Audi Symphony II Issues

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I recently bought a used stock Symphony II head unit for my Audi A4 and it's giving me three issues related to the audio output. Basically, my car's stock sound system has a 4 channel amp in the trunk powering all the speakers, this amp is driven by 4 pre-outs from the head unit. I have the rears tapped for a subwoofer amp, if that matters. With the original Concert unit unit I had no problems.

The first issue is that it distorts heavily at low volumes, if I turn it up past the 1/4 mark (ish) everything's fine again. The second is that the radio is for some reason quite a bit louder than the CD player, and it seems to have some serious distortion, but that might be because it's just louder than the CD at the same volume level making the distortion more noticeable. Finally, the biggest issue, is that after some time of playing (anywhere from 15 seconds to 15 minutes) I'll start to hear hissing, popping and crackling coming from the speakers. If I let it continue the actual audio will go very quiet suddenly and the noise will just get louder and louder gradually. If I turn it off then back on sometimes it'll get slightly better, and leaving it off for an hour or two brings it back to the original state.

Before the crackling the audio output seems perfectly fine, if I go past the volume at which it stops distorting I can't hear anything out of the ordinary. I've also noticed that it seems to happen sooner if I use the radio instead of CD player or mess with the tone and balance/fader controls.

I've never really dealt with diagnosing audio equipment, nor do I understand how the head unit actually functions on a hardware level so I'm at a loss here. The radio issue isn't really a big deal, I can live with it, but I mentioned it just in case it's related. The only thing I know for sure is that it's not the stock amp, I connected midrange speakers to my sub amp and turned off the LPF, I could still hear the low volume distortion. For some reason I couldn't get it to start crackling that last time, but I was in a bit of a rush and didn't have time to potentially wait 15 minutes for it to start again.

Could somebody give me some pointers on what to look out for or a starting point in diagnosing? I'm about to take the unit apart but I don't really know what to look out for or what circuit I should be inspecting.
What year is the car?

Have you confirmed the hissing/popping is coming from the speakers powered by the stock amp?

Are you sure the head unit isn't going bad? Can you put the old one back in to test or tap into the amp input wires and send an audio signal from a different source? You want to rule out the stock amp by confirming it can play audio from a different source without problems.

If I had to guess, it sounds like something is overheating. You can try opening up the head unit and seeing if anything looks burnt. If you have an IR temp gun you can also scan around for hot components.
The car's a '99, head unit is out of a '03 I believe. It's definitely the head unit though, the stock one works perfectly and connecting an aftermarket amp has the same issues. (It's all in the wall of text that is my first post, I don't blame you if you missed it) Overheating is a possibility, but just turning the head unit off for a few seconds makes the popping quieter for a bit, I wouldn't think that's enough time for anything to cool down.

Regardless I'm going to be tearing this thing down sometime soon, hopefully I'll be able to find something obviously wrong.
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.