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 Forum index » DIY Hardware and Software
i want to build a very simple vca!
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the19thbear



Joined: Apr 19, 2007
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PostPosted: Fri May 11, 2007 7:08 am    Post subject: i want to build a very simple vca!
Subject description: i want to build a very simple vca!
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i want to build a very simple vca to use in my modular ( formant elektor).. any good ideas how?? i´m not too much inot electronics, but i can build stuff from manuals etc..
the vca has to be 1v/octv... standard.

links suggestions etc are welcome!
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Blue Hell
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PostPosted: Fri May 11, 2007 7:13 am    Post subject: Reply with quote  Mark this post and the followings unread

Like http://hem.bredband.net/bersyn/VCA/lm13600%20vca%201.htm ?
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the19thbear



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PostPosted: Fri May 11, 2007 7:27 am    Post subject: thanks
Subject description: thanks
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thank you! did you build this?? do you have a pcb layout? ok i can do that myself, but if you have already done it Laughing

AND techical questions: is this +/- 15v powered? and, on the schematic, just before the plus voltages goes into the lm13600 i see two diodes?? do I have to put them there or are they build into the lm13600?? and what are the two circles on overlapping eachother right after the lm13600?? ( sorry im a newbie Embarassed ) i just havent seen that before..

thanks!
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Blue Hell
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PostPosted: Fri May 11, 2007 7:48 am    Post subject: Reply with quote  Mark this post and the followings unread

No I didn't build it, actually I thought I was going to try and find a reference to the elektor VCA design, as you have a formant and that one is not too complicated. Then I stumbled over this one, and it's a bit simpler even - not using the FET (I do have a formant myself although I don't really use it anymore and I did not build it myself - I did build some formant modules before I bought the one I have now. The thing was dirt cheap at the time, less than 250 Euro).

AFAIK both the opamps and the OTA can withstand +/- 18 V, so 15 V would be OK. The diodes are the built in linearization diodes from the OTA.

There is a CA3080 design as well somewhere on the same site, but the 13600 seems more current.

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richardc64



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PostPosted: Fri May 11, 2007 8:09 am    Post subject: Re: thanks
Subject description: thanks
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the19thbear wrote:
...on the schematic, just before the plus voltages goes into the lm13600 i see two diodes?? do I have to put them there or are they build into the lm13600?? and what are the two circles on overlapping eachother right after the lm13600??


The 2 diodes are part of the 13600 innards, as are the two circles. They are often omitted from the 13600/13700 symbol.

Here's an even simpler VCA. (I really need to start keeping track of where I find this stuff.)


vca13600.gif
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vca13600.gif


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fluxmonkey



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PostPosted: Fri May 11, 2007 8:41 am    Post subject: Re: i want to build a very simple vca!
Subject description: i want to build a very simple vca!
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the19thbear wrote:
i want to build a very simple vca to use in my modular ( formant elektor).. any good ideas how?? i´m not too much inot electronics, but i can build stuff from manuals etc..
the vca has to be 1v/octv... standard.

links suggestions etc are welcome!


umm... VCAs don't have octaves...
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Blue Hell
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PostPosted: Fri May 11, 2007 8:50 am    Post subject: Re: i want to build a very simple vca!
Subject description: i want to build a very simple vca!
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bbob wrote:
umm... VCAs don't have octaves...


Laughing

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frijitz



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PostPosted: Fri May 11, 2007 9:04 am    Post subject: Re: thanks
Subject description: thanks
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richardc64 wrote:
Here's an even simpler VCA. (I really need to start keeping track of where I find this stuff.)

It's simpler because it has a linear response. An expo response was what was requested. Smile
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frijitz



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PostPosted: Fri May 11, 2007 9:15 am    Post subject: Re: i want to build a very simple vca!
Subject description: i want to build a very simple vca!
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bbob wrote:
umm... VCAs don't have octaves...

OK. So how many dB/V do you like? Smile

If your EG's have expo shapes, then I think a linear VCA is a better idea. Otherwise you have an expo of an expo. I use that in my hybrid system where the VCA's can be controlled either from my wind controller or an EG. It works, but it is a different kind of envelope.

Ian
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richardc64



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PostPosted: Fri May 11, 2007 11:35 am    Post subject: Re: thanks
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frijitz wrote:
It's simpler because it has a linear response.


Really? How do the two differ sonically?
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frijitz



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PostPosted: Fri May 11, 2007 12:14 pm    Post subject: Re: thanks
Subject description: thanks
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richardc64 wrote:
frijitz wrote:
It's simpler because it has a linear response.
Really? How do the two differ sonically?

Most natural systems (strings, pipes, auditoriums...) have an exponential die out when their excitation stops. So most people like this response in a module. An exponential of an exponential doesn't have the nice tail-off of a natural system. They decay is too abrupt. You can make the decay time longer, but the shape is still too abrupt. Of course, you might prefer it in some situations ... up to you. Smile
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synthmonger



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PostPosted: Fri May 11, 2007 1:17 pm    Post subject: Reply with quote  Mark this post and the followings unread

There is an even simpler vca involving a single 2N2222 transistor around google somewhere. I had the image on my comp somewhere but I can't find it ;x
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fonik



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PostPosted: Fri May 11, 2007 1:36 pm    Post subject: Reply with quote  Mark this post and the followings unread

what about masa's modulation vca:
Posted Image, might have been reduced in size. Click Image to view fullscreen.
http://masa921.hp.infoseek.co.jp/

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Mooger5



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PostPosted: Fri May 11, 2007 5:03 pm    Post subject: Reply with quote  Mark this post and the followings unread

I learned with this thread too. Thanks guys.
Is there a reason for R2 /R3 value of 220R on all the schematics? Is it a 3080 thing? In the particular case of my synth I´ve found 120k to be more suitable.
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Blue Hell
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PostPosted: Fri May 11, 2007 5:18 pm    Post subject: Reply with quote  Mark this post and the followings unread

Mooger5 wrote:
Is there a reason for R2 /R3 value of 220R on all the schematics? Is it a 3080 thing?


