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max.spicer
2005-11-15, 14:47
Whilst I wait for my T-Amp to arrive, I've got my SB3 plugged into some Labtek computer speakers. The speakers themselves don't actually sound too bad, but they have a fairly loud hum when the SB3 is plugged in. When powered on, but with nothing plugged into them, they are fine. The moment I plug in the SB3, they start to hum. However, if I touch the exposed metal parts of the rca connectors, the hum goes away. I'm not suggesting that this is the fault of the SB3, but is there anything that people can suggest to sort this out (short of standing touching the connectors all the time!)? My limited electrical knowledge tells me that I have mains hum and that there is a grounding issue.

One thing of note, is that I'm fairly sure the speakers didn't hum when plugged into the SoundBlaster Live in my computer.

Thanks,

Max

Mark Lanctot
2005-11-15, 15:49
This is called a ground loop. It's caused by
audio equipment being at different ground
potentials. The potentials will balance
themselves out by sending a current over any
available connection. In this case, it's over the
analog interconnect cables to your powered
speakers. It's grounding itself through your
powered speaker amplifier.

Since the Squeezebox has a "floating ground" (note
the power supply has only two pins and does not
use the 3rd grounding pin) I don't know what to
tell you regarding its cause or possible
solutions. Usually the solution is to provide a
better path for the equipment to ground itself.
This involves making sure the units are plugged
into the same outlet and ensuring the outlet
ground is OK by using a circuit tester. As a last
resort, you can connect the two components' cases
together using speaker wire.

None of this won't do much in your case since the
Squeezebox has a floating ground, so I'll leave
that one to the more experienced.

However there's something you can do to minimize
this. The hum is coming from your computer
speaker amplifier. So if you turn it up it will
get worse. The idea is then to have as large a
gain on the Squeezebox as possible and as little
gain on the computer speaker amplifier as
possible. So set the SB's volume to maximum and
turn down the computer speakers' volume to suit.
This won't eliminate the problem, but it will
minimize it.

I'm at a loss as to where the ground loop would be
coming from. Since the SB has a floating ground,
it cannot "generate" its own ground loop, it
merely "passes it along" and acts like a link in a
chain. Is there anything else connected to the
computer speakers? Is the SB wired? If it is,
it's the Ethernet cable that's not grounded
properly. Try disconnecting the cable. The hum
should disappear immediately...along with the
music, unfortunately!

max.spicer wrote:
> Whilst I wait for my T-Amp to arrive, I've got my
SB3 plugged into some
> Labtek computer speakers. The speakers themselves
don't actually sound
> too bad, but they have a fairly loud hum when the
SB3 is plugged in.
> When powered on, but with nothing plugged into them,
they are fine.
> The moment I plug in the SB3, they start to hum.
However, if I touch
> the exposed metal parts of the rca connectors, the
hum goes away. I'm
> not suggesting that this is the fault of the SB3,
but is there anything
> that people can suggest to sort this out (short of
standing touching the
> connectors all the time!)? My limited electrical
knowledge tells me
> that I have mains hum and that there is a grounding
issue.
>
> One thing of note, is that I'm fairly sure the
speakers didn't hum when
> plugged into the SoundBlaster Live in my computer.
>
> Thanks,
>
> Max
>
>

--
___________________________________


Mark Lanctot, marklanctot (AT) yahoo (DOT) ca
___________________________________

pfarrell
2005-11-15, 15:56
On Tue, 2005-11-15 at 17:49 -0500, Mark Lanctot wrote:
> This is called a ground loop.

Ground loops are fairly frequent. Sometimes they can be fixed
easily, others are very hard to impossible to fix.

> As a last resort, you can connect the two components' cases
> together using speaker wire.

Altho sometimes you need very heavy speaker wire, maybe 4-ought :-)

The problem is so common that pro-audio stores sell isolation
transformers. But good transformers are expensive, so that
is usually not a good solution for a low buck, temporary problem.

--
Pat
http://www.pfarrell.com/music/slimserver/slimsoftware.html

Mark Lanctot
2005-11-15, 16:03
The question remains Pat, where could the ground
loop be coming from?

If there's nothing connected to the computer
speakers other than the SB and nothing connected
to the SB, where could it be coming from?

Usually in A/V systems, 95% of the time it's
coming from the cable TV connection.

But here all you have going to the SB is
(possibly) an Ethernet cable. But if it's
wireless...?

