# Controlling the Gain of THAT VCA 281x with the Voltage from a Digital Potentiometer.

Thread Starter

#### Lukas M Brenner

Joined Aug 5, 2015
11
The circuit I’m going off of is listed here http://www.thatcorp.com/datashts/dn02.pdf I had my circuit much closer to this and it wasn’t working, so I’ve strayed a bit by adding the divider for the control voltage in front of the buffer, and adding R7 and C8 (47R and 2.2n) which I got from another THAT design note.

So far I haven’t been able to get any signal from the output. I’ve tried biasing the VCA in a couple different ways but have had no luck. This and the single rail power that I’m using are maybe the two things I’m most suspicious of. I’ve also referenced their page with pedals made from their Analog Engine, which has a Voltage Reference going into EC- ; not sure how that's working, tried but failed.
( http://www.thatcorp.com/pedals/4316 Simple Limiter r00.1.pdf )

#### OBW0549

Joined Mar 2, 2015
3,566
You aren't getting any signal out because the THAT2181 is unpowered.

The bottom end of R5 absolutely MUST be connected to a negative voltage sufficient to bias the chip properly (see the data sheet). A positive supply alone, connected to pin 7, is not enough.

See the following for a working example:

Thread Starter

#### Lukas M Brenner

Joined Aug 5, 2015
11
OBW0549, thanks for the feedback ! I don't have much experience with negative voltage rails. Is there an easy way to bring 9v+ down to 4.5± ?

#### OBW0549

Joined Mar 2, 2015
3,566
That depends on what else your 9V source is connected to. If neither end of the 9V source is connected anywhere else in your circuit (e.g., the negative end isn't connected to circuit ground), you can use what's known as a "rail splitter" circuit, which will develop a voltage halfway between its (+) and (-) input terminals. By grounding that midway point, the (+) and (-) input terminals can be used as a bi-polar ±4.5V supply.

But if the negative end of your 9V source is connected to circuit ground and the +9V is used to power other stuff in your system, a rail splitter won't work; you'd be better off using a negative voltage converter such as a LTC1144 to generate a -9V supply from your +9V supply.

Thread Starter

#### Lukas M Brenner

Joined Aug 5, 2015
11
Thanks again OBW0549. There's plenty of other stuff happening with the circuit, so I think I'm going to try implementing an H11F1M to create a VCA that way; price point being lower and also immediately on hand. Will keep the LTC1144 in mind and report back if I take that route.