# 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, 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
• (1) Is there a DC offset on the guitar signal at this point? (Check with a multimeter.) (2) I suspect you need to turn on the FET a little more gently with an RC circuit into the gate. I'll let someone else confirm this and suggest some values. Commented Jul 28, 2016 at 20:12
• There's no "infinite current" here. There may be feedthrough from the high speed step on the gate voltage, via gate-drain capacitance, (Cgd) which would make quite a crack. Especially if you really are using a high power FET like the IRF530, there's about 1000pf there. Try a much smaller MOSFET - maybe even a JFET (J310) with a much smaller Cgd. Also, @transistor's idea of slowing down that step will help.
– user16324
Commented Jul 28, 2016 at 20:25
• I found recently that even when bypassed (so the opamp is still running but doesn't have any connection to the guitar signal asides from common ground), there's still a faint clicking and so it seems like it's a noisy opamp... possibly being amplified by the transistor/opamp? Which may be why it gets much louder when the effect is activated... Incidentally a voltage divider on the gate/base does reduce the clicking a little... Commented Jul 29, 2016 at 19:47

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.

simulate this circuit – 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.

• This looks great, i'll give it a go! Commented Jul 29, 2016 at 19:42
• Thanks. Unaccept the answer until after you've got it to work. That way you'll continue to encourage other answers. One thing: you're using a square wave to modulate the volume. This is always going to sound weird. A triangle wave would be better and is easy to generate with the addition of another opamp. Commented Jul 29, 2016 at 20:25
• Ah okay, thanks - fairly new to Stack so my bad! Int terms of the triangle wave, I was thinking that maybe the gradual increase in voltage would work better than the immediate jump by the square wave, i'll try this. One concern is that even without the guitar signal being affected by the circuit (when in bypass) there's still the faint clicking sound - suggesting that the issue is at the opamp/relaxation oscillator and since both guitar signal and the opamp are connected to a common ground, there could be some disturbance here. Again, may be helped by using a triangle wave instead. Thanks! Commented Jul 29, 2016 at 21:07
• For my schematic i've used a standard NPN BJT in place of a MOSFET (I don't have any MOSFETs yet). I connected the guitar signal to the emitter, with collector to ground. I then put a 1uf cap between base and collector and hey, no clicking! Makes me realise noise was from the transistor as even when bypassed the circuit's still complete and goes to the common ground, explaining why it was quieter when bypassed. For the LED/LDR approach, or with a triangle wave, i'm still going to try those to see if it makes for an even better circuit. Commented Aug 1, 2016 at 21:33
• Sorry for the late reply - just got back from holiday. I managed to try the LDR schematic, but there is still the clicking sound. I tried placing a 1uF cap in various places (since it worked in my previous comment) but no success. Commented Aug 20, 2016 at 17:02

If you want a gain-modulator-on-a-chip, why not use a made-for-purpose Voltage Controlled Amplifier, like the THAT2180?

A single-supply version of the following schematic with your tremolo oscillator's output applied to the Ec- terminal and a suitable op-amp in place of the OP275 should do the job: