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I'm using a Focusrite Scarlett as a preamplifier between my computer and amplifier. The preamp gets power only through a USB connection. Sometimes when I touch the cover of the preamp, I can hear a small "poof" from my speakers. I didn't realize those were mild electric shocks, until today, when I touched the cover and heard a loud "poof" at the same time as I felt that poof on my finger. So, the cover of my preamp is giving out small electric shocks.

The warranty on the preamp has expired and I'm happy with it otherwise, so I would prefer to keep it. On the other hand, I don't want to feel another shock like the one I felt (or burn down the house). I was thinking, is there an easy way to make the preamp safe? Like, can I go out and buy a grounding wire and connect that from the cover of the preamp to the radiator? Or something like that?

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  • \$\begingroup\$ Which amplifier are you using? Solid-state, tube, class AB, etc. \$\endgroup\$ – rdtsc May 26 '20 at 19:30
  • \$\begingroup\$ My amplifier is NAD C 316BEE. \$\endgroup\$ – Atte Juvonen May 26 '20 at 19:48
  • \$\begingroup\$ First thing you should state is grounding. Is it a desktop PC or a laptop PC? Does the PC or charger have a grounded plug? Is it connected to a grounded outlet? Were you touching any other equipment at the same time, grounded or ungrounded? \$\endgroup\$ – Justme May 26 '20 at 21:27
  • \$\begingroup\$ The C-316BEE appears to be a class-AB amp. My guess is, there is a voltage difference between the ground the computer is connected to (goes all the way to the Scarlett case) and the ground the amplifier is connected to. Try moving the amplifier power cord into the same outlet strip as the computer, just to test. If you have a multimeter, check for volts between the two enclosures. \$\endgroup\$ – rdtsc May 26 '20 at 22:29
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If it's a wall-wart USB power supply, it has a floating secondary. It will develop an offset with respect to ground which, while annoying, isn't harmful. It can lead to hum noise.

The loud 'poof' you felt the second time was probably static discharge from your own body.

Try grounding your preamp or power it from your PC and make sure it's grounded. This will address both issues.

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  • \$\begingroup\$ My preamp is powered from the PC, and my PC is grounded. So... this shouldn't happen? \$\endgroup\$ – Atte Juvonen May 26 '20 at 19:18
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    \$\begingroup\$ No, it shouldn't if your PC has a 3-pin line connection. If it doesn't, then the secondary floats, leading to the problems you're seeing. Maybe check the ground connection with a voltmeter if you have one. Also check the ground connection at your amp. \$\endgroup\$ – hacktastical May 26 '20 at 19:20

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