How to suppress ground loop hum without reducing audio gain?
I added an aux input to my car's factory head unit by soldering wires onto pin 4 (right channel) and pin 33 (left channel) of the M62490FP chip as per the forum instructions. For ground, I soldered a wire to a part of the metal casing, which I suspect is the problem. These wires then go to a female 3.5mm aux input.
Problem is the input is noisy and playing music from a phone while charging from the 12V DC output produces a terrible ground loop hum. To address this, I added a ground loop isolator component between my phone and the chip. This fixes the hum, but kills the gain, making it too soft. Circuit from the forum (except for ground):
Now I'm stuck with a loud noisy system, or a clean radio that's way too soft. As I understand it, a ground loop is caused by kind of resonance or improper grounding.
Does it matter where on the head unit different components are grounded? To my limited knowledge, it should not matter as long as there is continuity between circuit nodes.
The car is a Subaru Forester 2007. Complication is that I am essentially overriding the CD mode and need a silent audio CD to not interfere with the aux input. Other hobbyists report this working perfectly.
Circuit Diagram:
Best Answer
That's the reason why you can use the built in differential amp in this IC, the pins are 2, 3, 34 and 35. Connect your L and R signal to the cap of pin 2 and 35, Gnd to cap of pin 3 and 34. These pins seems to connect to somewhere, so be careful when you decided to cut trace to isolate them.
When you plug in your 3.5mm plug and charge at the same time, you should not hear any hum. However do note it needs to be grounded to work and currently it's grounded to the USB Gnd pin. I don't guarantee it to work if you decide not to charge and play. There would be a switch to use the differential amp, that you have to find out how to enable it.