Noise with battery power supply—can it be grounded?

[gfsark]

I just bought an amazing little power supply/inverter for an upcoming gig on Saturday (‘tis the season). Amazing because it weights less than 5 pounds, smaller than a toaster, and will power my Bose S1 with separate bass for hours at medium volume. (250 watt-hours, lithium ion, Rockpals is the brand name)

Unfortunately, the power supply produces some hum especially with my electric guitar. When I touch the strings on the guitar, most of the hum goes away, so I’m guessing that the problem is that the whole system is ungrounded, and sensitive to picking up noise.

Any ideas on how to eliminate this noise, or ground the power supply or guitar? Premium cables? Note that my acoustic K&K mini has less noise than the electric. Gigging on Saturday at an outdoor Xmas event that has no power where I’m playing, so don’t have much time to fuss with this. It’s not a show stopper but annoying. Thanks for your ideas.

[jonfields45]

New cables are not going to help.

Single coil or humbucking pickups in the guitar?

As a first test, try using an extension cord to get the power supply away from the rest of your gear.

[MikeBmusic]

Forget the inverter and get an S1 battery?

[OneThing]

The Ebtech Hum X Ground Loop Hum Exterminator might do the trick.

[James May]

First, make sure any nearby XMAS tree lights are powered by DC, not AC. Then, have all the local AM radio stations shut down their transmitters while you perform. If there are any overhead power lines, cut them.

But seriously, it does sound as if you are picking up noise in your pickups. Unless they are humbuckers, they don't know the difference between noise and music. (I'll refrain from making a musical taste editorial here.)

I assume you mean that you are running on battery power. You say that touching strings makes most of hum go away. That would give a clue as to one mitigation strategy: run a wire from your amp or guitar's ground to something that is more grounded than mid air - such as your skin or a patch of actual ground. It's not a real attractive approach I know. You could pick someone in the audience, hand them the wire, and say "here, hold this!".

It MIGHT be helpful to have an effects pedal, with a grounded case or a piece of copper tape on the bottom connected to system ground, touching the earth, in between your guitar and the amp. Or to a copper water pipe!

The goal is to somehow get your amp or guitar grounded to, well, er, earth GROUND. Since you don't have an AC power feed with its associated earth ground connection, you'll have to improvise.

[gfsark]

Interesting. The guitar does have humbuckers, and they are definitely vintage. Guitar is a 1975 Ibanez. I’m thinking that the volume and tone controls are probably picking up the noise, since wiggling them (not turning) seems to cause more noise. I suppose that there are better/more shielded pots for the guitar than what’s there.

Also, since there is no ground, I don’t see how I could be getting a ground loop. Could I? Hence an isolator such as the Ebtec Hum Eliminator wouldn’t work.

I’ll try a few more things and report back. Thanks.

[jonfields45]

Assuming no hum plugged into the wall outlet, I would suspect the Rockpals battery+inverter. The ones I looked up online are pure sine wave, but it might still emit noise (for example the switching frequency of the PSW inverter) that your magnetic pickups are picking up.

Since it is not your amp getting the noise from its AC power, but rather your guitar itself, I think getting that inverter further away using an extension cord is a worthy experiment.

[gfsark]

Surprise finding:

The noise from the electric guitar depends its tone control setting. Zero noise when the tone controls are at zero (maximum bass) and maximum noise when the controls are set to 10 (maximum treble). This is true for both pickups. And noise increases linearly with the tone control setting.

Any speculation on what would cause this?

And, to reiterate, when the Bose amp is plugged into the wall there is no noise on any of the guitar’s settings. Only when plugged into the battery power supply does the noise arise. And the acoustic guitar with K&K minis is noise free regardless of the power source.

[jonfields45]

Originally Posted by gfsark

Any speculation on what would cause this?

An insufficiently shielded inductor in the Rockpals switching inverter is emitting enough electromagnetic noise to be picked up by your guitar. Touching the strings or metal on the guitar helps change the strings (and much of the rest of the metal) from being an antenna to a shield.

The tone control helps because this is higher frequency noise, not 60Hz wall power noise and maybe its harmonics from some light dimmer, but rather the switching frequency of the Rockpals and possibly lower frequency components being demodulated through rectification by the protection diodes of whatever the guitar is plugged into.