A common hassle for mobile audio is induced noise. When audio grounds and power grounds are combined, there is a potential for noise to be injected into the signal. Several customers have noticed a noise issue, especially while their audio source is charging. Let’s say we are using an iPhone for our audio input, and we have the headphone jack connected to the aux input, the iPhone ground is considered to be floating, and we have minimal noise. This is because the iPhone ground is connected to the stereo’s audio ground.
However, our iPhone battery is running low, so we stick our inexpensive USB car charger on the phone, and suddenly we are treated to unwanted noise. To explain this, let’s look at the car charger first. It is likely using a switched voltage regulator to step down the 12 volts from your car to the 5 volts for the USB charger. A typical switching voltage regulator such as the MC34063 is inexpensive, but because of the low switching frequency requires the use of relatively expensive capacitors and inductors. Since the designer of the car charger didn’t know the exact amount of power that your iPhone might take from the charger, they made calculated tradeoffs with the values of components used. And since this charger only costs $1 at Target or Walmart, only the cheapest suitable components are used. The end result is that most USB car chargers are noisy devices, both in the electrical signal as well as unwanted RF interference.
To top that off, there is some level of resistance in the car charger wiring, and current is flowing through this wiring to your phone. This resistance means that the ground potential on your charger is not exactly the same as the audio ground potential, which means that current will flow through your audio ground, causing this noise issue you hate.
The solution to this problem is an audio filter, which uses transformers to pass only the differential audio signal to your stereo, rather than trying to maintain a common ground. Consider as well running power/USB cables away from your audio lines. I also ensure that my source volume is high without causing signal distortion, as the signal to noise ratio is higher, requiring less amplification. This will reduce the amount of noise amplified through your stereo.
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