Mic head amp with MOSFET gain stage and input buffer, makes sense or not?

[pasarski]

This thread http://www.groupdiy.com/index.php?topic=39091.0 and Gus challenging to think out of the box made me think of other solutions for SS mics than the usual suspects.

Reading through these http://www.passdiy.com/projects.htm articles (the "Penultimate Zen" especially) I came to think if a buffer described there would make the input impdeance high enough for a condenser mic capsule? I'm still an electrical idiot so what I'm asking is should I explore MOSFET's for mic head amp at all or am I in a completely wrong path? ???

 

[JohnRoberts]

While mosfets are capable of very high impedance, they are not noted for low noise (AFAIK). That circuit you referenced has an input impedance of 47.5k due to input resistor and  output noise of 35 uVolts divided by the gain of roughly 4x divides down to 8 uV at the input. 

If you can live with 8uV of input noise go for it... 

JR

 

[johnR]

With a 47k input impedance, noise would be the last of your worries

Condenser mic head amps typically have input impedances in the gigohm range at low frequencies.

MOSFETs have high input capacitance as well as bad noise figures. I suspect that someone would already be using them if they were as good as JFETs, but I've never tried one in such an application. The easiest way to find out is to try it...

 

[pasarski]

Yes I understand that the source resistor should be more like 1 gig. And that would mess things up propably..

I don't know why I came to think MOSFET's anyway. High noise is not a good thing. So JFET is the one I should concentrate. Should probably forget the input buffer too?

One other thing (this is again one "brainstorm/-fart" thread of mine that I regret later ) :

I've built one SS circuit, Oktava 012, and when I had the BJT follower connected wrong way the mic sounded muffled of course but it had a nice character I haven't heard in SS mics usually. It was not just the missing highs and lows but something else. When I got the BJT  right way the highs and lows came alive but the mic started to sound more like every other FET mic out there. I'm trying to think ways to lower the output impedance with out a BJT. And without a transformer. Why? I don't know.

 

[johnR]

So JFET is the one I should concentrate. Should probably forget the input buffer too?

You need an input buffer. That's the main function of the JFET. The fact that it also provides voltage gain in some circuits is incidental. The main thing it does is provide the extremely high input impedance required by the capsule.

Getting a low enough output impedance using only 48V phantom as a power source is a problem that has occupied mic designers for decades now. You could do worse than study schematics of established designs for ideas. The only way a single JFET would be able to do it without extra transistors is with a step down transformer, eg. as used in the U87.

Edit: There is another way, which is to add a high frequency oscillator and transformer (or a switching buck regulator, but that's likely to be noisy) to provide a lower voltage, higher current supply for the JFET.

 

[pasarski]

Thanks for he clarification. The term buffer was unclear to me.

 

[Samuel Groner]

I don't recall any noise measurements of contemporary MOSFETs; likely they are still noisier than good JFETs but it's a function of how much noisier. For sure MOSFETs have less current noise.

As far as I know at low frequencies the current noise of JFETs is not entirely negligible for mic amps. I've though about an architecture where a MOSFET source follower is followed by a JFET source follower and at high frequencies the former is bypassed with a feed-forward capacitor.

Samuel

 

[JohnRoberts]

I haven't done a new mic preamp design in almost 2 decades, but I have maintained a passing interest in device technology.

IC mic preamps are accepted as good enough for most of the industry.

I have seen some modern JFETs with input noise voltage comparable to very esoteric and expensive lab curiosities of old, and decent bipolar SOTA. I just ASSumed their noise current would be much better than bipolar devices, while their input capacitance might be an issue. This would have become apparent if I melted any solder, but these days I don't do that very much just for fun.

It seems the low hanging fruit in mic preamp design has already been harvested, so improvements from here are incremental and not very audible, while some are happy to pursue those fractions of a dB.

JR

 

[bockaudio]

I haven't done a new mic preamp design in almost 2 decades, but I have maintained a passing interest in device technology.
IC mic preamps are accepted as good enough for most of the industry.

He's talking about the microphone head amp that the capsule connects to, not the microphone preamplifier that follows the mic.
Most current production fet mics do NOT use IC's.
regards,
David
www.bockaudio.com

 

[pasarski]

I haven't done a new mic preamp design in almost 2 decades, but I have maintained a passing interest in device technology.
IC mic preamps are accepted as good enough for most of the industry.

He's talking about the microphone head amp that the capsule connects to, not the microphone preamplifier that follows the mic.
Most current production fet mics do NOT use IC's.
regards,
David
www.bockaudio.com

Yes I'm talking about mics and was only wondering about the use of MOSFET instead of the usual JFET, and wondering if the two MOSFET topology could be converted for mic use. I think I mixed up the terminology and made people confused

 

[JohnRoberts]

Gotcha... my general comments about mosfets still stand, while I haven't looked at them lately they generally "were not" very low noise for low level audio frequency applications.

JR