[TzeYang]

1) Wallwart 24VDC, touches pot (buzzz)

2) 2x 9V Batteries, No noise.


mind you, there is no metal/aluminum case, i tested them all naked.

Do you think it's due to the noisy wallwart?

 

[amb]

Connect the pot body to signal ground and it should cure the noise.

 

[TzeYang]

thanks!

i don't get it. (more technical explanation please lol)

i've built several LM7824 PSUs for bigger amps before and i never had to ground the pot with an extra strap. I opened up the wallwart and it had a 2200uF filter cap and an ON SEMI LM7824 regulator inside.

The pot's a RK097 btw.

 

[amb]

The pot body and shaft is a chunk of metal which comes very close in proximity to the signal inside. At anything less than full-volume, the resistance of the pot makes the signal path a high-Z node, which is susceptible to interference. If the pot body is not grounded, when you touch it, and there is an EMI field nearby (i.e., power supply), your body will act as an antenna and pass the interference into the signal. This is usually less of an issue in battery-powered gear because there is no onboard AC mains/transformer.

In a metal case where the case itself is already grounded, just having the pot mounted to the case also grounds the pot. But in a non-conductive case or front panel, the pot body won't be grounded, and thus a separate ground wire becomes necessary.

 

[TzeYang]

okay here's something even weirder.

I'm using a non conductive plastic case to case the desktop amp that i mentioned that was working great without the aid of a ground strap. The trafo is external though (it's an AC wallwart outputting 24VAC)

 

[Eokboy]

IME I get this problem in newly completed amps but they disappear after a few hours.

... weirded out

 

[PhaedrusX]

Quote:

Originally Posted by amb /img/forum/go_quote.gif The pot body and shaft is a chunk of metal which comes very close in proximity to the signal inside. At anything less than full-volume, the resistance of the pot makes the signal path a high-Z node, which is susceptible to interference. If the pot body is not grounded, when you touch it, and there is an EMI field nearby (i.e., power supply), your body will act as an antenna and pass the interference into the signal. This is usually less of an issue in battery-powered gear because there is no onboard AC mains/transformer. In a metal case where the case itself is already grounded, just having the pot mounted to the case also grounds the pot. But in a non-conductive case or front panel, the pot body won't be grounded, and thus a separate ground wire becomes necessary.

i'm having the same issue with your Mini^3, whether using the AC adapter or not.
it comes in the standard metal Hammond case, and as far as i know is built to spec with no modifications.
i assumed the case was grounded in your design, but maybe not? i'm just wondering if this is a design issue or a build issue, or possibly something else, although using it in different rooms with different gear attached yields the same results.

 

[TzeYang]

For your case above^^

The Case is grounded because the pot is attached to the metal plates.

Funny that both designs are more or less the same and the Mini3 doesnt require a ground strap but my amp does?

I doubt it's the ground strap thingy. I'm willing to bet something is acting up on the wallwart. I'm going to build a tread to see what happens lol.

 

[amb]

Mini³ is battery powered, so it's less susceptible to AC interference. Also, since Mini³'s Hammond casing is connected to signal ground via strips on the bottom side of the board, if the front panel is aluminum, and the hole for the volume pot is tight enough to touch the pot's threaded bushing, then the pot would be grounded and will be silent when you touch the knob. If the pot does not contact the case, then under certain circumstances, touching the volume knob could induce some hum/buzz.