Two computers cause hum at amp

[Bill]

I have two computers feeding audio into one receiver-amplifier.

Either computer has an audio output of approx 50 to 100 millivolts.

The voltage measured between the two computers audio output is about
300 miollivolts. I can plug in one computer, a dvd drive, a cassette
deck a turntable, etc., and there is no problem. If I try to plug in
both computers via any input port on the amplifier, I get a bad hum
from this 300mv difference between computers.

Both computers are on different breakers, but on the same 110v leg of
the main power into the panelboard. This voltage difference is
apparently somehow being generated inside the computers.

I expect there is no remedy except physically disconnect one computer
but thought is was worth asking for any suggestions.

--
Bill F

 

[$wannie]

i find the same problem, by sometimer using 2 computers for DJ'ing.
But what i've learned is to not connect the 2 computers together to try to
ground them together! that caused my one channel input on my mixing desk to
blew!

What i do now, is to connect both computers to my mixer (audio from
soundcard), and only then afterwards to connect the power cord to the
computers, ans switch them on. (and don't connect any other direct wires
between the pc's)

But when i connect one pc, and measure voltage from any point of my mixer,
or equipment to any other non connected running pc's audio output, i find a
voltage that actually chocks me by touching in between!!!




Bill <bill@nomail.invalid> wrote in message
news:060120041319091400%bill@nomail.invalid...

I have two computers feeding audio into one receiver-amplifier.

Either computer has an audio output of approx 50 to 100 millivolts.

The voltage measured between the two computers audio output is about
300 miollivolts. I can plug in one computer, a dvd drive, a cassette
deck a turntable, etc., and there is no problem. If I try to plug in
both computers via any input port on the amplifier, I get a bad hum
from this 300mv difference between computers.

Both computers are on different breakers, but on the same 110v leg of
the main power into the panelboard. This voltage difference is
apparently somehow being generated inside the computers.

I expect there is no remedy except physically disconnect one computer
but thought is was worth asking for any suggestions.

--
Bill F

 

[ToasterKing]

Measure the voltage difference between the signal ground of audio
output on one computer, and signal ground of audio output on the other.
I expect you'll find a difference. Your computers will have floating
grounds with respect to each other, so it's perfectly normal to have a
voltage difference. The voltage difference may be traveling between
the computers via the audio cables and the receiver/amplifier, creating
a ground loop and causing your hum.

I wouldn't suggest connecting the DC grounds of both computers
together, as this could just lead to further problems. I'd use a 1:1
isolation transformer for the audio of at least one of the computers.
Radio Crack sells an audio "Ground Loop Isolator" for around $20 US
which is an isolation transformer designed for audio use, with RCA
cables already attached. Disconnect the audio from the PC that is
causing the hum, then attach one of those thingies inline.

TK

In article <060120041319091400%bill@nomail.invalid>, Bill
<bill@nomail.invalid> wrote:

I have two computers feeding audio into one receiver-amplifier.

Either computer has an audio output of approx 50 to 100 millivolts.

The voltage measured between the two computers audio output is about
300 miollivolts. I can plug in one computer, a dvd drive, a cassette
deck a turntable, etc., and there is no problem. If I try to plug in
both computers via any input port on the amplifier, I get a bad hum
from this 300mv difference between computers.

Both computers are on different breakers, but on the same 110v leg of
the main power into the panelboard. This voltage difference is
apparently somehow being generated inside the computers.

I expect there is no remedy except physically disconnect one computer
but thought is was worth asking for any suggestions.

--
<<Please remove the word "SPLAT" from either below address to use it.>>
Email: mailto:ToasterKing@SPLATbigfoot.com
Visit ToasterKingdom at http://SPLATtoasterking.tripod.com/

 

[JURB6006]

I've read the other replies, and I think I have a better solution.

First of all it is very unlikely the voltage is generated by either of the PCs,
even if it was they would both have to defective UNLESS the amp is also
connected to a VCR or TV that is running off cable.

Thing to remember here goes back to the National Electric Code, that a CATV or
antenna line should be grounded as close to the point of entry (of the
building) as possible. Thus, the installer usually uses a cold water pipe. Now
on many newer electrical installations a 10 foot ground rod is driven into the
ground outside to provide a positive (assured, not opposite of negative) earth
connection. If there is appreciable distance between you water line and the
ground rod, that is probably the reason. I had a similar problem once and what
I did was is to rig an antenna isolator from a hot chassis TV to provide a coax
to coax connection.

If cable is hooked to NOTHING in this system, or if your house uses a cold
water pipe for the main electrical ground the former does not apply.

In that case you could have a "hot" check done on each of the components
involved. It can be ANYTHING hooked up to that amp, or the amp itself, or one
of the PCs and the amp, there are a few scenarios. In no case should you get a
shock and it is possible one of the PCs needs a power supply.

I would NOT disconnect the ground pins, but if in this (would be rare) case
that is nessecary use a two to three adapter and wire the provided ground lug
to the SAME ground as everything else connected.

IMO we are going to see more of this type of problem as the PC and the rest of
the home entertainment system integrates. Proper wiring in houses and slightly
modified techniques in installation will become quite important. Remember this
can be caused by ANYTHING connected.

I've also read of a problem with some Delll laptops having enough line leakage
to be about dangerous, so if one of the PCs is one,,,,,,,,,,,

At any rate, I for one would appreciate eventually knowing what cured the
problem.

You might consider seeing if you can make a "package deal" on estimates with a
local shop, but make sure they have the equipment to HOT CHECK for line
leakage. Tell them that they will probably wind up fixing the offending unit,
once they know which one. If you're in N.E. Ohio I could handle it, but if
you're not, as much as I hate to say it, you might want to use an authorised
shop, at least for the amp. (we are an independant) They are more likley to
already have the right equipment, but a competent independant shop might as
well. Also, anyone can build a simple test jig to do this. Many service manuals
have the instructions on how to do so.

Many otherwise competent techs might not actually know how to solve this
problem in the home, it just so happens I remodel houses and do custom installs
as well. If your close enough that is.

You may, for test purposes only, use a two to three adapter on each PC and see
for sure where the leakage is. You might be tempted just to leave it in place,
but don't, it might compromise safety. Also, if ANYTHING is connected to an
antenna or cable, try disconnecting that FIRST. It may tell all.

Let us know what's up after this step and we can get on to finding a safe and
effective solution.

JURB

 

[Vector Viper]

"Bill" <bill@nomail.invalid> wrote in message
news:060120041319091400%bill@nomail.invalid...

I have two computers feeding audio into one receiver-amplifier.

Either computer has an audio output of approx 50 to 100 millivolts.

The voltage measured between the two computers audio output is about
300 miollivolts. I can plug in one computer, a dvd drive, a cassette
deck a turntable, etc., and there is no problem. If I try to plug in
both computers via any input port on the amplifier, I get a bad hum
from this 300mv difference between computers.

Both computers are on different breakers, but on the same 110v leg of
the main power into the panelboard. This voltage difference is
apparently somehow being generated inside the computers.

I expect there is no remedy except physically disconnect one computer
but thought is was worth asking for any suggestions.

Ideally, all equipment wired together should share 1 outlet strip. This
keeps all
grounds "together" although ground loops can still happen...

One tip for any equipment without a 3-wire plug....disconnect it from ALL
other pieces;let its case electrically "float". Now measure the AC voltage
from
the case to the 3rd wire ground of an AC outlet. The voltage should be less
than
30 volts (5-15 is not unusual) If is it high, 50-90 volts, reverse the plug
of that
piece of equipment. Do this for all peices. This keeps the grounds from
"fighting"
each other....worth a try...

Viper

--
Bill F

 

[Steve(JazzHunter)]

On Tue, 6 Jan 2004 20:17:41 -0800, "Vector Viper" <ahowald@w-link.net>
wrote:

"Bill" <bill@nomail.invalid> wrote in message
news:060120041319091400%bill@nomail.invalid...
I have two computers feeding audio into one receiver-amplifier.

Either computer has an audio output of approx 50 to 100 millivolts.

The voltage measured between the two computers audio output is about
300 miollivolts. I can plug in one computer, a dvd drive, a cassette
deck a turntable, etc., and there is no problem. If I try to plug in
both computers via any input port on the amplifier, I get a bad hum
from this 300mv difference between computers.

Both computers are on different breakers, but on the same 110v leg of
the main power into the panelboard. This voltage difference is
apparently somehow being generated inside the computers.

I expect there is no remedy except physically disconnect one computer
but thought is was worth asking for any suggestions.

Ideally, all equipment wired together should share 1 outlet strip. This
keeps all
grounds "together" although ground loops can still happen...

One tip for any equipment without a 3-wire plug....disconnect it from ALL
other pieces;let its case electrically "float". Now measure the AC voltage
from
the case to the 3rd wire ground of an AC outlet. The voltage should be less
than
30 volts (5-15 is not unusual) If is it high, 50-90 volts, reverse the plug
of that
piece of equipment. Do this for all peices. This keeps the grounds from
"fighting"
each other....worth a try...

