Help with single supply VCA schematics

Hi All,

As a learning exercise, I’m trying to do a small vca inspired by Olivier’s document on the shruti. The goal is to hook it to a teensy using a 2-channel dac, one for an oscillator shape, the other for the gain control. One of the design changes - beside using a CA3080a that I have lying at home, it to make it work using single supply voltage since then I can power it from the teensy. I modelled using ltspice and came up with this:

It seems to work, except that the gain voltage isn’t effective unless it gets above 2.5v and I’m not to sure why. I could only generate signal above that threshold but would loose resolution and I also tried adding an offset voltage to the gain signal but it didn’t seem to work either.

Has anyone an idea of what is exactly happening or if I’m doing anything wrong ?

Thanks !

First, I would suggest creating a VCC/2 named port to make this mess more readable :slight_smile:

You’re going to get annoying DC offsets due to each VCC/2 being a different voltage due to the difference between each 33k resistor. You’d better have a single pair of 33k resistors as a divider, buffer it and distribute it everywhere. If your +5V is going to be crappy (OLED :D), you can also buffer a 2.5V Vref (like an LM4040) and get a supper clean half-rail! Don’t forget to put bypass capacitors everywhere!

I think the problem is in your current source. I would put U1’s + input to the half rail ; and send +5V to its - input through a 5k or 10k resistor…

TL072s are not rail to rail, so whenever you see their + input at the same potential as their negative supply rail there must be something wrong. Speaking of which… A TL072 powered by 5V has only 2V of usable headroom before it clips either ways. Which sucks. Use TLV247x (rail to rail I/O) ; or if you’re cheap, MCP600x.

Hi Olivier,

Thanks for the comments. I haven’t found how to use named ports to carry signal from point to points, hence the ugly mess indeed. I’ll try your suggestions and get back with more question if needed.

Thanks a lot !

Two named ports with the same name creates an invisible connection between them.

magic \o/