who needs jfets

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Gus

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I don't think you need to find JFETs anymore to build a good phantom powered transformer out microphone.
There are opamps(with fet inputs) that you can use with a good circuit design that do not need any selection.
You do need a good transformer.
You can also build nice transformerless circuits as well.

After all the discussions on the web about microphones over the years
Same old copy the old stuff Instead take what is good from the old stuff and add the good from the newer devices.

discuss
 

Khron

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With opamps, the biggest drag might be the quiescent current of opamps - many would like to suck almost all that phantom power can offer. Jfets, while also cheaper (?), can be made to work "well enough" with piddly (sub-mA) currents.
 

Matador

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It's also difficult to find an op-amp that can withstand over 40V: most tap out at 36V max when using a single supply.

I built a microphone prototype using a discrete op-amp that could take up to 100V, however it was using JFET's in the input differential pair. :) It seemed to work well, but sonically I wasn't convinced it was remarkably better than the plain JFET into a transformer.

If you don't mind sending clean, bipolar rail voltages to the mike (via a 5 or 7 pin XLR) and abandon phantom power, then it really opens up the possibilities.
 

rogs

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There's a DIY hobby project for FETless electrets HERE that uses Opamps instead of a JFET.... and there's a version with a DC voltage multiplier for LDC capsules HERE ..

Looks like they might be an interesting alternatives to using JFETs ? .....
 

Bo Deadly

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Discrete parts can be significantly better than integrated circuits in various ways. It has to do with things like dielectric isolation in an IC that prevents making matched N and P parts that are equally good. Noise is probably going to be the big difference. A discrete JFET can have noise that is a few nV/rHz. 2SK170 is under 1 (with perfect contitions). A low noise JFET amp is going to be 5-10 nV/rHz. That makes a difference in a mic pre.

But don't let me stop you from trying. Supply voltage isn't a problem. You can just add a zener to protect the amp. Then drop >18V or so over an RC. You're going to need a good reservoir of power anyway to drive the line without causing droop. That would be the real benefit of using an op amp. You can drive that transformer really well.

A transformerless circuit is less elegant since you really need two op amps to drive balanced.
 

Gus

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I built one of the circuits I simmed. It drives the transformer like a nice tube for a microphone.
Some nice opamps are under 2mA
 

Bo Deadly

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You have enough current for a 2mA for the op amp, 1mA for the Vg divider and 1mA for a JFET in front of the op amp. Then you get the low noise benefit of the discrete JFET and drive capability of the op amp. At 4mA total at say 24V, that 13.6V dropped across your phantom supply leaving 10.4 which is a dropper resistor of 2.6k (although presumably split into two 5.1k resistors from each signal line) into 22uF. But then you have a Vg divider of two 12k resistors into a big cap like 100uF so that you can connect the ground end of the transformer secondary to that and skip the output coupling capacitor. Then make another RC off of Vg for the 10V for the JFET input. Use a switch with two different coupling caps into the inverting op amp to implement a low cut. And make sure you have enough low cut to stop Vg from drifting.

Or you could leaving out the JFET. But if you do that, makes sure you use an SMD amp with a guard trace around the input to limit leakage. And since you have a nice low drive output from the op amp, you might run that back to the input to bootstrap it for high Z instead of using a 1G resistor which is sketchy.

Although there is one major problem with all of this which is that your capsule polarization voltage is less because you are drawing more current. And with prolonged signal, your supply might droop and reduce the capsule voltage further and output with it (which could be timed to make a type of compression I suppose).

The mind-numbingly simple KM84 circuit is hard to fault honestly and it leaves 46V for the capsule.
 

Gus

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As I posted you can build a nice circuit with an opamp.
There are FET input low noise rail to rail opamps
You don't need 1mA for the divider

You don't need a selected JFET you can drop in an opamp in a circuit with no adjustments

I built a circuit without a DC to DC remember you sometimes want to pad the microphone signal

Look at the links rogs posted for some ideas.

I have built a few microphone circuits with the basic circuit you can find in the KM84. I did not post I did not like the common 1 JFET circuit.
I have a few adjustments to that basic circuit I like
 

Recording Engineer

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I remember Christopher Fritz in New York was doing opamp-based mics under Fritz Soundlab with electrets and we’re electronically-balanced (all if I remember correctly). I never looked at them closely and I see his website is no longer up.
 

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