R3.1 hardware limitations and R3.2 changes

[vk2seb]

Some things to change/update for R3.2 hardware:

R3.2 hardware changes

• Fix routing error which requires 0 ohm resistor between GND/GNDD
  • Context: R3.1 boards are modified such that L4 is a 0 ohm resistor, otherwise there is too much ground bounce due to the LED PWM switching. This is caused by the decoupling caps around the PWM controller being (incorrectly) on GND instead of GNDD. Interestingly the board works even without this change and the noise isn't worse, but the CODEC sometimes will randomly shut down due to the ground bounce if L4 isn't swapped (it is already correctly reworked by hand on all units I have shipped).
• Move VCOM reference voltage closer to the statistical center of the AK4619 ADC inputs. At the moment the VCOM voltage is a few hundred mV away from the statistical zero of the DC center of the ADC inputs.
  • This has no effect after calibration, but it means that with no zero calibration, the raw counts are about 1V away from where they should be, and means we get less input swing in one direction on the inputs.
• General noise improvements. At the moment there is room for improvement on R3.1 noise performance. Noise is clearly audible if I turn up the gain on my Befaco OUT module into headphones all the way up to max. Qualitatively the noise sounds about the same as my MI Beads, but clearly worse than my ES Disting. Some ideas:
  • Removing the GNDD/GND tie should reduce noise
  • Switching to 48KHz sampling (this actually does audibly reduce noise a fair bit, I tried it)
  • Reduce input/output swing. At the moment +/- 10V capability is really hurting our noise performance. Maybe most people don't care and we can do +/- 7V or so which would quieten things down considerably
  • Investigate input/output op amp source/load impedances

R3.2 new features (maybe)

• add 3V3 regulator that can optionally power a connected FPGA board
• add some diodes such that the op-amps are powered from 3V3 if no +/-12V rails connected (so the analog sections still work if nothing is connected but the PMOD connector)

R3.2 cosmetic

• Move LEDs and V-scoring a bit further back so the front panel doesn't tilt as much after being screwed on
• Swap website out for apfelaudio.com
• Remove solder deposits on text on bottom side (I think this was caused by a solder bath at the MFG)
• Plate through holes on front panel

[vk2seb]

R3.3 has been brought up in #50 and as such this can be closed