Motherphaser under the scope

To continue on my phaser story , here are few oscillscope pictures dealing with the “sweep” pot on the Motherphaser As you’ll see the “sweep” control allow the LFO signal to move above and below VR, allowing the sweeping on the phaser from the bass to the treble (note that I use those terms maybe improperly ) Note that there’s some resistor to limit the voltage .  Beside , you can get rid of one op amp (the final mixer) easily by mixing and outputting the signal with one amp.


the sweep  “center biased” ,as I may say, delivers a nice triangle waveform


moved to the positive rail


moved down to the negative rail


As you can see on both extrems the waveform get chopped . Without the limiting resistors the waveform will be completely off into the power rail (negative or positive) giving a flat line.  If a bigger power supply (=bigger headroom) you’ll get a more larger amplitude .

I must say that I was quite happy with that , it was as I expected . Nothing fancy or spectacular , we’re only dealing with simple LFO for phaser effect . However, it gives more variations on the phaser sound, I don’t understand why any manufacters used this trick to expand the possibilities (actually I do ) . Synth wise though it could create a more complex waveform which could be interesting.

Therefore I try this with a VCO . I made the integrator cap of the LFO way smaller (to oscillate in the audio range) , kept the idea of the shape control to have a variable control to transform a triangle into a square with great succes , I added a PWM kind of control and an  germanium output stage for more harmonics. Interesting sound results appeared (and waveforms too). As my synth desire is growing everyday I keep the circuit for a VCO waveshaper .

once again, don’t mind the blurry pictures the Iphone was diying of its empty battery. However, I think you can get the idea.

thanks for reading


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