The DÆ v3.0 Attenuator has worked well but not perfectly. The main problem has been an occasional click heard through the speakers when adjusting the volume. While not loud, the click was definitely there at times.
The v3.0 attenuator is made of levels of twelve attenuation steps each. A single level is shown in the photograph below. A particular volume level is selected by positioning a magnet in front of the reed switch for that level. The reed switch are the tiny glass cylinders arranged around the perimeter of the level. Each reed switch is flanked by two precision resistors (blue parts).
v3.0 Attenuator Level.
I suspect the click was heard when the reed switch for a particular level opened before the reed switch for the next level closed. This is shown in the animation below where a closed reed switch is represented by a white cylinder. Notice how the reed switch at the far left opens (disappears) before the red yoke positions the magnet (black cylinder) in front of the middle reed switch. There are periods of time when none of the reed switches are closed and the attenuator presents an open circuit to the circuit following the attenuator which can result in an audible click.
My first attempt to correct this was to add a “muting” relay to the output of the attenuator that was activated when the yoke is in motion which ensures that the attenuator never presents an open circuit. The muting relay helped but did not totally solve the problem.
Single Magnet Yoke.
A more effective solution was to add a second magnetic to the yoke. This ensures that there is an overlap between the opening/closing of the reed switches. The animation below shows the dual magnet yoke and the “make before break” action of the reed switches. Notice that at least one and at times two reed switches are closed (visible).
Dual Magnet Yoke with Make Before Break.
For the time being I have left the muting relay in place to provide some insurance in case the tolerance in the strength of a particular pair of magnetics and the sensitivity of a reed switch stack up in the wrong way.
One final issue appears to be the switching between attenuator levels which can still result in an audible click. Engaging a particular level is handled by a reed relay on each level. I have updated the circuit design to provide a “make before break” action for the level switching relays. The printed circuit boards with this improved circuit design have use arrived and I should be able to test them within the next couple of weeks.