RCA input hum [solved]

[Paul Birkeland]

The highest steps of the attenuator are way higher than my usable listening level. Noise is only audible when no signal present at 6 highest steps.

Yeah, this is not particularly abnormal.  If the presence of this noise bothers you, your stepped attenuator can be padded so that it can go no higher than the last step or two that it can go to now.  In all honesty, these parameters will change as you change sources and headphones, so the effort does not seem to justify the outcome. 

-PB

[jabbr]

Next steps are:

1) build John Swenson's 5V power supply to supply my DAC. I'm making a dual headed USB A -> single USB B cable for this purpose.

2) upgrade power cable and power strip.

3) I was considering running the +/- lines of one of the balanced outputs (either R or L) into the Crack (R/L RCA in) just to see if that eliminated the hum but I'm on the fence as to whether that would be worth it.

[jabbr]

Some feedback (sic) on different RCA cables:
1) proper shorting plugs: no static/hum at highest gain
1a) open RCA inputs: static at highest gain (higher pitched than when RCA cables connected to DAC)

2) BJC/Belden 1505F:  clearly the lowest hum of the cables group

3a) BJC LC-1 (Belden's low capacitance shielded cable): seems to be position dependent -- hum increases when run near my 4K monitor
3b) silver plated copper teflon 20 AWG twisted pair/shielded -- switchcraft plugs
3c) Huber Suhner sucoform (microwave coax- 40 Ghz)  -- switchcraft plugs

4) silver plated copper teflon -- 24 AWG  x 4 circular braided - switchcraft plugs -- much more audible hum and graininess

[jabbr]

Problem solved! I installed the speedball upgrade. Initially I got a constant white/pink noise in the right > left channel which didn't vary with volume, nor input, nor shorting plugs. Swapped out the 12au7 and the crack is dead silent (without input) even with the volume at max. It's really sounding great