Tone Stacks - Part 2 - James & Baxandall

[m4268588]

Is it possible to easily "shift" a Baxandall tone stack? What I mean is, say the sweep for a frequency is -15 dB to +15 dB, and 0 dB with the pot in the middle position. What if wanted -9 dB to +21 dB, and +6 dB with the pot in the middle position? I.e. we rarely want to reduce, and only a little when we do; but we want a default (i.e. pot at noon) bump for that frequency, and a bigger boost range?

 

[aquataur]

This is very interesting. I never saw it mentioned anywhere, but this matter of putting a passive stack into a feedback loop reminds me of bootstrapping. Although the concern is not the outwardly "seen" impedance, the S/N ratio should be markedly improved.
For that matter, can I use a FMV Tonestack in a feedback loop?

 

[Chuck D. Bones]

Hmmmmm. Maybe.
Three concerns:
1. Putting a filter inside a negative feedback loop inverts its transfer function. Mid-cut becomes mid-boost, etc. In the F configuration, the feedback gain can go to zero, which means the close-loop gain becomes very very large.
2. If the filter's phase-shift is 180° at any freq (other than zero Hz) and the loop gain is greater than unity, then you have an oscillator.
3. Make sure you have a DC path in the feedback loop, otherwise you have no control over the bias point.

 

[aquataur]

Hm. I begin to see a light. So for something like an active James Stack boost and cut are inverted, but since it is boost and cut anyway, only the rotational sense on the pot would change?
Accordingly, an all-cut stack like the FMV becomes an all-boost.
And for a BMP it works out too.
Edit:

Make sure you have a DC path in the feedback loop, otherwise you have no control over the bias point.

Do you mean something like there is in the CC1? Most of the ones cited here don't have such a feedback resistor. And indeed, taming cap.