Spring Line Hum

Don Tillman don at till.com
Mon Jul 17 16:31:36 CEST 2000


   From: "J. Larry Hendry" <jlarryh at iquest.net>
   Date: Sun, 16 Jul 2000 10:39:12 -0500

      From: Paul Perry <pfperry at melbpc.org.au>

      I did hear once of someone using 2 lines next to each other &
      out of phase as a kind of 'humbucking' setup. Dont know how
      successful this wd be, quite apart from the fact that combining
      signals from 2 very similar lines might have some weird byproduct.

   The PAiA spring reverb uses this design.  

A small nit... the PAiA spring reverb actually has the *driver*
transducers out of phase instead of the pickup transducers.  (See
http://www.paia.com/hotspuse.htm) John Simonton posted about this to
synth-diy a couple of years ago; the goal was not to cancel hum but to
get that "weird byproduct" of subtracting two very similar reverb tank
signals, a sound more characteristic of a plate reverb.  Very clever.

Of course it's very easy to wire the PAiA Hot Springs to cancel hum.
I'm guessing that either hum was not a problem at the time it was
built or the physical placement of the reverb tanks effectively did
the job for them.

I've used a few proper professional spring reverb units and they have
hum too.  Given that hum is a disaster for audio work, I think the
DIYer who wants to build something better than what they can buy off
the shelf should seriously consider hum cancelling of some sort.
Either an extra dummy coil or a parallel tank.

I wonder if the Sound Enhancements will sell you extra pickup coils
for cheap to use for hum cancelling?  It's almost worth buying a
second tank to trash for the coil.  :-)

But I do like the Hot Springs approach in general.  You can even
improve on it; wire the pickup coils out of phase and add a phase
switch on the driver coils and you've got a choice of two very
different reverb sounds AND hum cancelling.

Or if you're building a stereo reverb you can use three tanks; one for
common hum cancelling and the other two for stereo reverb.

Also, the obvious stuff:
Use a torroidal power transformer, this is what they're for.
And drive the tanks with a very robust signal.  Sound Enhancements
recommends this and has a set  of application notes with great driver
circuits.  

  -- Don

-- 
Don Tillman
Palo Alto, California, USA
don at till.com
http://www.till.com




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