Motivation
I have had a closer look at my DC VCA with active feedthrough compensation. The goal was to have a still a good DC performance and THD with lower effort. The key was to use OpAmps with better DC specs. With for DC optimized OpAmps it is possible to stay under 2mV CV feedthrough over the whole CV range of 0..5V without active feedthrough compensation. With active CV feedthrough compensation like in my DC VCA it is possible to reach under 1mV. But i think it is not worth the effort. I added a AC/DC switch if you need to block DC in the audio path as well. So this VCA makes a good workhorse or general VCA.
Note: You have to select the LM13700 for best performance.
Specs and features:
- CV feedthrough below 2mV
- AC/DC coupled (switchable)
- 0,2% THD
- lin and log response tp CV
- Controll Voltage 0..+5V
- 5Vpp in- and output (adjustable)
- simple Level indicator
- PSU +/-15V - 25/20mA
- Schematic
Implementation
Screenshot VCA CV vs. feedthrough
As you can see is the CV feedthrough nearly down in the noise floor over the whole CV range of 0..5V (yellow line). The maximum VCA CV bleedthrough is around 2mV (blue line).
Schematic
Description
There are four blocks in the schematic. From the top: VCCS lin, VCCS log, DC or Audio path, Level display. The implementation for the VCA is plain forward. The key for lowest CV feedthrough is to lower the Iabc and use selected LM13700 OTA. Here i used around 250uA for Iabc. You can reach a CV bleedthrough from under 2mV over the whole range of 0..5V. Best DC performance is reached when for DC usage optimized OpAmps like OP27 are used. THD is still around 0,2%.
For more details of the VCCS lin look here.
For more details of the VCCS log look here.
For more details of the level indicator look here.
Calibration
- Allow the circuitry to warm up. Set mode switch to lin, gain pot to max, input pot's to ground. Adjust TR103/203 for zero volts at the output
- Apply 5Vpp signal to the input. Set input pot to max. Adjust TR102/202 for same ampltude at the output.
- Set mode switch to log. Gain pot to ground. Adjust TR101/201 for zero signal output.
Special parts
None. Use OP27 or equivalent for best DC performance. You might want to select the LM13700 for best performance as well. They differ quite a lot from chip to chip. It is hard to find an LM13700 OTA with the same good specs for both OTA on one chip.
