Understanding noise

Last night I plugged in just the preamp and a cable, no guitar. Holding the cable off the ground, not touching metal, I still had buzz that got better if I touched metal. It was repeatable with 3 different cables. I also tried with extension and went outside, no help. Plugging in just a 1/4” jack with no cable made no noise.

Is that normal?

Often a disconnected cable will pick up noise. They can act like antennas.

The preamp is all tube at 300v but runs off a 2-prong Ac/Dc adapter, sending 1a at 12v to the pre. So it’s not grounded to the house electrical system.

Lots of power adapters cause hum; switched mode adapters are notorious for this. Other power non-switching DC adapters can also cause hum. Whether your power adapter is regulated is another question. Unregulated adapters can be problematic.

F'rinstance, laptops connected via USB can even cause hum problems that go away once the power adapter is unplugged and the computer runs on batteries. Happens often enough to be a known issue in studio-land.

Buzzes can be location dependent, with pickups responding to EMI and RFI, but 60 cycle hum is a different story, unless the pickups are very close to your guitar amp's transformers. And don't forget about adapters radiating hum into pedals and cables.

By the way, you didn't describe what your preamp is going into, other than a cab sim. Where does it wind up? A guitar amp? A computer interface and computer? Powered speakers?

Still and all, when you describe a hum problem that is solved by taking the preamp out of the chain...something's going on that isn't pickups. Perhaps I'm missing something.
 
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@LSchefman Thanks for taking the time to respond with detail. Besides solving the problem I also want to learn, so I really appreciate it :)

I think the hum might have been location specific, so I'm just focusing on buzz at this point.

The buzz seems to go away when I remove the preamp or put it on bypass. When I touch metal I hear it in the signal, that also goes away when I remove or bypass the preamp. So either the preamp has an issue or it's just amplifying the issue. I suspected the preamp because if I touch anything that's metal it affects the buzz, EXCEPT for one particular footswitch on the preamp. It turns out that was just the switch housing not having good contact with the enclosure. I don't think the switch housing needs to be grounded since it's not grounded internally, BUT now I'm wondering if there are ground loops because of the jacks and other switches grounding to the case mechanically, and also through the circuit board to a case ground screw?

Preamp output goes to Cab Sim pedal input, and I have headphones plugged into Cab Sim headphone jack. Cab Sim and Preamp have their own AC/DC power supplies which are plugged into the same power strip. This also has me wondering - since they're 2-prong adapters. Does that mean the signal voltage is floating, and maybe changing when I touch metal and ground it?

The experiment with just the cable was a mistake, as I apparently was using headphones plugged into a balanced mono output.
 
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Good news: I made a new cable, changed preamp tubes, and plugged the preamp into a headphone amp I recently built - both plugged directly into the same wall socket - and no noise. That’s as far as I got but I’m happy. I’ll build all new cables and I have an isolating power supply on the way.

I previously thought each pedal having it’s own power supply was considered ‘isolated.’ But then I looked at the schematic for my headphone amp and saw the difference between ‘ground’ and ‘earth’, and realized these little lightweight power supplies probably don’t have true isolating transformers. Then the lightbulb went off :)

I ordered the Cioks DC7. It can combine outputs in parallel for 12v, 1a and power my preamp, plus everything else I have and plan on getting. It looks like a nice little unit.
 
Why does a sampler make noise when connected to DI with the ground switch lifted?

I understand that instruments such as electric guitars and basses needs the ground connection for effective shielding on the cable between the instrument and the DI to prevent noise from entering the signal.
But why I get noise when I connect my sampler to my DI and I lift the ground?
I thought that the sampler had its own ground connection and did not need an external one, but maybe I am very wrong.
 
^

Not sure how your specific sampler works, but many cases, when you lift the ground, you're not lifting the ground from the line input of the sampler, but the main connection to ground at the power supply/chassis. So you may be removing the ground altogether.
 
^

Not sure how your specific sampler works, but many cases, when you lift the ground, you're not lifting the ground from the line input of the sampler, but the main connection to ground at the power supply/chassis. So you may be removing the ground altogether.

Thanks for answering ✌️
The sampler is a Korg Electribe.
I lift the ground on the DI Box.
 
I once lived at a place where my guitar cable would pick up a local radio station, and the last place I lived even my humbucker equipped guitars would pick up terrible amounts of noise from the near by power lines. Where I live now, I can crank single coils through a high gain amp without noise.

All that to that my best guess is that by lifting the ground on your DI, you’re probably letting some noise picked up by you pickups or cabling (even balanced cabling) through.
 
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