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.

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

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).

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.

On Sunday, June 13, 2021 at 6:23:58 AM UTC-7, 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...

Buzz means harmonics from a sawtooth, typically. That indicates that the
speaker power source (a wall-wart power brick??) has a capacitor that has
gone high-impedance (high ESR, for instance). Speaker amplifiers aren't
critically dependent on exact-match power bricks, you can find another
(if necessary splice the cord from the old one to fit the socket) that'll work.

Sometimes, you can open and repair a power brick. Not often.

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.

OK. That would be a circa 2002 M-Audio Audiophile 2496 PCI card:
https://www.newegg.com/m-audio-audiophile-2496/p/N82E16829121120

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.

One speaker or both speakers?

Is the buzz/hum still there when you turn off the unspecified model
Altec Lansing (amplified) speakers? I'm wondering if the problem is
in the computah or in the amplified speakers?

Why is this and how to remedy? Thanks.

Instead of turning off your unspecified model PC, try unplugging the
AC power cable from the PC to insure that it's really turned off. If
the hum/buzz disappears, my guess(tm) is the WoL (wake-on-LAN) power
from the power supply is making its way somehow to the sound card.
I've fixed one similar "noisy when turned off" PC problem by replacing
the power supply. Unfortunately, I didn't bother trying to figure out
the exact cause.

Does your unspecified model PC also have a built in sound card on the
motherboard? If yes, unplug the 2496, and try running your
unspecified model Altec Lansing speakers from the motherboard sound
(green) output. If the hum/buzz goes away with motherboard sound,
then the problem might be something on the M-Audio 2496 card. To be
sure, try installing a different PCI sound card. If the replacement
PCI card doesn't buzz, then it's probably something on the M-Audio
2496 card.

Do you have a ground loop isolation audio transformer handy? Something
like one of these[1]?
https://www.amazon.com/s?k=ground+loop+isolator+3.5mm
https://www.ebay.com/sch/i.html?_nkw=ground+loop+noise+isolator+3.5mm
I'm not sure what it might mean if installing one of these fixes the
hum/buzz problem, but it would be an interesting test.

Good luck.


[1] I carry some of these in my toolbox. They mostly get used to
break the audio ground loop between the TV earphone jack and amplified
speakers.


--
Jeff Liebermann
PO Box 272 http://www.LearnByDestroying.com
Ben Lomond CA 95005-0272
Skype: JeffLiebermann AE6KS 831-336-2558