From: META000 <META@UNB.CA>
Subject: Re: Ibanez pickups hum
Organization: The University of New Brunswick
Date: Tue, 29 Sep 1992 22:53:41 GMT

In article <TILL.92Sep28103928@acid-rain.lucid.com> till@acid-rain.lucid.com (Don Tillman) writes:
>   Date: Sat, 26 Sep 1992 22:25:19 GMT
>   From: META000 <META@UNB.CA>
>   Organization: The University of New Brunswick
>
>   Althoughthe technology for producing pickup coils is pretty damn
>   good, there are sufficient differences that occasionally crop up,
>   such that the two coils in one "humbucker" are not matched enough to
>   buck the hum completely.  It's my understanding that the factory
>   folks often take coils off the line and sort them into bins with
>   specific impedance ranges.  Any two coils from the same bin will be
>   much more alike in impedance than two coils from separate bins.
>
>Not true.  Factories use a digital counter to count of the number of turns
>being wound on a coil.  Much easier, faster and more consistant.  If you
>wind coils yourself, a coil winder with a turns counter is a godsend.
>
>   Fine.  IF it happens that somebody ain't doing their job right, you
>   can end up with a pickup where the impedance of one coil is a bit
>   different from the other.  We're talking a few per cent here, but
>   not much more.  Let's give the folks who work for Larry and Seymour
>   a bit of credit!
>
>   If you are delicate and graceful with a soldering iron, you can
>   improve the tone and noise/hum of your humbucker in the following
>   manner:
>
>   1) Remove the pickup entirely from the guitar (sorry).
>
>   2) Detach all relevant leads so that you have exposed two contacts
>   for each pickup coil.
>
>   3) Get hold of a 4 1/2 digit ohm-meter (3 1/2 may not be precise
>   enough), and measure the electrical resistance of each coil.
>
>   4) If they are within 1% of each other (e.g., one coil is 5000 ohms,
>   and the other is 5050 ohms), then any annoyingly loud hum may not be
>   coming from your pickups (although there are exceptions to this).
>
>   5) If they are more than, say 0.5% different, take the coil with the
>   greater resistance, and GENTLY! lift the wire at the end of the coil
>   (i.e., on the outside) and unwind it approximately ONCE for each
>   ohm, until you have reduced the coil resistance by thedifference
>   between the two coils (e.g., if you needed to reduce the 5050-ohm
>   coil in the above example, take off about 46 turns) scrape the
>   insulation off at the appropriate part and measure the new
>   resistance.  When the two pickups are within 0.1% of each other,
>   you'll have done about the best that can conceivably be done.  I did
>   this with a home- brew "stacked humbucker" I made, and I can
>   practically stick this thing on the damn power transformer, and not
>   hear any hum.  We're talking VERY quiet.
>
>Sorry, but this won't work.  The pickup output has to do with the number of
>turns of the coil, not the impedance.
>
>"Yeah, but the impedance is proportional to the number of turns, isn't it."

Sorry, my goof with respect to terminology, but NOT my goof with
respectto results.

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.

Bear in mind that thereis a subtle distinction between how a pickup
behaves as an antenna for RF and 60hz hum, and how it behaves as an
antenna for signal from magnetically sensitive strings passing over
it.  The parameters that are optimal for the one may not be
identical for the other.

It bears noting that my pickup coils could have identical numbers of
windings, but unequal DC resistance, and have hum sensitivity as a
result.

I don't mean to be persnickety about this.  All I know is that coil
matching for DC resistance (over and above whatever altering the
number of turns does for toneand output) will give you a true hum-
rejecting pickup, and mismatched coils will produce a pseudo or at
best partially hum-rejecting pickup.

I'm sure thatthe commercial firms do their best to maintain consis-
tency.  If the wire they worked with were equally perfect, and the
bobbins similarly perfect, and everything else perfect, then all
pickups of a given design would sound the same, and be identically
hum-rejecting (if designed to be).  Fact is, they aren't, and I
accept that particular reality humbly.  Jeff Baxter's pickup fix is
simply a way to fix up reality a bit.

Thanks for the clarification, Don.  Mark Hammer
>
>Nope, the impedance has to do with the number of turns, the length of each
>turn, and the resistance per foot of the wire.  But the resistance per foot
>of the wire is hardly accurate to 0.1%.  It's more like 10%.  So it's only
>a rough measure of the number of turns.
>
>If you don't believe Maxwell's equations or if you want to try an
>experiment; build two coils with the exact same number of turns (say, 7600,
>like a Strat).  Wind one with 40 guage wire and wind the other with 42
>gauge wire.  Their outputs will be near identical, yet their impedances
>will be very different.
>
>  -- Don
>
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