PJK3

disconnect your RCA's from the amp. if the hum goes away which it likely will, it's RCA or HU.

double check your HU ground. since it varies w/ volume, i'm guessing the HU is your issue (it has to be a pre-amp loop)


if it stays, check amp grounds.

imdarkrider

I have to disagree with PJK , theres no sound going into the amp when you unhook the rcas therefore of course your not going to get a humming noise , and when your headunits volume is turned to zero , it has the same effect as disconnecting the rcas , it's not sending any voltage through the rcas when the radio is at zero. If you didn't have a humming noise before you put in the amp you currently have then it's not the radios ground.

Brennon

Correct...I've had the headunit in for over a year. I'll take a look at the amp grounds and try to separate the power from the rest of the cables.

PJK3

you can disagree w/ me all you like. you CAN have a ground hum w/ RCA's disconnected from the amp. i've seen it plenty times in person, and i can probably dig up a few threads here that had that exact circumstance.

in my opinion, troubleshooting ground loop hum problems when you have an external amp starts with 1 simple step: disconnect the RCA's from the amp.

if the hum stays, the ground loop hum (and thus questionable ground) is in the amp to speaker section. if the hum disappears, then the problem is in your RCA's and HU. this step effectively cuts your troubleshooting process in half. (which is what i said if you double check my post)


and turning your HU volume down DOES NOT in any way simulate disconnecting the RCA's. what it DOES do is remove the signal source (and probably the pre-amp) from the loop. if you've got a power wire leaking over to your RCA's, you'll still have a hum (volume down or not). whether the HU is sending a voltage/current down the RCAs, the EM field flux around the power wire will induce a voltage and current in the RCA's... that's simple physics. (especially if they are unshielded or the shielding is broken)
and if the insulation and shielding is penetrated and you're grounding to chassis... that will cause a hum as well (not covered by 'turning the volume down').


now, if the volume DOES control the ground loop hum, there are 2 choices... you've either got a pre-amp hum (which means the HU is the leak) OR if you're using an input source (such as a changer, MP3 player, etc) your hum could be coming from a ground loop issue via the accessory.

in his case, he hasn't mentioned an input source, therefore the HU is likely the problem.


i repeat my recommendation:
check the HU ground and reground if needed.

and if you want me to expound on it, verify that the outer connector on the HU's output RCA's (the reference) has a common ground and continuity w/ the HU chassis and ground. (make sure the resistance is very close to 0 ohms)


--
and to address the fact that the HU has been installed for a period of time and there hasn't been a hum...

there could be 2 reasons for that... he damaged the HU or altered the ground while installing, or that's simply because previously, his amp and pre-amp were grounded to the same location (b/c they were the same unit). there was no distance difference in which a ground loop could establish.

a major reason a ground loop hum occurs is due to a voltage drop caused by the inherent resistance that exists between the two grounding points (which causes grounding through the RCA's instead of the chassis) if his amp ground is solid (which my RCA test would verify), then his HU ground must be marginal.


here's another good troubleshooting site: http://www.bcae1.com/audiots.htm

*edited for clarity*