External Interference

To solve a problem requires to analyze the cause.

We are dealing with four different kinds of external interferences --

#1. The basic sixty cycle hum caused by single coil pickups.
#2. Humbuzz A caused by unshielded circuits
#3. Humbuzz B caused by inferior light dimmers.
#4. Electrostatics caused by electrically charged pickguards.

How can we solve these problems?

#1. Use a dual coil system to neutralize external magnetic fields. Shielding does not reduce the hum.
#2. Humbuzz A can be eliminated by shielding with either coppertape or conductive paint.
#3. Humbuzz B can be only eliminated by shielding the circuit including the cavities below the pickup with aluminum tape.
#4. Electrostatic charges are the cause of static cling or crackling noises. Just put a few drops of liquid Downey on your pick guard and polish it with a fine cloth. I use Downey for several purposes. I put it in a spray bottle and spray very little of it on my carpet floor and in my car, and I can barely remember when I got my last electric shock! Downey can even save your life. There were several cases in the news where static charges ignited the gasoline when they were pumping gas at the gas station.

Now let me address some possible side effects of shielding. Here again we have to ask the question--

What is their cause?

Changes in tone quality are caused by eddy current interferences while loss of highs are caused by capacitance. Losses of highs due to capacitance are virtually impossible. Only if you use extremely long shielded, high capacitance leads from the pickups to the controls, there may be some loss of treble. On the other hand, eddy current interference can change the tone dramatically. Shielding the cavities with copper or aluminum tape does not cause any eddy currents.

So, what causes eddy current interference?

#A. The Tele bridge mount.
#B. The covers and baseplates of the classic humbucker.
#C. Any metal plates below the coils.
#D Metal pick guards for Strats.
#E. In general, all metal parts in the vicinity of the pickups.

The magnitude of eddy currents depends on the thickness of the material, the proximity to the coil and the kind of metal. Copper or aluminum under the pick guard or shielding the cavities will not cause eddy currents, but wrapping any conductive tape around the coil will. Aluminum will cause higher eddies than copper, steel or brass, but alloys like nickel-silver or 300 stainless steels cause virtually no eddies.

Are eddies good or bad?

I compare eddies with salt in a soup. One of the sweetest sounding Strats I ever played was a 65 Strat which had a .025 stamped sheet aluminum plate as shielding under the pickquard. No loss of highs at all, but the higher midrange was slightly reduced. I use eddy current interferences to fine-tune pickups, but interpreting the specs of a pickup is as complicated than reading an EKG!



Copyright © 1996-2010 Bill Lawrence