Looks great. Initially I thought that you repurposed an old HP case.
What is the topology of the resistors/relays network? Trying to understand how the resistances of the relays factor in, especially in low settings.
Thanks! I thought about doing something like that, but buying old equipment and ripping it apart instead of repairing it doesn't feel right, generally speaking. It can be cheaper than buying a new case though. If I found a faulty HP unit damaged beyond repair, i think I would go for it. Actually, I just ordered an old non-working Agilent DC Mobile Communication power supply. So if I can't repair it...
In this series-style toplogy the contact resistance plays a significant role, as you can see in the first performance diagram in my first post: At 1 Ohm the deviation was about +5% .. 6% (4-wire measurement), meaning a total of 50-60mOhm for 2 relays in series + PCB traces etc. In these cases the resistor tolerances (0.1% spec) are negligible. The relative error drops very quickly though. For 2 Ohm it's already well below 2%, for 100 Ohm about 0.1% (all for 4W). With values above 10k it's the other way round - the resistor tolerance of (in my case) +-0.1% dominates the error. Then the calibration/adjustment improves the accuracy to a point where my 34401A's accuracy becomes a concern.
I implemented some measures to reduce the effective contact resistance, i. e. using both relay poles in parallel, shorting the upper decades for the three lower decades (i called that "bypass relays"). But at the time I didn't care too much about the accuracy of the really low values anyway. The lower decades are partially there to improve accuracy in the upper ranges...
I want to hear how this thing sounds in action. It must be really sweet.
The signal relays are reasonably quiet. Plus the case doesn't have any cooling vents. I made a short clip (will take a few minutes for the higher resolutions):
There they are.
Thanks, appreciate it! Will take a closer look at it! Did you do any measurements/graphs showing the accuracy? I might wanna know more
Which low EMF relays did you use?