To celeberate the 50th anniversary of the doctor and his daleks, here’s a circuit that can’t make a decent dalek voice but also a nice tremolo.
Allons y !
There’s an input buffer to accommodate any signal (the maximum amplitude will depend on the power supply used ) Then the signal is splitted in two pathes “dry” , “wet” which goes into the LM13700 wired as an amplitude modulator and a third path going into a peak detector and a comparator to open or close the transistor in the integrator part of the LFO . This avoid annoying oscillations in abscence of any input signals.
The LFO (or VCO ) goes into a really crude waveshaper (IC7A) giving a gain of 2 for the triangle wave and a gain of 4 to make the op amp saturated and to have a square wave at the output. This feeds the Iabc pin of the OTA. The switch in the LFO is made to access different frequency ranges (don’t trust the cap values, try yours) . if made lower enough, it will create a tremolo effect (and the INT pot will be used as a intensity pot). Actually it’s still a VCO but you can’t hear these low frequencies.
The LFO op-amps are LM324 (rail-to-rail op amps) not TL072.
The LFO rate can also be controlled by an expression pedal (or any CV control you want)
Signals wet and dry mix at the output stage , both having a pot to determinate their importance in the output signal.
As in the Moog ring modulator , we could also add a second LFO to modulate the frequency of the VCO. We could also add a low pass filter in the VCO path to tame harsh sound , I tried that with the unused OTA of the LM13700 and it worked perfectly good ! (also make the low pass filter variable via another LFO). Possibilities are endless if you don’t care about space and cost. but I think the circuit is complex enough and wiring time will be massive !
Ok, it’s not a really dalek voice modulator per say , Ring modulators are really hard to use and making them capable of doing other effects is nice to justify their presence in a gear collection. Ring modulator must be the hardest musical circuit to sell (unless you have a modular synth guy in front of you….) .