ThHeEeeee ChHhHoPpPeErR – duty cycle tremolo

I always wanted to make a tremolo with a duty cycle control. This control gives the possibilities to modifiy the symetry of the waveform. I tried few things but I always had a ticking noise or the FLO bleeding through the audio which is a bad thing of course. I must admin that I also have an flat with a bad electricity (no earth connection in some rooms).   One other issue  is to obtain  an useable speed range (not too fast but not too long either) .


On this one I got pretty lazy. I took the Ray wilson variable skew LFO and made some changes to suit the guitar tremolo effect. And ….Here’s the schematic :


Although it’s pretty basic I’ll give few words about it.

As you can see there’s a kind of very simple “waveshaper” after the oscillator (the op-amp with the  soft square/triangle switch) . The idea behind the square wave is simple, I wanted to avoid ticking noise as much as possible hence this op-amp configuration instead of classic square wave.

the 10K tail resistor with the speed pot. gives a really slow speed (for a tremolo) , you can increase for making it faster.

the 1K trim is to reduce noise, again, set it with ears (but to be honest it doesn’t seem to make a lot…)

the final X2 gain stage is here to allow a volume pot. Notice that it’s not really necessary because the OTA gives a good volume boost on its own. But I had one part of the TL072 left so I used it for the output stage. So no complains about “volume drop”

the two bias sections are also here to reduce noises and ticking .

the transistor after the OTA is the darlington contained in the LM13700 of course. The schematic software didn’t have a model for a LM13700.

Last thing , you can note two 100K in the integrator part of the oscillator. It’s because I only use and stock “standard” values . Obviously 200K resistor can be used. The same method was used for the 5K in the waveshaper op-amp , 4K7+1K .

If you don’t know Ray Wilson’s work , I highly recommend you to visit his site ! ok it isn’t guitar related , it’s synth diy , but you’ll find more intersting ideas in this erea than in effect pedals !


not as neat as my other circuits but I ran out of cable ties so I made some DIY ones with cut resistor legs ! The room left in the pedal was a challenge . No big issues to make the board but you should have a good idea of the layout before starting to solder .


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