Reddit reviews Korg CM100L Clip-On Contact microphone For Tuners

The bass I've heard of belonged to Roy Mitchell-Cardenas from Mutemath. Sadly the only pictures of it I can find are from after it was severely damaged in the Nashville floods a year and a half ago (link). I heard he got a professional luthier to install it, but I don't know any more specific info than that.

The biggest problem with magnetic pickups is the strings. Magnetic pickups work because the string vibrates through a magnetic field, and the pickup can detect this. For this to work, the pickups need to be some sort of magnet-conducting material. This is why electric guitar strings are always made of nickel and/or steel. Other magnetic metals can work too, but most would give a very weak signal.

The reason this works on a bass is because you don't (at least in this case) need to bow it. If you used electric bass strings on a cello, I'm guessing they wouldn't bow very well. You might be able to get cello strings with a metal core, but I'm not sure how well they would work. If you wanted to test this out before retrofitting a cello, you could get a bass and put cello strings on it (if they're long enough), and see how strong its output is.

Of course, http://en.wikipedia.org/wiki/Electric_cello says that some electric cellos use magnetic pickups and steel strings, so maybe it would be doable after all.

Now, don't think that piezo pickup means acoustic sound. You can get a pretty non-acoustic sound by putting any pickup through distortion and/or an amp. You can also try just using a cheap piezo pickup, like this or even something really cheap like this or this.

If you do still want to go with magnetic pickups, I would recommend a Fender Precision Bass style pickup (like the one on Roy Mitchell-Cardenas' bass). A P-Bass pickup sounds great, it's hum-cancelling, and I think the sound would be a good fit for a cello (though that's entirely speculating on my part). The biggest reason to use one though: you want the pickup to be approximately the same distance from each string. Most guitar or bass pickups wouldn't work because they're designed for relatively flat fretboards, not the extreme curvature of a cello. A flat pickup on a cello would pick up the end strings very well, but the middle strings would be really far from the pickup so they would be really quiet. A P-Bass pickup is split, so you could angle each half differently in order to get the poles close to each string (hopefully the string spacing works for this, too).

I'm working on building a pickup using a piezoelectric contact microphone originally intended to be clipped to the neck of a guitar. I haven't been able to pick up a clear signal even in direct contact with the mainplate of my watch, so I'm going to cut open the body and see if I can't make it more sensitive. I'll post about it if I can get it working; this would certainly beat trial and error, particularly if I'm trying to regulate in multiple positions.

The Korg CM-100 or CM-200 contact mic is perfect for this kind of project. I bought a few of the CM-100Ls when they were $10, and they sound just fine. Piezos aren't easy to solder to, and these Korgs have a nice cable already built in, so it isn't really worth the effort to try and DIY it.

As far as I can tell, there is no functional difference between the 100 and 200, so get whatever is less expensive or more readily available to you.