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 Forum index » DIY Hardware and Software » Lunettas - circuits inspired by Stanley Lunetta
Single supply VCA?
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commathe



Joined: Jul 26, 2013
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PostPosted: Tue Dec 02, 2014 9:35 pm    Post subject: Single supply VCA? Reply with quote  Mark this post and the followings unread

Has anyone got any good ideas for something along these lines? I've tried making ones using a Vref/virtual ground but it never quite works out. I need something that will not just pass digital signals and won't clip things. I need a "true" vca.

I'm totally lost. It seems the only options left are some sort of OTA as a voltage controlled resistor design or to use a vactrol (not so great).

Log & linear response would be nice, but not essential.
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Cynosure
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PostPosted: Thu Dec 04, 2014 10:57 am    Post subject: Reply with quote  Mark this post and the followings unread

I would recommend an OTA. You can get two VCA's with one LM13700. I have done it by replicating the configuration used in the JH Wasp filter schematic.

You will need to make sure you attenuate the voltage on the input. OTA's distort easily and the distortion does not sound nice. Also, add a 15k resistor to +V from the diode bias input.
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commathe



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PostPosted: Thu Dec 04, 2014 11:25 pm    Post subject: Reply with quote  Mark this post and the followings unread

Thanks! OTAs definitely appear to be the way to go. I'm worried about how close they will get to the rails though. I'm considering trying them in the feedback loop of an op-amp...
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PHOBoS



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PostPosted: Fri Dec 05, 2014 3:51 am    Post subject: Reply with quote  Mark this post and the followings unread

an OTA will probably give you the best results,.but I have seen some VCA's using a transitor/FET on the input of an opamp.
either in series with the signal path like here:
Posted Image, might have been reduced in size. Click Image to view fullscreen.
from this thread.

or to pull the input to ground like this:
Posted Image, might have been reduced in size. Click Image to view fullscreen.
from this page.

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LFLab



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PostPosted: Fri Dec 05, 2014 6:10 am    Post subject: Reply with quote  Mark this post and the followings unread

THAT4315? No experience whatsoever, but seems appropriate.
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synaesthesia



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PostPosted: Fri Dec 05, 2014 9:47 am    Post subject: Reply with quote  Mark this post and the followings unread

For single supply, there is also this one, mentioned in this thread: http://electro-music.com/forum/topic-42337.html
I believe it is a variation of the super simple diode VCA plus AR described by richardc64 in this thread: http://electro-music.com/forum/topic-23960.html.
Did anybody try it and can let us know how it works?


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richardc64



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PostPosted: Fri Dec 05, 2014 4:28 pm    Post subject: Reply with quote  Mark this post and the followings unread

synaesthesia wrote:
For single supply, there is also this one, mentioned in this thread: http://electro-music.com/forum/topic-42337.html
I believe it is a variation of the super simple diode VCA plus AR described by richardc64 in this thread: http://electro-music.com/forum/topic-23960.html.

It started out that way, but without the 2nd diode of the 2-diode design, it more resembles the "1-transistor VCA" used in the TR-808, TR-606 and DR-110: The output is taken from the point where the CV controls the amplitude.

Using an opamp instead of a transistor as the active element allows for a larger input amplitude. In the 1-transistor circuit the input must be limited to less than the transistor's base-emitter saturation voltage -- about 850mV for a 2N3904.

(In that circuit Gate In needs a resistor of 2K or so to Gnd, otherwise Release will be longer than it should be.)

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commathe



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PostPosted: Sun Dec 07, 2014 12:03 am    Post subject: Reply with quote  Mark this post and the followings unread

I want to be passing tri/saw/sine signals without too much distortion and definitely not clipping so the diode/transistor method is out. Looks like I'm going to have no choice but to go OTA. Bummer because I also wanted rail-to-rail output (was hoping to do some CV processing) but to be honest it serves me right being so fussy.

Hopefully I'll come back with my own solution some time soon.

I've got my eye on this guitar compressor schematic at the moment. I think using the OTA as a voltage controlled resistor in an op-amp feedback loop would hopefully work well.

Posted Image, might have been reduced in size. Click Image to view fullscreen.[/url]
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LFLab



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PostPosted: Sun Dec 07, 2014 8:18 am    Post subject: Reply with quote  Mark this post and the followings unread

Seems like quite a lot of parts. Have you checked out that4315? 3,60euro at mouser.
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richardc64



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PostPosted: Sun Dec 07, 2014 11:56 am    Post subject: Reply with quote  Mark this post and the followings unread

commathe wrote:
I want to be passing tri/saw/sine signals without too much distortion and definitely not clipping so the diode/transistor method is out.

