Schoeps/Dorsey circuit theoretical questions

[abbey road d enfer]

OK, I've looked at the schemo. The FET is configured as a cathodyne phase-splitter, so the input capacitance is a tiny fraction of Cgs + 2x Cgd.
Anyone can tell me why it's not suitable for an SDC?
is it just because the bias voltage is only 30V or so? Or is there anything else?

 

[Gus]

Something to try is to build the circuits.  I have built the Dorsey and and the 012 circuit with good parts.  I like the 012 circuit better than the Dorsey. 

Look around the Neumann site for "Microphones". Search the Yahoo microphone group a very good thread about the the Schoeps circuit.

 

[Rossi]

My main concern with the Dorsey variant is noise. 35V is not enough to get good S/N from most SD capsules. At 60V a Schoeps mic gives self noise of about 15 dB A. That's pretty good. At 35V the same mic would be around 20-21 dB A. That's not so good.

In my experience the 2SK170 performs no better in this hi Z phase spitter application than a humble 2N 3819 or a BF245A.

 

[Silvas]

The second is the choice of the input FET. The schoeps uses a BC264 which has an input capacitance of 4pF, and the dorsey uses a 2SK170 with an input capacitance of 30 pF. Considering the very high impedance at that point, isn't such a large input capacitance a bit of a problem?

At 16kHz the BC264 should have a input reactance of 2.5M (already quite small compared to the 1G precharge resistor), the 2SK170 on the other hand only 330k. Wouldn't it be better to use the BC264, despite the slightly higher noise figure?

The choice of 2SK170 is completely wrong, as used in Dorsey mod (as well as in many commercial mics). Having said that, it might be a very good choice, once all the voltages and operating points set right. Just look carefully at its data sheet to find the answers, otherwise, use what Schoeps does (and leave in the biasing pot). Look for Zapnspark messages concerning the right procedure of biasing.

Best, M

Can you elaborate on that? I have no idea what the Dorsey mod is, but I don't see why the 2SK170 would be such a bad choice.

I tested 2SK170, BC264C and 2N3819 with good results on my pcb. I didn´t noticed any headroom or sound differences, but i didn´t tested too long.

 

[bitman]

My main concern with the Dorsey variant is noise. 35V is not enough to get good S/N from most SD capsules. At 60V a Schoeps mic gives self noise of about 15 dB A. That's pretty good. At 35V the same mic would be around 20-21 dB A. That's not so good.

In my experience the 2SK170 performs no better in this hi Z phase spitter application than a humble 2N 3819 or a BF245A.

Interesting.

Some years ago I wanted to make an ambisonic mic. I got 4 mxl603 SD capsules, some perfboard and the Dorsey article and went for it. It worked well but each amp was on the hissy side. I cleaned the boards until you could see yourself in the solder but nothing ever changed. Knowing nothing outside of the recording article, I was always suspicious of the 35v polarizing voltage as all 4 head amps exhibited identical s/n ratio which is inline with what you cite. Not unusable, but not good either. I'll have to now see if I can cram a doubler in that already crowded mic body.

 

[leswatts]

In my experience the 2SK170 performs no better in this hi Z phase spitter application than a humble 2N 3819 or a BF245A.

Right , Rossi... non inverting section output impedance is gm, but inverting section it's the drain resistor.. what's that ? 2K2? 6 nV/sqrt hz?

Les

 

[l0calh05t]

Something to try is to build the circuits.   I have built the Dorsey and and the 012 circuit with good parts.  I like the 012 circuit better than the Dorsey. 

Look around the Neumann site for "Microphones". Search the Yahoo microphone group a very good thread about the the Schoeps circuit.

I'd really like to, but I have no spare SD capsules lying around (and as I said... I'm not going to hack up my Oktavas just yet)... what would be the cheapest source for true condenser capsules? Of course I could also get a spare -10dB pad and hack that up as a connector... hrmmm...

 

[Rossi]

Right , Rossi... non inverting section output impedance is gm, but inverting section it's the drain resistor.. what's that ? 2K2? 6 nV/sqrt hz?

Never having studied ee, I lack the knowledge to compute exact figures, so I mostly use spice to get ballpark figures. Or acutally build the circuit, which I did in this case.

But I was actually referring to source Z which is very high in any condenser mic. I'm sure at low source Z the 170 outperforms a 2N 3819 or a BF245A, but even at medium source Z, I couldn't hear much of a difference (which surprised me). I experimented with active ribbon mics for a while, and with the transformers I used I had a source Z of several kohms.

 

[Rossi]

what would be the cheapest source for true condenser capsules?

Get a t.bone SC140 from Thomann. The capsule is pretty good, actually.

 

[l0calh05t]

Get a t.bone SC140 from Thomann. The capsule is pretty good, actually.

Those are fairly cheap, yeah. But what about the even cheaper t.bone EM700, is that a true condenser, or back electret? Same question for the SC300.

 

[Rossi]

You're on your own there, Bruce Campbell. The 140s are the ones I recommend. They're insanely cheap and you'd have to be insane to go cheaper. BTW they have a DC converter inside.