From: META000 <META@UNB.CA>
Subject: Re: Ibanez pickups hum
Organization: The University of New Brunswick
Date: Thu, 1 Oct 1992 20:30:52 GMT

In article <TILL.92Sep29174802@acid-rain.lucid.com> till@acid-rain.lucid.com (Don Tillman) writes:
>   From: META000 <META@UNB.CA>
>   Organization: The University of New Brunswick
>   Date: Tue, 29 Sep 1992 22:53:41 GMT
>
>   YES, impedance IS a function of all those things, INCLUDING
>   variations in resistance per-foot over thelength of the entire coil.
>   Indeed, it is precisely the variation in
>   resistance-per-unit-of-length which can produce less than optimal
>   hum rejection IF you're simply counting turns of wire, and end up
>   with two coils that have the same number of turns but different DC
>   resistance.
>
>   Personally, I have no idea how many turns I have on my pickups.  I
>   just wind 'em until I have no more room left on the bobbin, measure
>   the DC resistance, and match the DC resistance of the higher value
>   coil to that ofthe lower value coil by taking off some windings.
>   When the coils arematched for DC resistance (which, of course, is
>   not the same thing as impedance, strictly speaking), the hum
>   rejection is just about perfect.  When they're more than a few per
>   cent off from each other, the hum is discernible, although not as
>   bad as single coil pickups.
>
>   It bears noting that my pickup coils could have identical numbers of
>   windings, but unequal DC resistance, and have hum sensitivity as a
>   result.
>
>Sorry, this is just not correct.  The induced voltage from a magnetic field
>(hum, noise, screaming high Bb, whatever) is determined by (among other
>things) the number of turns of the coil but it is *not* determined by the
>dc resistance of the coil.  Maxwell's equations have been proven pretty
>accurate about this sort of thing for some time now.
>
>The reason your experiment works as well as it does is because the measured
>dc resistance tracks the number of turns tracks closely, likely within 10%,
>and maybe even right on occasionally.  Since you said the resulting hum
>rejection was not perfect, but "just about" perfect, it seems that your
>experiment disproves your theory.
>
>Executive Summary: Matching the number of turns is accurate, matching the
>dc resistance is close.  There are, of course, other effects.
>
>  -- Don
>.
>.
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).

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.

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.

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

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.

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.

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.).  On the other hand, high impedance
with fewer windings OUGHT to result in lower inductance, and so MORE
high end.  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?

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.

Mark

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