Spring Line Hum

Harry Bissell harrybissell at prodigy.net
Sun Jul 16 18:25:34 CEST 2000


Hi Ken (et al)

The Hum is usually from stray magnetic fields getting into the input
pickup coil...
which is a (ta-da) 'single' coil and has all the problems of the similar
guitar pickup.
Here are some practical and not so practical tips...

HARRY'S "TOP 10"  LIST

1) If the spring has an open bottom, close it with a heavy (like 18 - 12
ga) STEEL
plate.  Iron would be even better.

2) Encase the entire spring unit in similar totally sealed steel box...
better iron...

3)  Surround the transducer with a shield made of Mu-Metal... an nickel
cobalt alloy
that is extremely permeable... search the web for Mu-metal or Co-netic
which is a brand
name. Expensive but very effective

4) (serious hackers)  Design a recovery coil that is a dual coil
(humbucking) design

5) (ditto) Design a recovery tranducer based on a piezoelectric
detector, which are
much less hum sensitive.

6) make SURE the drive amp sends ZERO hum at the output (or you WILL
pick it up)

7) Make the drive amp send the biggest signal possible to improve S/N
ratio

8) Companding ???

9) Location of spring unit away from magnetic fields.  semi-easy in a
studio... impossible
live....

10) use Toroid transformers in your power supply so they won't radiate
magnetic fields.

The idea of shielding for magnetic fields is to give the lines of flux
an easier path around your unit than through it. So you need a permeable
material (steel, iron, mu-metal and not
aluminum copper etc.)  The shield has to allowthe flux an easy path so
there can not be
air gaps (flux will leak).  A bent tube shape shield (only one seam) is
better.  Thin metal
wrapped around and around (laminated like a transformer core) can be
very effective too.

The lower the frequency, the more metal you need... we are talking 50-60
Hz.

Advice from a guy who once got a BIG SURPRISE when a completed chemical
analysis
system was first turned on... and "mr. monitor" was introduced to "ms.
rotating magnetic
stirrer". Made his head spin.

Ken MacBeth wrote:

> Hi, all. Once again, I am going to include VC Reverb
> into a design I'm working on. I've built the the poly
> fusion VC Reverb a couple of times before successfully
> but have always encountered the inevitable background
> hum, sadly in the past, this factor has always stopped
> me using springlines in general. Anyone one with any
> experience of this out there?
>
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