Inductive pickup
Suggestion for a directional electromagnetic pickup project.
Contents
Introduction
This project describes a simple device for detecting audio-frequency electromagnetic signals, such as those emanating from a loudspeaker, or it's cable, whilst playing music. It can also be used to enable the recording of various types of sound otherwise inaudible to us most of the time, such as the electromagnetic whine of an electric motor. The most likely application, though, is the recording of musical sounds, for example an electric guitar played through an amplifier, with the signal being obtained from either the loudspeaker cable (if accessible) or from the voice-coil of the speaker itself.
In principle this is a DI device with the added feature of being positionable in the same manner as a microphone.
If you do undertake this project then you should be able to obtain useable results by simple experimentation with the winding of the inductor coil (described below), but bear in mind that there is no rigorous information provided on this, and therefore the technical performance of the coil is almost guaranteed to wildly off. For that reason I would recommend that you investigate the calculation of coil parameters and the construction of inductors and pickups if you wish to have better performance.
Parts required
Ferrite rod
This is just a standard ferrite rod that can be bought from any electronic components supplier, or taken from a non-working AM radio. A short length of rod, approx. 6 cm, is all that is required for the purposes of this project.
The ferrite rod is used to intensify the electromagnetic field around the coil windings.
Insulated wire
Almost any insulated single-strand copper wire can be used. The only requirement is that it is of narrow enough gauge to allow for the required number of turns for the coil without becoming too bulky.
In the version that I built I used 30 AWG wire-wrapping wire with an overall diameter of 0.5 mm, which I had left over from another project. I would not recommend using anything of a smaller gauge (i.e. thicker), as the coil will become too bulky and will be difficult to wind.
Fixative, potting compound, or glue
Although the coil itself may be lightly taped to keep the turns tightly packed, it is advisable to fix the whole assembly into a single unit to prevent the coil turns working free over time and to prevent stressing the connections to the XLR connector as single-strand wire is prone to breakage.
If you plan to leave the coil section exposed then you can use epoxy-resin, hot-glue, or some similar fixative to hold the whole assembly together. You should bear in mind though that this arrangement is not inherently strong, and the use of some kind of overall casing is recommended if you want to use this project as more than just an experiment.
If you do enclose the assembly in a protective cover or sleeve of some kind, then you have the option of using potting-compound instead of, or in addition to, the adhesives mentioned above.
Make sure that you test the unit thoroughly before fixing it firmly as you may be unable to disassemble it once the fixative has hardened.
Male XLR connector
The connector is also the holder for the inductor assembly.
It is recommended that you use the Switchcraft® type XLR rather than the Neutrik® type, because of the narrow cable exit hole. This exit-hole is used to support the ferrite rod. The Neutrik XLR connector has a screw-on clamp/strain-relief assembly which may be difficult to adapt for this application.
Assembling the parts
Winding the coil
The information given here in no way represents the correct method of designing and constructing a good coil, and is intended only to give a starting point for further investigation.
The coil is wound onto the ferrite rod in evenly laid turns, changing the direction of lay at the end of the row. The starting point for the coil winding should be far enough along the ferrite rod to allow a reasonable length to be slid inside the XLR connector without touching it's solder terminals. Remember to leave enough wire as a flying-lead when beginning to wind the coils, so that you can solder the lead to the XLR's terminals.
Once winding is complete you may wish to tape the coil, or apply a thin layer of fixative, to keep everything in place.
Finishing
The pickup that I constructed was essentially unfinished, as I intended it only for experimentation. This makes it relatively easy to alter the project, but the resulting device is rather fragile and extremely ugly.
A better solution may be to fit a short length of plastic tubing over the pickup assembly. Alternatively a plastic box of the type you can get from any electronics hobbyist supplier may work just as well. In that case you would want to substitute a chassis mounting XLR connector for the in-line one in my description.
Testing
Testing essentially means connecting the device to a microphone pre-amplifier via a balanced microphone lead, and bringing the pickup into proximity with an appropriate electromagnetic field. The pickup that I made worked well when connected to either a high or low impedance preamplifier. If you hear audio output then it works, if you can't hear anything then it doesn't work. Not very technical, is it. This is a very rough and ready project, and if you intend to go further with the design then you will probably want to perform more rigorous design and testing than that described here.
