good sound but low level interference crackle

[filmshow]

I am showing DVD and BluRay films through a Del laptop and a projector, sound going into an external 'simple' amp (so I can alter treble and base but that's it). Everything is great apart from a level of interference when there is no (or very little) sound on the film... it sounds like there's something on the laptop being picked up by the amp. If you listen through headphones it's clear. Is there a piece of hardware or software I can run to identify and remove the problem?

Thanks

[quaxo]

Exactly what kind of amp is this? Are you talking a full blown receiver with large speakers or is this like a subwoofer with speakers?

Also, are you sure the speakers/amp/subwoofer are shielded?

How is it connected to the computer? Through what kind of cables?

[filmshow]

OK there's going to be a limit to how much detail I can give you as it's not all my kit, but here goes:

Using the 3.5mm headphone jack from the laptop, which is the SigmaTel High Definition Audio jack as defined through the HDMI properties control panel.

The amp is a little unknown, the venue will not let me play around with it.

cables are 15m RCA->RCA stereo audio cable

I've no idea if there is shielding, can I put something in place to negate it if there is none?

Speakers are 1 left 1 right, I would call them large speakers, 70cm high, suspended from the ceiling...

Is that enough info ??

[westom]

cables are 15m RCA->RCA stereo audio cable

I've no idea if there is shielding, can I put something in place to negate it if there is none?

  Is noise picked up by cables?  Disconnect cables from laptop end.  Short the two conductors at the laptop end.  Turn up amp volume to maximum.  Does noise exist?  Then cables may be the source.  If not, cable is not a reason for noise.

  How is this amp powered?  AC?  Notice that headphones have a different power source.  A concept called ground loop is why laptop and amp must share the same power source.

In a recent confernce of EMI/RFI engineers, noted was so many were bald.  These kind of problems are more difficiult, require patience and are made more difficult if just wildy replacing things.  Expect to do much head scratching. 

[filmshow]

if I unplug the cables the sound goes away, so I think it's the laptop... they have separate power sources.

I just want to run a box in between the two to eliminate the noise... but which box???

[westom]

if I unplug the cables the sound goes away, so I think it's the laptop... they have separate power sources.

I just want to run a box in between the two to eliminate the noise... but which box???

  I said disconnect the cable and short the cable at its far end with amp on highest setting.  If you only disconnected cable, that says almost nothing (headphone test already did all that).

  Separate power source?  They you must connect to the same power source.

  Which device?  Prepare to be balding.  From what you have posted, nobody can say which.

   Is the power to each two wire or three wire?  What exactly is this amp?  Electrically connected how?  Does it have a chassis ground stud?  Notice what I am doing.  Desperately trying to extract details that were not provided.  Appreciate the perspective.  Everything connected to both amp and computer are relevant.

You must list everything.  Any USB devices?  Internet connection? Etc.  Appreciate the concept called ground loop.

[filmshow]

I can feel my hair falling out as I type, there's no way of knowing what you're asking, they guy at the venue won't let me into the office where there are a few knobs a whistles to play with, which was why I was hoping to bring a box into the room under my control and negate anything thereafter. I've no idea how many wires power the devices, I assume my laptop has an earth.

I don't think I'm going to get an answer here, I know too little about the venue... I'll have to go back and try and get a discussion going with them again, I'll have to wear a hat though...

[westom]

I've no idea how many wires power the devices, I assume my laptop has an earth.

The laptop would not have an earth.  It might have a safety ground - that third prong.  Due to wire distance between that wall receptacle and earth, then the wall receptacle is electrically different from earth.

  Same is also why all devices best shar a common power source.  Distance between two receptacles in the same room but on different walls means receptacles are electrically different; could therefore create a ground loop.

[filmshow]

OK well that's not impossible to fix, I can run a lead from the socket box as the amp...

[quaxo]

If it is indeed a ground loop problem, there are solutions for that:
http://www.radioshack.com/search/index.jsp?kwCatId=&kw=ground%20loop&origkw=ground%20loop&sr=1

...should you not be able to get them on the same power source.

[filmshow]

could I test the ground loop problem by running the laptop from battery power?

[Gizmologist]

I have the same issue on my computers. The noise is NOT a ground loop. Ground loops will be at a 60hz frequency. The noise floor of the audio amp is relatively high but with source material such as music, the noise is negligible.

I design systems for use with computers that balance the output and match the earphone or line out impedance to either 150 ohm mic level or 600 ohm line level for coupling to commercial sound systems. I have run into this several times- more with PCs than laptops. The noise we hear is induced from the motors of the fans. They run on the same rail as the op-amp that drives audio output section.

One solution I found is to take small ferrite sleeves and run the fan power wiring around the ferrite cores at least once. Due to the short length of the wires, these may need to have short extensions spliced into them.
You should never tie the output of a computer audio card directly to Hi-Z lines (RCA style cables) feeding an amp. Audio  isolation transformers should always be used. These prevent any common mode noise between the units.

Hi-Z audio cables should also NEVER be more than 20 ft long. The capacitance in the cable degrades the audio and the cable itself can pick up all kinds of racket including AM radio. Cable runs longer than 20 ft should be converted to balanced transformer isolated runs than back to Hi-z if necessary.

[westom]

I have the same issue on my computers. The noise is NOT a ground loop. Ground loops will be at a 60hz frequency. The noise floor of the audio amp is relatively high but with source material such as music, the noise is negligible.

  The most common ground loop is 60 Hz.  But that fan noise is another example of ground loops.  Most common ground loop create a 60 Hz hum - but not all.

  Gizmologist has described solving ground noise (also a ground loop) inside the box.  Discussed previously was addressing the problem, first, outside the box.  But again, this can be a balding experience.  Noise sources that never cause a problem can suddenly be problematic by circuits changes far from the noise source.  Therefore one might be totally confused by noise eliminated over here that was really created by a fan over there.

  Generally, such problems begin by suspecting problems outside the box.  Then moving inside the box as information dictates so.  These problems are not solved quickly.  Many times, the solution is to fix the symptom - not cure the problem.

[Gizmologist]

If the noise changes pitch rapidly or sounds "rough" it is definitely an induced or generated noise on the rail.

If it is a steady humming sound that is either a "dull" sound or somehwhat "sharper", that is a ground hum usually. The difference in tonal quality is a function of the type of input impedance matching.

Does the sound change as the drive engages but before music starts?

[filmshow]

the sound is a crackle, not a clean humm or anything consistent, as soon as there is sound in the film you can't hear the background noise. It doesn't seem to be linked to drives starting up or the DVD playing - if you eject the DVD it still makes the noise...