Pro Audio Design Forum

... had enough range, but plenty more was available. There are two problems using base injection that we have all collectively figured out: 1) The servo Fc varies with preamp signal gain. 2) The servo response can be undesirably second order and peaking when it potentially interacts with the mic's ...

... long as we have DC gain in there we have to get the correction on the input just right, and not just approximately. Otherwise the proposed second servo would need to do way more than just correcting the mismatch caused by Vbe differences.

... outside a predefined window. A calibration procedure ? Initiated by the user and a warning led to tell when out of window. I thought of this for servo loops that could be used to adjust, then frozen to keep the adjustment. I see such a technic for a mike preamp: Half way between balancing with ...

I started counting and there are three servos if we count the common mode servo operating in feed-forward. Instead of a "twin servo" (already taken) it's a triple servo. LOL. I think the time constant of the servo that corrects microphone ...

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 ...

... We did discuss in an email varying the top of the phantom resistors similar to what Paul Frindle did. You are correct that if it was the only servo it would be no different then servo'ing the bases. What I was proposing was dual servo loops. One which samples the offset at the input terminals ...

... - I don't see much difference between varying the voltage at the top of the phantom resistors compared to the very traditional way of injecting servo current through resistors into the bases. Sinking/sourcing current using extra resitors will cause some current through the phantom resitors resulting ...

... now. Updated January 25, 2022 : A new PCB version, assembly instructions and Project Manager link are available. Improvements include a simpler servo, selected components to improve CCIF IMD2 distortion and an output pad for head amp applications. Product links: Assembled and Tested PC Board: ...

Take a look here at some servo bandwidth tests I did with an OP07: https://proaudiodesignforum.com/forum/p ... t=20#p4999

You might want to read the full thread.

I compared using a differential Deboo integrator, and a non inverting two caps integrator. Integrating capacitor(s) insulation resistor does result in DC offset error in both cases. With the Deboo the resistor mismatch brings a similar effect. With the non inverting two caps, there is no such resist...