Amp install - EMF Noise?

[jules]

Hey guys,

I just finished installing a new amp in Malibu and I'm now getting noise whenever the amp is on with the engine running. Doesn't matter if the head unit volume is up or right down.

Any of you guys with some stereo experience know if I need to get some sort of filter to cancel out the noise?

Ta
Jules

[Locutus]

physically disconnect all inputs to the amp and make sure you have a good solid earth. if the problem persists you may need to buy a noise filter.

[jules]

Thanks. I think it's the RCA leads or the head unit earth. I unplugged the RCA inputs to the amp and the noise went away.

Jules

[Locutus]

no probs.
if either of those don't fix your problem, try extending your head unit earth to share the same earthing point as the amp (aka single point earth). an old audio electronics trick to eliminate hum.

[jules]

Yeah, I was just reading about ground loops. Sounded like a myth! Interestingly when I pulled the head unit to redo the wiring I noticed the earth lead had come fully out of the plug, so I soldered up a new connector and attached it to to the rear mounting bolt of the head unit. That may be the culprit?

In regards to the RCA leads being fed into the amp. There are 4 channels, and the hum goes away if you only have 2 channels plugged in. As soon as you plug all 4 in, presto here comes a quiet hum. It doesn't matter which 2 channels are plugged in (1-2 or 3-4) the hum is only three when all 4 are connected.

Jules

[Jeo]

Sounds exactly like an earthing issue. I'd be most suspicious of the earth for the amp itself.

Failing that you could be getting interference through the RCAs. Are they run near any power cables?

[jules]

Yes the RCA's are running close to the amp power lead (8 gauge). I don't think the amp earth is the issue, as when the RCA leads are unplugged the noise goes away.

What do you guys think about me doing the following?

Pull the head unit and then unplugging the RCA's from the head unit (whist still plugged into the amp). If the noise remains, then I guess it could be the RCA leads, if the noise goes away, then I guess it could be the head unit earth?

Sound like a plan?

[Jeo]

You can get interference from having power and signal cables run right next to each other. You said the RCA cables work independently so I doubt they'd be the problem. If you've got another set lying around (any quality will do, cheapest nastiest household ones will be fine) you could try running them straight through the cabin to see if power cable interference is the cause. My girlfriend's Prelude had a horrible whine when we got it and to test it we ended up running a cable out the window and to the boot. Would have looked really stupid to anyone who saw us.

Does the pitch of noise change with RPM? Is there any noise when the engine is off?

I'm guessing there wasn't any noise before you installed the amp? It is possible that the headunit ground could be the problem but it's very unlikely if it wasn't there before the amp was added (assuming you didn't damage the ground in the install process).

[jules]

You can get interference from having power and signal cables run right next to each other. You said the RCA cables work independently so I doubt they'd be the problem.
>>>Yes, it seems weird to me that running it in 2 channel is quiet but 4 channel is noisy. I can even swap from front to rear connections on the amp, and it makes no difference. Given that if I unplug the RCA leads from the amp the noise goes away I would assume the amp is OK. It is new BTW.

If you've got another set lying around (any quality will do, cheapest nastiest household ones will be fine) you could try running them straight through the cabin to see if power cable interference is the cause.
>>>OK, I'll do this whilst I've got the head unit out.


Does the pitch of noise change with RPM? Is there any noise when the engine is off?
>>>Yes pitch changes with revs, no noise with engine off.

I'm guessing there wasn't any noise before you installed the amp?
>>>Correct

It is possible that the headunit ground could be the problem but it's very unlikely if it wasn't there before the amp was added (assuming you didn't damage the ground in the install process).
>>>As mentioned when the head unit was out I noticed the ground wire had popped out of the head unit plug. Figuring this was a bit dodgy, I cut the tiny lead connector off and soldered a new eyelet on, and then attached it to the rear mounting/earth chassis bolt.

Jules

[Locutus]

the loose/disconnected head unit earth could have caused your issues. give it another go, i guess.
as for only buzz/hum only when all 4 inputs are connected, not sure about that one. theoretically all headunit outputs & amp inputs should share a common earth, so not sure what's going on there.

i don't think only unplugging the RCA cables at the head unit is a reliable way to determine if said cables are the culprit. i would test by temporarily running another set of RCA cables through the cabin as Jeo suggested.

by the way, are you testing by playing a CD/MP3 or the radio? i occasionally had alternator buzz in my NA, but only while listening to the radio and only when the engine was cold. i never bothered to track it down since typically listen to my own music.

[jules]

Hey,

I am testing by turning the head unit volume down to zero. I have tried this in all modes Radio/CD/USB and can still hear a faint sound. The RCA leads I have are from an amp wiring kit (Rampage brand) from SuperCheap bought a couple of weeks ago.

Jules

[jules]

Fixed it!

1/ I pulled the head unit and unplugged the RCA leads. No noise.
2/ Plugged in some cheapies I had lying around direct from head unit to amp. No noise.
3/ I then "un-routed" the leads that I was using that went near the main amp power lead and plugged them back in. No noise.

OK, re-route making sure to keep then away from any source of power and test. No noise!

I guess one of the sets of RCA's didn't like getting close and personal with an 8 gauge power cable.

Thanks for all your help tracking this down guys. Much appreciated.


Jules

[Jeo]

Yeah, some of the more expensive RCA cables have really good shielding and you can run them close to power leads, but I always just ran them apart anyway.

Good to hear you got it sorted.