a pickup for my Little Harp

 Sat 24 Aug 2024 08:03:14 PM PDT

A single-coil pickup for my "Little Harp" (actually, a 15-string diatonic
zither).  Maureen and I want to use this instrument for accompaniment in
performing some Sufi love songs.  Rather than trying to mic-up the harp,
I want to see how it sounds with a pickup.

A secondary motivation for building this pickup is to experiment with and develop techniques for making pickups for my Pandalon and other harpsichords which I plan to build.  The size and shape of this pickup is roughly the same as what I'll be using on those keyboard instruments (one pickup per octave, I hope).

I already have the fixtures built, which attach to a sewing machine to
turn it into a pickup winder, so I'll see if I can make that work here.
I'm first going to try one long bobbin (7+3/4").  If that doesn't work
(i.e., if the tension variations are too extreme and the wire can't be
prevented from snapping), I will try a series of shorter bobbins, such
as three covering 5 strings each.  Although I may also try using
thicker wire before going to multiple bobbins.

There's not much to be done on computer to implement this project; mainly,
I'm just creating a PostScript image of the pole-piece positions.
Maybe I'll put some pictures here of the construction process, etc...
And of course I'll write out the design, as it develops.

----
The design.

The harp has 15 strings, usually tuned more or less diatonically.  Actually,
for the music we will be playing, it's tuned to a harmonic minor scale
in A minor, starting at E4 and spanning E4..E6 (so the tonic A4 is at string
number 4).

    15  E6
    14  D6
    13  C6
    12  B5
    11  A5
    10  G#5
    9   F5
    8   E5
    7   D5
    6   C5
    5   B4
    4   A4   = A-440Hz
    3   G#4
    2   F4
    1   E4

The pickup will be positioned close to one end of the strings on the harp
(probably the left end).  The harp is trapezoidal in shape, so the pickup
will thus be positioned at a broad acute angle relative to the strings,
not perpendicular.  Therefore, the spacing of the pole-pieces is not the
same as the spacing of the strings, it is slightly larger.

The total length, center-to-center, from string 1 to string 15 at the
proper angle, is 7+3/4".  Thus, the spacing between polepieces is
7.75 / 14 == 0.55357" (about 35/64").

The height of the strings above the soundboard is 1cm (about 3/8").
Therefore, I'd like the total height of the pickup to be no more than 1/4",
so that there is adequate clearance below the strings, even with the pickup
mounted on (sonic) insulating standoffs (such as rubber washers).

The maximum pickup height is determined by the length of the polepiece
screws.  Since Lowe's was out of stock on all of their several small steel
machine screws of length 1/4", I had to go with the next smallest, which
are metric, 8mm long.  These are closer to 5/16".

The basic form of the bobbin will be built up from two layers of clear
acrylic plastic.  The sheets are 0.08" thick each, so total winding height
of 0.16", or about 5/32".  The #42 wire will be wrapped around this form.
A strip of paper containing the printed diagram of the polepiece positions
(poles.ps) is epoxied in between the two acrylic sheets.  It will
later serve as the drilling template.

On the top and bottom, wider flanges made of thin clear plastic (cut from
suitably wide and flat "bubble" type product packaging) will contain the
wire coils, while adding very little to the overall height of the bobbin.
So the bobbin will consist of a sandwich of the two thick layers in the
middle, between the thin layers on top and bottom.

The bobbin sandwich will be attached to a thin sheet-steel bottom plate.
The polepiece screws will pass through holes in this plate and screw into the
plastic bobbin, thus holding the plate in place.  The plate will be about
the same length as the bobbin (possibly a bit longer to create room for
mounting holes), but it will extend wider on one side; several small magnets
will be epoxied in place along this side.  The polepiece screws will thus
conduct the magnetic field through the middle of the coil windings and up
to the strings.  (I will have to figure out a good way to ground this plate.)
It may be necessary to experiment with the number and positions of the magnets,
before gluing them into place.

Because the vertical space for the coil is so narrow, to get a decent
number of windings (at least 1000, I'm thinking; pretty low impedance
is desirable and I don't want it too "hot"), the coil will have to be
short and wide, similar to a P-90.  Particularly, if it turns out that the
#42 wire is too fragile to hand-wind onto the long bobbin at any sane
rate of rotation, I may have to order some thicker-guage pickup wire,
and then it'll of course take even more volume for the windings.

----
Actually, looking more carefully at the situation, there is not room to
put mounting holes in the bottom plate, beyond the natural ends of the
bobbin.  The pickup will have to be attached to the harp in some other
way, yet to be determined.  (Maybe angle brackets, around the sides
(front and back) of the harp.  Actually, even double-stick foam tape
would work and there would be advantages to using it, despite its
klugy presentation.)

Even the extension on the side, where the bottom plate extends beyond
the footprint of the bobbin to make room for the magnets, will need to
be shaped with angled corners so that it doesn't extend out to the full
length of the bobbin: otherwise, due to the angle of the pickup, it would
extend outside the perimeter of the footprint of the harp itself,
a very undesirable condition.

This is because there is only a small margin of space on the surface of
the harp outside the string band, below the lowest string and above the
highest: the margins are the same as the string spacing, or even a
little less.

The bobbin itself will surely extend to the full available length
within this footprint, allowing for a reasonable width of coil windings.

----
I see Lowe's advertises ceramic magnets, several types, pretty cheap.
E.g.:
    Item #176644 | Model #542001
    Project Source 3/4-in Magnetic Ceramic Discs- 8 pc
    $3.98

I like the sound of these better than the new high-power rare-earth
type magnets, because I assume these will be weaker, and too much flux
is not what we want.

So the extension region of the bottom plate should be 3/4" wide, or a little
more, beyond the outer extent of the bobbin flange.  (Some of the other
rectangular magnets were smaller than the 3/4" discs.  I especially like the
discs though, because I presume the magnetization will be "N on one face,
S on the other"; the rectangulars would probably be "N one end, S the other",
I'd guess?  I really want only one pole in contact with the plate.)

----