# Voltage control decay

Hey! I have been looking at this skull and circuits claque:

And seeing the two unused channels I f the v2164 I was wondering if I could use one of them to add voltage control to the decay. I looked at the voltage controlled decay of the befaco kickall

But I don’t really understand how that works… it is different from the voltage controlled resistor in the lm13700 datasheet, but a little similar.
Does anyone know how it works (or has other good ideas for this or ideas to use the 2 channels on the v2164?) I think the Befaco thing is pretty near the same as the datasheet one. It lacks an AC coupling capacitor and the resistors to ground are a little larger. Main difference is it lacks the -15 V bias in the datasheet version.

How (or if) you’d translate that to a v2164, though, I don’t know.

1 Like

I think it may be hard to implement a voltage controlled resistor with only the V2164 because it is a non-inverting current amplifier. (The current flows into the resistor when a positive voltage is applied to it e.g. in the claque circuit when capacitor C14 is getting discharged through the decay pot, so an inverting amplifier is required to change the positive voltage into a negative current.)

You could use the V2164 to create a voltage controlled current sink, but for any given control voltage the produced current would be constant and then the capacitor discharge would be linear, unlike the exponential decay of the original RC circuit in the claque. And for audio decay, an exponential curve usually sounds much better.

If you also have a spare opamp, you could combine it with the V2164 to make the inverting amplifier required to implement a voltage controlled resistor, but the additional difficulty in this circuit is that the potentiometer that we want to replace is not just connected to ground, it must only conducts when C14 is not getting charged through C6, D5 and D7.

Something like this might work (with the right choice of values for the three resistors).
Connecting the non-inverting input of the opamp to the cathode of D5 should cause the output of the opamp and the V2164 to go positive when the C14 is getting charged through C6, D5 and D7. The diode between the V2164 and C14 will block the V2164 output current. The rest of the time the circuit should act like a voltage controlled resistor between C14 and ground.

You’d probably also need a bit of circuitry to adjust the control voltage to a 0 to 3.4V range before applying it to the VC input. A quirk of the V2164 is that the gain decreases as the control voltage increases, in this case it would mean that the higher the control voltage, the longer the decay.

3 Likes

Thanks for the detailed analysis and suggestion! Greatly appreciated! I will test this on breadboard once I am back from holiday Since you mention that it’s more complicated because the pot does not simply go to ground, do you have an idea why that is? Why is it going back through the diode and then to the resistor to ground and not simply resistor to ground?

The D8 diode is necessary because this is only the discharge path for the C14 capacitor, it is charged through D7.
R15 is to avoid a short to ground if the DECAY pot is set at minimum value. The short to ground would completely prevent C14 from charging at all.

I’m not sure if R15 and D8 actually need to be connected to D5 and D7, it doesn’t seem so.
You could try disconnecting them and then you could move the DECAY pot to between R15 and ground.
With R15 and D8 disconnected from D5 and D7 you could also get rid of one of D5 or D7.
So that this:

becomes this:

You could then replace the DECAY pot by a voltage controlled resistor to ground circuit.

2 Likes