The circuit detail presented is a gyrator filter that is one of two that hang out in the feedback loop of an opamp, creating a "low" and "high" tone control for audio. (The "low" section is shown) I'd like to have two available boost/cut frequencies for each control, though. It's easy enough to switch an extra set of paralleled caps as shown and breadboard tests are so far successful, but I've never seen it done this way. Is there a demon lurking in the floating capacitors?

And does it really matter which "side" of the caps the switch is on?


simulate this circuit – Schematic created using CircuitLab

and here is where i found my example in it's natural habitat: http://electricdruid.net/wp-content/uploads/2016/01/Boss-MT-2-Metal-Zone-Schematic.pdf

I know, I know, it's a guitar stompbox...but just in case it helps to see the "low" and "high" tone section in some sort of context. In this schematic, the "low" section is built around an opamp, whereas my example circuit above shows a BJT as the active. I did this to draw the example faster.

  • \$\begingroup\$ It might work in a simulator but high impedances and poor PSRR and stray capacitance might be an issue. Try CMOS switch with Op Amps \$\endgroup\$ Sep 19, 2017 at 2:08
  • \$\begingroup\$ OK..got it. I haven't done any simulation or analysis other than listening to it, and it sounds fine so far. Obviously, in order to switch frequencies, those lines would have to either run off the PCB to a panel mounted switch, or be switched "locally" with a relay or...CMOS? Really? \$\endgroup\$
    – user156429
    Sep 19, 2017 at 2:12
  • \$\begingroup\$ the source pot changes the breakpoint , too high impedance. \$\endgroup\$ Sep 19, 2017 at 2:22
  • \$\begingroup\$ Just for curiosity. Will SW1A/B be mechanical switches, or they are just a placeholder for a much better circuitry? (e.g. a FET, with a slow rising/falling ramp voltage at the gate, to avoid thums?) \$\endgroup\$
    – next-hack
    Sep 19, 2017 at 6:53
  • \$\begingroup\$ @TylerStone Please don't take much note of "helpful answers" posted in the comment section. The comment section is a bad place to give answers, and most high-rep users know this. \$\endgroup\$
    – pipe
    Sep 19, 2017 at 7:17

2 Answers 2


Certainly a feasible concept. Only downside might be that if there were long wires to the switches could pickup noise and cause the circuit to not be stable.


Define "Bad".

I'd put some high value resistors (several megohms) across the switches to eliminate any DC.

That would tend to reduce the clicks (or thumps) when you close the switches, if that's a problem.

  • \$\begingroup\$ by "bad" i suppose i was looking for either a scientific reason one would not want a cap floating, or if it may be bad "form" or etiquette. \$\endgroup\$
    – user156429
    Sep 19, 2017 at 14:59

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