Errata/ Notes on Revision 0.3.3

[jbeyerstedt]

Loose collection of things which I noticed when using prototype 2 (v0.3.3) for the first time:

Positive:

• Microphone and headphones noise floor seems to be quite a lot lower than at prototype 1
• vu meter low-pass filtering is not really noticeable (I like the quite snappy response)
• It still works

To improve:

• vu meter does not reach the top LED before the microphone clips at the headphones and line output
  • TODO: Increase amplification or change reference voltages
• low-pass filter of vu meter amplifier/ rectifier has a DC offset. Lowest LED is always lit, bottom 5 LEDs when microphone is "on"
• headphones have a bit too harsh high-end for my taste
• muting delay is too long (takes about 1 second to fade down, should be more like 100 ms max)
• microphone gain might be a bit too high (1/8 turn (should be about 10 dB) is fine for SM58 and HMD660)

EDIT: More to improve:

• Replace line input trim with a 10k resistor (empirical value for 0 dBu) or just make the op-amp stage unity gain. Question: Is the op-amp stage needed at all then?
• Add some low-pass filtering on the microphone input
• Maybe: Reduce line out by 6 dB (it clips my audio interface quite easily)

[jbeyerstedt]

Required rework:

• Channel Mixer:
  • Replace C314 with 100nF (less muting delay)
  • Replace C301 with 680pF (11 kHz low-pass on headphone)
  • Replace R309 with 8.2K, replace R232 with 220R, replace R335 with 18K (adjust vu meter scale to -27 to +6 dBu)
  • optional: Replace R303 with 1K (reduce microphone gain range to 34 dB instead of 45 dB)
• Channel I/O:
  • Add 220pF in parallel to R602 (10kHz low-pass on microphone to reduce high-frequency noise)

[jbeyerstedt]

Remaining points (which require layout changes or more changed components) are moved to #9