Electrical – Voltage gated switch issues for audio application (guitar pedal)


So i'm a newbie, and with the assistance of my A-Level in electronics step brother we're trying to make a tremolo pedal for guitar. The concept is that an opamp (powered by ±9v) is used as a square wave generator, which will open and close a voltage gated switch which in turn disconnects and reconnects the signal (turns it on and off), at a rate determined by the frequency of the square wave. The circuit below is what i've been using so far.


simulate this circuit – Schematic created using CircuitLab

The issue is that every time the signal cuts out as it's supposed to, there's a loud crack which is what I assume is the shorting of the circuit (the guitar signal, due to 'infinite' current going to ground).

I'm hoping someone has a suggestion how I could resolve this, or alternative suggestions for a voltage gated switch that won't be shorting the circuit. I tried putting the mosfet in series with the guitar signal but this didn't work. If there's a simple way of just getting rid of this crack (which could possibly be solved by softening the edges of the square wave generated resulting in a gradual increase/decrease of voltage above/below threshold potential), this would be hugely appreciated!

Many Thanks,

  • EDIT: Representation with output scope of the relaxation oscillator seen here (please copy and paste as doesn't display link nicely):

Best Answer

I have used the H11F1 opto-isolator as a volume control. The output section is a JFET, the input is an LED driven by just a few milliamps. Use it to replace the mosfet to 'short' the signal or in series with the signal. If used to short the signal insert a 10K to 100K resistor to limit the signal current, and a 100 ohm to 1K resistor in series with the JFET drain pin (pin 6) to limit the short current. Use a 1K resistor on the output of the op-amp to limit drive current to the LED side (pins 1 and 2) of the H11F1.

I found this link to DIY projects using the H11F1 as a gate or gain control.