Car Stereos: audio interference, rca cables, resistance measurements

Question
Hi,

After alot of messing around I still cannot find out how to solve this,
Ive got a 05 t-sport yaris and using the standard headunit along with a toxic 500w amp and two 6x9s.
I get a humm sound though the 6x9s and it gets bad if the gain is on full on the amp, Im getting the humm when the engines on and off, with the volume off thats when u can hear it most. I took the earth wire off the headunit today and tried putting it in a better place but still the same, When the radio was off the ground measured 0.04ohms on the multimeter but when the radio was on it came upto 0.6.7ohms! but when i turned the headlights on it went to 0.8.7ohms , Is it the rca's that could be messing the signal up or is it a earth problem? thanx

Answer
Hi Danny,

From what you're describing, I doubt that the noise you're experiencing is caused by a ground issue.  It's possible that it's induced noise in the RCA's, generated by some nearby "noisy" vehicle component or conductor; but I'm more inclined to suspect a defect in some part of your audio system.

The first thing I'd try to do is isolate the problem.  First of all, make sure that the noise is coming from both of your 6x9's, not just one. If you're hearing noise from both speakers, then try disconnecting the RCA connectors from the amplifier and see if the noise continues.  If you still have a hum, it's likely that there's a problem with the amp; if not, then the trouble may be in the RCA cable itself, or whatever it's connected to.  

You said you're using the standard head unit.  Do you mean the stock radio?  If so, where are the RCA cables connected?  Does the head unit have RCA pre-outs, or are you using a line-out converter?

Don't put too much faith in the resistance measurements of your ground point.  Very few multimeters are accurate at measuring resistances below 1 ohm.  When you're measuring a conductor that has current flowing through it--which is the case when you turn on the head unit or headlights--then the resistance measurements are even less trustworthy.  

Thanks for your question.

Brian