Solid State Microphone Circuit Idea

Hi,
Gus sent me this schemo for uploading. It's an idea for a solid state, transformerless mic schematic:



Can anyone see any problems? I'm sure Gus will be along later with some comments on his design.
:thumb:

Gus,

I am sure you will get back and comment on your HT supply.
I think 50 Ohm from ground in the output stage should be recalculated as an output impedance of the buffer plus 50 Ohm in series with it, for better impedance balance, and probably another 470 MF (why so high?) should be added from ground in series with calculated R.
Have you tested it yet?
Once I made a mic output as a FET source follwer, with another FET as a CCS, with balanced impedance output. Then the input stage could be made as an amplifier, and then directly couple both stages , using DC for SF bias.
It would be very interesting to compare these two schematics.

Excuse me if I appear a bit 'dim' here, but why not use a more traditional arrangement, where the fet is powered from the phantom voltage?

That would save needing to have a PSU box... ???

Alan

You could take the output off the source instead of the junction - that would eliminate one capacitor from the circuit. You could phantom-power this sort of circuit - I looked at doing that and you could get about 40 volts or so on the capsule. Unless there's a specific reason to use a separate power supply, using phantom makes a lot of sense to me. I had a similar design one time, it's attached. Feedback to the drain of the JFET will tend to reduce distortion even more than a source follower does on its own. I haven't tried it in a microphone yet, though it does bias properly on the bench. R4 will need to be adjusted to match the FET and bipolar to get a correct voltage drop across Q2.
http://www.telusplanet.net/public/dulan/newoktavasch.jpg

POWER

I don't think phantom has enought power. Look at the current in the output stage.

I was using ideas from the jensen web site, mackies, AKG, Neumann microphones. They get a balanced drive with what looks like a unbalanced circuit.

The two 50 ohms would go to 2 and 3. Marik yes the 50 ohm would have to be adjusted to a lower value from the EF with constant current sink to balance it the best.

I think the "problem" with solid state microphones is the limited power from phantom.

I need to do the math for the load that the base of a the tip50 places on the source of the fet in this circuit.

FET curves are not as linear as a good triode, so the fet needs feedback to help it out. A SF is a linear way to use a fet.

Dale

That circuit looks a little like the oktava 012. I like circuits like that.

I was going for a different idea. My ear/brain seems to like devices run at higher currents.

Gus

Yea, it is similar to an Oktava - it re-uses the FET current in the buffer stage. I tried to get rid of the coupling cap and one of the 1G resistors, but the rest of the circuit is similar. The other change is coupling the output up to the drain of the JFET to help linearize it. Many new mics do that sort of trick - that's what the twin JFET at the front of a TLM103 or the newer C414's do.

The new "Oktava-like" circuit has a 35 volt polarizing voltage, runs the buffer at 1.8mA, and the JFET at about .9 mA. The total current is 2mA. The output will be about 2dB quieter than the original Oktava circuit. It will probably deliver 1.4 volts P-P into 1200 ohms or 350mV P-P into 200 ohms without introducing obvious distortion.

[quote author="dale116dot7"]The new "Oktava-like" circuit has a 35 volt polarizing voltage, runs the buffer at 1.8mA, and the JFET at about .9 mA. The total current is 2mA. The output will be about 2dB quieter than the original Oktava circuit. It will probably deliver 1.4 volts P-P into 1200 ohms or 350mV P-P into 200 ohms without introducing obvious distortion.[/quote]Looks like a neat idea. Have you tried this on any of the Oktava mics yet?

Not yet. I've fiddled with the circuit on a breadboard but not on one of my mics yet. I might try that a bit later. I've been concentrating on my variable-mu compressor lately.

Could i use a 100megs resistor at the input?...that's the highest i find here...thanks!

Yes, in this circuit you can. That's because that 1 gig resistor shows up as an impedance that's almost infinite because it is amplified by the gain of the FET. The B+K measurement mics do this with a 6 gig resistor to achieve a frequency response that's flat down to somewhere around 0.1 Hz.

Thanks!...again :thumb: :thumb:

Maybe we could get together on a few mics... A totally handmade DIY mic would be great...


http://www.groupdiy.com/index.php?topic=7370

Yes!
that's what i'm dreaming...put together a mic,from the ground....
It would be a big challenge for me,not only for all the technical/skills needed,but for the lack of sources of parts here in my country...
And i'm also thinking of my own transformers...
BUt first,i'll build the capsule,and a simple fet preamp to test it...

This is turning me on more than Liv Tyler :razz: :razz: :green: ....