Ground Loop Hum

Hello everyone,

I recently bought a Lanikai Mahogany Concert Ukulele MAHCE with a Fishman Kula Preamp. Both the instrument and the preamp sound great, but unfortunaly there is an audible ground loop hum from the preamp. I have experimented with different PA setups, cables, battery change etc., the hum is always there. The vendor of the instrument (Thomann.de) informs me that the piezo is not grounded and that this is standard practice in electro acoustic ukulele construction. Is this really true, and if so, why?

I play different other electro acoustic string instruments that do not produce any hum at all, and I find very disturbing for the set, that whenever I switch to the uke the hum starts. It is not annoying as long as I play, but it's really a problem during instrument swapping, breaks, quiet ballads etc. The hum stops when I touch a metal part in the signal path, but this is obviously not a practical solution.

Do any you have some advice about how to deal with this problem? Would it be possible to modify the instrument so the pickup becomes grounded? Or could the PA be grounded somehow?

Thanks!

Yeah, first and only idea I have is to try a DI with a ground lift switch.

Hippie Guy said:

Yeah, first and only idea I have is to try a DI with a ground lift switch.

Click to expand...

Yes, what Brad and Kekani said about getting a DI box - this is going to be the easiest and cheapest solution, which will also set you up for running the output via a balanced XLR cable, which if you want to grab an XLR-to-1/4" cable will allow you to ground lift the signal on output, which should fix the issue.

I have a handful of these DI boxes, which are the cheapest I've found, but they get the job done:

https://smile.amazon.com/Premium-Direct-Injection-Audio-Box/dp/B0027V760M/

These are also sold under a dozen different brand names for a lot more money (the Behringer 'version' is like $25 and the same exact unit except for the brand name printed on the case) and are all made in China, and having opened them up, I can confirm that the soldering is cleanly done inside, and in fact this does have the proper impedance matching transformer inside, which will ALSO help with that issue as well. So dont let the low price fool you, these cheapo units do work.

However, if you've got money to burn, you can get something from Radial or CloudLifter which have lots of bells and whistles, which are duplicated if you already have an active preamp inside the uke.

Otherwise, troubleshooting ground issues with the piezo element itself is going to be a long trip down a dark path since they are prone to pickup all kinds of EM/RF interference which also manifests as hum and can fool even the best of us into thinking it's a ground loop.

This or any other DI box fixes the hum, regardless of the source due to the isolation provided by the internal transformer, which breaks the physical connection of the wires, and instead carries the connection from the input to the output via induction.

I am surprised that the Fishman Kula preamp does not filter out the hum, and actually the first thing I would do is that I would open the preamp cavity and remove and re-plug the connections, just in case something got loose or oxidized, and with your output jack plugged into the amp or PA (at a low volume) rotate the plug in the socket (both the one that comes from the pickup element, as well as the one that goes to the output jack) and see if there is any 'scratchiness' or 'crackling' in the sound. If so, with everything disconnected, a shot of contact cleaner, or a healthy 10-15 ml of isopropyl alcohol dropped into the slot, and re-inserting the plug while still wet and turning it back and forth, and removing and re-inserting it a few times is usually all you need to generate enough friction to clean the contacts.

Isopropyl alcohol (90% is preferred, but 70% is ok too) is perfectly safe to use to clean electronics, and will evaporate into nothing and leave zero residue after about 2-3 mins.

Hope this helps!

Hello Kekani, Brad, & Booli,

Thanks for your answers. I have tried the uke with various setups, directly into combo amp (e.g. Roland Micro Cube), through mixer (e.g. Yamaha MG series) to monitor speakers or FOH etc., always using short, high quality cables, but with no external DI. The setup is not causing the noise, this I know for sure because when I switch to a different instrument, the hum goes away.

I play several other instruments with piezo pickups, some passive and some active, some are Fishman equipped, some with pickups and preamps from other manufacturers. None of them have ever produced any hum, I have never needed to use a DI since max cable run I use is 3m. In fact, I own several DI's that I used years ago when playing on large stages. But since I do all my own sound now and therefore sit right next to the mixer, I haven't needed them for some time.

Even my cheapest guitar, an Ibanez GA6CE-AM classical guitar with an AEQ-2T preamp, has no hum at all (using the exact same amp and cables as with the Lanikai). The Lanikai ukulele is not a cheap instrument and the Fishman brand has quite a reputation for quality, so I was somewhat shocked at the noise when I plugged it in the first time. Especially because it is a brand new instrument, straight from the factory (I can't believe there is anything corroded inside).

The vendor (Thomann) took back the instrument and checked it, but said there is nothing wrong with it and send it back to me. Thomann insists, that all ukuleles produce a hum when amplified, as opposed to guitars. Is this really true? Have you guys had the same problem with all or most electro acoustic ukes you have ever owned? Or has Thomann sold me a faulty model (which is kind of what Booli suggests)? It is my first electro acoustic uke, so I have no frame of reference.

If it is a general problem, how do other players generally deal with it? DI is not a good solution for me. It would add weight to my gear (bad, since I fly a lot), and more time to set up (already too long). Also, I often run the instrument through my TC-Helicon Voicelive 3 (jack in, XLR out either to mixer or straight into powered FOH speakers). I don't really see how the DI would fit into this scenario, since I want to kill the hum before the signal goes into the Voicelive (which by the way does not have an XLR input, except the one for my mic).

Thanks for your inputs!

I don't recall ever having a uke that hummed.

Looks like that pickup is mainly a factory-only kind of unit. Something that Lanikai gets for cheap in bulk, but not so much meant to be a high-quality, stand-alone solution. That said, it souldn't hum. I'd ping Fishman and see if they have any thoughts.

The Voicelive has a ground lift, doesn't it? Try that.

Looks like with that unit you're kind of stuck with some specialized hole cuts in the instrument. Otherwise, I'd personally jump ship and put a different pickup in. I've spent enough time troubleshooting to be pretty over pickups that don't work perfectly well.

Good luck!

If you have a friend with an electric ukulele you can borrow that and try it, just to be sure. Ukuleles should not be any more prone to hum than guitars. Both are prone to hum. Perhaps that's what the dealer meant to say.

I know how frustrating amp hum can be, and it sounds like you are being thoughtful and analytical while trying to remedy the situation. You are not alone.

I recently received an electro-acoustic instrument and wrote the manufacturer because the electronics did not work. They had me remove the pre-amp (equalizer?) from the body, and unplug, then re-plug the lead from the pickup to the box. That immediately fixed the problem. Seems that the plug had jostled loose during shipping (or was never properly seated) and the manufacturer knew just where to point me.

For the OP, if you are willing to do some 'light' technical reading, the following links might be helpful for you:

- Understanding, finding, and eliminating ground loops (2003) [pdf] (mit.edu)
(this ^ link is a direct download to save the PDF file to your device)

- Bonding, Grounding, and other Techniques for Electromagnetic Compatibility, Electromagnetic Pulse (EMP) Mitigation, and Safety (Military spec)

- Sound System Interconnection (Rane Corp)

sourced via recent post on Ycombinator's Hacker News here:

https://news.ycombinator.com/item?id=17640674
some discussion here ^ as well