Buzzing when I connect external speakers

[Ludatha]

Hey guys,
I have got a 120watt amp for my guitar, and I love playing music through it with my iPhone, it works perfectly and sounds great, it uses a standard headphone jack.

When I connect the amp to my PC, in either the front headphone or rear headphone jack it also works, but I get a loud buzzing through the amp, I have normal PC speakers connected all the time and they don't make that sound when plugged into my PC.

I am running Windows 7 Ultimate 64-Bit and my sound card is built in RealTek HD Audio.

Any suggestions?

[Major Payne]

You need to provide a ground between the ammp and computer and use cables which are highly shielded. In fact, you need to check to make sure there is no potential difference between the metal case of your computer and the amp as you might just blow something.

An isolation transformer usually is used to isolate connections like you want and to prevent ground loops which cause the buzzing you hear. Maybe try to use the line inputs/outputs instead of the higher level microphone/headset connections you are using.

I remember having to repair a Peavy amp one time because the a/c lines they were plugged into was incorrect and it literally blew pieces of the ics out in one amp when someone tried connecting two amps together (a big no-no the way they tried).

[Apostropartheid]

Although first put your phone well away from the speakers. Not sure whether this will work because you hook them up to your phone already but it's worth a shot.