Transforming the super simple VCO into a synth

exactly the same like this (the 2 leg of the LDR go to the pin pot (with the stripboard))

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@Dud Thanks man, big help!
I’ll do this in a bit and let u know if it works; as LFO I was thinking to do the same oscillator circuit with a big cap and connect the out to the CV in of the filter. Does is make sense?

yes ti’s that
I would rather advise you to try an LFO in the CV of the pitch pot of the SSO, to better realize you, because I am not sure that a CV on this kind of filter is very convincing

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I noticed that on my circuit I’m going out directly from the 10k pot central pin (transistor, cap and resistor are plugged on that same row, and not on the third pin), unlike in the layout here. Could it be because of that?
Also, I’m connecting the 2nd oscillator audio out into the CV, is that correct?
The space on my breadboard is finished and I already have too many flying cables and alligator clips around. I guess it’s better to wait for the other breadboards that are coming soon and try to make everything more clear using more space

yes make like in the schematic

that’s right !

stripboard you mean

look here Glossary


Nono I meant Breadboard, I’ m still working on that for the synth. I just ordered 2 other small bread boards to have more space :slight_smile:

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ok sorry, but you plan to make the final circuit after test on stripboard?

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Yes of course! I plan to do a wooden case like I did for the controller

and I did a prototype of the synth with cardboard

I am considering if to make two individual mono outputs (3.5 mm) or a common stereo output (6.3 mm), but I’ll probably end up doing both :slight_smile:


you have space in your box, adding a 3rd oscillator could be good I think to be able to make chords to 3 sound

Oh yes I was considering that as well!

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At this point I’ m thinking to not put filters here and build a separate filter module with maybe 4 inputs, do u guys have some idea on a fairly simple project?

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a MS20filter clone (on 9 V battery)
I made several because simple and they work really well !

it’s a real filter with resonance and cut off frequencies not like the RC soft one

1 filter on the general mixer output is enough I think

like here with the SSO : (The sound crack a little bit at times, it comes from the microphone of the camera which saturates slightly, sorry!)

more infos :

it’s writ LM13600 on the stripboard but you can replace it by LM13700


Thanks! I’ll definitely give it a try. The MS20 filter sound good!


Hey @Dud, I did this but it doesn’t seem to work :confused:

  • if your oscillator work
  • if your pitch pot on your oscillator work
    so the vactrol must work (it’s just a LDR with a led together in the black connected to this pot)
  • what value have you test for the led of the vactrol ? (1k is good)
  • and after with what have you send curent in the CV in ? (if you have a resistance of 1k on the positive leg of the led : you can check it with a 9V battery)

have you a pic plz ?

  1. The oscillator works
  2. Both oscillators work. I did the vactrol with an LDR and an LED and made made everything dark with insulation tape (which also holds them together)
  3. Unfortunately idk the value of the LED, is there a way to test it? (I’m using the same red LED that u see in the picture)
  4. I’ m using a 470 Ohm resistor, the LFO is basically the second oscillator, the capacitor is 470 uF. The overall circuit is powered with 18v.

    Here is a picture of the circuit but it’ s super messy, not sure whether it can help or not. The oscillator that I want to modulate is the one with the green LED, the LFO is the red one :slight_smile:
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with what you say it’s seems good, with the pic is difficult !

  • remove the vactrol

  • take juste the LDR and put it on the pitch pot of the first osc, and test it with a light (your phone for exemple) and your hand for hide the LDR

for testing the LDR like that you must put the pot to zero (completely at the left when you are in front off the knob = low pitch

the light must do the same thing like when you turn the pot to the right (the pitch more high)

test only that and say me if it’s ok


Works when I only use the LDR! But it doesn’t when I try to plug the rest again

  • test only the led (+ resistor) without the LDR :wink:

if your led bright test that
( if not try another led maybe it dead (or you put it upside down +/- or the resistor was not strong enough do it with 1k)