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 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.