of it.

From Mr. Borbely's famous article "JFETs The New Frontiers Part 2" (a balanced pair version of this cascode arrangement) ... 2SK389 is basically a dual packaged 2SK170. Slice this circuit in half and put that B1 buffer's JFET current source in place of the J511 CRD shown, and you have something pretty close to the A-T front end:

So Erno was a good friend of mine. I have letters from him when I was in college 1980-81. Meet him at Dynaco when he was working on the Hafler stuff.
Besides the LSK389 I would also take a look at the LSK489 it's become my fav over the 389 lower c and higher bias area.
Also note the current source in the source coupling needs to have a really high Z to work best for balance.

 

[pasarski]

If you omit R3 (i.e. set it to zero), the drain current for both FETs will be the Idss value of J2. The 2N3819 will be in the 10mA range, which is going to be too much for a phantom-powered circuit.

Regarding clipping, looks like we could do with a capacitor from J1's drain to ground, otherwise the voltage drop across R4 will increase dramatically for positive output current swings.

Upsy daisy, guess it's the negative halve clipping after all. Forgot that source follower is inverting.

 

[hifizen]

Alright... cracked open an old AT3032 and traced out the circuit - attached. It's a lovely circuit to my eyes. The "Borbely" cascode arrangement here is J2.

I've previously opened up the AT3035 and found a nearly identical circuit, maybe just a few small component value differences, but all the active devices and the circuit topology were identical. The AT4000 series mics also have a very nearly identical circuit... small differences, some more expensive caps for example, but still using the same active devices.

[Edit] oops... typo in the schematic: those 2SK208's should be gain group GR, not R. Schematic fixed & re-uploaded.

 

[k brown]

Alright... cracked open an old AT3032 and traced out the circuit - attached. It's a lovely circuit to my eyes. The "Borbely" cascode arrangement here is J2.

I've previously opened up the AT3035 and found a nearly identical circuit, maybe just a few small component value differences, but all the active devices and the circuit topology were identical. The AT4000 series mics also have a very nearly identical circuit... small differences, some more expensive caps for example, but still using the same active devices.

AT is very fond of that output topology; very similar to that in the AT871R boundary mic.

 

[hifizen]

I also recently looked inside the Bai Fei Li (ProAr) C 414 mic that folks are talking about, and noticed that it uses an almost exact clone of the capacitance multiplier voltage filter and the push-pull output buffer stage. The input stage on that mic is a different arrangement with a single JFET and a PNP emitter follower cleverly bootstrapping both the JFET source (DC coupled via it's bias resistor into the emitter) and drain (AC coupled to the emitter).

The output stage seems to work well, and sounds good to me. I'm fond of it too.

 

[jp8]

I also recently looked inside the Bai Fei Li (ProAr) C 414 mic that folks are talking about, and noticed that it uses an almost exact clone of the capacitance multiplier voltage filter and the push-pull output buffer stage. The input stage on that mic is a different arrangement with a single JFET and a PNP emitter follower cleverly bootstrapping both the JFET source (DC coupled via it's bias resistor into the emitter) and drain (AC coupled to the emitter).

The output stage seems to work well, and sounds good to me. I'm fond of it too.

Interesting. AFAIK, that JFET + PNP bootstrapping combo is from an AT patent. Probably expired. Nothing new, but still clever. In another arrangement, it's also being used here: https://www.edn.com/condenser-microphone-uses-dc-coupled-impedance-converter/

I simulated the EDN circuit in LTspice. Huge headroom, more than one will ever need (except for maybe @kingkorg, hitting his snare drums ). But simulated noise was quite high. I did not further investigate the root cause of this noise, but it was about double the noise from a typical Schoeps style circuit.

Did you draw up a schematic of the BaiFei 414?

Jan

 

[abbey road d enfer]

I simulated the EDN circuit in LTspice. Huge headroom, more than one will ever need (except for maybe @kingkorg, hitting his snare drums ). But simulated noise was quite high. I did not further investigate the root cause of this noise, but it was about double the noise from a typical Schoeps style circuit.

Noise coming from Q4, added to the TL061 noise, comes attenuated by 40dB to the drain of the FET.
Q1/Q3 have a rather high gain, determined by the FET's Gm and the load impedance.
I have a strong suspicion it's the cause of excess noise.
I believe filtering the output of Q4 and using a much quieter opamp would help.

 

[Migs 31]

Alright... cracked open an old AT3032 and traced out the circuit - attached. It's a lovely circuit to my eyes. The "Borbely" cascode arrangement here is J2.

I've previously opened up the AT3035 and found a nearly identical circuit, maybe just a few small component value differences, but all the active devices and the circuit topology were identical. The AT4000 series mics also have a very nearly identical circuit... small differences, some more expensive caps for example, but still using the same active devices.

[Edit] oops... typo in the schematic: those 2SK208's should be gain group GR, not R. Schematic fixed & re-uploaded.

Interesting circuit indeed, from AT.
Can you remember what type of diodes are D1 to D8 and D9-D10?
Anything written on them or any info on those ones. that you can share?

Thanks for cracking up that mic and sharing your findings here, much appreciated.

M

 

[hifizen]

Did you draw up a schematic of the BaiFei 414?

I didn't take the time to sketch it out, but here's a close-up photo of the board for anyone interested.

It's a little hard to make out, but the marking on the input JFET is 3T.

 

[hifizen]

Interesting circuit indeed, from AT.
Can you remember what type of diodes are D1 to D8 and D9-D10?
Anything written on them or any info on those ones. that you can share?

Thanks for cracking up that mic and sharing your findings here, much appreciated.

M

I didn't try too hard to identify the diodes - they're all the same, marked 3. Loads of matches on the SMD code search engines. As expected, they measure the usual silicon junction forward voltage.

 

[jp8]

Noise coming from Q4, added to the TL061 noise, comes attenuated by 40dB to the drain of the FET.
Q1/Q3 have a rather high gain, determined by the FET's Gm and the load impedance.
I have a strong suspicion it's the cause of excess noise.
I believe filtering the output of Q4 and using a much quieter opamp would help.

My bad, I connected C2 to the emitter of Q2 instead of the source of Q1, which resulted in the high noise. But even with the correct circuit and IC1 being a noiseless voltage-controlled voltage source in my simulations as a noiseless op-amp, the circuit is still 1-3 dB noisier than other circuits I simulated.

Your suspicion that the excess noise is mainly coming from Q3 and Q4 was right: selecting the individual noise sources in the schematic revealed that these were the main contributors. Not sure about that op-amp noise. R2 is a huge noise source, but its contribution to the output noise is negligible due to the attenuation caused by the low output impedance of Q1.

Anyway, even if one were able to improve the noise performance, it would still not be a circuit that I would use in a microphone build. The circuit remains simple without an additional output buffer, but it deprives you of the option to build in a low-cut filter between the JFET impedance converter and the output buffer. And the gain is a fixed 0dB. In many cases, you won't need more than that, but if you do want the circuit to have some gain, you're out of luck.

Jan