Pat Farrell wrote:
> On Tue, 2005-11-15 at 17:49 -0500, Mark Lanctot
wrote:
>
>>This is called a ground loop.
>
>
> Ground loops are fairly frequent. Sometimes they can
be fixed
> easily, others are very hard to impossible to fix.
>
>
>> As a last resort, you can connect the two
components' cases
>>together using speaker wire.
>
>
> Altho sometimes you need very heavy speaker wire,
maybe 4-ought :-)
>
> The problem is so common that pro-audio stores sell
isolation
> transformers. But good transformers are expensive,
so that
> is usually not a good solution for a low buck,
temporary problem.
>

--
___________________________________


Mark Lanctot
___________________________________

Alex Twisleton-Wykeham-Fiennes
2005-11-15, 16:08
On Tuesday 15 November 2005 21:47, max.spicer wrote:
> Whilst I wait for my T-Amp to arrive, I've got my SB3 plugged into some
> Labtek computer speakers. The speakers themselves don't actually sound
> too bad, but they have a fairly loud hum when the SB3 is plugged in.
> When powered on, but with nothing plugged into them, they are fine.
> The moment I plug in the SB3, they start to hum. However, if I touch
> the exposed metal parts of the rca connectors, the hum goes away. I'm
> not suggesting that this is the fault of the SB3, but is there anything
> that people can suggest to sort this out (short of standing touching the
> connectors all the time!)? My limited electrical knowledge tells me
> that I have mains hum and that there is a grounding issue.

does this occur only when both speakers are connected to the squeeze box?
i.e. if you only have either the R or the L speaker connected and the other
speaker left unconnected to anything then do you still have a hum.

Also, how are the speakers powered? Is there a separate supply for each
speaker or are they daisy-chained together in some way?

Alex

pfarrell
2005-11-15, 16:11
On Tue, 2005-11-15 at 18:03 -0500, Mark Lanctot wrote:
> The question remains Pat, where could the ground
> loop be coming from?

Kinda hard to tell from outside the room.

> If there's nothing connected to the computer
> speakers other than the SB and nothing connected
> to the SB, where could it be coming from?

Are the speakers powered by the SqueezeBox? or
are they self powered using AC power. If there is
any connection from the wall jack to the speakers
and also from the walljack to the SqueezeBox to
the speakers, then there is a potential for a ground
loop. Or wall to amp to speakers and also wall to SB to
speakers.

It doesn't take much of a signal when you are amplifying
it for the speakers.


--
Pat
http://www.pfarrell.com/music/slimserver/slimsoftware.html

max.spicer
2005-11-16, 05:57
does this occur only when both speakers are connected to the squeeze box?

I'm not sure. I'll experiment tonight.


Also, how are the speakers powered? Is there a separate supply for each
speaker or are they daisy-chained together in some way?
The left speaker is daisy-chained from the right speaker. The right speaker has a input from a separate power supply.

Max

max.spicer
2005-11-16, 06:03
The speakers have an external power supply. This is plugged into a four way plug, which is plugged into the wall socket. The power supply plugs into the right speaker (which has the amp inside it) and a lead goes from the right speaker to the left speaker. My SB3 is plugged into the same four way plug as the speakers' power supply. Nothing else is plugged into the four way adapter. The SB3 is wireless, so has no other connections going into it. The SB3 is plugged into the right speaker via an rca interconnect, with an rca-minijack converter at the speaker end. I've tried using a cable with 2 rca at one end and a minijack at the other, and got the same hum.

After speaking to a techy guy at work, I'm going to connect one of the speakers audio inputs to ground (a radiator or the earth pin on a socket) via a resistor. This will hopefully have the same effect as me standing there with my finger on the connector.

Max


On Tue, 2005-11-15 at 18:03 -0500, Mark Lanctot wrote:
> The question remains Pat, where could the ground
> loop be coming from?

Kinda hard to tell from outside the room.

> If there's nothing connected to the computer
> speakers other than the SB and nothing connected
> to the SB, where could it be coming from?

Are the speakers powered by the SqueezeBox? or
are they self powered using AC power. If there is
any connection from the wall jack to the speakers
and also from the walljack to the SqueezeBox to
the speakers, then there is a potential for a ground
loop. Or wall to amp to speakers and also wall to SB to
speakers.

It doesn't take much of a signal when you are amplifying
it for the speakers.


--
Pat
http://www.pfarrell.com/music/slimserver/slimsoftware.html

Alex Twisleton-Wykeham-Fiennes
2005-11-16, 06:11
On Wednesday 16 November 2005 12:57, max.spicer wrote:
> Alex Twisleton-Wykeham-Fiennes Wrote:
> > does this occur only when both speakers are connected to the squeeze
> > box?
>
> I'm not sure. I'll experiment tonight.
>
> > Also, how are the speakers powered? Is there a separate supply for each
> >
> > speaker or are they daisy-chained together in some way?
>
> The left speaker is daisy-chained from the right speaker. The right
> speaker has a input from a separate power supply.

Is the speaker power supply earthed?

