It's likely because radio broadcast is not flat, but tailored to sound better in a car. It also depends on the car and the stereo in it of course. I find different stations sound different in their mix in general--rock will boost the bass usually, etc. In your case it seems like you have the right "mix" with your car/stereo and broadcast and not so much with the CD.

Then of course, maybe the CD player just isn't that great. Poor converters maybe, output stage sub-par. Again, it's not easy to figure it out with car stereo as it's all just crammed in there, but that's what I'd guess if it were my situation.

Hope that helps a little, though aside from upgrading I'm not sure I have any solutions for you.

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I have a Kenwood KDC-215S CD player in my car. When I listen to the radio on it, flat settings sound great (bass:0 treble:0). For some reason though, when I switch to CD mode, the flat settings sound terrible. It sounds very bright, and the vocals often sound piercing. There is also nowhere near enough bass. First of all, any idea why this might be? It doesn't make sense to me that it sound fine flat on radio mode, but not on CD mode. Secondly, what would you suggest I do to fix it?

First, it is unlikely that what is labeled as 'flat' really is flat since the source, speakers and car acoustics are coloring the sound. Numbers on a dial are only relative.

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To specify, I know how to adjust bass and treble. My question was more on the theory side. What should I listen for when adjusting each settings (bass, treble)? I especially have trouble working with the treble. What is a good thing to reference when adjusting it? Lastly, I was wondering if it's better to boost levels or reduce? For example, if there is more treble than bass, would it be better to boost the bass or reduce the treble? Thank you in advance.

Listen to what you normally listen to and, apparently, find somewhat lacking. Adjust to taste as there are no absolutes here (as you have already found out.) If possible, and if it suits your taste, it is better to cut than to boost since the latter will also usually entail a reduction of power available in the rest of the spectrum and a consequent reduction of dynamic range potential.

Kal

Kal and db gave good advice.

The nice thing about this unit is each source can be programmed for the desired tone adjustment you like.

If ALL your cd's sound a little thin you could adjust the controls to what you like and set it for that source, which won't affect the radio setting.

My experience with car stereos is that cd's are just as variable in sound quality as they are on my home system. Kenwood has a pretty good rep with the car crowd, btw.

Good luck.