On Sun, 13 Jun 2021 09:23:53 -0400, W Pulaska wrote:

Pretty simple set up. I have my phono output from my desktop Audiofile
2496 sound card driving an Altec Lansing speaker set. Works fine when
everything is on, but when the desktop is turned off I am getting a
buzz/ hum from the speakers and loud enough that I power them off. Why
is this and how to remedy? Thanks.

I think your desk-top has a soft and a hard power switch. There
may also be other peripherals that are still running plugged
into the desktop.

If the humming is still there when the hard power switch is off,
try terminating the signal lines from the PC with 600R.
The PC may go high-Z when turned off - becoming a hum antenna.

Fool with cables to see if any movement modulates the amplitude.

Check grounding on the PA - are these active loudpeakers?

The source volume should be high, the amplifier's gain low, for
best S/N performance.

RL