dfiction
Member
- Joined
- Feb 8, 2016
- Messages
- 14
Hey everyone!
I found this wonderful forum and I've been lurking for a year or so. I have a question about a VCA circuit I'm designing, and I think that I am far enough along to ask for help.
I'm doing some work with the THAT2162, and in my experiments and trials I've managed to get a very good sounding, quiet VCA design that I'm quite happy with. I've designed a circuit that controls the gain sent to 6 power amps with a single potentiometer, by using 6 different VCAs, implementing the THAT2162. I'm using a single 12v supply, and the whole vca section of the circuit is running in reference to vcc/2. I opted to use differential drive because I wanted to maintain a wide dynamic range in my circuit. This might have been overkill, but I ended up with a pretty great circuit, using some rather expensive semiconductors.
While I have a great sounding circuit with wide dynamic range and low noise, I am having a lot of trouble controlling the contour of the volume pot, which should not scale linearly, but rather with an approximately log curve, so that I can use the single potentiometer to control some "long fadeouts" with the volume knob. I did measure the gain at the Ec- port My circuit tracks linearly, scaling between -120 -> +6 dB at the full extremes of the volume pot (the extremes are configurable by two resistors in the control circuitry).
But the volume pot doesn't track correctly, the way a master fader VCA should, and I don't know how to modify my circuit accordingly. At first I kept increasing the resistor R5, which lowers the threshold of the "off" position of the volume knob P2, and I thought that the right approach would be to raise the minimum threshold to around around -70 db or so (R5 to 100k). But finally I have learned that I want a log response of the pot, which would enable me to keep my dynamic range, and give the circuit a "good response" when the user adjusts the volume control.
So, here's my circuit. I have already read the THATCORP design note 116, which details the diode breakpoint application, but I'm not sure how I should adapt that principle to my current circuit, for P2 is controlling a very small span of approx 300 mV. Where to interrupt, or stick a diode?
I don't really need a perfectly logarithmic response in this circuit. I just need the volume knob to be "useful" in the lower positions—right now the linear response I'm getting means that most of the action is in the top rotation of the pot, not very good for fades.
Any wisdom you might have would be much appreciated.
best,
Daniel
I found this wonderful forum and I've been lurking for a year or so. I have a question about a VCA circuit I'm designing, and I think that I am far enough along to ask for help.
I'm doing some work with the THAT2162, and in my experiments and trials I've managed to get a very good sounding, quiet VCA design that I'm quite happy with. I've designed a circuit that controls the gain sent to 6 power amps with a single potentiometer, by using 6 different VCAs, implementing the THAT2162. I'm using a single 12v supply, and the whole vca section of the circuit is running in reference to vcc/2. I opted to use differential drive because I wanted to maintain a wide dynamic range in my circuit. This might have been overkill, but I ended up with a pretty great circuit, using some rather expensive semiconductors.
While I have a great sounding circuit with wide dynamic range and low noise, I am having a lot of trouble controlling the contour of the volume pot, which should not scale linearly, but rather with an approximately log curve, so that I can use the single potentiometer to control some "long fadeouts" with the volume knob. I did measure the gain at the Ec- port My circuit tracks linearly, scaling between -120 -> +6 dB at the full extremes of the volume pot (the extremes are configurable by two resistors in the control circuitry).
But the volume pot doesn't track correctly, the way a master fader VCA should, and I don't know how to modify my circuit accordingly. At first I kept increasing the resistor R5, which lowers the threshold of the "off" position of the volume knob P2, and I thought that the right approach would be to raise the minimum threshold to around around -70 db or so (R5 to 100k). But finally I have learned that I want a log response of the pot, which would enable me to keep my dynamic range, and give the circuit a "good response" when the user adjusts the volume control.
So, here's my circuit. I have already read the THATCORP design note 116, which details the diode breakpoint application, but I'm not sure how I should adapt that principle to my current circuit, for P2 is controlling a very small span of approx 300 mV. Where to interrupt, or stick a diode?
I don't really need a perfectly logarithmic response in this circuit. I just need the volume knob to be "useful" in the lower positions—right now the linear response I'm getting means that most of the action is in the top rotation of the pot, not very good for fades.
Any wisdom you might have would be much appreciated.
best,
Daniel