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North Facing Magnets

Started by Oliver Sheen, May 22, 2017, 03:11:23 PM

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Oliver Sheen

Just built a little jig to try and rewind some of my 19 dead pickups. VV told me that they might be the 1970s north facing magnet types. Can these be replaced with later south facing magnet types or do I need to keep them all the same?

Also my first pickup wire snapped with just a few turns left (well maybe 100 or so) Can it be soldered back together?

Thanks!

sean

#1
Short answer:  It won't matter a bit.  Well, maybe it does, see my last-minute realization in the PPS at bottom.
[Hey, it turns out it does matter.  See the PPS below and see the video linked by Jenzz.]

It won't matter a bit for the sound of the piano.  The phase relationship of each individual note is inconsequential, and scrambled by differences in note attack and criss-cross pickup wiring.  The notes with reversed magnets may generate current in the oposite direction for a given direction of tine movement, but we don't care.


However, the collection of pickups is wired in a way that will cancel out noise induced by stray electromagnetic fields.  The groups-of-threes wiring ensures that noise induced in three pickups is cancelled out by hoise induced in the three pickups nearby (which have their coils oriented the same way in space, but have their terminals connected in opposite "polarity" or direction.)  The opposite "polarity" is due to the way the terminals are jumpered together in the groups-of-threes series (alternating back-to-front and front-to-back ins and outs), not due to constructing the coil differently on each pickup.  The pickups are all identically wound, with the coil connected to the terminals on the pickup identically.

Since we have an odd number of pickups in a 73-key piano, the whole rail is no noisier than one single pickup.  (The 54-key, and 88-key have nice even number of coils, and may be theoretically quieter.)

All the pickups I have dismantled have their magnets oriented the same way, but the polarity of the magnet inside the coil plays absolutely no part in the phase relationship of the induced noise.

So, long answer is also "it won't matter a bit, as long as you wind your new pickups in the same direction as the old pickups."

Sean


P.S. - You may note that a humbucking guitar pickup does need magnets physically oriented in opposite polarity.  The humbucking does not depend on the magnets; the hum-cancellation is created by having the coils wound in different direction (clockwise vs. counterclockwise, or right-handed vs left-handed).  One coil is wound clockwise, and the other is wound counter-clockwise.  The magnets in a humbucking pickup are oriented with poles pointing in opposite directions to make the intentional noise (I mean, music) go round and round and come out "in phase" - reinforcing instead of cancelling.    EDIT:  I just got an education on yewtoob.  Sometimes, instead of winding the coils in opposite directions, it turns out that they can wind both coils in the same direction, but attach the ends of the wires in opposing direction (or "out of phase").  So it looks like they build humbuckers both ways.

https://www.youtube.com/watch?v=7vn414CJYzc (Not Tommy Dorsey and Edythe Wright, but my favorite version)
https://en.wikipedia.org/wiki/The_Music_Goes_Round_and_Round


OH WAIT!  P.P.S. - The magnets should probably all be oriented in the same direction for minimized interaction with pickups on either side!  If one magnet sits between two other magnets, and the center magnet has reversed polarity, then there will be some reduction in the magnetic field strength where their "magnetic field lines" overlap!  The interaction might be large enough to reduce the magnetic field strength at the tip of the pickup, and this would cause reduced volume for the notes involved.  This could be easily tested on a working harp by laying a pickup on top of the group in the wrong direction.  I will try this and report back.

Also, over time (like years and years and years) having the magnets in close proximity with reversed polarity could weaken the strength of the "permanent" magnet.  Like a really weak de-magnetizer.

P.P.P.S. - Before I bother to experimentally disprove my objections raised in the PPS above, it is amusing to note that having magnets in close proximity to each other with poles pointing in opposite directions for a long time is pretty exhaustively proven safe by... the kabillions of ancient guitars with humbuckers that still sound great and loud.

Sean


How many edits?  Hopefully, this is the last.

Oliver Sheen

Thanks for that detailed reply, Sean. Can you explain again why the winding direction is important?

