iPhone Lightning cable vs. Aux output.

[birdy67]

I just discovered something while referencing a mix in a car yesterday. I was playing the mix through Dropbox streaming on an iPhone 6 using the lightning/USB cable to plug into the car stereo system. The mix sounded very compressed/limited. I could actually hear "pumping"... the whole mix ducked down when a loud snare came in for the first chorus. I didn't think I had squashed the mix that much so I checked it in my studio and the mix didn't sound compressed at all. I then went back to the car and played the very same file through Dropbox but used the AUX input/headphone out from the iPhone. The mix sounded way different...much clearer and less compressed. A bit brighter than using the lightning cable...but totally free of that limited pumping sound.
Anybody have any insight on this? It seems to me that if the car stereo itself had some sort of limiting algorithm built in, it would affect both the AUX input and the lightning cable...but maybe I'm wrong about that. Has anyone else out there experienced this?

[markjazzbassist]

when using the 1/8" jack out you're using a cheap Chinese DAC (digital to analog converter) from the apple iphone. When using the lighnting cable to you are bypassing the built in apple DAC and your using the DAC in the car stereo. Based on your listening it appears the DAC in the stereo is an even cheaper Chinese component.

Best case is if you check mixes a lot in your car, to get a small DAC of quality converters and run that DAC straight into the line in jacks on the back of the car stereo. you will need to rip out the car stereo to find the line in jacks.