Why are electric guitar pickups designed the way they are?

February 15, 2005 12:52 AM

Why are electric guitar pickups designed the way they are? Wouldn't it be a lot more effective to run a weak current through the strings and have single coil pickups without magnets? The wiring would be slightly more complicated, but I'd think the increased signal to noise ratio would justify this. I'm assume I'm missing some big drawback or inviability though.
posted by fvw to media & arts (9 comments total)

What about nylon strung guitars? (like the Chet Atkins style solid body affairs)
posted by the cuban at 3:17 AM on February 15, 2005

Yeah, it wouldn't work for all guitars, it couldn't replace accoustic or optical pickups. But it would greatly boost signal strength on coil pickups.
posted by fvw at 4:55 AM on February 15, 2005

Pickups already don't work with nylon string guitars. Pickups are a bit like microphones. A microphone is a magnet wound with wire, with a vibrating diaphram. In the case of a guitar pickup, the string is the diaphram. Not only do strings have to be metal, they have to have some ferrous content.

I think one of the prime difficulties might be that you'd be operating a semi-complex electrical system in a rather harsh environment, where the strings might be dirty, corroded, old, sweaty, etc. To the extent that all the string-bearing surfaces of my guitar have become worn with use I bet you'd be replacing and resoldering all the time.

I've almost convinved myself that the scheme would work under ideal circumstances but I'm not entirely sure why you think it would yield a cleaner signal -- if a clean signal is in fact what most people want out of a guitar (generally it's not)
posted by RustyBrooks at 5:51 AM on February 15, 2005

Sure -- for years you've always been able to find pickups with power--active pickups. And with an active pickup you can have an EQ and other on-board effects, but they do a few other things that passive pickups never do: they can run out of power during the middle of that kick-ass solo or if stored too long, spill battery guts inside your guitar.
posted by plinth at 5:53 AM on February 15, 2005

An electric guitar pickup consists of a coil of fine wire wound around a permanent magnet, placed close to the steel strings. Because steel is a magnetic material, it disturbs the field around the permanent magnet as it vibrates, and this generates an electrical signal in the coil by electromagnetic induction (Wikipedia has more detail).

Nylon strings don't work with this kind of pickup; for a nylon stringed guitar you need to use an acoustic pickup (which is essentially a contact microphone).

It's the steel string's magnetic properties, not its electrical conductivity, that makes a pickup work. If you wanted to use a wire coil pickup without a permanent magnet in it, you'd need another way to make the string vary a magnetic field as it vibrates. You suggest running a current through the strings, but unless my physics intuition fails me badly, to match a decent permanent magnet's field strength you'd need to use enough current to detune the strings and burn the player's fingers.

You could perhaps exploit a steel string's conductivity instead of its magnetic quality by using a capacitive pickup (just a couple of conductors placed close to, but not touching, the string) but the signal strength you'd get this way would be several orders of magnitude smaller than that obtainable by a magnetic pickup and therefore much more vulnerable to noise. Using the string itself as one of the capacitor "plates" would make things worse, as it would put a large electrically conductive guitar player on the front end of a sensitive amplifier. I'm sure you've heard what happens when you touch the tip of a plug before sticking it in your guitar.

In any case, it isn't the magnet per se that makes pickups sensitive to electrical noise - it's the fact that if the pickup coil has enough turns to make it generate a usable signal from the small magnetic field variation you get from a vibrating steel string, it also has enough turns to generate a usable signal from stray magnetic signals from anywhere (stage light wiring, power transformers in amps, etc). To improve the s/n ratio you'd need not a missing magnet but a stronger one - then you could get away with fewer turns in your pickup coil.

Apart from the basic physics, there's a lot of black art in pickup design. Optimizing the signal to noise ratio will do you no good if you end up with a pickup whose tone you just don't like.
posted by flabdablet at 5:56 AM on February 15, 2005

Hmm yes, I'd assumed the magnetism of the strings was all due to the current induced in them by moving them through the magnetic field, but the magnetic field makes the strings themselves magnetic ofcourse. Thanks, flab!
posted by fvw at 8:29 AM on February 15, 2005

Running current through the strings would also involve some hardcore UL/CSA certification and expensive failsafes, since in the worst case the user could get electrocuted by a short from power -> strings.
posted by shepd at 8:51 AM on February 15, 2005

Guitars without ferrous strings, like the Chet Atkins Gibson model (Gibson site seems to be dodgy at the moment), can use piezo contact pickups, which make use of the piezoelectric effect: some crystals will acquire a charge when pressure is applied. Piezo pickups contain ceramic crystals which respond to the pressure from the vibrating strings (and resonating body) by producing a weak current, which is then amplified. The Chet Atkins guitar gets its unique sound from the combination of a solid body, nylon strings, and piezo pickups. I've wanted one for years.
posted by TimeFactor at 9:16 AM on February 15, 2005

Yeah, piezo pickups are nifty, and though they're immune to EM interference they do tend to pick up more vibrational hum off the guitar body. Still, definately worth playing with if you've got the money (which I haven't, which is why my current setup picks up all sorts of unpleasant noise, which is why I've been thinking about this stuff).
posted by fvw at 11:13 AM on February 15, 2005