Amplifying instruments by measuring current in conductive strings

Question

The normal musical instrument pickup uses a magnet inside a coil, and the movement of a ferromagnetic string causes changes in the magnetic field, which then induce currents in the coil.

Another possibility would be to use just a strong magnet near a string, and measuring induced currents in the string itself. This would only require a conductive string, not a ferromagnetic one.

Does anyone actually use this method? Is there a name for it, or is it useless, so no one talks about it? I tried it last night, and it works, but the signal is very low (-50 dBV, for instance, vs -5 dBV for a magnetic electric guitar pickup).

Answer 1

Does anyone actually use this method?

For stringed instruments, no, but the principle is used in many, many places.

is it useless, so no one talks about it?

It's not useless, but it has some significant disadvantages, and other methods are simply far superior.

I tried it last night, and it works, but the signal is very low.

That is the main disadvantage. Others are:

• You are measuring low induced voltages in the string that is in contact with a conductor (human) which messes up the measurement
• More advanced measuring techniques induce a voltage in the string, but again the human conductor is in the way
• The strings are more variable in terms of electrical properties (resistance, capacitance, inductance) all of which affect the sound, and mean that two instruments might sound very different
• The strings, being long, are not unlike antennas and inductors and are very good at picking up stray noise in electrical and magnetic fields. You can be much closer to a lamp ballast with a coil pickup before picking up the AC hum than you can with a string pickup.
• There's no galvanic isolation, so the amp has to be isolated from the AC line and high voltage sources, which pretty much rules out tube based amps.
• Electrostatic discharge from the user into the string and conducted into the equipment is damaging, but even if it's suppressed you can't easily filter out the additional noise it causes.
• Getting a good electrical connection to the string in a performance environment is difficult since it's hard to tin the metals used in many strings, and vibration, corrosion, and other environmental factors complicate it.

That's just the start. Don't misunderstand, coil pickups have disadvantages too, but they are superior in many areas.

Answer 2

Yes, this method IS in use and has been for quite some time. stringamp.com is a Danish company that uses the technique and carried the design forward, after another company went under (in the 1930s, I believe).

They have multiple leads from the instrument, to a belt-hung controller/pre-amp (6 controls for a 4 string violin), which I presume to be 4 tone controls and a balancing volume between the top and bottom pairs of strings (Presumption based on conventional 2-pickup, 3-potentiometer controls of 2-pickup guitars).

Answer 3

Moving coil and moving magnet phonograph cartridges (and other similar transducers) embody essentially the same principles as what you are describing. As you would expect from your experiments, moving coil cartridges have a much lower output than moving magnet. However, the fidelity is presumably better with moving coil cartridges.

If you can get an acceptable signal to noise ratio, I don't see any reason why this wouldn't work. I would expect the timbre to be somewhat different, however. You will most likely need an additional 10 to 20 db of voltage gain. You might (carefully) try a high-impedance microphone input, or placing an op-amp in the signal path to get a bit more gain.

Given the eclectic nature and history of pickup coils (such as humbuckers) and the demands put on musical instruments by the stage environment, I would characterize your application as a "specialized use." There's no way to know ahead of time if the strings will be more sensitive to feedback, for example, without testing it first.

I once put a condenser microphone on a pillow, set an autoharp on top of it, and cranked up the treble. The resulting recording was awesome. Keep experimenting!

Answer 4

See this link for details on using a moving string as a pickup. http://music-electronics-forum.com/t14952-2/

The most practicle way is to use an 8 ohm to 20K ohm or higher miniature transformer (8 ohm side) attached across a string behind the nut and the bridge. The string acts like the ribbon in a ribbon microphone.

Attach the high impedance side of the transformer to the amp input. Also, attach one leg of the low impedance side to the high impedance ground side to minimize noise.

Place one or more magnets in the space between the neck and the bridge and listen for tonal changes as the magnet position is changed.

You can also pass a string or strings through miniatute toroid current transformers to obtain sound from each independent string or the combined strings.

Transformers are the most practicle way to boost the very low impedance of a guitar string in the .5 to 1 ohm range up to a higher more useable level without introducing too much noise.

The sound quality with this setup can be more acoustic sounding than the traditional sound of a high impedance guitar pickup that all have a resonant hump in the 2Khz to 5Khz range.