mediatechnology wrote: ↑Mon May 21, 2018 12:44 pmOne of the major considerations is servo pulling imbalanced phantom currents.
A quick experiment shows that Olaf's servo used in the MC preamp will have enough range to correct mic imbalance.
Yes, that servo can probably be made to have enough correction range... I'm just not sure whether the preamp circuit topology (where the opamps source/sink the current through the input transistors, i.e. "current feedback") is the the right one to use.
No matter which servo arrangement you use with that preamp circuit, it boils down to this: When you servo so that the output has 0VDC offset, then there is 0VDC across Rgain. Since the inputs sit Vbe below (or above, depending on transistor's polarity) the emitters/Rgain, you basically servo away any DC offset on the inputs (assuming identical Vbe for the two input transistors).
My personal rule-of-thumb is to never "leak" out any DC servo correction voltage/current through the input. But without DC blocking caps this circuit does. Of course, in this case it is meant to do so, more on this below. In a line-level circuit I'd put a unity gain buffer (non-inverting opamp) in front of it to stop my servo's correction action being visible through the input. With a microphone input that's of course not a good idea.
If we assume that the phantom resistors (the 2x 6k8) in the preamp are perfectly matched, then any DC offset on the input (before our servo corrects it) is caused by inequal current draw of the microphone. For whatever reason that is. Once our servo is used, it artificially changes the voltage on the input, which means that the microphone operates under different DC conditions now.
I'm not saying it does not work (remember, I also build a DC-coupled micpre with a flying frontend), just that it changes operating conditions for the microphone. And from preamps with phantom blocking caps and servos we know that we can get interaction between those phantom-blocking caps and the servo. Basically peaking in the frequency response or worst case even osciallation. Now we don't have caps of kown value in the preamp but persumably caps of unkown value in the microphone.
Again, it does work... usually. I haven't found any microphone that seemed to care. But that doesn't mean they don't exist. And "conceptually" it is wrong to alter the DC voltage on the input that the microphone created there.
The only advantage my servo circuit brings to the table is that it's Fc doesn't vary with gain (as long as the transistors have high enough beta and you pick the resistor values wisely). In order to not affect the microphone with your servo, you need a different preamp circuit.
Normally (i.e. with phantom blocking caps or if there is known to be 0VDC offset on the input) that preamp topology you are suggesting to use has many advantages. Just in this case it has one big disadvantage, in that it doesn't allow you to "decouple" the input DC offset from the output DC offset. At some point I have thought about using a folded cascode on the input, differentially modifying the current through the cascode transistors in order to DC-servo... but as long as the opamps source/sink the current through the input transistors you will end up in the exact same place.
From building my own preamp with a "flying" front-end, I know that it is easy to just take an existing preamp circuit and "fly it". But what's normally best is not the best circuit to "fly". - Sorry, I have no suggestion which preamp circuit to actually use... because all of them that have a DC gain of one (which should be fine for normal use, i.e. unless someboy shorts one input to ground) have caps in them.