From: till@acid-rain.lucid.com (Don Tillman)
Subject: Re: Ibanez pickups hum
Reply-To: till@lucid.com
Organization: Lucid, Inc.
	<TILL.92Sep29174802@acid-rain.lucid.com>
	<01OCT92.17835628.0171@UNBVM1.CSD.UNB.CA>
Date: 3 Oct 92 09:33:19

[Resending -- the pitiful excuse for software I'm using indicates that it
didn't get out the first time.  Apologies if you're seeing this twice.]

   From: META@UNB.CA (META000)
   Date: 1 Oct 92 20:30:52 GMT
   Organization: The University of New Brunswick

   (Don Tillman) writes:
   >Executive Summary: Matching the number of turns is accurate, matching the
   >dc resistance is close.  There are, of course, other effects.

   Seems sensible to me. I'll bite.  The question remains as to how
   accurate commercially produced coils are, with respect to number of
   turns, and whether trimming them back, using DC resistance as a
   guide, is the optimum way to reduce hum (assuming that unwinding the
   pickup and counting the number of turns on each coil is NOT a viable
   option).

Commercially produced coils use digital turns counters; anything else would
be painfully inefficient and less accuarate.

   In my own case, since the coils are handwound, and vary
   dramatically, compared to machine-wound coils, in terms of # ohms
   per turn, DC resistance does not accurately reflect # turns on each
   coil, nearly as much as it would on a machine-wound coil.  I suppose
   I could keep scrupulous records of how many turns I put on, and lord
   knows I've triedto do that, but hand-winding tends to preclude such
   accuracy.

I found it a trivial exercise to build an LED/phototransistor/CMOS turns
counter.  I heartily recommend it if you're going to do any pickup
experimentation.

   Consequently, I feel that DC resistance matching, in real-world
   realistic terms (as opposed to ideal theoretical ones), is, as Jeff
   Baxter suggested, a reasonably good way of improving the
   hum-rejection in a two coil hum-rejecting pickup, whether they
   happen to be the standard Gibson type, stacked Strat types, or
   side-by-side (Evans) types.

If you know that the wire gauge and physical size is the same, and you
don't know the number of turns, then sure, dc resistance matching is
probably your best bet.  But don't expect it to match any better than, say,
10% or so.

   Now, matching pole strength in pickup coils...that's another thing!!

Right; well that was the original question that started this thread. 

   Incidentally, Don, I'vealways wondered what the relationship is
   between distance of the wire from the poles themselves.  For
   instance, a Strat type pickup and a Fender Jazzmaster pickup could
   be designed with the same typesof polepieces, the same overall
   impedance (bear in mind this is hypothetical, not actual), and
   probably sound radically different.

Wire wrapped a distance from the poles will be missing that many lines of
flux, so the sensitivity will be down.  Unfortunately I'm not really
familiar with the internal construction of the Jazzmaster pickup.

   I assume that a "flat" coil form, like a Jazzmaster, or the P-90
   "soap-bar" pickups, results in a higher impedance pickup for the
   same number of turns, relative to a Strat-type coil, since turns on
   the outsideclearly have to travel a greater distance to go all
   theway around the existing, underlying, coil turns.  On the other
   hand, I always had the impression (rightly or wrongly) that coils
   wound closely around their pole-pieces, would have more high end.

Mostly true, except that the reason the wider coil has a higher impedance
is because the wider coil has a higher inductance, not because the
lengthier wire has a higher dc resistance.  (DC resistance is *really*
unimportant.)  Higher inductance means that the electrical resonant
frequency of the pickup will be lower (all else being equal).

There's also a mechanical issue; the width the coil picks up, called the
"magnetic aperature", is a major factor in determining the high end
response.  When the wavelength of the note on the string (either fretted
note or harmonic, makes no difference) approaches the magnetic aperture
width, you'll lose a substantial amount of signal because the pickup picks
up just as much upsy as it does downsy.  What frequency is that?  I'll
leave that as an excercise for the reader.  (Hint: it's different on each
string and it's dependant on the scale length.)  This is one reason why
wider pickups like Gibson humbuckers and P-90's have less highs than thin
pickups like Strats.

   Presumably, the higherthe pickup impedance, the more high end one
   would lose via the usual means (cable capacitance, impedance
   matching with the next stage, etc.).  

(...but most importantly, interwinding capacitance.)
					 
					 On the other hand, high impedance
   with fewer windings OUGHT to result in lower inductance, and so MORE
   high end.  

I can't parse this.

	      Given what you know about the math behind this stuff,
   what kind of coil (flat-type, or tall-thin type)ought to result in
   more high end frequency response,in real world conditions?

More high end?  Tall-thin.  But of course more high end isn't necessarily
better.  (We can talk about the frequency response all you want, but when
it comes down to music...)

   If the flat-type has some advantages, I wonder why it is that no-one
   has produced a "stacked Jazzmaster" hum-rejecting pickup to fit
   inside a standard Gibson hum-bucker cavity.  That is, one set of
   pole-pieces running through two wide, flat coils, stacked on each
   other.  Seems to me thiswould be an ideal alternative for someone
   who wants the reverse retrofit (i.e., they started outwith a
   humbucker-sized hole, and now want a "single-coil sound", with
   hum-rejection) to what people usually ask for.  Admittedly a small
   market, but a market nonetheless.

The modern version of the Gibson P-90's do this (P-100's?)  Note that many
folks don't like the sound of stacked humbuckers.  I've never played one,
so I can't comment much.  My personal preference is for single coil pickups
(cuz I like the sound of 'em), individually preamped, with a dummy coil to
cancel hum.

  -- Don