Dumb idea for a mic circuit?

[tk@halmi]

Forgive me for pointing this out, but to me the asymmetry is visible if you compare the top and bottom half of the output sine wave. I am referring to the shape, not the magnitude.

 

[OneRoomStudio]

Forgive me for pointing this out, but to me the asymmetry is visible if you compare the top and bottom half of the output sine wave. I am referring to the shape, not the magnitude.

That's fair. There may be some asymmetry resulting in some even-order harmonics, but the same could be said about almost every "classic" microphone. As long as it's subtle and no hard-clipping, I'm fine with it - especially since this is with a 500mV input, which is much higher than the circuit would normally see.

 

[inkster]

As a young engineer/tech/tinkerer I marveled at the beauty of phantom power...an elegant solution to the problem of powering a condensor mic without an inconvenient power supply to set up, only problem back then I couldn't afford any of the mics I dreamed of owning but I was able to get my hands on a Senn T-powered mic.
That helped jump start my fascination with mics once I built my own 48V to 12V adapter box thingy.

How would a +/- voltage potential safely and reliably work utilizing a 3 pin XLR?...seems to me an out of phase cable or connection would cause issues resulting in a need for protection diodes or a circuit to resolve the potential for external wiring errors inside the mic.

I'm a huge fan of well designed higher voltage circuits like those found in the 5088 console and some older solid state designs but my 2 cents would include a DC-DC circuit to bring the 48V up to the desired voltages, which of course is already widely used but there is always room for innovation and experimentation.

Build a working model, I would love to see the results!

 

[jp8]

That's fair. There may be some asymmetry resulting in some even-order harmonics,

Did you make an FFT of this waveform already? For most professional condenser mics, max input and output levels are specified at 0.5 % THD. On an oscillogram, THD often becomes visible only at levels above 1-2%. As the distortion is very obvious, I'd expect way higher THD. Could be exactly what you want if you'd like to add some "color", but if you want to compare apples to apples, you should specify the input and output levels at 0.5% THD.

I'd suggest to take a more realistic load to the circuit. Most preamps have an input impedance of 1-3k. A 10k load gives too rosy a picture of the distortion and max input- and output levels.

resulting in some even-order harmonics, but the same could be said about almost every "classic" microphone.

Sure, any circuit will generate lots of distortion if you drive it hard enough. But to say that every classic microphone generates the same amount of distortion at the same input level is just not true. There are many clasdic designs, from properly designed and biased Schoeps circuits to OPA Alice or Rode NT5 that generate less than 0.5% THD at 500mV input. Some can handle several Volts at their input. If you add a feedback capacitor as I mentioned in one of previous posts, I'm sure you'll improve the performance considerably!

Jan

 

[abbey road d enfer]

For some reason, I get different results.
Here I compare the outputs with 0.5V and 1V at the input?
With 0.5V distortion is visible, and with 1V clipping is evident.
Too much uncontrolled gain.

 

[jp8]

For some reason, I get different results.
Here I compare the outputs with 0.5V and 1V at the input?
With 0.5V distortion is visible, and with 1V clipping is evident.
Too much uncontrolled gain.
View attachment 140871

Yes, without any feedback applied, it's just an open-loop CS amplifier with Gain governed by gm and load impedance only. With gm all over the place ( device-to-device variations and as function of bias current) and a frequency dependant load, gain will be ill-defined.

Jan

 

[moamps]

View attachment 140871

Maybe I'm not seeing well or I'm already that old, or it's too late, but considering that IMO the gain of the circuit cannot be calculated if the active parts are not biased into the active region where they work as some kind of amplifiers.
So, if the gate voltage is 0V, for this type of FET, the source cannot be at a more negative potential if we want to place it in the active linear region.
At the same time, if the base of the BJT is at 0V, the emitter must be at a lower potential, around -0.65V.
These are two contradictory requirements.
This means that IMO this circuit as it is does not work as a linear amplifier at all. Not to mention what the input impedance of the circuit is if VGS is positive.
But I may be wrong... Also, forcing the current of that FET to about 13mA does not seem optimal to me.

 

[OneRoomStudio]

Well, if nothing else, I now know that the answer to my topic question is a resounding "Yes."

A few things to answer some questions though:
-I never intended to feed this power via 3-pin XLR. This would need at least 5 pins, like a tube mic.
-This wasn't intended to be as linear and distortion-free as an Alice or Schoeps (I'm personally a fan of mics with a bit more color)
-I wasn't shooting for "perfect," just something different.

Clearly not a direction worth pursuing though. Thank you all for taking a look.

 

[k brown]

I've really liked this circuit for "something different" in a mic (constant current source).

B1 Buffer, designed by Nelson Pass as a no-gain HiFi 'preamp'.
https://www.passdiy.com/project/preamplifiers/b1-buffer-preamp

I adapted it for use in microphones by replacing R103/203 with 1G, eliminating everything before the FETs (replacing with 1G +820pf, for capsule polarization) , adding an impedance-balanced output, and providing power from a pack of five 9v batteries supplying polarization, and tapping off at 18v for the FETs.

Despite many saying that 2SK170 is ill-suited for use in microphones, this circuit has performed beautifully with Primo EM21/23, and Oktava '012 capsules.

 

[tk@halmi]

There are changes you could make to have better control over biasing and gain.
- Bias Q2 using a common two resistor voltage divider.
- Tame gain with partial bypass of the source resistor (just add a smaller resistor in series).
- Reduce current a bit to give those transistors a little break.