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More thoughts on home-made guitar pickups

Alongside other projects, I am busy with a stradella-type chord keyboard based loosely around my MIDI-synthesizer work (see connectable.org.uk). The idea is (at the moment) to provide a flip-over keypad which will then sit over the guitar fretboard. As well as being able to play a wide selection of chords, each with a single key-press, the voicing for the chord-production (which will be MIDI) can be controlled with the right hand playing on the strings. The work is far from finished, but it involves the use of a pickup for each string, and I’ve used a simple technique in the manufacture of the pickups which may be of interest to those who have read, or currently are reading, the article(s) on High Quality Guitar Pickups On a Shoe-string.

A Bifilar-wound low-inductance string pickup made using a plastic sewing-machine spool machine and 2 neodymium magnets.
The prototype individual-string pickup

The prototype individual-string pickup

The spool on the left is as bought from sewing-machine accessory suppliers. It contains a convenient slot to anchor the winding into. Look carefully and you can see I’ve reduced the cheek diameter to the ridge on the original spool. I did this (carefully) on a lathe, but ad-hoc measures involving a small grinding wheel should also give satisfactory results. Dimensions are as follows:

(Original)
Spool length: 11.5mm (0.45 inch)
Cheek diam: 20mm (0.8 inch)
Centre diam (o/s): 7.8mm (0.3 inch)
Centre diam (inside): 6.4mm (0.27 inch)

(Modified)
Cheek diameter after reduction: 16mm (0.63 inch)

Rod Magnet
Length: 6.3mm (0.25 inch)
Diameter: 6.3mm (0.25 inch)

There are plenty of suppliers of magnets on the web, one suggestion is: http://www.first4magnets.com/bar–rod-magnets-1-c.asp

I fitted 2 magnets, leaving one end slightly-proud of the former. A single-rod 1/2 inch long would be equally as good. Using 26swg wire, a total coil resistance of 2.3 ohms resulted. This works well with the balanced amplifier in the main topic.
My design uses six such coils, and each has a separate pre-amplifier and is further processed. Making a former to accept the six-magnets, but winding around the whole set together as in a ‘normal’ pickup could be done using a bifilar winding and then fed into a balanced pre-amplifier, but as the coil former will now be very eccentric, would require much more care (and clearance under the winding chuck) in it’s construction.
Of course six individual coils as per my setup could be used, and the coil outputs combined in a mixer circuit after balanced amplification, resulting in a much more managable and tidy arrangement, though with the requirement to find space, ideally within the guitar body, for the extra electronics required.
My article, when finished, will feature both a full-size (DIL-package op-amps) and an SMD solution (surface-mount devices), so that the output from the guitar is that old familiar 1/4 inch jack socket. I recently purchased a Cheetah ‘Stratolike’ for £25.00, which I will be installing the prototype electronics into. This is ongoing work though, so don’t hold your breath!

Picture shows original size of spool and the diameter Ive reduced it to.

Picture shows original size of spool and the diameter I've reduced it to.

Every guitar is different, and the finished coil dimensions meant I would have to stagger the coils across the strings. You should ascertain what the finished coil diameter should be for your guitar and maybe reduce the cheek diameter before making 6 of these!
The prototype blu-tacked onto my Les Paul

The prototype blu-tacked onto my Les Paul

This crude, but effective setup enables easy winding of biliar coils. Two reels of the same diameter wire are mounted on the 6mm bar in the foreground. The coil former is mounted on a 6mm screw and held in the chuck of the hand-drill. After anchoring each wire firmly in the spool slot, the hand-drill is turned clock-wise whilst guiding the 2 wires using the fingers of your left hand. (folks that are left-handed should build a mirror-image of the arrangement!) For counting turns (I don’t always bother) I’ve mounted a burglar-alarm type reed-switch on a strip of plastic, and the small neodymium magnet attached, reliably produces switch closures for a counter as the drill shaft revolves. The hand-drill is easily detached from the terry-clips for use elsewhere if required.
Coil winding lash-up

Coil winding lash-up

Close-up of turns-counter pickup and magnet

Close-up of turns-counter pickup and magnet

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