Base audio system - want to kill the built-in bass/treble boost

[kscott]

This is for my 2009 Altima sedan with the base audio system. I was using the Aux In jack, and it sounds like the head unit is applying a lot of boost to the treble frequencies and the very lowest bass frequencies. There seems to be no way to adjust the Bass/Mid/Treble controls to get a flat frequency response from the unit.

For the treble part of the problem (the more bothersome of the two), initially I thought it was just that the dash tweeters were too loud, and started trying some resistors to put in series with them. For one of my other cars, I have a set of component speakers for front doors & dash, and their crossovers are provided with a jumper to adjust the tweeter volume; reducing their tweeter volume by 2 dB made a world of difference to getting them to sound good. But with the Altima's stock system, you can completely unplug the dash tweeters, and still, the excessive treble is coming out of the front-door speakers. The treble can be made to sound its best (i.e. reasonably OK) by using my Ipod's "Treble Reducer" EQ setting, along with turning the Altima radio's Treble control to around -7 or -8. But that's a really excessive amount of signal processing that's needed, and probably ends up still having a peaky / non-natural response.

For the bass part of the problem, the Altima radio's Bass control is pretty useless. If you turn it down enough to get the lowest bass under control (and it's really not quite able to do that), you've lost all of the mid-bass.

Now that I've noticed this with the Aux In jack, it's apparent that the same built-in equalization is being applied to the radio. Haven't tried the CD player yet; I assume it's affected also.

Does anyone know if there's a way to set the Altima's audio system to not apply this built-in equalization?

Or, is there a way to condition the audio that you feed into the Aux In jack? I have an old Alpine 7-band EQ that I could wire into the middle of an Ipod-to-Altima cable, but that's probably not narrow-enough bands to compensate for what the Altima radio is doing. Maybe a patch cable could be made with some sort of RC filter to roll off the lowest bass frequencies. It seems doubtful that a passive circuit like that would have a sharp-enough cutoff, but at least its frequency could be somewhat matched to the boost that the Altima radio is applying.