looking for simple VCA for 5V powersupply circuit

[cypher]

Hi all!
I am looking for some very simple voltage controlled amp circuit (or better just attenuator).
The "problem" is, that the circuit should work well at rather low power supply (5-6 V). The most of the typical designs is unfortunatelly for 15 V or so.

I am thinking of something really simple maybe based on LM13700 (maybe the reference VCA design should work in low voltage circuit?) or transistor based.

The control voltage (comming from EG or LFO) should be also quite low - just about 3V. The maximal VCA output should not be too much amplified (not to fry the following chain) - I think only lowering the incomming volume will be fine.
The negative current wave is not required (so probably no dual voltage (voltage dividor) is needed. Some lo-fi distortion will be no problem.
Thanks for advise.

[bod]

the LM13700 will be fine for this purpose on 5v dc supply, i use it in my wee DCO arduino synth with a virtual ground reference at 2.5v dc.

in fact, the VCA circuit for the DS7 would be perfect for this. nice and cheap, low parts count and reliable.

cheers,

bod.

[cypher]

[JovianPyx]

You've probably already seen this, but if not...

http://electro-music.com/forum/topic-33190.html

The VCA schematic near the end should also run on 5 volts (but ground would be zero volts, not the virtual ground at 2.5 volts mentioned). I bring this up because cypher wrote "some lo-fi distortion would be no problem" - the single transistor VCA will have a thump/click/pop added to the audio output when the modulation source is a transient generating envelope generator.

My personal preference, however, would be the LM13700.

I would wonder why you couldn't write the VCA (or NCA as Numerically Controlled Attentuator/Amplifier) into the Arduino code...

[richardc64]

[JovianPyx]

A resistor used as a current source is a rather poor current source, that is, it doesn't always do what one might think from Ohm's law because the driving source can't supply infinite current due to it's own internal resistance. The use of a simple resistor is appropriate when the relationship between the control voltage and the control current input to Iabc doesn't need to be strictly linear or particularly accurate.

A current mirror would be used when you already have a current available and want to copy it for the OTA Iabc current. There are also other types of current mirror circuits which can, for example, output a current which is double that of the input current. Current mirrors are often used when it is desired to distribute precise (as in precisely the same) control current to more than one OTA.

To evaluate the single transistor method, I'd have to see the schematic to determine what the designer did and why.

There's also the linear voltage to current converter circuit used in linear applications (such as the Fatman, but there are others as well). This is accomplished with an opamp and high gain transistor (such as darlington pair) with the base-emitter junction in it's feedback loop and the collector as current output. If made with a PNP transistor, then this is a current source at ground potential which delivers it's current into the negative rail and not ground (hence why some VCOs and other modules are powered below ground). Note that OTAs require that the current be sourced with respect to the negative rail and not ground.

[bod]

You know, I was actually pondering the same thing Richard!! And thanks for the answer Scott(?)!!

The arduino doesn't have a vca, but you can code it to create the same effect. I use an lm13700 vca circuit similar to the ds7 clone (in this thread http://electro-music.com/forum/topic-6386.html ) for my arduino synth and analogue dub sirens.

Try the one in that thread, it'll do exactly what you need I recon...

[Clack]

for an OTA example what about the wasp?

Here is elby designs drawing of the original on the left and their interpretation on the right. (only difference being a larger signal input cap)

http://www.elby-designs.com/pixie/pixie-about.htm

the CMOS buffer could be replaced with the LM13700's buffer?