Problems Connecting Laptop To PA / Amp

[mrchu]

When connecting my laptop to various guitar amps and PA's I've had the same problem each time, I get feedback which seems to be the equivalent of PC background noise. It changes when you open folders and carry out tasks on the laptop...it's as if you can hear the laptop think....I've had this problem when using the onboard laptop chipset, an Audigy2 PCMCIA and an M-Audio FireWire solo....do I need to connect the laptop to a pre-amp or something prior to connecting to any form of amplified speakers or PA? Or is it something to do with cables? Or perhaps I need a very specific type of amplifier for the laptop?

This problem does not occur on typical PC speakers (GigaWorks S750, headphones etc..) so I know the system is working fine...it's just a case of me doing something wrong or missing a required piece of hardware.

Any help is greatly appreciated

[peterb]

You are picking up electrical noise from the internals of the PC. You don't hear it on typical PC speakers because they are pretty low quality and pretty insensitive, but you are feeding it into a sensitive amplifier. You aren't feeding it into a guitar pick up input are you? If you are, then it is no wonder you are getting problems - the impedance matching will be wrong for a start, and it will be FAR too sensitive. You should be using an 'ordinary' auxiliary input on the guitar amp, or feeding it via a mixing desk with an aux input.

[mrchu]

Cheers, I've managed to resolve the issue by running the audio signal through a numark mixer before it hits the Amplifier, there's no excess background noise now

thanks again

[peterb]

Excellent!

("A satisfied customer - we should have him stuffed" ... Basil Fawlty)

[Rastinny]

Greetings
I seem to have a simmilar problem connecting my dell latt and sony viao to a PA. Howling noise of hard disk and other electrical noise goes comes trough as soon as the jack is pluged in the laptop. The noise reduces in loudness when power cable is plugged out the laptop and it is running on batteries but the sound stays bad. No problem with a pc or with any other devices. Asked in sony shop and have been adviced to buy external soundcard. The external sound card is plugged in to the usb drive but this does not remove or reduce the noise at all. As soon as the jack is plugged into the external sound card the noise reappears. I have not tried to connect the external SC though optical connection to the PA because my PA has no opttical input. I was also suggested to buy a special extension lead that is supposed to 'ground' better but have not yet tried it. If any of ye have an Idea or have the same experience (specially DJ's) I'd be grateful if ye would share this.

All blessings,

Rastinny

[peterb]

It is interference generated by the laptop internals (anything ifrom the screen inverter - does the noise reduce if the screen blanks? - to other internal inverters, timer chips etc. There is noise being generated by the external PSU, as the noiser reduces when that is disconnected.

One way to tackle it would be with a special audio transformer that would electrically isolate the laptop output from the PA input - but they aren't cheap., and may not cure the problem if it is radiated interference. (One way to determine that is if the lead is unplugged from the laptop, but held near it - but that isn't conclusive on its own).

The PA probably has a balanced input, compputer do not have balanv]ced output. Some form of audio balun might also work (cheaper than a high quality audio transformer. You also need to look at the impedance of the PA input. Generally these are low, but if you are using a high impedance input, that can cause problems.

As I said in an earlier post on this revived thread, these problems can be very difficult to track down and fix.

[Rastinny]

Thank you very much for your response! I agree the problem is very difficult to source and to solve. Last saturday I went out and bought a €65 extension lead supposed to 'ground' better and eliminate the noise but this has also failed to produce an improvement. The noise is relentless and I'm now out of idea's. The last thing I may try is to buy a new power adapter cable. This will put me back about €150 so I'd rather not but...
Any other suggestions are still welcome

Question? Is there a certain measurable range of noise which an audio jack from a laptop is expected to produce? How do you measure this, what is the unit of measurement and should this be specified in the computers technical specifications.
Could it be that the amp and the speakers I use are simply too sensitive and are made to process audio of a different input range?

How expensive is the transformer you suggest and how does it actually work.

The audio input at my mixer is analog (red and white).

Thank you for responding!

All Blessings!

[peterb]

There is no spec (afaik) that states how much noise an audio output should produce. Laptops are particularly bad - plastic cases which reduce shielding, lots of high frequency components in a small space, onborad soundcards - again with poor shielding. Sadly it is difficult to tell waht make or model will be troublesome or not without trying. (I have a panasonic that is pretty good - just pot luck)

It could be that you are overdriving the amplifier, particularly if the sound is distorted. You need a line output (not often found on laptops) and a line in at the mixer. Worst combination is headphone out, microphone in on the mixer. If you have to use a headphone output, the volume on the laptop may need to be reduced.

An audio transformer (you need one per channel if you using stereo) isolates the signal earth from the amplifier signal earth, so it can reduce the risk of earth loops that can be a source of interference (series mode interference) Using balanced lines (which it seems your mixer doesn't have) reduces common mode interference.

as for cost, anything from a couple of punds upwards - professional quality (very flat frequency response etc) are very expensive.

For a selection, look here

Maplin > audio

Note that these are components and will need mounting on some sort of circuit oard, and enclosing in a case. You would need one (per channel) of the more expensive ones (about £7.00 or £10,00). The cheap ones are for use in audio amplifiers.

Googling Audio transformers will give a few other sources of supply and more information.