I have built a TVR controlled by a pro mini using relays to switch volume and source.
Source switching is quiet, but the volume relays are not. The puzzling thing is that the noise is not consistent; it may be loud, or virtually inaudible from the same relay. I had suspected contact bounce as the source of the noise, but it is only generated when there is a signal present on the input; if I mute the source, no pops or clicks. I can also switch the source while music is playing and that doesn't cause any pops.
Because it is a transformer volume control, I don't think there is a DC offset on the seconday. (where the volume relays are) There are snubber diodes on all of the relays, which are Takamisawa RY5W-Ks. I prototyped the setup with a manual rotary switch, and that was silent, so I don't think any other bit of equipment is introducing a DC offset.
I have read of a possible capacitive coupling from relay to switch contacts, but I would have thought that this would occur even if there was no (or a very low) signal present on the contacts. People have also talked of a high-pass filter on the input, or a 1M resistor on the output, but I would prefer to fix this on the coil side of the relays if possible, not in the signal path.
Any help would be appreciated.
