Why does my head unit distort at such low volume?

So I just picked up this head unit last Friday (http://www.crutchfield.com/p_105KDHDR60/鈥?/a> and installed it. I fired up my test tone CD(and played a sine wave @ 1kHz, normalized to 0dbfs) and was only able to make it to about 40(of 50) volume before hearing distortion.



Now, the setup of my car is very non-standard. It's changed three times:

At first I had a low level signal run direct from an Ipod to the trunk through RCA cables, but I had to crank up the gain on the amps so much that all of the interference and alternator whine was more prominent than my music.

To overcome the problem of all this interference, I installed an equalizer in the dash, which had a built in 20W RMS amplifier(normally you would connect your speakers to this.) I set it up to where the speaker level (high voltage) signal was going to the trunk(running through the same type of wire used for RCA cables - braided outside with a small wire inside) then through a line level converter(http://www.crutchfield.com/p_142SLC4/Sco鈥?/a> , and finally through very short wires into the power amp. This allowed me to keep the amplifier gain all the way down, maximizing the ratio of signal to noise. This setup was able to get extremely loud without distorting or having any interference. The down-side was that I was feeding the EQ/first amp stage with an Ipod(which is made to feed headphones), and the impedance mismatch was detracting from the quality and frequency response of the sound. (most low end and midrange was gone - sounded like when you plug a pair of headphones in half way.)



And thus I thought "Hey I should just replace the EQ with a head unit that plays MP3s."

So I did, and much to my dismay, it sounds WORSE AND NOT AS LOUD as it did before. I attribute the loudness issue to the fact that I cannot turn the head unit up all the way without it distorting. The "Worse" part is the distortion itself - after all, what company would put out a $170 head unit that distorts before even hitting full volume?



To test the distortion, and rule out that any other component could be causing it, I turned the gain on the line level converter all the way down, and the gain on the outboard amplifier all the way down as well. I then turned up the head unit until I heard distortion(starts around 38/50 volume - totally unbearable past 42/50 volume.) Note that the sound coming out of the speakers was VERY very low in volume(since the amp gain was down,) but still distorted. Just to be sure, I turned up the gain on the amplifier, as well as the line level converter, and still heard the same thing - the same distorted sine wave, just a bit louder. The tone(harmonics) of the distortion did NOT change - which is a dead giveaway that neither of the two knobs I adjusted are responsible for it - they are just magnifying the distorted signal being passed through them.



So, at this point, I have a few ideas as to what could be causing this distortion:

1. The cables that carry the speaker level signal to the trunk are designed to carry a ground(0) and hot(+) signal(unbalanced) and not two actively driven signals(+ and -). Possibly the difference in resistance between the two conductors of the cable are causing the head unit's amplifier to freak out as it is easier for it to push on one pole than the other. I doubt this is it, because the equalizer I had in there before was dirt cheap (no name Japanese product from a flea market) and did not have any issues. With that being said, maybe the EQ was so cheap that it did not have two actively driven terminals - and the cable was actually being used as intended - with a ground and a positive connector. I doubt this is the case, as the left front and left rear, as well as the right front and right rear shared a ground on the wiring harness - note that the front left and front right(or rear left/rear right) did NOT share a ground - only the speakers on the same side of the car did.

2. The head unit is expecting to see a 4 ohm load (a regular car speaker) but is instead seeing the resistance of the line level adapter, and is thus passing more current than it is designed to - causing distortion. I doubt this is the case, as I wired up a 4 ohm 25w resistor across the + and - speaker wire, and did not get any different of results. I have yet to try hooking up a regular speaker to the head unit.

3. This head unit is designed for people who want everyone else at the stop-light to hear what they are listening to, regardless of how badly clipped and distorted it sounds, and love nothing more than to feel like they have "out bumped" someone else. You guys know who I am talking about...



Tomorrow I am going to try connecting the HU directly to my speakers to see if the results are any different.



What do you guys think?Why does my head unit distort at such low volume?
1. is there any chance the speakers are wired out of phase.. or reverse polarity?



2. is there any chance the stereo head is damaged or not good enough?



3. check it out.. and replace components as needed.