I think JURB6006 has the coreect solution. There is one ground on the
computers, the wall socket, there is another ground on the CATV cable
to the receiver. That's where your hum loop is coming from, I
guarantee it. Remove the CATV cable and the hum will go away. To
solve this purchase two baluns, 300 -> 75 Ohm transformers. Connect
the two 300 Ohm sides to each other, put the cable through the device,
and the gorund has been separated - no more hum. Always, always this
is the problem in a case like this, and it's surprising how few
knowledgeable techs know about it. There might ALSO be an inadvertent
ground, something touching a heating radiator, grounded metal stereo
rack, etc, just as a scarce possibility.


. Steve .

Viper



--
Bill F

 

[JURB6006]

Remove the CATV cable and the hum will go away. To
solve this purchase two baluns, 300 -> 75 Ohm transformers. Connect
the two 300 Ohm sides to each other, put the cable through the device,

Although you are right that it will, this would degrade quality, at least the
isolation. If it's from a CATV box, fine, channel 4, 2 whatever, but if you got
raw cable it it FUBARS the signal on some channels, especially those that share
with local pager companies. (annoying)

A much better solution is to use those isolators I mentioned, unlike the balun
they ARE made specifically to isolate line voltage from the input.Also if local
VHF channels are repeated you get ghosts if your cable is "leaky". Any twinlead
is a leak. You're talking about taking it to twinlead and back to coax to
isolate the ground. I believe the isolator is a much better solution.

This is only for RF, A/V is different, but for RF any TV manufacturer's
isolator should do the trick, transparently. Actually in the long run it is
better to deal with it at the RF end than the video or audio end. This is
because it is much easier to build a filter for frequencies that are farther
apart.

Of course this guy could still have a ground fault, but let's hear the results
first.

JURB

 

[Dave Cole]

in article ooaovv8sjte9dlcnukgbv5nsufio0ap73q@4ax.com, Steve(JazzHunter) at
jazzhunterNotHere@internet.com wrote on 1/7/04 09:58:

I

To
solve this purchase two baluns, 300 -> 75 Ohm transformers. Connect
the two 300 Ohm sides to each other, put the cable through the device,
and the gorund has been separated - no more hum.
. Steve .

Good idea, but most of the cheap baluns I've seen don't even isolate D.C.,
much less 60 Hz A.C. You might have to pay more for a true isolator.
Good luck.
Dave Cole

 

[Bill F]

One tip for any equipment without a 3-wire plug....disconnect it from ALL
other pieces;let its case electrically "float". Now measure the AC voltage
from the case to the 3rd wire ground of an AC outlet. The voltage should be less
than 30 volts (5-15 is not unusual) If is it high, 50-90 volts, reverse the plug
of that piece of equipment. Do this for all peices. This keeps the grounds from
"fighting" each other....worth a try...

Reversing the plugs was a good suggestion, I did find one feeding a
cassette deck that was wrong way, but it didn't solve the problem in
this case.

I think JURB6006 has the coreect solution. There is one ground on the
computers, the wall socket, there is another ground on the CATV cable
to the receiver. That's where your hum loop is coming from, I
guarantee it. Remove the CATV cable and the hum will go away. To
solve this purchase two baluns, 300 -> 75 Ohm transformers. Connect
the two 300 Ohm sides to each other, put the cable through the device,
and the gorund has been separated - no more hum. Always, always this
is the problem in a case like this, and it's surprising how few
knowledgeable techs know about it. There might ALSO be an inadvertent
ground, something touching a heating radiator, grounded metal stereo
rack, etc, just as a scarce possibility.

A while ago I had a hum problem with the CATV cable and the receiver

and I tried the two baluns back to back scheme then. I couldn't get
enough signal through it for my cable modem to lock on to the signal.

There presently isn't any CATV cable to the receiver except via the
VCR. There may be a path through the ethernet cables back to the cable
modem but both computers are tied together through the same ethernet
loop. I disconnected the audio cables to the receiver from the VCR,
which has a CATV connection, and that did lower the voltage
differential a little but not enough to eliminate the hum.

The 300mv I am measuring is between the audio grounds of the two
computers and it's there even with the receiver completely out of the
loop. I ran an extension cord from the power strip on one computer,
into the room with the other computer and measured a 300mv difference
between each 110v hot side feeding the two computers. Maybe that is
the problem, the two power leads to the computers are not exactly
balanced because of different loading and resistances of the house
wiring.

Next thing I'm going to try (today) is a power extension cord
connection one computer to the other to see if that eliminates the
input voltage difference and hopefully the audio output voltage
difference. If that doesn't work I'll try the audio isolation
transformers.

