Pro Audio Design Forum

Sorry for getting this thread back to live.... but it was really worrying me that reality worked better than simulation... :roll: For reference I'll link in the image from my first post again: http://www.analoglabor.de/DIY/instamp_new.jpg My proposal was to inject the DC servo's (differential) corre...

I'm not too clear about the "differential receiver on the other end" concept, practically speaking. You mean adding another active stage at the other end? You (most likely) don't have to add an extra stage. For example, if you run your signal into an inverting opamp, just build that opamp...

That's because at high gain the cutoff is higher - I chose 7 Hz at 60dB. The cuttoff frequency of my caps and Rbias is about 0.7 Hz (since I need to record signals down to 8Hz*). So the servo cutoff can only be below that at max. gain. And with a gain range of 60dB this results in the servo being s...

Option C however does not bootstrap your input resistors. No, obviously not, because it just explains how to servo the circuit without having to touch the bases. So in order to have a bootstrap for Rbias, you'd still need a cap and two matched resistors for that. And yes, I am working with a discre...

Hhm, this thounds all rather complicated. Especially considering the precision neded so that no imbalance (due to not so perfectly matched resistors) is added which would mess up the effect of the CM bootstrap. Since both alternatives have in common that they do not sample the DC error across Rg, wh...

Making RaDC and RbDC large has an upper limit since there is the collector current across them. So at some point you'll have too much voltage drop across them (i.e. the inst. amps output sitting at an extreme DC offset eating your headroom). But the THAT1570 in JRs example shows internal constant cu...

But I think there's a differential input/differential output integrator out there than can have an AC common mode signal impressed on the outputs. One could use a Diff-Boo followed by a fully differential opamp (like OPA1632), driving that opamp's Vcm pin with the AC common mode signal. Or maybe ev...

Ok, please ignore the question where to sample the DC offset for a while... The circuit you drew show both differential servo correction and CM bootstrapping. I think you can however combine both functions into the servo and have it produce both differential DC correction and common mode bias resist...

It seems like combining the functions of both servo and bootstrap using a combination of differential and common mode drive feeding bias resistors is a possibility. So would that work? Maybe... but isn't that almost what my circuit a few posts above does? Except that I have separate resistors for C...

If I understand correctly Olaf's main concern is both how you inject servo correction back into the preamp input without creating an added common mode load. Yes, that's exactly whyat I'm trying to do. I've done a bootstrap from the emitters for two different reasons and it works pretty well. The pr...