As I wait to receive the next batch of PCBs for the v2.2 attenuator I started thinking about how to further improve my design. I went back to consider my objectives which are:

Make the attenuator feel like a high quality potentiometer volume control. I didn’t want a stepped attenuator based on a rotary switch with detents. The stepper motor I use as an input device satisfies this objective;
To build a stepped attenuator for better accuracy than what is available with a conventional potentiometer based volume control. A potentiometer uses two resistor traces for the left and right channel. A stepped attenuator uses a string of resistors and is known to have much better left/right channel balance and can follow a desired taper (attenuation vs rotary position) with much greater accuracy. Stepped attenuators often have one resistor for each ladder step. I decided to use two parallel resistors for each ladder step to further enhance accuracy so I think this objective is achieved;
The volume control should be motorized so that the knob would actually rotate in response to commands from a remote control. I like the aesthetics of this;
I wanted two totally separate attenuators for the left and right channel so that either could be rotated and the other attenuator would follow. I think this looks cool and the feature has been realized by a Bluetooth link to coordinate the two attenuators;
Make it silent, meaning no electrical glitches/switching noise, no audible clicks from the relays and also no noise from the motor operation. The Trinamic motor driver with StealthChop mode has made the stepper motor movement silent - so ✔ on that objective. The software makes sure that at least one relay is energized at all times in a make-before-break feature so the electrical glitches/switching are eliminated when switching between steps - so ✔ on that objective. And of course the vacuum chamber is there to reduce audible clicking noise - almost ✔ on that objective too - just need to pull it all together.

It seems I achieved my objectives.

None-the-less the world is not perfect yet and further improvements are possible.

One improvement might be to reduce the size so I started to search the web for smaller relays because the relays along with the ladder resistors take up the most board space. I think I am already using one of the smallest electromechanical relays available at 10.6 mm x 7.4 mm x 10 mm.

During the search I found some very small reed relays at 3.9 mm x 3.9 mm x 15.5 mm which would certainly be interesting. The reed relays also have the benefit of being virtually silent without the vacuum chamber. The downside appears to be cost because a reed relay is about three times the price of the electromechanical relays. The total cost of relays adds up because my current design uses one relay per ladder step. I will look at this further at another time but I thought of a way to use a simple reed switch and a rotating magnetic instead of a reed relay. The reed switch is much less expensive than a reed relay because there is no coil or associated enclosure.

It turns out this is not a unique idea (nothing is really new under the sun) but it may be possible to implement this idea using some of the techniques I learnt for the vacuum stepped attenuator to design a new version of this idea.

v3.0 of the DÆ stepped attenuator of the on the horizon?