Pro Audio Design Forum

Ah, dual servo loops, now I understand. Will have to think about this, whether it makes any difference in reality. What I realized is that if you have a servo that is correcting the offset across Rg, then this correction will also show up on the transistor's bases. Not in a totally linear way (becau...

Just checking in very infrequently, so noticed this just now... Vary the voltage on the top-side of the phantom pull-up resistors slightly to bring the mic output into balance. Duh, should've thought of that one before - it has the advantage of being able to correct large errors (tens of millivolts)...

To me this is not unique to an input-capacitorless topology - its a concern anyone should have using the input-correcting servos we seen thus far. Yes. I'm sorry if I didn't make it clear enough that this is a general concern and does not just apply to an input-capacitorless micpre. - An input (any...

Yes, with a transformer input the low(-ish) DC resistance of the primary winding basically shorts the two inputs together at DC. What I'm worried about is having an active feedback system controlling the DC offset. Because with an active system you can get things like overshoot which leads to peakin...

What about just servo'ing the top side of the phantom resistors? That would work and be quite similar to injecting the servo correction into the bases. I think our problem is that we want zero VOLTAGE offset on the input and don't really care whether the microphone consumes unequal CURRENT. Just th...

Just thinking aloud now: With that current imbalance spec of +/-4%, we'd need to deal with a max. of 2V DC offset of either polarity. So if we would split the gain into two stages, we could have a first gain stage (with differential output) with a fixed gain of 20dB and not need a servo. Assuming th...

Some caps are microphonic, so even if you don't find a lot of THD on a silent lab bench, you'd probably get some if you used them in a passive speaker crossover. Probably less of a problem in a micpre, but that really depends on physical placement of the micpre. What I find amazing with those listen...

Wouldn't "conventional" servo's at the bases also have an issue about who was in control even with AC-coupled inputs? There are two servos operating against (or with) each other on opposite sides of the input coupling cap. That's a good question. - And it is not just about "conventio...

What do your instincts tell you about which topology would be affected the most by moving DC conditions? What would be the audible effects? With microphones with caps on the output you can get interaction between the servo and those caps. You can look around and find the mic with the worst-case cap...

From experience, what value do you see in the technique? Lower noise, increased transparency? What are the audible benefits realized in practice? This is tricky to answer because I never compared the exactly same preamp with flying frontend and no caps to how it sounds with flying disabled and caps...