markitzero said:
A Furman won't fix your problem. Make sure all your gear is on the same circuit, and ground lift the power strip or whatever is powering your whole rig. Use those cheap-ass little 99 cent ground lifters from Home Depot. They are a must-have in audio. You should be fine.
Whoa there. Always lift shields on audio cables if you have to lift a ground. You should NEVER lift the mains ground on a piece of gear. There are two reasons why lifting a ground on a power outlet is a bad idea:
1. It is a safety hazard. Ignoring the trouble you could be in if you got caught doing such a sloppy wiring job, you could also get a nasty electrical shock when you touch the gear. That hum you're hearing is AC current being dumped into the ground plane of your gear. Only a trickle of it is ending up in your audio, but the amount of leakage in whatever piece of gear is responsible could be significant. Lifting the ground isn't getting rid of all that hum. It is simply denying it a place to go. That means when you touch the right piece of gear, it goes through you.
2. Most gear was not designed to work that way, and you can cause all sorts of problems, particularly for computers. You'll be completely wrecking the EMI/RFI certification of the machine (probably violating dozens of FCC regulations in the process), and you may significantly increase the frequency of crashes and data corruption as a result.
You're probably right that the gear is on multiple circuits, but even if it isn't, a lot of devices have poor grounding in their power supply designs. It's not particularly uncommon. (See my rant about this below.)
I am assuming that the headphone amp is an HP4 that came free with the FIREPOD. If so, the ground is floating (wall wart). The input is likely coupled through a resistor to that floating ground. If your computer's ground plane is noisy (and what computer's ground plane isn't?), you're probably dumping a good deal of noise into the headphone amp.
Don't get me started on wall warts. Digital gear notwithstanding, they are pure evil, as they basically mean that the gear cannot be properly grounded. If the gear is hooked up to a dozen things, you're probably okay. For a headphone amp hooked up to only a single device, it's asking for trouble.
As for the amplifier, it is probably responsible for dumping tons of hum when you hook it up. The ground lift switch is a good fix for that. The mere fact that this works tells me that some bonehead who worked on the amp's design thought that it would be a good idea to make the signal ground be separate from the chassis ground.
Yes, I know that separate grounds are common in audio gear, and it is F*&(ing idiotic. The result of such a design is that any hum induced on the ground like will consistently get carried through the shield on your signal cable to any device whose ground isn't broken by design, during which time it is basically inducing hum on the signal line. (This is my #1 rant about hardware design. Ground your bloody grounds, people. The excuse that you don't want power supply noise to leak from the chassis ground into the signal ground is a copout, and indicates that the chassis ground should simply be grounded better.
)
Sadly, most gear these days doesn't have a ground tab, and the HP4 is no exception. Same with the FIREPOD. But there is hope.
ANYWAY... Back to the question.....
The easiest way to ground these devices properly is to wire yourself a 6" pigtail with a 1/4" plug on one end and a jack at the other end. On the
plug end, add a 10AWG wire alongside the audio cable. Solder this to the ground lug on the plug. Put the cable together and plug it into one of the jacks. Build one of these grounding pigtails for the FIREPOD, the HP4, and your amp.
Once you have built these pigtails, for each physical location, tie the pigtail to a true earth ground by hooking the 10AWG wire to the ground prong on an electrical plug and plugging it into a wall outlet. If two pieces of gear are nearby, you can connect the pigtails together.
By hooking these devices to a common ground, you are basically ensuring that any electrical noise in the ground place of the device will take the path of least resistance out of your rig instead of ending up in your audio.