Alex

Roy Owen
2005-11-16, 08:04
On 11/16/05, Alex Twisleton-Wykeham-Fiennes <alex (AT) fiennes (DOT) org> wrote:
>
> On Wednesday 16 November 2005 12:57, max.spicer wrote:
> > Alex Twisleton-Wykeham-Fiennes Wrote:
> > > does this occur only when both speakers are connected to the squeeze
> > > box?
> >
> > I'm not sure. I'll experiment tonight.
> >
> > > Also, how are the speakers powered? Is there a separate supply for
> each
> > >
> > > speaker or are they daisy-chained together in some way?
> >
> > The left speaker is daisy-chained from the right speaker. The right
> > speaker has a input from a separate power supply.
>
> Is the speaker power supply earthed?
>
> Alex


Good point. You might try using a 3 prong adapter or ground lift. I've seen
this problem many times at live music events. Usulally it happens when the
mixer is at a differnt ground ponential than the instruments. Using a ground
lift almost always solves thsi problem.

pfarrell
2005-11-16, 08:09
On Wed, 2005-11-16 at 10:04 -0500, Roy Owen wrote:
> Good point. You might try using a 3 prong adapter or ground lift.
> I've seen this problem many times at live music events. Usulally it
> happens when the mixer is at a differnt ground ponential than the
> instruments. Using a ground lift almost always solves thsi problem.

Testing a ground lift is OK. Using a ground lift violates the electrical
code for a reason, it defeats a safety design.

Running your mains power without the third wire ground is a really
bad idea.

--
Pat
http://www.pfarrell.com/music/slimserver/slimsoftware.html

radish
2005-11-16, 08:38
I think Max is in the UK. Over there it's much harder to disable earth on an earthed plug, you'd have to take the plug apart and unwire it. In some cases even that isn't possible.

Alex Twisleton-Wykeham-Fiennes
2005-11-16, 08:58
On Wednesday 16 November 2005 15:38, radish wrote:
> I think Max is in the UK. Over there it's much harder to disable earth
> on an earthed plug, you'd have to take the plug apart and unwire it. In
> some cases even that isn't possible.

If you have a sealed plug, then use an extension cable that doesn't have a
sealed plug. Lift the earth on the extension cable and then place this
between your sealed plug and the mains.

Just don't forget which cable you did it to and not put it back as this should
be for testing only - once you have isolated the problem then you can try and
work out a safe(r) way of making a sustainable solution.

Alex

Deaf Cat
2005-11-16, 09:01
Hummmm

Maybe way off subject, but I borrowed a Nordost Blue Heaven interconnect to try at home, fab sounding cable although it seemed to generated a hummmm and quite loud at that.

Would you believe I wrapped it in tin foil and the humm lessened, could not believe I was sitting on the floor in my lounge foil wrapping a stereo cable.........

I did think I could try PTFE tape, but thought it would be a nightmare getting it off and the shop would probably not like their Blue Heaven fully wrapped in Plumbers Tape For Everything.

Hummm just a thought...

docbee
2005-11-16, 09:27
I think the sb is grounded indirectly by the ethernet. You should use a not-shielded ethernet-cable to avoid this. Have a look at the ethernet-cable, if there is metal around the plug it is shielded.

Look at the jpeg attachment. The grey one is schielded, the yellow one not.

fuzzyT
2005-11-16, 09:30
Deaf Cat wrote:

> Would you believe I wrapped it in tin foil and the humm lessened,

That hum probably wasn't from a grounding issue, it was probably RFI
(Radio Frequency Interference). An unshielded cable can act as an
antenna of sorts and pick up all sorts of stuff. The foil acted as a
shield. Grounding one end of the foil might work even better.

--rt

max.spicer
2005-11-16, 12:45
I've just run a cable from the left interconnect to the earth pin on the plug for the speakers power supply. I've also put a resistor in the middle of the cable. There's probably lots of reasons why this is a very stupid thing to do (which is why it's currently unplugged!), but it cured the hum nicely. I suppose my problems come when the cable becomes detached from the earth pin and wanders over to live or neutral...

Max


On 11/16/05, Alex Twisleton-Wykeham-Fiennes <alex (AT) fiennes (DOT) org> wrote:
>
> On Wednesday 16 November 2005 12:57, max.spicer wrote:
> > Alex Twisleton-Wykeham-Fiennes Wrote:
> > > does this occur only when both speakers are connected to the squeeze
> > > box?
> >
> > I'm not sure. I'll experiment tonight.
> >
> > > Also, how are the speakers powered? Is there a separate supply for
> each
> > >
> > > speaker or are they daisy-chained together in some way?
> >
> > The left speaker is daisy-chained from the right speaker. The right
> > speaker has a input from a separate power supply.
>
> Is the speaker power supply earthed?
>
> Alex


Good point. You might try using a 3 prong adapter or ground lift. I've seen
this problem many times at live music events. Usulally it happens when the
mixer is at a differnt ground ponential than the instruments. Using a ground
lift almost always solves thsi problem.

BBobley
2005-11-16, 14:46
I used to use a pair of "computer" powered speakers with my Squeezebox. It hummed the same way as you described. They were very cheap speakers that came with my Dell computer. Last month, I replaced the computer speakers with a much nicer pair of Edirol powered speakers and the problem disappeared.

Not sure if this helps, but I thought I'd bring it up!