Why does minitaur lose "oomph" when resonance is increased?

[botski]

Is this phenomenon a symptom of the circuitry, which implies the issue will always be there? Or is there a magic setting somewhere, and we just have to check "Dont allow Minitaur to lose "oomph" upon resonance introduction"? Or is this just all in my head ?

[GregAE]

Is this phenomenon a symptom of the circuitry, which implies the issue will always be there? Or is there a magic setting somewhere, and we just have to check "Dont allow Minitaur to lose "oomph" upon resonance introduction"? Or is this just all in my head ?

From the KnobTweak Database:

Q: Why does the output level change when I crank up the Resonance?

A: The Moog filter design provides a kind of automatic gain compensation (AGC). By this, I mean as you raise the resonance level, you will notice that the output level of the filter is reduced. The idea is to keep the overall signal level constant. Without AGC, high resonance values could significantly increase the signal level at the filter output, causing an overload condition within the synth's signal path. Not all LPF designs provide AGC. AGC can be either a good thing or a bad thing - it depends on your musical situation. If you are trying to create a strong bass sound that contains resonance, for example, AGC could be working against you by reducing the bottom end. AGC is not programmable and can not be switched off in the Voyager. Think of it as a design feature that creates the 'Moog sound'."

- Greg

[MC]

AGC is not really the correct term.

Resonance is implemented in the Moog ladder filter by inverting the filtered output and feeding it back to the filter. Those frequencies that approach the cutoff are already phase shifted and the inversion in the feedback network puts them nearly in phase, so a boost action for those frequencies nearest the cutoff takes place, thus resonance. By contrast, passband frequencies are out of phase and are attenuated due to phase cancellation.

[botski]

Thanks for the answers, and I am glad to know about the KnobTweak database now.

[GregAE]

AGC is not really the correct term.

Perhaps not, but the information in the KT database about this particular subject was derived from Jim Aiken's "Power Tools for Synthesizer Programming" (pages 88 and 89), where he uses the term 'automatic gain compensation' to describe the effect of lowering the level of frequency components in the passband as the resonance is increased in order to keep the filter output to a reasonable level.

Specifically, from page 88:

"If you are using a low-pass filter to create a bass tone for pop music, gain compensation can work against you, because as you increase the resonance the bottom will tend to drop out of the tone."

[sunny pedaal]

on the other end the ms20 using live or the crumar ds 2 sometimes needs two hands to tweak as the filter and resonance can seriously kill your speakers otherwise. with the mini or espec. the multimoog this is much easier and safer thanks to the effect described above