Ground loop problem ("digital" noise) with an external Firewire interface: how to sol

[negopus]

I have the following setup at home: PC with external Firewire interface, Yamaha S90 ES, Mixer.

I have a "digital" noise when both the PC audio interface and the S90 ES are powered on and connected to the mixer. The noise I call "digital" is clearly from the PC circuitry, as it changes if I load a program or move the mouse (it is not the noise that appears if you connect a digital audio cable into an analog input). The noise disappears if I disconnect the PC interface from the mixer or just power the PC off. The noise also disappears if I disconnect the S90 ES from the mixer. If I power off the S90 ES while the computer is on, the noise continues.

I suspect it is a ground problem. So I connected the AC power of all my gear in a "star" topology and added an AC stabilizer before the "star". But I still have that "digital" noise.

I suspect I have to resort to some form of decoupling (DI box or decoupling the FireWire interface from the PC using a 4-wire cable). I don't like the idea, because it removes the symptoms only, and does not solve the whole problem.

Any suggestions?

[negopus]

My audio setup is now made up of FireWire devices only:

• Windows XP PC with a FireWire card using a Texas Instruments chipset
• M-Audio FireWire 410 audio interface (with external power supply)
• Yamaha S90 ES synth with mLAN card (powered from mains)

All the peripherals are powered from the same AC outlet.

When I connect all the peripherals to an audio mixer I hear, in the audio output, a “digital hash” noise. A ticking or tweeting noise, especially when I move the mouse or start a new program. This noise is due to a “ground loop”.

I read that to in order to solve “ground loop” problems one has to “break the loop”, preventing ground levels in the different devices from contacting each other.

In order to “break the loop” I created a FireWire 6M-6M cable with just 4 wires. I cascaded a FireWire 6M-4M cable (with just 4 wires) and a FireWire 4F-6M adapter. Using this cable I connect FireWire devices with a 6 pin connector, but only with the 4 signal ping. The DC voltage and the ground don’t go from a device to another. All the devices are powered, so they don’t need to get the DC voltage from the FireWire cable.

When I connect the devices using this cable I no longer hear the “digital hash” noise. So the “ground loop” problem seems solved. But the FireWire devices do not work with cable. The PC doesn’t even recognize them.

Is it possibile that FireWire devices, when connected to each other, are able to “expose” their features and tell if they have a 4 or 6 pin connector? If two devices at the ends of a cable have a 6 pin connector, but they don’t see the DC voltage because they are connected using only 4 wires, can they stop working? I think this is not likely to occur. If this happened, all the FireWire 4 to 6 pin adapters would make no sense.

Why do FireWire devices, having 6 pin connectors, not work when they are connected with only 4 wires, so that DC voltage and ground are not connected?