iPod nano 4 ground loop between headphone out and USB charger - Page 1
Forum section: electronics / general

author: RoGeorge
author_rank: Super Contributor
timestamp: December 08, 2018, 08:29:03 pm
content: There is some strange ground loop that happens if the power amp and the iPod are powered from the same power supply (+5V for USB and +12V for external power amplifier, with common GND). I couldn't find and break the ground loop, and ended by supplying the audio signal through an isolation transformer. I guess the ground loop is somewhere inside the iPod. It does not happen when the USB charging cable is plugged, but only when the iPod recognizes the USB as a charger, which makes me thing something shitty happens with the audio GND and the digital GND inside the iPod. Also, I found online many iPod "ground loop breakers", which are nothing more but an isolation transformer for each audio chanel, transformer placed between the headphones out and the external power amplifier. I suspect some switches inside the iPod, but couldn't find any iPod nano 4 schematics, and had no will to investigate proprietary Apple design, so just added an isolation transformer and moved on.The setup was:SMPS for 220V AC to +12V (amplif), +5V (USB) with common GND for both +5V and +12V2 power wires (GND and +12V) from the SMPS out to the external audio amplifier4 wires USB iPod charging cable plugged into SMPS (+5V, GND, and 2 resistor dividers for USB D+ and D- in order to make the iPod accepting the +5V SMPS as an iPod charger)stereo Jack cable plugged into the iPod's headphone out and the external amplifier input (GND, shield and signal connected to the audio input of the external power amplifier)When I plug the USB cable, for the first 1-2 seconds all is OK, then the iPod recognizes the USB as a charger and then terrible digital noises happens. I can hear the processor, and the display very, very strong, the audio became unusable.If I disconnect the USB cable, or if I connect the USB cable to another power source (e.g. to the laptop) the audio stays clean.Anybody had the same problem trying to amplify the headphone out signal while the iPod is USB charging from the same power source as the external audio amplifier, please?

author: RoGeorge
author_rank: Super Contributor
timestamp: December 09, 2018, 07:31:11 am
content: Anybody knows where I could find the schematic of the iPod nano 4, the model from this pic? The only schematics I could find online were from an older model (probably iPod 2, looked more bulky than mine).I want to see how the headphone out ground is made inside the iPod, if it's a real GND or a virtual one, and how it is separated from the USB's GND.

author: tom66
author_rank: Super Contributor
timestamp: December 10, 2018, 10:47:03 pm
content: I've encountered the exact same issue with an Android phone and an iPod Touch 2nd generation.The issue is purely down to ground currents from the charging port returning through the headphone ground. There are two ways to resolve this. You can either isolate the USB with an isolated DC-DC converter, which was the approach I took, using a little 5V DC-DC module rated at 5 watts. Or, you can isolate the headphone output, I tried this first, but the isolators available despite claiming to support stereo, are mono only, at least for one of the devices I tried.