One of my guitar students just built a guitar from parts he bought online. He used a Duncan Jazz at the neck and a DiMarzio Super Distortion at the bridge. The issue is only with the Super Distortion. He and his dad did the wiring, and from what they told me about it (and from looking at their wiring diagram), I don't see how their wiring could cause this issue, though I didn't take the pickguard off and look at it myself yet.

The problem is that when the SD is heavily distorted, there are some very noticeable harmonic artifacts. It sounds almost like it is going through a ring modulator. The harmonic artifacts are very inharmonic/dissonant, so the sound is definitely not desirable and I haven't noticed it on other SDs I have used in the past. It is definitely not a case of any of us not being used to using a high-output harmonically rich humbucker.

The Jazz p/u sounds perfect, and there is nothing funny about how the SD sounds when clean. It is just at high gain (which is what he bought it for) that this ugly inharmonic sound is there.

We already ruled out magnet/string pull and fret buzz as the issue. We also tried multiple amps and distortion boxes. Tomorrow they are going to try bypassing the tone pot to see if that is the issue. Each p/u has its own pot, and they are just the cheap ones that came with the kit he bought, so could that be the issue?

Like I said, the best way I can describe it is like the inharmonic sound of running through a ring modulator. Anyone know what this can be? A defective pickup? Improper wiring? Crappy pots?

Thanks in advance.
The pickup sounds like it went microphonic. If this is the case, then the only solution is a new pickup. If they are in the USA send them here www.guitarfetish.com they have some really good pickups for cheap. I actually replaced an old Dimarzio Super Distortion with their fat Pat and could not be happer.
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I thought that when a pickup went microphonic it meant it would give microphonic feedback at high gain? I will check with the student and see if it does indeed feedback at higher volumes (it didn't while I was there). If it doesn't start microphonic feedback, does that mean something else is the cause, or not necessarily? I've had microphonic pickups before, so I definitely know that terrible high-end squeal when they feedback, but mine didn't have the messy harmonic ring modulator tone his p/u does. But of course different pickups may have different symptoms of going microphonic.

I'll also recommend that URL for his future p/u shopping.

Thanks for the reply.