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I have recently had a piezo pickup added to a nylon string guitar. There is no onboard preamp, just an undersaddle piezo running to a 1/4 jack in the bottom of the guitar body.

I noticed a problem with 50 Hz hum, and through a process of elimination, have narrowed the cause down to my own body. When I touch the jack, a pedal on my pedal board, a metal electrical item that is completely disconnected from my rig, or even a water pipe, the hum is eliminated or reduced (good video on this here).

I've looked at anti-static wrist straps like this one (I'd attach the clip to the jack plug), but surely there has to be a better solution than this?

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    Just to expand upon @leftaroundabout ‘s answer, the problem is probably not that your body is acting like an antenna, it’s not a particularly good antenna. The guitar is a fairly decent antenna however and likely THAT’S what’s picking up the 50hz hum. When you touch something your body is large enough to ground the interference. With a grounding wrist strap you are grounding the guitar to YOU, not the other way round, the guy in the YouTube video hasn’t quite got it there. You can show this by getting the guitar to hum and then going to the next room, or noting that you can stay in the…
    – OwenM
    Apr 9, 2022 at 22:43
  • …same position but move the guitar around and get more/less interference as it goes nearer/further from various electronics you might find in your room. A piezo is a very extreme device to interface with, electrically (not unusual, but ultra high impedance and low signal means it is very susceptible to problems) as such you really need a piezo preamp, are you using one? That may help things, but as mentioned some kind of proper approach to shielding/early preamplification may also help.
    – OwenM
    Apr 9, 2022 at 22:46
  • @OwenM thanks. Yes I'm using a Radial Tonebone pedal with the piezo buffer engaged. It's built for exactly this application. Tried very short cable between jack and pedal with almost no effect. Apr 10, 2022 at 5:47
  • that’s a very good pedal! I have one for my double bass. Ok then I would try moving to a few different locations, trying to minimise EMF sources and see if it gets better/worse, just to get a handle on the problem. If the noise alters then you can be sure you’re picking up emf and some kind of shielding/improved grounding is in order. I’m sure this isn’t the case but accidentally using a speaker rather than shielded instrument cable can cause third. There’s a slim chance it’s in the amp (bad instrument ground>circuit ground) but tbh, I think it’s location and shielding needed!
    – OwenM
    Apr 10, 2022 at 8:29
  • @OwenM thanks. My studio is in the eves of my house. I get a noticeable reduction if I move the instrument to one side of the room which definitely supports your theory. I'm running into an Audient iD44 and I see the issue even if I disconnect speakers. I've already been to hell and back solving a ground loop issue and have a USB lift on the cable between the interface and my Mac, so as sure as I can be that the issue is specific to the instrument. I'll give the guy who installed the piezo a shout and see if he can do anything as far as shielding goes. Apr 10, 2022 at 11:57

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Forget it, this is all just bodging around the real problem. A piëzo has way too high impedance to be reliably shielded with body-grounding. The only proper solution is to ensure all components are shielded in the guitar. A decently made PU system should have all components well wrapped in metal and using good coaxial cables between the parts. Of course, it also helps a lot to have a preamp very close to the PU, to minimise the hum-susceptible parts, which is why better piëzos for guitar tend to come with preamps. (BTW a preamp is also a very sensible thing to have even in a magnetic-PU guitar, but there it isn't quite as critical.)

You'd probably be best of ditching that PU and getting a better one (or in fact another guitar that already has a piëzo built in). Short of that, I'd recommend wiring a simple buffer preamp directly to the PU you have, and taping some grounded tin foil over the inside of the guitar's top so it covers the PU and preamp. It probably won't eliminate the hum completely, but certainly better than any body-grounding hacks.

Additionally, hum can actually be tackled quite well with noise-cancelling algorithms; I think many newer noise-gate pedals use that nowadays, so that may be worth a try as well.

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  • Thanks. The instrument is actually a Ronroco (I simplified the question by calling it a nylon string guitar: it has the same characteristics including nylon strings), which are hard to come by and rarer still with pickups. Sound-holes won't take a pickup so either piezo or piezo / mic combo is the only option. I'm using a Radial Tonebone (which is designed specifically for this) with it's piezo buffer engaged. Using a short cable between jack and pedal, so don't think ai can make much improvement there. Apr 10, 2022 at 5:52
  • External preamps like the Tonebone can't really do anything to help against interference that's already introduced in the instrument, that would be like buying expensive studio monitors in the hope to make a badly made recording sound better. Such preamps work indeed fine with well-made (specifically well-shielded) passive piëzos. The point of the extremely high input impedance is to avoid the piëzo from acting as a low-cut filter or other distortions, but it actually makes any hum interference even worse (a lower-impedance preamp would short part of the hum to ground). Apr 11, 2022 at 21:48

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