I think wires over/near the PCM54 DAC produce some crosstalk (noise).
http://en.wikipedia.org/wiki/Crosstalk ---------------------------------------------------------------------------------------------------
From the datasheet:
http://focus.ti.com/lit/ds/symlink/pcm54.pdfINSTALLATION CONSIDERATIONS
If the optional external MSB error circuitry is used (PCM54),
a potentiometer with adequate resolution and a TCR of
100ppm/°C or less is required. Also, extra care must be
taken to insure that no leakage path (either AC or DC) exists
to pin 27 (PCM54). If circuit is not used, pin 1 (PCM54)
should be terminated to common with a 0.01mF capacitor.
The PCM converter and the wiring to its connectors should
be located to provide the optimum isolation from sources of
RFI and EMI.
The important consideration in the elimination
of RF radiation or pickup is loop area; therefore, signal
leads and their return conductors should be kept close
together.This reduces the external magnetic field along with
any radiation. Also, if a signal lead and its return conductor
are wired close together, they represent a small flux-capture
cross section for any external field. This reduces radiation
pickup in the circuit."
----------------------------------------------------------------------------------------------
A big and good grounded metal housing/plate over the DAC section (PCM54 and signal traces to the PCM)
may help (shielding with coppertape for example) . I don't put my fingers into fire that it works but it's a common technique to reduce crosstalk and EMI probs. shielding shielding shielding ...
Digital signals have very fast changes between their states (0=0V, 1=~5V).
These fast potential change (0->5V; 5V->0V) produce some overshoot.
http://en.wikipedia.org/wiki/OvershootThis produces higher frequencies (some sort of a little HF transmitter) which interact with
the DAC (or other signal traces). So long wires act like antennas sending and receiving a lot of garbage from
outside and interact between each other.
PS:
cool bent! that switchbox is cool.