VolCtrlDR - Balanced Stepped Volume Control
This is a stepped volume control circuit with relays which was very much inspired by Bruno Putzeys Article The G Word, or How to Get Your Audio off the Ground.
It features a balanced input with a very high common mode input impedance and a rather high balanced imput impedance. The output section is configurable for balanced and unbalanced output. Volume control is done by 6 relays per channel, offering 64 steps. We used 1dB steps, though that is fully configurable by changing the attenuator resistors.
I quick note on the attenuator resistors. There's a python script on gitlab which I used to simulate all possible resistor combinations in the E-series to get away with the least possible deviation from the desired 1/2/4/8/16/32dB steps. The 16 and 32 dB steps are even combined by using two resistors in parallel to fine tune them. On paper the resulting precision using 0.1% resistors is astonishing! First measurements seem to fully back this...
Opamp wise we used the best available parts, at least to out knowledge, i.e. the OPA1612 for signal and OPA2192 for the DC servo. The PSU is on the board using LT3042 and LT3093 ultra low noise regulators. The section with the first pair of filtering capacitors and the transformer can be separated from the audio/regulator pcb easiy.
All PCB design was done in kicad, all the design files are available on gitlab as ususal.
Finally some pics...