Substitute a vactrol?

[semisight]

Hey, I'm new here and new to synths in general. The other day I was reading about electric pianos on wikipedia, which lead to synths, and now a few days later I find myself wanting to build one.

I have some informal experience in analog/digital circuit design, and the first synth I came across (for beginners) was the WSG. The problem was, it only has a few knobs. No buttons, no way to create any sort of coherent music. So, I decided to scrounge around some more to see what was required for a basic, "real" synth. Along the way I found schematics for an extremely simple AR envelope generator, a one transistor VCA, and most importantly, this thread here, in which doctor vague posted a very simple VCF circuit using a vactrol.

My problem is I don't have a lot of money for this, and vactrols are kind of expensive. I could build a vactrol by wrapping some black electrical tape around an LED and an LDR, but the only LDRs I've been able to find have bad response times.

Could use an optoisolator with a phototransistor to the same effect? Or, could I somehow lower the response times of LDRs by using multiple LDRs, or LEDs, or something?

Much thanks,
Semi

[andrewF]

= Welcome
Hi
Many people roll their own vactrols.
Funnily enough the led colour can also affect the behaviour of the LDR

Using a 'vactrol' means you are not too concerned about accuracy, so don't worry too much about response times. If you find you are not happy with a circuits behaviour, try a larger/smaller LDR or brighter/weaker led. For me all the ones I have made have worked fine, maybe they are different from real vactrols but let's just call it 'character'.

Some pages to get you started

Grant
Takeda

[semisight]

Thanks, yeah accuracy is not a big deal as long as sound comes out . I've been reading about vactrols and LDRs, and it seems the best response times of LDRs (maybe 15ms) can't compare to a good vactrol which has response times of maybe 3.5ms.

I think I'll probably end up rolling my own anyway, but I wonder if I couldn't get the same effect off a phototransistor; they're definitely faster. Futurlec has a quad optocoupler for $.60, which might be worth playing around with.

Those links are interesting, especially the first one.

[blue hell]

semisight.

A photo transistor would not be a direct substitute for a resistor (an LDR can be seen a resistor), for it to work it should be biased properly (current will only flow in one direction and not in both as with a resistor). While this might be a solution for some situations, in general it would not work.

[semisight]

Oh, that seems obvious now. This is probably another stupid question, but what if I had two parallel phototransistors wired in opposite directions?

[blue hell]

You'd probably get some pretty funny non linear behavior

(the transistors likely would not properly be biased)

[semisight]

Ooo, fun .

Just one last question, if I used a phototransistor with a base pin, could I bias it correctly?

[blue hell]

I'd have to experiment

Maybe http://electro-music.com/forum/topic-33446.html&postorder=asc would be of interest too, there is some vactrol replacement going on there.

[panda30y]

To be honest, I haven't been able to find where to buy a vactrol in the first place. I don't want to hijack this thread, but where does one buy a vactrol in the first place?

[Dave Kendall]

Hi.

Try here
Other useful synthy type parts there too.....

cheers,
Dave

[panda30y]

Oh nice, thanks alot for the tip. I've just never explored that section too much, just a quick glance over.

[semisight]

I'll bookmark that site. Still kind of expensive, but then they are under "hard to find" .

[RKHobbs]

You can buy them from Allied Electronics (alliedelec.com)
They have the Perkins Elmer vactrols and the Silonex analog optocouplers.