Low level hum?

I was just curious.  When you hook up an amp and sub to a factory headunit through an LOC and use the vehicles accessory wire as an amp turn on, why do you get a low level hum when the car is on and radio turned off?  It only happens sometimes from what I can gather but i've never known why it does that.  My only line of thought is that since the radio is off tne there is no input voltage to the LOC, hence no input signal at the amp and maybe there is some kind of feedback problem.  Anybody who can explain this to me would be very much appreciated

its not an always thing. i've done many setups with LOC's and never gotten a hum. its getting some kind of interferance somewhere.

Its not on a car that I am working on now or anything, I have just seen it in the past and wanted to know why it happens.  Most recently I saw it on a car that I did a remote start, sub and amp in off of the factory deck.  I didn't hook up the accessory wire that powers the radio at first so when I remote started, the car was on meaning the amp was on but the radio wasn't  and there was that noise, once you put the key in or once I hooked up that wire, it worked fine with no noise.  Every other time I have seen it that has been the case, radio off = noise, radio on = no noise.  If there was interference due to poor cables or wiring than it would show itself all the time right?

my bad!Prove your connections, use a meter!

I promise, I'll behave!

Now you're getting a bit out of my realm. I will defer to the older...oops, I mean more experienced members The Flakman

I feel strange. I have deja vu and amnesia at the same time.

John | Manteca, CA

ha ha, nice.  I tend to think a bit too deep in to things, can't help it.  By the way your signature is now my headline on myspace, thats funny.

All I know for sure is that I have never had this problem using the exact same equipment with an aftermarket radio.  Thats why i think it has to have something to do with either the LOC or using accessory as an amp turn on.

Close, but I believe it has to do with the regular speaker wire that connects the LOC to the factory wiring.  These wires, if not twisted, will act as antennae (so to speak).  Use wires that are twisted together to prevent this interference.  The area behind the deck is a hot spot for picking up noise, so even short lengths of connecting wire should be twisted, even if they are just the little wires that came attached to the LOC.  These are input signal wires prior to amplification, not speaker wires, and that is why they should be twisted and regular speaker wires from the amp don't need to be.  (Twist + and - around each other.)Build the box so that it performs well in the worst case scenario and, in return, it will reward you at all times.