Roughtest VCA possible

umschmitt

Hey all. I discovered only recently the use of transistors as simple controllable resistances (it's never too late my friend). Among other things, I came up with this idea, which, quickly tested, actually works (well OK, there should be an inverter on the CV path)… It seems to be an easy way to add control over VCA to devices. What are the drawbacks (and there must be plenty !) of such a piece of circuitry ?

roughtest_VCArghh.png

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add an inverter for non inverted behavior !

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synthesist · Posted: Thu Jan 19, 2012 12:35 pm Post subject:

I didnt try your circuit, but I am sure possible drawbacks are that you can hear som clicking bleed through. Or that it doesnt mute all the way when the CV is low.
I build different VCAs now. One is a one-transistor-vca from ray wilson. This one didnt work for me. I had all the troubles that I mentioned above plus distortions in the audio.

Do you plan to use it to process CV signals or Audio?

umschmitt · Posted: Thu Jan 19, 2012 1:55 pm Post subject:

Yep, "from 1" means nothing. I actually wrote "from circuit" (so it's Eeschema's fault), and the pot labeled "volume" is the output of it. My idea with this transistor thing is to be able to modulate the level of signal, since the first thing not represented on the drawing is a 3080, I should have specified it. I'm thinking tremolo and such. So yes it's meant for audio. (I also tried this at the bias input (? = pin 5, to modulate the bias current) of the OTA, but somehow, something somewhere didn't appreciate the manoeuvre and as a result I had seconds of silence…)
Well drawbacks : clicks, no zero (actually this is not a problem for just a modulation)… As is, it's also "backwards", ie large CVs induce (relative) silence. Problems with impedance maybe ? (no idea !)
You're talking about this one transistor VCA, right ? Thanks, that looks very interesting, I need to read it thoroughly.

synthesist · Posted: Thu Jan 19, 2012 2:53 pm Post subject:

The single transistor circuit I tried was from the wp20. I'll post a link later when I have found it.

But the only thing I can really recommend is the VCA from the Soundlab Mini Synth. It needs a half of an Lm 13700 Chip. So I made 2 VCAs out of one Chip.
First I had problems because it didnt Zero with a regular Gate 0-5V. But I figured a way just with some trial and error now I can zero it with the secon potentiometer of the circuit. If you are interested I can write down the simple changes I made.
And with this one there is almost no bleedthrouh.

umschmitt · Posted: Sat Jan 21, 2012 4:46 am Post subject:

Found the WP20 VCA schem. This one is a proper single transistor VCA, with signal flowing from collector to emitter and control through base. Makes me realize that what I posted is actually… a VCA ! As in Voltage Controlled Attenuator. So the title is somehow OK. I'll try to solder some at the output of non CV instruments and see how playable / usefull it turns out…
Besides, synthetist, I'd be glad to see your arrangement on this 13700 VCA. I happen to have ordered four of them !

synthesist · Posted: Sat Jan 21, 2012 7:26 am Post subject:

you can find the original on the mfos site at the soundlab minisynth schematics VCA
my changes are:

R22 is 27K instead of 1K

R18 is 10K instead of 470K

R23 is 27K instead of 1K, in my setup the output distortes with only 1K at higher volumes or higher input. Also it would filter the original input signal at lower volumes. So I changed the resistor.

R19 is the actual trick. I just disconnect the pin that should go to the positive power rail. In this case I think it is relevant to really use a 100K potentiometer since it doesnt act as a real potentiometer anymore.

So now the VCA gets a Gate Signal or a envelope signal and I can always adjust the output so it mutes all the way when the gate signal is !off!
In the original schematic I could only make it even more louder.

There is a VCA from the mfos page that is actually supposed to do what this circuit now makes for me. But it uses many components and I didnt get it working.

PS: It sounds great too if I "gate" this one with an audio range signal for lofi ringmodulation robot sounds.

umschmitt · Posted: Sat Jan 21, 2012 7:52 am Post subject:

OK thanks for your explanations. I may try this one of these days !