Ok I know I'm not the only one with this issue, but yall know that humming when an ipod (or another device) is plugged into the aux in AND a a power source from the 12v outlet? Well the cause for that is a ground loop, in otherwords, the voltages at the radio/auxin/12v outlet are different, and that allows a small dc current to creep into the AC audio signal, ie the hum.
I've been trying to solve it for a few weeks now, but now I'm stumpt. I've been using a ground isolator (aka a small transformer) to break up the dc current in the signal, but I'm looking to fix the problem, not put a band-aid over it. They actually issued a TSB for the navigation equiped HL because of the hum. All they did was install an inline transformer to mask it. So here is where I'm at:
I'm trying to get everything equal to the radio main 12v+ and ground. So:
1) I disconnected the aux in panel 12v outlet power connections. I tapped into the main headunit 12v +/ground, and used a relay to feed that to that 12v outlet only when ACC is activated. So now that outlet matches radio voltage since they are technically the same wire.
2) I took a look at the aux's in signal sheild, and I have isolated the shield as the cause of the hum, I'm just having a hard time fixing it. Here is the circuit between the radio and aux in:
The top abbreviations (*1) are for nav units, the lower ones (*2) are for non-nav. This is for JBL systems:
AUXl: cable detection wire
ALl: Left + signal
ARl: Right + signal
ASGN: signal shield
AGND: shield ground
Now if you look at the circuit, the non-nav headunits ground the shield to the headunit it self. But for navigation units, the shield is grounded to an OEM ground on the chassis (and its not the same one as the navigation chassis ground). This is why only the navigation headunits had the TSB issued. So I took out my multi-meter to compare the radio ground and the shield ground to see if there were any issues, and there were. There is a 3.3 mV drop and a 17.4 ohm resistance between the two grounds, in theory, they should be 0 V and 0 ohm (or close to zero ohm). There is the source of your hum. Now I tried to ground the shield to the radio ground, but it actually made the hum louder. So I'm kinda scratching my head as to what to do now. If yall have any ideas, I'm all ears. This has been on my nerves for 2 years now and I'd like to fix it.