Spring Line Hum

[jhaible]

> So the hum problem certainly must be preventable

Sure it's possible. The spring reverb I've built for my CX-3 organ
does not produce any noticable hum at all. But only after I've removed
all these ugly 19" 1U boxes with internal low profile pcb mount transformers
from the same rack. Replaced transformers with toroidal ones, "outsouced"
the transformer of one particularly bad device (Miditemp PMM88) - yes
making a wall wart from an internal PSU !
If you don't have a complete Mu-Metal shield around the pickup, hum
or no hum is mostly determined by the strength of the stray field of nearby
transformers. That Miditemp box produced a stray field to interfere with
the reverb tank over a whole meter !

JH.