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.

Voltage gated switch issues for audio application (guitar pedal)

Voltage gated switch issues for audio application (guitar pedal)

– 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, Zach

  • EDIT: Representation with output scope of the relaxation oscillator seen here (please copy and paste as doesn't display link nicely): http://www.falstad.com/circuit/circuitjs.html?cct=$+1+0.000005+159.55918304990493+60+10+50%0Aa+304+208+400+208+0+9+-9+1000000%0Aw+304+192+304+160+0%0Ar+336+128+384+128+0+100000%0Aw+400+128+400+208+0%0Aw+400+208+400+272+0%0Ar+400+272+304+272+0+100000%0Aw+304+224+304+272+0%0Ar+304+272+240+272+0+100000%0Ag+240+272+240+288+0%0Ac+304+160+240+160+0+0.0000022+0.25657397239391444%0Ag+240+160+240+176+0%0AO+464+208+464+176+0%0A174+304+160+368+128+0+500000+0.005+Resistance%0Aw+384+128+400+128+0%0Af+464+208+512+208+0+1.5+0.02%0Aw+512+224+512+240+0%0Ag+512+240+512+256+0%0Aw+512+192+512+176+0%0AR+512+176+512+144+0+1+450+1.5+0+0+0.5%0Aw+400+208+464+208+0%0Ao+9+64+0+35+0.3125+0.00009765625+0+-1%0Ao+11+64+0+42+0.625+0.00009765625+1+-1%0A


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.

Another option is to couple up an LED to an LDR (light dependent resistor) if you have one to hand.

Voltage gated switch issues for audio application (guitar pedal)

Voltage gated switch issues for audio application (guitar pedal)

– Schematic created using CircuitLab

The LDR with R6 forms a potential divider. It will always be in circuit but should give a softer turn on and off. R7 (experiment here) would allow adjustment of the depth of the effect.

Just check the maximum source or sink current for your opamp and size R5 to limit the current to that value.

Category: voltage Time: 2016-07-28 Views: 0

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