Jenzz

Look at this vid from Chris at VV:

https://www.youtube.com/watch?v=MozJadTEcsY&t=261s

It shows the importance of having consistend magnet pole direction throughout the pickup rail...

Jenzz
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sean

#4

That video is great!  That shows that the magnets pointing the opposing directions in neighboring pickups do indeed cause enough interaction to be heard.  So that should be corrected by flipping the magnets around so that they all align.

Drill a 3/8" hole in a lump of wood,  stick the pickup tip in the hole, and push the tip and magnet out from the back side.  Use a drill press, arbor, or a dull nail and hammer.  (The nail and a gentle tap works easiest for me.)  Later production pickups seem to have a bigger glob of obstinate glue in there.
______________________________________


The direction of the winding is important, because we need the induced noise to be generated in a uniform direction across all the pickups (so that we can use the groups-of-threes front-to-back back-to-front method to cancel the noise).

In the discussion below, I am discussing only the current caused by EMI (Electro-Magnetic Interference or Radio waves: undesirable electric and magnetic fields).

Thanks to Ørsted, Ampere, Faraday, Maxwell, the right hand rule, and the way that electrons like to move in a magnetic field, there is a pretty detailed understanding of what happens in a solenoid (coil of wire) when there is an incident magnetic field.  However, I can't find a satisfying writeup on the web.

See the diagrams at https://en.wikipedia.org/wiki/Solenoid.  The magnetic field causes the electrons to move in a circle around the core of the pickup, but we want those electrons to move in a predictable direction (they do, according to Faraday's law and the right hand rule).  In the diagram showing the magnetic field around the cross-section of a solenoid, the wires with a dot have current flowing OUT of the page toward your face; wires with an X have current flowing IN to the page.  The current is moving in circles around the magnetic field.

We want the current in the wire to move in a predictable direction, so we wrap all the pickup wires in a helix that is uniformly oriented with respect to the tip of the pickup, and also to the wire terminals.  (We do this by always pointing the pickup in the same direction when winding, turning the winder in the same direction every time, and always connecting the start/inner coils to the tail terminal, and the end/outer coils to the tip terminal.)  This way, current moving in a circle around the pickup core will eventually spiral around the helix and arrive at the end of the coil at one of the pickup terminals.

Imagine that at one instant, all the EMI noise is pushing electrons toward the tip of the pickup, we want to have this electron flow cancel itself out.  We do this with the way we connect the pickup terminals.  In the groups-of-threes series connection, half of the pickups have their terminals wired tip-toward-ground, half of the pickups have their terminals connected tail-toward-ground.  So the net current at the RCA jack is nothing.  EMI noise is cancelled.  Nice.

If there is a uniform magnetic field created by an ancient fluorescent light ballast, or a refrigerator compressor, or whatever; and that magnetic field wanders across our Rhodes pickup rail, the magnetic field will cause a current in all of the pickup coils.  This current will travel around the pickup coils headed for the same end of the pickups.  But since we have the pickups electrically connected in a way that this flow of electrons is opposed and cancelled by current from other pickups, we get no noise.  Then an instant later, when the offensive magnetic field points in the opposite direction (the other half of the cycle in the wave), the noise will travel toward the other end of the pickups - where it is again cancelled out.

Groups-of-sixes and criss-cross pickup rail wiring works the same way.

This noise-cancelling strategy is generally very effective, but it can be defeated if the magnetic field is not uniform across the whole harp (or a big enough region of the harp).  If the source of the magnetic field is particulary strong and is particularly close to the piano, then the magentic field can be non-uniform enough to be heard.  If you lay your cell phone on top of the piano, and the phone tries to excite the ringer or vibrator, you may hear the induced noise in the Rhodes output.  If you lay an un-shielded electronic device (maybe a synth, or a fluorescent sheet music light) on top of the piano, you may hear a buzz in the Rhodes output.

That's is the physics as I understand it.

Sean

Ben Bove

That's a pretty comprehensive explanation, great read
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