Circuitbenders Forum > DIY Synths
LDR causing pitch of a squarewave to bend
michael page:
Hiya,
I'm putting a squarewave from a 40106 oscilator through a vactrol to fade it up and down. It works well except the pitch seems to bend a little bit.
The power for the LED is a separate power supply to the synth.
heres the circuit :
http://mpanimation.blogspot.co.uk/2015/06/ldr-causes-pitch-to-bend-i-cant-workout.html
Anyone know why this might be?
Cheers
Mike Page
Circuitbenders:
Possibly because thats not how you wire up a 40106 oscillator. The cap should be going to ground.
Like this:
The LDR will be changing the discharge rate of the cap and therefore the frequency if its wired like it is in your diagram, although thats not how you'd normally wire up an LDR to control the pitch.
When you have it wired right, you'll probably need some kind of buffer between the output of the 40106 and the LDR, or the LDR is always going to change the pitch.
michael page:
Ah apologies! yes thanks, I drew my diagram too hastily (I have corrected it now). The cap goes to ground and its buzzing away happily. So the idea is that the LDR fades the oscillator volume up and down as the LED fades up and down. This aspect of it works well but there is a subtle pitch bend too.
I'll try putting a diode between pin 2 and the LDR.
Circuitbenders:
i don't think a diode will work as it'll either be blocking the output or still letting the cap drain to ground via the LDR depending on which way round you put it. Try a transistor buffer, or an opamp.
http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/buffer.html
michael page:
Cheers for the suggestion. Ive got 12 oscillators being mixed in this way so simplicity is a priority in this design.
I've seen schematics with of the outputs of a 40106 synth being mixed with diodes so I recon I'll give that a pop and report back. http://electro-music.com/forum/topic-61732-0.html&postdays=0&postorder=asc&highlight=
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