parametric eq frequency switching question

[blackieNYC]

rather then duplicate the whole equalizer, I'm thinking about switching between two frequency knobs.  The pedal will be a dual fuzz, and the selection of fuzz A will switch in frequency pot A, and the selection of fuzz B will choose freq pot B. 

Are there any foreseeable problems here?  Like maybe, if Vb is disconnected, will there be a harmless moment of non-eq audio, or will there be a moment of violent squeal?
If it matters, I think I'll be using a 4066 to do this switch.  A 3p2t stomp will insert the audio path of fuzz A or B, and the third pole of that stomp will drive The Tone God's Wicked Switch, using the 4066 to swap frequency pots A or B.  There will be two eq bands - 4 switches will be configured to chose 1 of 2 pots at each band, for each fuzz. 
(Another idea i'm considering, to get the pot count down, is to have only one frequency pot (per band) and have the fuzz selector switch simply add a 4.7Meg resistor in parallel to the pot, just to give fuzz A a different flavor than fuzz B.  the pot would work for both, but when you switch between them there should be a slight change of the eq.)

[garcho]

what two fuzz circuits are you switching between?

[blackieNYC]

I think I've settled on the Krank/Simple two Knob Distortion "collaboration" as Fuzz A, and the 49er/Tube Sound Fuzz as Fuzz B.  Both are great sounding and easy.
I've found a metal sound I like - a narrow Q low frequency boost.  After Fuzz A-or-B, variable between 125Hz and 600Hz, +6dB or so.  Like a low-freq-range parked wah.  Very modern metal sound to my ears.   Another narrow little bump between 1.2K and 4K for a little bite.  I don't feel the need for adjustable Q for this project.
Two fuzzes and a parametric EQ. But I want to stuff it in one pedal.  And make it easy to use.  For once.  So I think the two will not only share the EQ, but probably share one gain pot for each band of the EQ, which will be fine with me.  From there I could have 4 frequency controls - bass and treble for A, bass and treble for B. But that's too much. 
The frequency pot is 1M. instead of switching in a 2nd pot, I found that if I were to add a 4.7M resistor in parallel to that pot when fuzz A is engaged, I'll get a slightly different center frequency on fuzz A than fuzz B when I switch between the two.  Thru half the pot's travel anyway.  But I"m not switching in a preset, the pot still works - just a different response.
In the end, it's really just a dual fuzz, where each one has an unusual Bass and Treble control.  I'm concealing the fact that there is a fully functioning two-band parametric eq in there.  Just some interesting tone controls.

[blackieNYC]

Switching Vbias between two pots is ok?  Looks odd.  I'll try it out.

[samhay]

They pass DC - they are not caps - so you won't have as much of a problem with popping.
However, the op-amp bias will momentarily disappear, which may be audible. If it is a concern, string a 10M resistor parallel to the pots.

[FiveseveN]

There's no DC path through the gyrator, U1b gets its bias from U1a.
Leaving the pot floating will disable the filter (might even be useful for a "tone bypass" option). Not sure about pops, the 10M suggestion should work.

[samhay]

Bias (or not) of U2A - the gyrator op-amp is the problem.

[FiveseveN]

Ah sorry, I misunderstood then. But now I remember doing a similar mod on an Ibanez SM7 and don't think it had any issues.

[blackieNYC]

Got the eq on the breadboard. There is quite a crunch when I switch from one pot it Vb to the other pot to Vb. used a 10Meg to 2.2 meg resistor in parallel (and always present during the switching) and it really didn't make a difference.
What is silent is to separate the Gyrator leg from the audio path. You just lose the eq with no audible unwanted noise of any kind. Instead of just the two pots, I could build two gyrators. Not that muc in parts. But another op amp stage I wasn't planning on.
Btw, the 5532 is superior to the TL072 as the gyrators.  The TL0 is prone to oscillation on power up if your Q is narrow.

[garcho]

^ FET vs BJT

I could build two gyrators.

how many times are you going to build this pedal? might as well have two tone controls, no?

[blackieNYC]

How many? One! Even if I should sell it! I really should give full controls to each. An exercise in knob&switch count self discipline. Double dirt - just 2 gain, 2 volume.  Parametric- 2 gain, 2 center freq. if I don't share any controls between the two fuzzes. Q I don't need.  8 knobs.  And now 4 gyrators - best way to avoid the switching noise. But I don't need to duplicate the in and out of the eq.
Why do you suppose the FET op amp goes into oscillation and the BJT op amp does not? What spec might that be related to?

[garcho]

input impedance, slew rate are different, but i don't know why one would squeal and the other not