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=

[Circuitbenders]

Yes, but in those schematics the diode connects to one side of a pot acting as a voltage divider and the other side is connected to ground. Theres a constant 100K resistance between the diode and ground regardless of the pot position.
In your design one side of the voltage divider is an LDR so the resistance between the diode and ground is changing and the pitch will change.

If you scroll down that thread theres a much better design with an opamp buffer acting as a mixer. You only need one.

http://electro-music.com/forum/phpbb-files/oscillators_with_diode_mixer_138.gif

[michael page]

Thanks for the reply,
yes you are totally right im going to have to work somethign else out.