On 6/13/2021 4:54 PM, Dave Platt 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.

When the PC is powered on, there's DC power going to the sound-card
output circuitry. The output amplifiers (when powered on) should have
a relatively low output impedance - typically a couple of hundred ohms
for a "line level" output, quite possibly less. Since the speaker
set's input circuitry has a high impedance (at a guess, 47k ohms or
higher), the voltage on the signal conductor will be dominated by the
output amplifier - and when you aren't playing music, it'll be right
about at zero volts ("dead silence").

When you power off the computer, the sound card loses power and its
output amplifiers shut down... they go to a "high impedance"
open-circuit state. At this point, the sound card isn't holding the
signal line at zero volts. Instead, the signal line will start acting
like a simple antenna, picking up 60-cycle buzz and hum from magnetic
and electrical fields around the PC. With its high input impedance,
the speaker set will be sensitive to even small noise currents and it
will amplify the resulting voltage.

If you were to unplug the cable from the PC, and touch the end of the
plug with your finger, you'd get a similar (possibly much-louder) buzz.

If this really bothers you, you can probably make a "noise stopper"
device, wired between the sound-card output and speaker set. You
would need a small (sensitive-coil) 5-volt-DC double pole,
double-throw relay. The relay coil would be wired to a USB plug which
would go into the PC, so that the relay would be energized when the PC
was turned on. You would wire one set of the normally-closed relay
contacts, and a 47-ohm resistor, across each of the two audio signal
lines and audio ground.

When the PC is off, the relay would be in its normally-closed
position, connecting the 47-ohm relay across the audio signal. This
would silence the audio, muting the buzz.

When the PC is on, the relay would operate, opening the
normally-closed contacts, disconnecting the resistors and unmuting the
audio.

A similar unmute-upon-power-on system is used in many commercial audio
components (e.g. tuners and preamps).


You could just put a several K ohm resister across the audio signals. It
might drop the signal level some, but would pull the noise level down.
Adjust the ohms to opimize results.