Piezos and hum

You’ve probably heard this before: you mount a piezo pickup (one of the round, cheap ones) into something and you end up with lots of hum when you amplify the signal.
There’s already lots of threads, articles and opinions about it on the net, but nothing that seems to solve the problem unfortunately.
Now to get to the point: I have a cigar box guitar, and it has one of those piezos in it going through a volume pot into a jack plug.
When I connect that piezo to anything (audio interface, IO Dock, Mixer… you name it) I get lots of hum. I didn’t measure the frequency but it sounds like 50Hz and its harmonics.
I usually have patch cords with metal casing around the jack, when I touch that part the hum goes away, so it must be some grounding issue. I checked all solder joints and they all look good, even re-did some and nothing changed, so I’d rule that out.
Does anybody have experience with dealing with these kind of problems? Any input on this would be greatly appreciated!
btw. I have the same problem with all piezos I have. it’s not just this one.

how have you shielded and grounded it?

You need a symmetric Piezo Pick Up, just like a Humbucker :wink:

I think piezo’s are typically run through a pre-amplifier inside the instrument and then the cable to the amplifier plugs into that. Matching impedance and boosting signal levels and that sort of thing.

Before you run into the next endless DIY-Project, how about this

I have some cello pickups which work without a special preamp… They require a piece of conductive strip under the piezo… And I also made a few piezo myself but I don’t remember if they had a hum problem…

MicMicMan: how do I properly shield and ground it?fcd72: you know what… I think I’ll give it a try!

Yes, as much as i am into DIY, somewhere is a point where i can get to work to earn some money and have a better product in less time than trying to make it myselves.

totally with you on this!

You have to use shielded wire everywhere you can, the shield being grounded.
If you have some embedded preamp, you have to shield it as well : either you put it in a small metal box, either you glue some aluminum foil or even some copper tape to the cavity. The closer the preamp is, the less risks of hum you take, that’s why virtually any guitar using a piezzo is stuffed with a preamp that runs on battery (a good opamp is enough for an OK piezzo preamp).

Your shield has to be grounded, you use the - (ring/sleeve) point of your audio jack for that purpose.
It can be useful to ground your strings as well (well, ok, it’s mostly useful with single coil guitar pickups). Usually, the bridge is used for that purpose.

I experimented with piezos in the past and didn’t have that problem.
I didn’t use anything special. Just piezzo - wire - jack.

Yes that’s right : single coils pickup are more sensitive to “hum”.
I built a lapsteel guitar with a dualcoil ( or humbucker) , grounded all components :
I connected all grounds together, jack ring, potentiometers body, ground piezzo, bridge ( strings ), switchs etc … and no hum ! ( like this link )
You can also lining the pickup cavities with self-adhesive copper foil : like this

I think cheapest are the piezo , more “hum” you get …
You can find plenty of wiring here

You said “(one of the round, cheap ones)” what is this ?

this is the piezo I have:

tha’t what I mean by “cheap and round” :slight_smile:

Anyway. thanks for all the suggestions! I’ll try a couple things and report back. Adding a little preamp seems to be the way.
Over at Muff’s they suggested me to try this circuit here: http://www.zachpoff.com/diy-resources/alex-rice-piezo-preamplifier/

or this one: http://www.megalithia.com/sounds/tech/piezo/opamp.html

You sure those are not piezo speakers instead of pickups?

There is not really a difference for a speaker or a pickup.

Tell this to people that spent 5400€ on a pseudovintage TELEFUNKEN :wink:

There’s not really a difference between me and my dog either, we have over 85% of our DNA in common. There are a few differences in performance in various areas though. I suck at chasing rabbits and my dog is not very good at soldering.

Piezos are quite different from mammals in the way that they both work well (or bad, depends on the perspective) as mics and speakers. They are actually speakers, but can be used quite effectively as contact mics.
They are cheap and while not very hi-fi, they do the job… except when you get hum.