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Single op-amp ~vco~
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nicolas3141



Joined: May 25, 2007
Posts: 139
Location: Christchurch, New Zealand

PostPosted: Sat Nov 14, 2009 6:15 pm    Post subject: Single op-amp ~vco~ Reply with quote  Mark this post and the followings unread

Design goal - super simple vco with very low parts count, no uncommon parts, running off a single 9V battery. Four vcos from one LM324 on a couple of square inches of stripboard. This is what I have come up with so far. Not built yet, but works in the circuit simulator - hey what could possibly go wrong Smile

The limitations: -

Firstly its not V/Hz or V/octave. From 0-1V you get about half an octave. From 1V-2V about one octave. From 2V-3V about two octaves. Think I'll be using other VCOs for pitch critical stuff.

Secondly the waveshape changes. At the bass end more or less square, at the high end more of a narrow pulse type wave. This is a feature I think, not a bug Wink

So thats based on playing with the sim. I'll try building it in the real world and let you know how I get on.

And why I want lots of simple VCOs? Add a single transistor VCA to each one (easy for with square waves). Some LFOs and/or ADSRs. And then I can have a go at 4/6/8 op FM. Yeah!!

Will keep you posted. If anyone else tries building it, let me know how you get on.

Cheers,
Nicolas


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jnuaury



Joined: Feb 28, 2008
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PostPosted: Sat Nov 14, 2009 7:25 pm    Post subject: Reply with quote  Mark this post and the followings unread

interesting!
i was playing around with a similar idea after i breadboard a variation of this...

http://www.jiggawoo.eclipse.co.uk/guitarhq/Circuitsnippets/simplevcf.gif

uses one op-amp stage and only a few more components, but you can some rounder waveforms
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slacker



Joined: Nov 18, 2007
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PostPosted: Sun Nov 15, 2009 4:54 am    Post subject: Reply with quote  Mark this post and the followings unread

Very cool, thanks for sharing that.
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doctorvague



Joined: Mar 14, 2007
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PostPosted: Sun Nov 15, 2009 7:04 am    Post subject: Re: Single op-amp ~vco~ Reply with quote  Mark this post and the followings unread

nicolas3141 wrote:


And why I want lots of simple VCOs? Add a single transistor VCA to each one (easy for with square waves). Some LFOs and/or ADSRs. And then I can have a go at 4/6/8 op FM. Yeah!!


Maybe we're talking apples and oranges here, but I've always thought FM in that style requires extreme pitch accuracy, which is why, with notable exceptions like the Zeroscillator, it's usually been generated digitally, like the Yamaha DX series and the Modcan VCDO - so the ratios are exact multiples. Maybe you're not after those kinds of ratios the same way I'm looking at it. Could be my personal bias- I used to sell programs for the DX series and see it as a pretty exact science in most cases, other than noise or intentionally dissonant patches

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nicolas3141



Joined: May 25, 2007
Posts: 139
Location: Christchurch, New Zealand

PostPosted: Sun Nov 15, 2009 1:32 pm    Post subject: Reply with quote  Mark this post and the followings unread

Quote:
I've always thought FM in that style requires extreme pitch accuracy


Yes I think you are right, but when patching an analogue those kind of ratios are tuned in by ear while twisting knobs which is a little bit different from the DX7 way of doing things. I am hoping that it will work even without good linearity as long as all the oscillators are built identically and have matching responses. At the end of the day I don't think I will be getting the same range of sounds as a DX7 can do, but I am sure I will get something interesting out of it. And the idea of doing it with super low parts count is that even if it turns out to be of limited usefulness I won't have spent too much money or time on it.

Cheers,
Nicolas
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metal_head_82



Joined: Dec 27, 2009
Posts: 34
Location: Germany
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PostPosted: Thu Oct 06, 2011 2:59 am    Post subject: Reply with quote  Mark this post and the followings unread

Just in order to revive this old thred Wink

I just posted a single op amp vco in a thread here:
http://electro-music.com/forum/topic-49901.html

Perhaps this might be interesting Wink

Cheers
Hendrik

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