The VC-1 is.
On the left, the Silonex Audiohm opto-couplers (OC) with the electronic drivers. it has been described here. On the right, the line-buffer/impedance-controller described there. It works as expected, using two digital pins on the Arduino micro-controller to change the volume.
I use 2.2 Mohm metal film resitors as a voltage divider on the analog input. Using 1% metal film supposedly lowers the distortion (if any) caused by fluctuating voltages on the op-amp input. Gain = 1. Input impedance = 1.1 Mohm, which should be enough to plug in any guitar. Output impedance is a few ohms, which is what Silonex recommends to drive the OCs. Simple, no distortion, relatively cheap to build.
The Silonex OCs have to be calibrated by hand. Eventually, a matched pair would alleviate the problem, but really well matched pairs are hard to find, and expensive. So I included a function in the code to alter the resistance curve of the OCs digitally. To calibrate, I use a multimeter to measure the actual resistance and try different values in the curve function to obtain the specs I want. This also means that the VC-1 has to be tied to a micro-controller (and Arduino in this case, of course).
The design is sensitive to its environment. Having a micro-controller driving an analog audio signal is difficult. I spent many hours in research and testing mode to find a way to prevent “digital leakage” on the audio side. Also, the OCs have to be very well calibrated, or the device will pick up various radio channels…
After testing is done, I will have the circuit board produced in quantity. There will be more than one version possible, probably on a “universal” board. So it will be available as a “product”.
Right now, the input driving the VC-1 has to be 0-5 volts, with the two channels balanced to an optimal “curve”. The next version will have two trim potentiometers on board to allow for unbalanced 0-5 volts input/outputs. It will also allow the VC to be controlled by only one control voltage. This will make the VC controllable by ANY 0-5 volt signal. So you’ll be able to attach any old scratchy potentiometer volume pedal, most expression pedals, a midi volume control or any other source to the distortion-free, low impedance VC.
Talking about midi: this version is midi compatible, if attached to a micro-controller that translates incoming midi messages to 0-5 volts outputs. It might be a first…