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the19thbear
Joined: Apr 19, 2007
Posts: 151
Location: denmark
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 Posted: 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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the19thbear
Joined: Apr 19, 2007
Posts: 151
Location: denmark
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Posted: 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 
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  ) i just havent seen that before..
thanks! |
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blue hell
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| Posted: Fri May 11, 2007 7:48 am Post subject:
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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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Jan
also .. could someone please turn down the thermostat a bit.
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richardc64
Joined: Jun 01, 2006
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fluxmonkey
Joined: Jun 24, 2005
Posts: 708
Location: cleve
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Posted: 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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Posted: 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... |
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Jan
also .. could someone please turn down the thermostat a bit.
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frijitz
Joined: May 04, 2007
Posts: 1734
Location: NM USA
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Posted: 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.  |
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frijitz
Joined: May 04, 2007
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Location: NM USA
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Posted: 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?
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
Joined: Jun 01, 2006
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Posted: Fri May 11, 2007 11:35 am Post subject:
Re: thanks
Subject description: 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
Joined: May 04, 2007
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Posted: 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. |
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synthmonger
Joined: Nov 16, 2006
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Location: flada
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| Posted: Fri May 11, 2007 1:17 pm Post subject:
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| 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
Joined: Jun 07, 2006
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Mooger5
Joined: May 02, 2007
Posts: 199
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| Posted: Fri May 11, 2007 5:03 pm Post subject:
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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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| Posted: Fri May 11, 2007 5:18 pm Post subject:
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| 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.
_________________
Jan
also .. could someone please turn down the thermostat a bit.
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Mooger5
Joined: May 02, 2007
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| Posted: Sat May 12, 2007 7:27 am Post subject:
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Awesome explanation! Thanks! 
[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.
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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
Site Admin
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| Posted: Sat May 12, 2007 7:47 am Post subject:
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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
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Jan
also .. could someone please turn down the thermostat a bit.
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doctorvague
Joined: Mar 14, 2007
Posts: 281
Location: new mexico
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| Posted: Sat May 12, 2007 10:57 am Post subject:
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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
Joined: Apr 19, 2007
Posts: 151
Location: denmark
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Posted: 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
Joined: Dec 30, 2006
Posts: 286
Location: Barcelona & Emporda, Spain
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| Posted: Sun May 13, 2007 2:04 pm Post subject:
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| 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
Joined: May 02, 2007
Posts: 199
Location: Portugal
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| Posted: Mon May 14, 2007 7:30 am Post subject:
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| 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 |
So true. No measuring data could counter-argument that  |
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Coriolis
Joined: Apr 11, 2005
Posts: 616
Location: Stilling, Denmark
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| Posted: Tue May 15, 2007 9:50 am Post subject:
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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
Joined: Jun 07, 2006
Posts: 3950
Location: Germany
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| Posted: Tue May 15, 2007 10:30 am Post subject:
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here is the (inverting) VCA from the MS20 (or did i miss something?):
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matthias
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Big Boss at fonitronik
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Scott Stites
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