The simple way is to connect a resistor from DC supply at junction of r4/r5 to add or subtract current to that node. You could connect R to top of C4 to get a little more filtering.
JR
VCA theory question
Re: VCA theory question
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juniorhifikit
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Re: VCA theory question
Wouldn't that simply lower the voltage going into the buffers? I have the right voltage swing (0V to 5V down to 0V to ±390mV).
Maybe I misstated my goal. I need to take that 0V to ±390mV swing at the output of each buffer amp, and offset it so I get positive down through negative voltage. For example, on the positive CV port I want to take it's 0 to 390mV swing and make it -355mV to +35mV, passing through 0V (unity gain). This hopefully will yield a maximum of 12dB VCA gain; 0V will be unity gain; and the rest will be attenuating down below noise...
Maybe I misstated my goal. I need to take that 0V to ±390mV swing at the output of each buffer amp, and offset it so I get positive down through negative voltage. For example, on the positive CV port I want to take it's 0 to 390mV swing and make it -355mV to +35mV, passing through 0V (unity gain). This hopefully will yield a maximum of 12dB VCA gain; 0V will be unity gain; and the rest will be attenuating down below noise...
Re: VCA theory question
Think about what I said...
The output of the d/a is 0-5V, you want to add a negative V offset where that sums into CV path make it look like say -4 to +1 or whatever your precise range. A resistor from -15V to top of C4 will shift the range in that direction.
Note, the cap there will swing positive and negative so shouldn't be a polar cap.
Resistor to -15V will slightly reduce gain range so you may need to tweak value of r5 slightly lower to get full range...
JR
The output of the d/a is 0-5V, you want to add a negative V offset where that sums into CV path make it look like say -4 to +1 or whatever your precise range. A resistor from -15V to top of C4 will shift the range in that direction.
Note, the cap there will swing positive and negative so shouldn't be a polar cap.
Resistor to -15V will slightly reduce gain range so you may need to tweak value of r5 slightly lower to get full range...
JR
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juniorhifikit
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Re: VCA theory question
Thanks - I'm going to chew on that and then hit the bread board... I'll report back.
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juniorhifikit
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Re: VCA theory question
I have some 3.3uF poly film caps - can those stand in for the DAC output filtering? Any starting suggestion for an R value for the negative bias (it's ±18V I'm supplying)?
Re: VCA theory question
I could tell you but then I'd have to keep giving you fish...
use ohms law and some logic..
At 0dB you want the node (top of cap) to be 0V dc, and everything to the right of it will also be 0V so only current comeing into or leaving that node will be the current coming in from R6, and the current being sucked out by the new resistor going to -V supply.
So figure out what voltage from the D/A you want to be 0dB and use ohms law to calculate the current generated in R6 (I=E/R where E= D/A voltage for 0dB). The working backwards, we know we want the exact same current to be pulled out by the new resistor so that R will equal E/, where E equals the power supply voltage and I equals current from previous calculation.
If you reduce the value of r6 to get the gain range back up, you will need to reduce the value of new resistor a little too.
JR
use ohms law and some logic..
At 0dB you want the node (top of cap) to be 0V dc, and everything to the right of it will also be 0V so only current comeing into or leaving that node will be the current coming in from R6, and the current being sucked out by the new resistor going to -V supply.
So figure out what voltage from the D/A you want to be 0dB and use ohms law to calculate the current generated in R6 (I=E/R where E= D/A voltage for 0dB). The working backwards, we know we want the exact same current to be pulled out by the new resistor so that R will equal E/, where E equals the power supply voltage and I equals current from previous calculation.
If you reduce the value of r6 to get the gain range back up, you will need to reduce the value of new resistor a little too.
JR
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juniorhifikit
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Re: VCA theory question
And I can't thank you enough for your help and infinite patience! I've learned so much cobbling together this idea - from the internet, manufacturer's data sheets, but mostly from you all on this forum and especially JR! Big group hug!JR. wrote:I could tell you but then I'd have to keep giving you fish...
I'm happy to report that 26.8K was the magic number that gave me the correct bias to yield 11.9dB of gain from 4.98V of CV from the DAC. I couldn't be happier! Now on to the other parts of my circuit, which seemingly are a bit less complicated. Here's the schematic that I ended up with for the bipolar CV if anyone's interested:
