RE: (TV) On topic content > mags, guitar, hum

[Maurice Rickard <maurice@mac.com>]

At 10:05 AM -0700 4/2/06, robin dunn wrote:
> there is a bit of hum.  has anyone here foiled their
> pickguard?

Yes.  Not a bad idea.  Also consider foiling the body cavity.  I've 
done this before to some guitars (though not the K'zoo, for reasons 
I'll explain in a moment).  It does help.

>  is it as simple as applying heavy duty
> foil to the back of the pickguard?

That's basically it.  It doesn't need to be that heavy, though--make 
sure you can still get it to hold with glue stick.  Same with the 
body cavity, of course.  And attach this to the ground (sleeve) of 
the output jack.

> is there anything
> special to be done with the ground wire (i.e. does it
> need to touch, or not touch, the foil)?

This depends.  If you're playing through a tube amp, and you're 
singing through a separately amplified mic, or playing with someone 
else playing through a tube amp (particularly old two-prong amps), 
you have to watch out for ground loops:

http://www.google.com/search?q=%22Leslie+Harvey%22
or http://elvispelvis.com/electrocuted.htm

In my case, I added a small electrolytic capacitor in line with the 
string ground.  Cut the string ground and solder in a 0.1uF 
capacitor.  (If it's electrolytic, - side goes toward the sleeve of 
your output jack.)  In this case, you'd still get a zap, but more of 
a startling one than a fatal one.  (Yes, it's happened to me.)  The 
idea is to let enough current through to ground the guitar, but not 
enough to do damage to you if the ground goes live.

In my case, I didn't foil the Kalamazoo, because I already had a 
metal pickguard.  I don't recall my rationale for not foiling the 
body cavity (maybe the eventual rattle from another guitar when the 
glue stick dried out), but I instead rewired the Kalamazoo with 
coaxial cable.  In this case, the shielding is immediately around the 
wires instead of around the whole cavity.  The downside to this is 
that it gets pretty snug in there around the potentiometers.

> and for the
> most scientific amongst us, what causes hum, and why
> does foiling reduce it?

If you checked out the frequency of the hum, you'd find it's 60Hz. 
Yep--it's ambient electrical noise from our power supply.  An 
electromagnetic field surrounds all power lines, unless they're 
shielded.  (Most household lines aren't.)  Foiling (and all 
shielding) excludes it by conducting that electromagnetic energy 
along the surface of the shielding (and ultimately to ground).  See 
http://en.wikipedia.org/wiki/Faraday_cage since foiling is 
essentially the process of building a little Faraday cage.

-M
--

Maurice Rickard
http://mauricerickard.com/   |   http://onezeromusic.com/
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