Nexus 7 Head Unit Installation – Part 9 – Distortion

Complete Circuit

About a week ago I finally had all of the components arrive (with a couple extras I had to pick up) to get me full circle and complete what I think would be the first revision of a fully functioning Nexus 7 head unit. The last things to arrive were a left-angle USB cable (since the down-angle cable was slightly too large) and I had to pick up an add-a-circuit fuse tap for my car. There were a couple of little items too, like heat wrap, but no other major components.

With everything connected as per my latest revision of the wiring diagram:

Attempt 2

I decided to plug it in to my PC power supply test bench (which… is just an old PC cut open and the power supply completely cut apart). I wanted to test as much as the final system as possible, but because I don’t have the other end of the wiring harness (the end that would be attached to my vehicle), I kind of had to bypass the Enfig harness for the most part. Things that wouldn’t be tested with this setup

  • Steering wheel controls (must be provided over harness)
  • 12V turn on signal over Enfig harness
  • Ground line over Enfig harness
  • Audio through my car speakers (… obviously, but worth mentioning because it doesn’t answer whether or not I need a pre-amp)

Annnnnnd it actually all worked. My relay successfully turned on, powered up the headunit and peripherals, and I was able to play music through the DAC. I tested the music directly from the DAC with a set of headphones, but because i had to splice a cable to connect the DAC to the Enfig harness, I figured I should probably test that too. The cable going to the harness looks like:

Y Audio Cable

I couldn’t quite get my headphones connected to the end of the Enfig harness, because obviously it doesn’t have a headphone jack on it… But I *WAS* able to hear audio if I positioned the connector the right way on the pins on the harness. And yeah, I did test for continuity with a multi-meter before doing all of this 😉

Time to move to the car!

First In-Car Trial Run

With confidence that my system works on my test bench, it was time to try it in the car.

  • I ripped the headunit out with the Enfig stereo keys (getting way more comfortable with this)
  • I only disconnected the airbag light and the main wiring harness from the headunit (just to make things quick)
  • I added my add-a-circuit fuse tap to a 10A fuse at the bottom of the panel
  • That’s it 🙂

Great! Time to add some power and see if I start any fired.

Turned the key to accessory…

Power! Just like on the test bench, things started lighting up for the peripherals and the tablet came to life. The first thing I wanted to test was the steering wheel controls since those had never been tested from the actual vehicle through to the tablet.

  • Volume up: Check. Android interface shows the volume slider changing.
  • Volume down: Check. Android interface shows the volume slider changing.
  • Mode up: Check. Poweramp switched to next track.
  • Mode down: Check. Poweramp switched to previous track.
  • I didn’t try the other buttons for voice yet.

Next, was audio! I’d be covering a bit more untested ground with this because I couldn’t guarantee the audio was perfect coming off the Enfig harness… I just knew that I could get some sound.

Silence.

The first disappointment. Damn. I checked the volume and it certainly looked like it should be audible from the tablet. So… I took my headphones and connected to the Y-cable I made (I knew there was a good reason for that!).

Distortion.

What?! Why was it distorted? Was it too loud? A quick check proved no. Okay, so maybe my crappy cable I butchered got a bit mangled in the move down to my car. So I connected the headphones directly to the DAC.

Distortion.

Uh oh. This has been a can of worms for me now for the past week (albeit not actively working on it in the car during the week, but pondering it).

Ground Loop Distortion?

From my DJing days, one thing seemed familiar about this… distortion caused by a ground loop. Now, I couldn’t for the life of me figure out WHY there would be a ground loop in my setup in my car versus in my kitchen on my test bench, because the only thing that changes is power coming from the fuse panel (instead of the power supply) and ground on the Enfig harness (instead of the power supply).

But maybe that was it! Maybe the ground on the Enfig harness wasn’t totally right. Again, entirely speculative because I don’t even know if this is a ground loop in the first place. I decided to modify the wiring so that I could avoid the Enfig ground and use ground from the vehicle’s chassis. Once I proved it worked on the test bench, I brought it allll back down to the car. Powered it up. Got my headphones directly in the DAC.

Distortion.

So… It’s not the ground from the Enfig harness (which was hard for me to believe in the first place). I’ll likely connect back to that because running another long cable for ground is crappy.

Insanity…

I’ll admit that when I started writing this post I didn’t have the solution to the problem I was looking at, and I was hoping to record my research here and finish off with a nice explanation of what was going on. From the time I started writing this post though, I spent around two full days over the weekend troubleshooting what was up. I had scenarios that would work perfectly, and then suddenly go back to being distorted. I’d try different inverters, completely remove inverters, change up grounds, remove components, etc… Some times it would work, and sometimes it wouldn’t.

So did I find a solution after a whole weekend of troubleshooting? Not quite… Sorry. Did I figure out what was causing the problem?

Yup. And the secret? It has nothing to do with the inverter.

JoyCon. This was the problem the ENTIRE time. When my tablet boots up and JoyCon is plugged into my USB hub, my audio goes completely distorted. If I unplug the JoyCon and plug it back in, things work perfectly.

What’s The Next Step?

Well now that I know the JoyCon is the root of the issue, I’m going to go do some more research. My first thought is using a time delay relay to make the hub come on after the tablet is fired up. I’ll get one of these ordered (because I’m pretty sure it will resolve the problem) and I’ll get an update posted once I make some more progress!

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