Any audio/hi-fi experts out there?

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Active Member
May 16, 2004
South Devon UK
C250 TD
Hi folks,

Can anyone tell me why when I connect my pc to either of my home audio systems, (Quad and B&O) the sound quality is superb, however if I connect, using the same leads, cables plugs etc to a public address system or group type amplifier there is lots of background noise that sounds like the processor, ie the noise varies when you press keys etc. This makes it totally un-listenable!

Iv'e been asked to provide the music for a friends 60's type bash soon but having tried a couple of PA type systems I can't get them to work.

Any comment / suggestions appreciated.


most PA's use balanced line signals - a three pin system.

i assume that the amp you are using also has a two 'pin' system (jack or phono)?

i have rigged my pc up to our syetem ( - go to insanity rally and scoll down) and it worked fine.

a suggestion is that the pa amp isnt shielded rigth - try earthing the case of the amp using a separate cable? the 'third' pin of pa amps is a shield wire...

earth to ground first - if that doesnt work, earth to the case of the pooter (remove HDD first just in case the amp is playing up and blows up pooter at least you will still have the data on the hdd - very very unlinkely - but not worth the risk :eek: )

or go buy a wireless media controller...
Hi GuyD yes I am trying a two pin system over a three pin sys, I'll try that and report back. Thanks for your help


Hi Guy,

Problem sussed, thanks to you, but not sure why! Given your comments re earthing tried case to case plug to plug etc but no improvement, however remove earth from amplifier . . . silence so removing earth from mains plug lead to amp cures the noise but does this make the main amp dangerous?

Will try running amp via, "floating" rcd wil see if noise still missing.

Thanks for your suggestions


oh the joys of ground loops.

link up the case of the amp to the case of the player and the shield of the cable...?
I'd put the earth back If I were you. I used to do a lot of what you might call "gigging" in my youth, Claptonesque and Led Zeppelin stuff. Removing the earth was a way of removing a thing called "earth loops", it worked but you have no protection. Is the noise low freq hum or loads of hiss and distortion?

If you are going straight into a PA amp the inputs on those are for microphone (verrrry low) level signals, your PC will be outputting (relatively high) line level thats why it sounds nice of the home HI-FI which is designed for such levels. See if there is a mic/line level swithch for the channel you are using and switch it to "line" or see if there is a specific LINE input.

DONT attempt to use the XLR plugs/sockets in any way as sometimes they send whats called phantom power back up to the input device, 48 VOLTS, so you would kiss goodbye to your PC.

If you put a nice line level signal into a music amp, say Marshall or Vox, again they are designed for low level signals, guitar pickups, the gain section in the amp is massive and results in distortion and bags of noise, buts that sometimes what you are after with Jimmy Page stuff. Nee d to know if its hum or hiss/distortion though.
it's going to be down to one of several things but I'd suggest you check the leads you are using.

Presumably you will be running from the line out on your computer to two channels on the PA (assuming it is a stereo PA you'll pan one channel left and one right) then use the channel gain to match the pre-amplifiers to the level your computer is sending.

Many PA amps/mixers will have a dedicated tape/line in, this will probably be the best channel for you to use - check for "return" or "tape" channels on your mixer :)


Hi guys,
thanks for you suggestions etc.

From the earphone socket on my laptop I am using 3.5mm jack plug to RCA type phono plugs into a dedicated cd/tape input on a Dynamax 125 watt pa amp.
Without changing the lead just changing the amps causes the problem, earthing case to case or plug to plug or plug to case makes no difference.
It is definately the cpu that you can hear until disconnecting the earth on the amp!
I don't know if I have a line out on my laptop, (Compaq Presario 2100 bought Sep 2003) that of course may make a difference!

you probably don't have a line out on the laptop, so turn the volume down to about 10%.

The dedicated CD input won't like the levels on headphone out so you are probably better using the normal channels which will give you the ability to trim the input level.


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