--
Bill F

 

[Steve(JazzHunter)]

On 08 Jan 2004 07:13:26 GMT, jurb6006@aol.com (JURB6006) wrote:

Remove the CATV cable and the hum will go away. To
solve this purchase two baluns, 300 -> 75 Ohm transformers. Connect
the two 300 Ohm sides to each other, put the cable through the device,

Although you are right that it will, this would degrade quality, at least the
isolation. If it's from a CATV box, fine, channel 4, 2 whatever, but if you got
raw cable it it FUBARS the signal on some channels, especially those that share
with local pager companies. (annoying)

I've not experienced any signal lost on a wideband cable connection,
and no incursion. There is a DC path but it's through a 47k resistor
or such, hardly an issue. I'm using very old baluns, maybe they don't
make them like they used to <g> However it's true that 75 Ohm
isolators are now available, this wasn't true decades ago when I
started dealing with this issue.


. Steve ..

A much better solution is to use those isolators I mentioned, unlike the balun
they ARE made specifically to isolate line voltage from the input.Also if local
VHF channels are repeated you get ghosts if your cable is "leaky". Any twinlead
is a leak. You're talking about taking it to twinlead and back to coax to
isolate the ground. I believe the isolator is a much better solution.

This is only for RF, A/V is different, but for RF any TV manufacturer's
isolator should do the trick, transparently. Actually in the long run it is
better to deal with it at the RF end than the video or audio end. This is
because it is much easier to build a filter for frequencies that are farther
apart.

Of course this guy could still have a ground fault, but let's hear the results
first.

JURB

 

[Bill F]

In article <080120040800127049%flipflapco@mchsi.com.invalid>, Bill F
<flipflapco@mchsi.com.invalid> wrote:

Next thing I'm going to try (today) is a power extension cord
connection one computer to the other to see if that eliminates the
input voltage difference and hopefully the audio output voltage
difference.

I did this, both computers fed power from the same UPS. One via a 30
foot extension cord, which probably caused some small voltage drop due
to it's length. The voltage difference between audio grounds dropped
from 300mv down to 3mv. The hum was reduced, but it still had a quite
objectionable amount of hum. I'll pick up a Radio Shack isolation
transformer next time I'm in town and see if that helps. Meanwhile
guess I'll just have to plug in one computer and unplug the other
whenever I need to swap inputs.

--
Bill F

 

[çĺÝ]

One tip for any equipment without a 3-wire plug....disconnect it from ALL
other pieces;let its case electrically "float". Now measure the AC voltage
from the case to the 3rd wire ground of an AC outlet. The voltage should be less
than 30 volts (5-15 is not unusual) If is it high, 50-90 volts, reverse the plug
of that piece of equipment. Do this for all peices. This keeps the grounds from
"fighting" each other....worth a try...

Good suggestion but didn't solve the problem in this case.

I think JURB6006 has the coreect solution. There is one ground on the
computers, the wall socket, there is another ground on the CATV cable
to the receiver. That's where your hum loop is coming from, I
guarantee it. Remove the CATV cable and the hum will go away. To
solve this purchase two baluns, 300 -> 75 Ohm transformers. Connect
the two 300 Ohm sides to each other, put the cable through the device,
and the gorund has been separated - no more hum. Always, always this
is the problem in a case like this, and it's surprising how few
knowledgeable techs know about it. There might ALSO be an inadvertent
ground, something touching a heating radiator, grounded metal stereo
rack, etc, just as a scarce possibility.

There isn't any CATV cable to the receiver except via the VCR. I

disconnected the audio cables from VCR to receiver and that did lower
the voltage differential a little but not enough.

The 300mv I am measuring is between the audio grounds of the two
computers, even with the receiver out of the loop. I ran an extension
cord from the power strip on one computer, into the room with the other
computer and measured a 300mv difference between each 110v hot side
feeding the two computers. Maybe that is the problem, the two power
leads to the computers are not exactly balanced because of different
loading and resistances of the wiring.

 

[Bill]

In article <060120041848143763%toasterking@SPLATbigfoot.com>,
ToasterKing <toasterking@SPLATbigfoot.com> wrote:

I wouldn't suggest connecting the DC grounds of both computers
together, as this could just lead to further problems. I'd use a 1:1
isolation transformer for the audio of at least one of the computers.
Radio Crack sells an audio "Ground Loop Isolator" for around $20 US
which is an isolation transformer designed for audio use, with RCA
cables already attached. Disconnect the audio from the PC that is
causing the hum, then attach one of those thingies inline.

TK

Toasterking had the winning suggestion, and the hum is finally gone.

One $15 Radio Schlock audio isolation transformer completely solved my
hum problem with the two computers connected to the amplifier.

--
Bill