Axiom Home Page Forums General Discussion Technical Questions the dreaded vibration is back on the ep800

How about turning everything off at the circuit breaker box except the circuit the Subwoofer is on. If the hum goes away, turn them on one at a time until the hum returns, then it has to be something on that last circuit turned back on.

Just to clear up what that screw actually does. It isn't a "ground loop screw", it's a point of connection between the chassis (safety) ground, and the signal ground.

A device that has a 3-prong electrical connection connects the chassis to the third prong, so any shorts are shunted to ground rather than finding the path there through a person touching it.

RCA cables need their external shield connected to the chassis ground for best noise rejection. But that shield is also used as the signal ground for the audio being carried on the cable.

If everything is working right, there should be no problem tying the two grounds together. But quite often things aren't right. So it worth trading the lesser noise rejection for separating the signal ground from the chassis ground to break a possible ground loop.

XLR cables solve this problem. They have an external shield which should always be connected to the chassis ground, and a separate ground pin for the signal ground. When using an XLR cable, the connected devices should never tie the two grounds together (thus why Axiom has the removable screw).

I'd also like to take a moment to rant about so called "double isolated" electronic equipment with only 2-prong connections. I hate this trend to suppose that what ever design the engineer has come up with will always fail safe, and thus not need a safety ground. It is also not able to provide a true chassis ground for shielding, making any XLR jacks less useful (like in Onkyo's pre-pro line).

EDIT: Of course none of this gets you any closer to solving your problem. I just like to hear myself type sometimes. (It is a nice sound on this Unicomp, buckling spring keyboard.)

I have unplugged everything that could be on the same circuit. still a low hum and vibrating subs.

no dimmers nor any fluorescent lights.

The fact that the problem did not go away when I moved the equipment to another room and again with trying a different receiver is (mis?)leading me to believe that it has something to do with the sub...

Just wish I could find my ground chassis screw so I could check that.

Did you try what I suggested, 3 messages back?

Catbrat - Only saw the message just after submitting my last post. I'm going to try that now and see what happens.

Do you get any humming or vibrations from the sub with it just plugged into the outlet (turned on) without the cable connected to the sub and AVR?

so I've turned everything off except the sub and receiver... and the hum is still there. so I tried changing outlets: plugged the receiver and sub to another outlet that had its own circuit (the fridge), and still there.

then I unplugged the receiver from that outlet and plugged it in another one and still there...

I'm about to throw in the towel!!

what about the vibrating subs? is that normal when power is on and connected to receiver?

okay, I just disconnected the cable from the sub and when I touch the cones there is still vibration felt(albeit quite a bit less, but it's still there.. the level of vibration seems to be related to crossover setting: 150 will vibrate more than at 100) but if I connect the sub cable it's much stronger with a louder hum... could that mean it's the sub then? because there is no power anywhere else in the house... The only thing receiving current is the sub.

I would have to wait for Axiom's response on that one.

You can't diagnose a ground loop just by shutting off the components. You have to physically disconnect each component, one at a time. As an example with my most recent ground loop, I diagnosed it/eliminated the hum by disconnecting the component video cable that was hooked up from the DVD player to the television.

Any type of audio/video player you should disconnect one by one as well.

Maj, you've gotten good info on ground loops, but as I indicated in my previous reply, from your description it appears that there can't be a ground loop. For a ground loop to be formed there has to be at least one power connection path to the sub besides the direct connection through its power cord. This could be be through a plugged-in receiver or something connected to the receiver which in turn is plugged in to the wall, such as a player. Neither this nor any other subwoofer connection to the power other than the subwoofer power cord is present, and one connection can't make a loop. There have to be grounds to two areas having different electrical potentials so that the difference in voltage between them causes a small current to flow, with a resulting hum.

No, when the sub is plugged in and turned on, but not connected to any signal source there should be no cone movement or sound audible. The rectifier and filter capacitors in the amplifier power supply section should remove any audible noise coming in through the power cord. Possibly the repair to the sub amp was ineffective, or it's been somehow damaged since then.

As to the sub cable itself(not connected to the receiver at the time?)causing the problem to worsen, this may be reaching, but possibly it's acting as an antenna picking up airborne electrical interference to add to that coming in through the power cord and not being adequately suppressed by the amp power supply section. Tough one!