Model 12: Direct-in guitar hum and 2 pronged power plug

[chvario]

Hi,
I noticed that when I go directly into the Model12 with my electric guitar there is a loud hum when I am not touching the strings. I figure this is a grounding issue, that because the metal parts aren't connected to ground, they need to connected to my ground by me touching the strings. I have an even worse hum issue with my Hohner Pianet, and there aren't any strings for me to touch to ground them

So to troubleshoot I first double-checked the wiring of the guitar, and found that there is nothing wrong with it, that the ground of its output has continuity with the metal parts (strings, bridge, etc.) of the guitar.
To troubleshoot further I checked whether my outlet might have a floating ground, which it doesn't. But then I remembered that the Model12 only has a 2 pronged power plug, and therefore isn't even connected to the ground of the receptacle. So I wonder: Shouldn't that result in the Model12 to have a floating ground by default?

I am aware that Tascam's designers are smarter and more experienced than I am, so I am probably not seeing the whole picture.
And since I wasn't able to craft a Google search that resulted in a helpful answer, could someone here with a bit more electrical engineering knowledge explain to me:
a) why the Tascam Model12 doesn't need the third (ground) prong, and
b) how I could solve my hum issue?

Thank you!

 

[-mjk-]

The wall wort is outputting low voltage DC. There is no need for a safety ground for that.

 

[chvario]

Thank you. I understand that the low voltage isn't dangerous.
My question was more about the hum being caused by the "floating ground", since the circuit isn't able to "see" the actual ground.

 

[-mjk-]

One cannot I assume that not having a power grid ground connection is causing a hum. I explained to you why the power supply doesn't really need a ground connection. That also means that you're unlikely to get a ground loop with other pieces of equipment through the power supply, which is often a big problem. Your hum may it be entirely environmental. With your guitar plugged in try slowly spinning around the room and see if the direction in which the guitar is pointing changes the hum. If that's the case you may be able to find a null.

Professional installations have what they call a star ground which is a central point to which all other pieces of equipment are grounded by a home run from the star ground directly to the individual pieces of equipment. Those individual pieces of equipment are floating off the power grid ground. Shields are usually connected at one end only on balanced cables. You can experiment by taking a piece of wire and touching it on bare metal parts of the various pieces of equipment and see if connecting their chassis together will eliminate the hum, or do something else. Troubleshooting a noise in the audio takes a serious, methodical approach. You can try running a ground wire to the power grid ground and see the effect of that. From there you can branch out to connecting other pieces of equipment. Often just moving a piece of equipment can make a difference. My old computer had to be ground lifted or it made everything hum. My new computer needs the ground. You can't make any assumptions and you have to try everything.

 

[chvario]

Thank you, this helps a lot! I wasn't aware that star ground would be detached from power grid ground, but it makes sense.
I have found positions where the humming is less. I had hoped there might be a more stable, definitive solution, but obviously hum depends on many factors, and there's no one solution for every setup. That's probably also why sometimes switching DIs to ground lift helps, sometimes it makes things worse.
Interestingly, when I touch power grid ground while holding the guitar the humming gets louder. I will do some more experiments and see what I can come up with.

 

[Arjan P]

A very simple solution/ quick workaround: make sure you always touch the strings, by wearing an anti-static wrist band (or ESD wrist strap) connected to your strings at the tuning mechanism.

 

[chvario]

That's a good idea. I went ahead and ordered one. Thank you.

 

[-mjk-]

@Arjan P I've actually worked with the Charleswater company to eliminate ESD in PCB assembly lines. I've also professionally designed and implemented ground systems in broadcast sites and studios, as well as professionally investigated RFI complaints and eliminated them in consumer and professional equipment. In other words, I know about wrist straps and that never occurred to me. That genius suggestion about a wrist strap sent me back to school, man. Oh yeah, now got a lump on my forehead too. Thanks!

 

[-mjk-]

I wasn't aware that star ground would be detached from power grid ground, but it makes sense.

It really has to be. The "Edison" (as we used to call it) ground is not clean. Everyone and their neighbor is connected to it and it carries all kinds of nasty stuff that can potentially get into your audio. I've had excellent results with a 000 welding cable clamped on the street side of the water main coming into the building (the input side of the shut-0ff valve). Another good one is a very long copper rod or pipe. I know a guy who drove a pipe and filled it with rock salt. It contributed to the eventual demise of the pipe but it helped with the earth coupling. You have to make sure the clamp is tight and check it for corrosion. There are some coatings that can help with that once you make the connection.

I used to go into a new facility before they put the floor tiles in, and I'd snap chalk lines where I wanted to put the copper strapping. A long bead of industrial glue and there you have it. The contractor would tile right over it.

 

[chvario]

@Arjan P I found out that the cables of those Anti Static wrist bands include a 1M resistors. So I needed to use my own cable, but now it works, the humming is greatly reduced.

What I couldn't fix with it was the hum on my Hohner Pianet e-piano. In order to eliminate the huming there, I need not only connect myself to the ground of the output, but I also need to move my body close to the spot where that connection is made, so proximity seems to be a factor.
I wonder what the physics behind this phenomenon are.

 

[-mjk-]

Interelectrode capacitance.

 

[Arjan P]

@Arjan P I found out that the cables of those Anti Static wrist bands include a 1M resistors. So I needed to use my own cable, but now it works, the humming is greatly reduced.

That's right, I should have mentioned that. It's there for safety of the user (if with a wrist strap you would touch a live wire, the resistor reduces greatly the amount of current through your body), but is not really necessary in the guitar type of use we discussed. Glad it works, BTW!

In other uses (like with your Hohner Pianet) I would hesitate to connect myself to the instrument's output ground - depending on how the instrument is powered, and grounded. Be careful here!

 

[chvario]

The Hohner Pianet is completely passive (just a bunch of keys, reeds and pickups), it doesn't have a power supply, so it should be safe.

 

[Arjan P]

Right. Should've looked it up, but I had a different instrument in mind.. All good then!