Spring Line Hum

[Paul R. Higgins]

Regarding those flat-pack transformers: I have a slightly OT story to relate, 
although still in the musical electronics field.  I was building a tube preamp 
years ago, and thought it would be pretty neat to stuff everything into a single
rack space enclosure by using one of those PC-mounted flat-pack "Low Boy" 
transformers.  The manufacturer claimed that it had "semi-toroidal 
construction", so I figured stray fields wouldn't be a problem.  Wrong.  It had 
a strong enough field to inject hum into even fairly low-gain, downstream parts 
of the circuitry.  Replacing the flat-pack with a chassis-mounted transformer 
resulted in much better performance.

Clearly, this was a very critical design with respect to noise and hum (a six 
stage high gain channel-switching preamp), but I would venture to say that a 
spring reverb is an equally critical circuit.  I would never use a "Low Boy" in 
a spring reverb design, and I would keep the tanks far away from any power 
supply circuitry.

In addition to mu-metal shielding, another approach to consider might be to use 
a shielding paint that is used on control cavities in electric guitars.  It's a 
gray-black paint which I think is essentially powdered nickel/silver in a 
water-base epoxy resin.  You can get it from Stewart-Macdonald Guitar Shop 
Supply (www.stewmac.com).  I can vouch for the fact that it works great in 
guitars.  It might be possible to put the reverb tanks in a sub-enclosure and 
paint the inside. 

BTW, my Mesa/Boogie Quad Preamp has an unusual reverb design; there is one short
and one long tank in parallel.  They're driven by one half of a 12AX7; the other
half is the recovery amp.  The 12AX7 is a pretty anemic reverb driver 
(Mesa/Boogie really should have used a 12AT7) but the sound is really excellent,
with no hum--although still a little "sproingy".  I would bet that the 
out-of-phase trick used in the Paia Hot Springs sounds even better.

-PRH
 
> > 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.


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Paul Higgins
email: higg0008 at tc.umn.edu
University College, University of Minnesota
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