Yes it is, but I guess it's not so much the 220R but more that you need a small input signal to keep the OTA operating in a near linear way. So you'll see a large R1 and a small R2 (in the schema above) to divide the input voltage to something small. Next thing is that R2 and R3 should be equal to let comparable currents flow into or out of each input, this will keep the offset as low as possible so there is as little as possible DC offset at the output.

On the other hand you don't want the input signal to be too small for noise reasons, so it's a trade off between distortion and noise.

for a normal opamp circuit these considerations also play a role, but the small input voltage can then be made using feedback, which is not so easy to do for an OTA as it has a current output and not a voltage output. It is not unusual though to same this "same resistor value" thing around the inputs of a normal opamp as well.

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Mooger5



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PostPosted: Sat May 12, 2007 7:27 am    Post subject: Reply with quote  Mark this post and the followings unread

Awesome explanation! Thanks! Very Happy

[quote="Blue Hell"]
Mooger5 wrote:

On the other hand you don't want the input signal to be too small for noise reasons, so it's a trade off between distortion and noise.

So that´s why with 220R I couldn´t set the VCA without getting some noise.
Now maybe 120k is too high... overdrive is good for filters, not VCAs...
I´ll try a stereo pot to find the optimal value.
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Blue Hell
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PostPosted: Sat May 12, 2007 7:47 am    Post subject: Reply with quote  Mark this post and the followings unread

It depends on your signal levels, when you go in with low levels things may get a bit noisy. It would be best then to apply some amplification before you go into the VCA.

For a low pass filter a bit of noise is less of a problem (just as for distortion) as the filter itself will get rid of most.

On experimenting with the influence of your resistor on distortion maybe try to use a saw or triangle wave, that is if you have a scope to judge the results. If you don't have a scope you could try with complex input signals (like music or a mix of some oscillators) and listen how muddy that gets. It's a bit hard to hear (harmonic) distortion on a simple wave form.

In the end though, when 120 k sounds best ... well .. then it sounds best Very Happy

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doctorvague



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PostPosted: Sat May 12, 2007 10:57 am    Post subject: Reply with quote  Mark this post and the followings unread

Jan
thanks very much for your excellent and clear explanations. They are quite educational! Oh and the schematics too, everyone. I'm about to launch into OTA-land so this was perfect timing.
best regards
Phil
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the19thbear



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PostPosted: Sun May 13, 2007 8:13 am    Post subject: thanks
Subject description: thanks
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wow... thats alot of schematics and good explenations!

thanks to all of you! .. let me get this rigth.. someone tested masas mod. vca??

i think i will go with that then, if i can get my hands on a ca3080


thanks again
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Fernando



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PostPosted: Sun May 13, 2007 2:04 pm    Post subject: Reply with quote  Mark this post and the followings unread

Also, take a look to Rene's VCA circuits. Ken Stone has a pcb for Rene's VCA3. It is very simple and transistor based. It just gets more complex because of the CV mixer etc, but having a pcb re-simplifies anyhow
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Mooger5



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PostPosted: Mon May 14, 2007 7:30 am    Post subject: Reply with quote  Mark this post and the followings unread

Blue Hell wrote:
It depends on your signal levels, when you go in with low levels things may get a bit noisy. It would be best then to apply some amplification before you go into the VCA.

I just ordered the SSM2044s to go with the 3080s. They have built-in amplification.

Quote:
For a low pass filter a bit of noise is less of a problem (just as for distortion) as the filter itself will get rid of most.

Nevertheless I intend to overdrive the filter input (not the VCA´s) a bit and see if it will warm the Juno´s sound up.

Quote:
On experimenting with the influence of your resistor on distortion maybe try to use a saw or triangle wave, that is if you have a scope to judge the results. If you don't have a scope you could try with complex input signals (like music or a mix of some oscillators) and listen how muddy that gets. It's a bit hard to hear (harmonic) distortion on a simple wave form.

I use Zelscope (www.Zelscope.com). May not be a "real" scope, but it´s a great tool to visualize what you´re doing. That´s how I discovered the 80017a clones I had bought invert the signal, which input of the 3080 is the right one to feed, and it also helped me tweaking the Moog Rogue to sound better.
Testing the VCA with music is a very wise procedure. Thank you for the idea.
Quote:
In the end though, when 120 k sounds best ... well .. then it sounds best Very Happy

So true. No measuring data could counter-argument that Wink
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Coriolis



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PostPosted: Tue May 15, 2007 9:50 am    Post subject: Reply with quote  Mark this post and the followings unread

Wasn't there talk about a one-transistor vca from some polyphonic Korg synth? PS-series? MS?
It was something very simple...

C
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fonik



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PostPosted: Tue May 15, 2007 10:30 am    Post subject: Reply with quote  Mark this post and the followings unread

here is the (inverting) VCA from the MS20 (or did i miss something?):
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Scott Stites
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PostPosted: Tue May 15, 2007 10:56 am    Post subject: Reply with quote  Mark this post and the followings unread

Yep, and PS series - Juergen Haible goes into some detail here:

http://jhaible.heim.at/polykorg/jh_polykorg_clone.html
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