I failed to mention that omitting the 2nd diode from the 2-diode VCA the circuit handles analog as well digital inputs. Replacing the transistor in the "1-transistor" VCA with a single supply opamp makes it even better, as it allows bigger input swings.

Quote:
I've got my eye on this guitar compressor schematic at the moment. I think using the OTA as a voltage controlled resistor in an op-amp feedback loop would hopefully work well.

I agree with LFLab that looks like too many parts for a single-supply VCA, unless you have your heart set on the compression capability.

Here's a couple more core circuits:


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what's missing is the V/I converter.
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syntom_ota.gif



3series.gif
 Description:
Same operation as what PHOBoS posted. The 909 circuit is dual supply, but that's just for the opamp. Substitute a '324 or '358 for single-supply.
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3series.gif



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commathe



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PostPosted: Sun Dec 07, 2014 6:44 pm    Post subject: Reply with quote  Mark this post and the followings unread

Thanks again guys! It is time to get bread boarding and see what gets closest. Simulations of the MFOS one are looking ok. More distortion than I'd like but beggars can't be choosers!

EDIT: I just discovered this 4007 based VCA that I would have just glanced over if it wasn't for the fact that I have a note that the schematic as it is drawn here is WRONG. The body connections of the internal MOSFETs are tied to Vss and Vdd, meaning that this may actually work!


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Vanwonky



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PostPosted: Sun Dec 07, 2014 9:58 pm    Post subject: Reply with quote  Mark this post and the followings unread

Hi - I built this and it worked brilliantly for my needs. Output is a little quiter than I would like but I am sure that could be fixed easily by anybody on this forum but me! Very Happy

synaesthesia wrote:
For single supply, there is also this one, mentioned in this thread: http://electro-music.com/forum/topic-42337.html
I believe it is a variation of the super simple diode VCA plus AR described by richardc64 in this thread: http://electro-music.com/forum/topic-23960.html.
Did anybody try it and can let us know how it works?
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elektrouwe



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PostPosted: Tue Dec 09, 2014 1:26 am    Post subject: Reply with quote  Mark this post and the followings unread

here is my favourite single supply VCA: cheap (50 cent), exponential control, 4,5..18V supply, low component count, inverting or noninverting output, can drive low ohm loads. cons: ac coupled, DIP obsolete (some shops still have it) but SMD in production


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hexagon5un



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PostPosted: Wed Mar 25, 2015 12:17 pm    Post subject: Reply with quote  Mark this post and the followings unread

synaesthesia wrote:
For single supply, there is also this one, mentioned in this thread: http://electro-music.com/forum/topic-42337.html
I believe it is a variation of the super simple diode VCA plus AR described by richardc64 in this thread: http://electro-music.com/forum/topic-23960.html.
Did anybody try it and can let us know how it works?


Man, that brings back memories. I've used that and loved it. It's super-minimalistic but gets the job done. I have a lunetta-esque synth with 4 of them in it.

I've attached are my eagle design files, and a mirror-imaged layout that you can use to toner-transfer to the bottom of a board, if anyone wants to make these at home or get them fabbed.


vca_eagle_and_gerbers.zip
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Eagle design files and gerbers for LM358 EG/AR.

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PDF of traces for toner-transfer PCB production, if you're into that sort of thing.

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Components and layout for the top of the PCB

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okelk



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PostPosted: Thu Mar 26, 2015 8:20 am    Post subject: Reply with quote  Mark this post and the followings unread

I find the tda 7052 vca interesting! ...never heard of this chip.

the scott gravenhorst 4007-based VCA reminds me if this:
(it's a 4007 based VCA by Osamu Hoshuyama)

Posted Image, might have been reduced in size. Click Image to view fullscreen.
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commathe



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PostPosted: Fri Mar 27, 2015 10:38 pm    Post subject: Reply with quote  Mark this post and the followings unread

I've seen that one too, though I had actually forgotten about it. Definitely going to be trying both of them out!
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okelk



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PostPosted: Sat Apr 04, 2015 11:38 am    Post subject: Reply with quote  Mark this post and the followings unread

Also, I just remembered seeing another 4007 based VCA as part as rene schmitz' 4007 VCF. (Has anybody tried it? I'd love to hear it...)
...It probably also would be possible to replace the opamp with a cd4069...


http://www.schmitzbits.de/vcf4007.html

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commathe



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PostPosted: Mon Apr 06, 2015 7:34 am    Post subject: Reply with quote  Mark this post and the followings unread

Nice find! Could be perfect for building a wasp clone without a CA3080 or such. Definitely going to experiment with this too.

