I have a high-end acoustic twelve string built by the Santa Cruz guitar company. I would like to be able to amplify it when I sing with a mike. I am not willing to carve, glue, or screw into this beautiful instrument in any way, and I find playing into a separate "top mike" restrictive when performing. What I would dearly like is a sound-hole pickup which can be installed and removed without tools -- like the Seymour Duncan "Woodies". But so far as I can find out, no such thing exists for twelve strings. I have been told that the double course of strings -- a wound string next to a solid string -- confuses the pickup and causes distortion. Has anyone but me wrestled with this and come up with a reasonable approach?
I have been told that the double course of strings -- a wound string next to a solid string -- confuses the pickup and causes distortion.
That's nonsense, people evidently don't know what they're talking. PUs aren't sentient beings that could be “confused” by this. A magnetic PU is just a coil of wire wound around either six individual magnets or one wide magnet†. In either case, there's not much difference whether you use this with six or twelve strings: at the point where the picking-up of the vibration happens, the magnetic field is to a good approximation two-dimensional, so the two strings in each pair get basically the same polarisation and, when vibrating, each causes the same pertubation to the field as if the string were alone. These pertubation, when measuring the fluctuations as induced voltage in the coils, add up independently and hence without problems.
I said “to good approximation”. Actually, because the strings of each pair are so close, they will to a small degree couple magnetically. This leads to a bit of nonlinear intermodulation which could be called “distortion”. However it's a very “smooth” distortion like in an extremely clean tube amp, you probably wouldn't notice this at all, at least not if you tune the strings in exact octaves (because in that case all sum- and difference frequencies match with already present string frequencies; see this post for some explanation).
(If that was too theoretical for your tastes: just consider that twelvestring electric guitars usually use the same pickups as their sixstring pendants.)
At any rate, these twelvestring-specific issues will be neglectable to the colourations you get with any magnetic pickup even on a sixstring guitar, namely the comb-filtering caused by picking up at some spot in the middle of the string and the resonant low-pass effect of high-inductance coils. The latter is out of the equation in good modern acoustic PUs because these have low-impedance coils (ideally with low-capacity active preamp which I would highly recommend; the Woody actually lacks this), and the former is inevitable with any magnetic design but not really detrimental for a clear acoustic sound.
Whatever pickup you use, make sure you plug the guitar into a suitable input: electric-guitar amps suck for amplifying acoustic guitar because they trim away most of the transient clarity. Use either a dedicated “acoustic amp”, or a good DI straight into the PA.
†In case you're starting to object: a humbucker is just two seperate single-coils with opposite polarity.
There is an old DeArmond 1200 soundhole pickup made for 12-string guitars. You could buy this one right now!
(To be fair, this might sound more "electric" than you want, and you need to use old-school electric-style strings, I think. But for the price, it's worth a try...)