Spring Line Hum

[Hairy Harry]

I doubt very much if the conductive shield paint will be effective
at all much below radio frequencies. The only way to stop low frequencies is 
with lots of permeable material, which copper, aluminum ,silver and carbon 
based paints are pretty (hehheh) "thin" on.

The "low boy" transformer, in smaller VA sizes is more crippled by
the lack of enough core material. Most small transformers are run at higher 
flux levels in the core to make them smaller. The lack of enough core forces 
them to be run hotter and with more flux leakage.

A good rule of thumb is to look at the load regulation specs... 20%
is an awful number, almost sure to mean insufficient core and flux leakage. 
The bigger the transformer, the better that regulation spec.

Of course the true TOROID is the best performer.

H^)  harry


>From: "Paul R. Higgins" <higg0008 at tc.umn.edu>
>To: synth-diy at node12b53.a2000.nl
>Subject: Re: Spring Line Hum
>Date: Tue, 18 Jul 2000 22:46:16 -0500
>
>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.
>
>
>_____________________________________________
>Paul Higgins
>email: higg0008 at tc.umn.edu
>University College, University of Minnesota
>_____________________________________________
>

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