I'm slightly confused though because pin 7 is also the body connection for the N-channel MOSFETS on the chip? Obviously, Rene is smarter than me Laughing
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DGTom



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PostPosted: Mon Apr 06, 2015 5:07 pm    Post subject: Reply with quote  Mark this post and the followings unread

I assumed the 4007 is being used as a 'passive' device in that set-up, tho I've not tried it.

Did have a play around with the 4007 + 4069 combo the way Scott Gs drawing above suggested, using the 4069 in linear mode as a gain stage after the 4007.

seemed to work OK, tho the control voltage range was very small & careful trimming of the CV is needed as per the single transistor VCA - 4069 works well for that as well, lots of distortion, but that is kinda the point right?

Don't overlook the 2164 quad VCA chip for single supply applications. I'm using it with a +10V & +5V supply. 0V goes to what would usually be the negative supply pin & +5V goes to the ground pin. Works great with the 4069.

I'm a fan of 4069s for audio (in case that isn't obvious) It is noisy but I love the way it bends waveforms Very Happy

I've been meaning to try it with an OTA as well... maybe tomorrow.
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tokyomatik



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PostPosted: Mon Apr 20, 2015 2:28 am    Post subject: Reply with quote  Mark this post and the followings unread

hexagon5un wrote:
synaesthesia wrote:
For single supply, there is also this one, mentioned in this thread: http://electro-music.com/forum/topic-42337.html
I believe it is a variation of the super simple diode VCA plus AR described by richardc64 in this thread: http://electro-music.com/forum/topic-23960.html.
Did anybody try it and can let us know how it works?


Man, that brings back memories. I've used that and loved it. It's super-minimalistic but gets the job done. I have a lunetta-esque synth with 4 of them in it.

I've attached are my eagle design files, and a mirror-imaged layout that you can use to toner-transfer to the bottom of a board, if anyone wants to make these at home or get them fabbed.

I'm checking this board and looks realy usefull
still trying to understand, we have here an AD envelope and a VCA
the trimmer are Attack & Decay pots right??
but the CV connector is an input or an output? Question
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DUBmatze



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PostPosted: Mon Apr 20, 2015 3:18 am    Post subject: Reply with quote  Mark this post and the followings unread

the trick here is you dont use the positive pulse of the Squarewave. what you hear is the CV that is muted by Squarewave (when the output is low == gnd).

you see evry time the Squre is going low it mutes the CV at the output. so its a "digitalmuter"... Wink

so if you feed a audiosignal to the cvin it is just hacked by the Squarewave - (feed a padsound in and a rythmical square you got your "lofi trance gate sound")


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tokyomatik



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PostPosted: Mon Apr 20, 2015 5:28 am    Post subject: Reply with quote  Mark this post and the followings unread

perfect! now i'm a bit more confused
i thought the pcb made by hexagon5un were refering to this schematics
posted by synaesthesia on december 5 2014.
so to be precise, would be a GATE and not CV INPUT

synaesthesia wrote:
For single supply, there is also this one, mentioned in this thread: http://electro-music.com/forum/topic-42337.html
I believe it is a variation of the super simple diode VCA plus AR described by richardc64 in this thread: http://electro-music.com/forum/topic-23960.html.
Did anybody try it and can let us know how it works?


another question is: does it work only with square waves or i can feed also sines for example?
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DUBmatze



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PostPosted: Mon Apr 20, 2015 6:39 am    Post subject: Reply with quote  Mark this post and the followings unread

tokyomatik wrote:
another question is: does it work only with square waves or i can feed also sines for example?
iam not shure wich pcbs you have and what kind of vca is on them. but the simple diode vca works only with squarewaves whre the low lvl is = gnd.
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tokyomatik



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PostPosted: Mon Apr 20, 2015 7:05 am    Post subject: Reply with quote  Mark this post and the followings unread

i mean the PCB posted here 9 posts before on this page by hexagon5un, referering to the schematic of 20 posts ago also on this page by synaesthesia.....better if u check both so we can understand each other Wink


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Last edited by tokyomatik on Mon Apr 20, 2015 1:14 pm; edited 2 times in total
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tokyomatik



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PostPosted: Mon Apr 20, 2015 7:56 am    Post subject: Reply with quote  Mark this post and the followings unread

-
Last edited by tokyomatik on Mon Apr 20, 2015 1:20 pm; edited 2 times in total
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