On another board I had posted an article summarizing and detailing the amazing work of about a dozen people in creating a unique guitar pickup. That article was removed along with dozens of other valuable articles because of a severe misunderstanding on the part of a few people, tempers flaring, and angry reactions rather than calm solutions.
I can't do anything about that situation or the people and attitudes behind it... but I can restore this bit of knowledge to the Handmade Music community. Although I no longer have the names of the people originally involved in this project, to the best of my ability, following is a re-creation of the original article.
The term induction pickup is simple way to say "transference of electricity". The concept is simple:
1. Ferrous guitar strings (steel or nickel or chrome) disrupt the pickup's magnetic field.
2. Copper within that field converts the energy generated into micro-voltage.
3. A pass-through or step-up micro-transformer reduces amperage and increases voltage to a level that can be recognized and increased by a guitar amplifier.
No battery or external power source of any kind is required, nor is a large coil required. The only coil exists within the transformer and it is very small, largely impervious to RF or EM interference. That fact makes the pickup highly resistant to hiss and noise. It is a "single-coil, clear pickup". I don't really like to call it a coil, as in my mind one piece of wire soldered to itself forms more of a loop. So a more accurate name for this may be a "loop induction pickup" or "loop pickup" for short. Minor technicality and unimportant to the issue that it works. :D
Following are four different designs detailing how to make a loop pickup. If you have any questions I can answer them in the comments.
DESIGN #1: BASIC SIMPLIFIED
Surround a thin bar magnet with a copper wire. The N/S poles of the magnet need to be on the wide faces. (The above diagram shows "North up" but either pole can face upward.) Wrap the magnet in packing tape to prevent shorting the wire, then tape it to the wire. The wire should be a gauge that fits the hole in a pass-through micro-transformer (available on Amazon or Ebay). The ends of the wire will be soldered near the far end of the magnet prior to assembly, as you do not wish to get the magnet hot. Use either an industrial soldering iron or pen torch to solder the joint, as the thickness of the wire can be tricky to solder.
The side diagram shows how the wire passes down through the lid to the interior of the box. You can use any micro transformer you wish; I've tried several different kinds of varying types and strengths... and they all sound good. Since sound is subjective, some may prefer one transformer over another.
The wires run to the volume control, just as with a normal pickup. If you wish, you can bend the thick copper wire so that the transformer sits underneath the magnet / wire area.
METHOD #2: 4-prong Transformer
This is made similarly to method #1 but rather than using a pass-through transformer, uses a step-up transformer (4 poles, 2-in 2-out). The large copper wire attaches to the two input poles. Guitar wire runs from the two output poles to the volume control. Other than that the concept is the same.
Method #3: Full wire pass-through transformer
With this method a copper loop on a thin wood backing surrounds disc magnets. (Alternately you can use a bar magnet as in the prior two methods.) The magnets are insulated by tape as before. The copper wire is soldered at one end (or a side) prior to installation. The other end of the wire runs into a pass-through transformer with a large hole in the center, allowing both sides of the wire to run through the same hole. This creates current as the wire runs in and out of the transformer. The transformer itself is quite a bit blockier, but can be made of items such as disassembled power adapters (wall warts) with satisfying results. This is the "scavenged and found" parts version of an induction pickup and would work well on a steampunk-themed git.
Method #4: Plate Diagram
Unlike the others, this is made from an aluminum or copper plate rather than copper wire. This is a bit more time-consuming to build but offers a different sound. In this design the plate is cut part way up the middle (to the end of the magnet) to separate sides of the plate, in effect creating a C-shape loop. It is then bent so that the end of it drops down and underneath the box lid. (Alternately you can leave it flat and have the whole thing showing above the box, with only the transformer itself hidden below the lid.)
The tape-insulated bar magnet can be put on the top or bottom of the plate (I recommend on top). Two bolts and nuts run down through the sides of the plate and clamp a copper wire, which runs through a pass-through transformer as used in the first diagram. The wire from the transformer runs to the volume control.
As can be seen in all of these methods, the basic principle is the same: surround the magnet with a copper or aluminum loop, which converts the magnetic distortion field to voltage, increases that voltage at the micro-transformer and sends it out to your amplifier.
The result is a pickup that produces crystal clear, noise-free and hum-free sound. Just about anyone can make these; the designs are public-domain. They are relatively easy to build once you have collected the parts. Note that compared to other pickups these are relatively low-volume and don't have the rich body of other pickups. These are clean-signal without distortion or typical noise issues.
PS: It has been claimed that this design was invented years before, but evidently was never marketed. Kudos to the original genius of designing it... but that is of historic interest and for some reason never became widespread or in general use. This post documents the work of a group of individuals flying by the seat of our pants and figuring out one step at a time how to make a different kind of pickup that would be easier to build than standard pickups. The effort took some 95+ pages of research, documentation, discussion and experimentation, then was continued on 15+ pages of a second thread. The resulting designs are what you see above, are public domain for anyone to use, is fully documented above, and can be built and/or sold by anyone who so desires, without restriction from anyone-- which is as things should be in the field of cigar box guitars.
thanks SO much for this, i've been building instruments i"m calling Viola Chittara ( guitar tuned fretted viols) and i've been fighting the universe for a magnetic pickup solution of the solid bodies, and this is the best explanation of inductive pickups I've seen. not only that but the bass pickup currently on the bench will cost me a total of 10 bucks as everything used to mount and protect it are remnants of the build it'self. the method i'm using is 6 1/4" x 1/8" neos, 8 guage copper wire, and a pass through transformer coil... to fight the "String pull" issues common with neos, i'm adding a flame maple pickup cover ( the tailpiece and fingerboard on the bass are both flamed maple). I'll also be using this tenchique on the harp guitar main neck that is on my next bench project.
You're most welcome. The results above were the combined effort of about a dozen people over 100+ pages of discussions that resulted in four relatively easy-to-build designs. Each design will produce a different sound because of differences in wire /plate style and hookup and type of transformer used. I tried all four and each one "sounded good". : )
The volume on these isn't a great as coil-based flat pickups, but the sound is as clean as you get, with no noise.