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 Forum index » DIY Hardware and Software » Developers' Corner
Substitute a vactrol?
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semisight



Joined: Apr 07, 2009
Posts: 6
Location: USA

PostPosted: Tue Apr 07, 2009 2:01 pm    Post subject: Substitute a vactrol? Reply with quote  Mark this post and the followings unread

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
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andrewF



Joined: Dec 29, 2006
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Location: australia
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PostPosted: Tue Apr 07, 2009 6:12 pm    Post subject: Reply with quote  Mark this post and the followings unread

hello welcome hello welcome = 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
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semisight



Joined: Apr 07, 2009
Posts: 6
Location: USA

PostPosted: Tue Apr 07, 2009 9:04 pm    Post subject: Reply with quote  Mark this post and the followings unread

Thanks, yeah accuracy is not a big deal as long as sound comes out Mr. Green. 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.
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Blue Hell
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PostPosted: Wed Apr 08, 2009 9:59 am    Post subject: Reply with quote  Mark this post and the followings unread

welcome 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.

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semisight



Joined: Apr 07, 2009
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PostPosted: Wed Apr 08, 2009 11:13 am    Post subject: Reply with quote  Mark this post and the followings unread

Oh, that seems obvious now. This is probably another stupid question, but what if I had two parallel phototransistors wired in opposite directions?
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Blue Hell
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PostPosted: Wed Apr 08, 2009 12:21 pm    Post subject: Reply with quote  Mark this post and the followings unread

You'd probably get some pretty funny non linear behavior Wink

(the transistors likely would not properly be biased)

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semisight



Joined: Apr 07, 2009
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PostPosted: Wed Apr 08, 2009 4:38 pm    Post subject: Reply with quote  Mark this post and the followings unread

Ooo, fun Very Happy.

Just one last question, if I used a phototransistor with a base pin, could I bias it correctly?
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Blue Hell
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PostPosted: Thu Apr 09, 2009 5:52 pm    Post subject: Reply with quote  Mark this post and the followings unread

I'd have to experiment Laughing

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

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panda30y



Joined: Apr 12, 2009
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Location: Detroit, Michigan

PostPosted: Fri Apr 17, 2009 12:16 am    Post subject: Reply with quote  Mark this post and the followings unread

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?
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Dave Kendall



Joined: May 26, 2007
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PostPosted: Fri Apr 17, 2009 5:26 am    Post subject: Reply with quote  Mark this post and the followings unread

Hi.

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

cheers,
Dave
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panda30y



Joined: Apr 12, 2009
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PostPosted: Fri Apr 17, 2009 10:09 am    Post subject: Reply with quote  Mark this post and the followings unread

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



Joined: Apr 07, 2009
Posts: 6
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PostPosted: Fri Apr 17, 2009 4:11 pm    Post subject: Reply with quote  Mark this post and the followings unread

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



Joined: Jun 10, 2008
Posts: 19
Location: Maryland

PostPosted: Fri Apr 17, 2009 8:12 pm    Post subject: Reply with quote  Mark this post and the followings unread

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