opamp-based mic circuit idea

The cleanest, simplest, lowest consumption polarizing voltage circuit is Dip. Ing. Wuttke's as used in the Schoeps and its myriad copies.

Yes, that would be much simpler!  I'll give that one a try.

With the CMOS inverter -> multipliers, it's possible to get current draws under 200uA, but at the expense of very high part counts and low frequency operation.

Here's a modified version of the Schoeps converter.  Many thanks to Ricardo for the suggestion;  this is much better than what I was trying before.  Current consumption is a little worse but good enough.  The oscillator's output is a clean-looking sine wave around 800-900kHz.  My plan is to have the input voltage adjustable with a trim pot - on the breadboard, it took about 10V in @ 900uA to produce +/- 60V out.

I still need to take a moment and make sure I understand how the oscillator works (I don't think I do, to be honest), but this seems like the way to go!  Planning to go ahead and start on the schematic for the complete mic.

Decouple Q1 collector with a 100n ceramic.  I know Dip Ing Wuttke doesn't have it on the old CM5 but it makes the circuit more reliable.

Current consumption is determined by R1 but don't make it too large or it will stop oscillating.

Be careful how you measure these voltages.  You need to allow for the loading of the DVM.

Physical placement of the inductors is critical.

Excellent tips, thanks again!  Are there any specific things to watch out for with the inductor placement?  I just tossed it together on the breadboard and it works, but I imagine cutting down on rf emissions is a big goal.

I measured the voltages with an electrometer.  Input impedance is somewhere higher than 10^14 ohms.

dfuruta said:

Are there any specific things to watch out for with the inductor placement?  I just tossed it together on the breadboard and it works, but I imagine cutting down on rf emissions is a big goal.

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If the inductors are too close together, the mutual inductance can work against oscillation or help it  Depends on loadsa stuff like which way the inductances are wound.  Better to have them at right angles.  If you get good results with a particular layout, copy it for later designs with the same inductors.

I measured the voltages with an electrometer.  Input impedance is somewhere higher than 10^14 ohms.

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Jealous! 

dfuruta said:

I still need to take a moment and make sure I understand how the oscillator works

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This circuit works because the inductors are magnetically coupled; in other words they make a transformer. With the right conjunction of stars and your shakras being aligned, this transformer is a step-up, which means it is capable of delivering a higher voltage to the base of the transistor than the voltage delivered by the emitter. The transistor is in voltage-follower mode so has no voltage gain. In order to oscillate the circuit must have some gain. That's the xfmr that provides gain.
As Ricardo says, proper orientation of the inductors is paramount.

Interesting.  On my breadboard, the inductors are almost an inch apart.  I'll try some different arrangements and see what happens...

I thought this was a variation of the Hartley oscillator, but maybe it's not quite the same.

Jealous! 

Click to expand...

It's amazing what one can get off of eBay .  Old test gear doesn't have much resale value, I guess.

Here's a first draft.  Still need to experiment with inductor placement.

Ive always found this an interesting circuit.

Got PCBs and I'm starting to put them together.  The oscillator works fine, haven't fully tested the amplifier yet.  I am trying to fit this into an MCA SP-1 body, to start with, and it's going to be a bit tight.

In my earlier draft, C13 and C24 shouldn't be there;  I was going on auto-pilot, I guess, and didn't even notice that the LT3014 is ground-referenced.  Having those caps will result in high voltages hitting the opamp & fet upon startup.

Once I get something working and have a chance to try it out, I'll post an updated schematic and layouts.