vis_croceus wrote:
Contrary to the conventional wisdom, the factory HU/CDM provides a good signal source for aftermarket amps and speakers.

With the appropriate amp, you can use either the Bose or non-Bose CDM as a source for an aftermarket amp (with the Bose HU).

To use a Bose CDM you need an amp that can handle low-level inputs (off the top of my head I think it was around 0.6V) and to use a non-Bose CDM you need an amp that can handle speaker-level inputs.

I chose to go with a non-Bose CDM because the HU controls respond in a more normal/linear manner with this CDM.

Klaus wrote:
I have a theory about the need for the resistor mod on later model C4s to prevent SYS codes due to the dimmer circuits becoming open when an aftermarket stereo is installed.

I have owned 2 1996 models. One was a base model with the manual A/C. When I replaced the stereo with an aftermarket stereo the flashing SYS warning started, and it took the resistor mod to fix it.

My current 1996 CE is equipped with the electronic A/C. I recently replaced the stereo with an aftermarket unit. The SYS warning is NOT happening, and I did not need the resistor mod.

Looking at the FSM wiring diagrams, the electronic A/C also uses the dimmer circuits that the head unit uses. I suspect that is the reason that I did not need the resistor mod on this car.

So, my theory is if you replace the factory stereo with an aftermarket unit in a late model C4 Corvette:

If the car is equipped with the manual A/C option, you will need the resistor mod.
If the car is equipped with the electronic A/C option, you will not need the resistor mod.

vis_croceus wrote:
(from an old post of mine on another forum...)

I finally got around to putting a scope on these things.

Based on what I've read here about using the Bose CDM output into aftermarket amps, as well as the difference I can hear between the two, I was expecting to see some ugly equalization in the Bose CDM.


This is what I found















('MaxV' is at the highest voume setting that doesn't clip. For Bose this is at 100% volume. For non-Bose at about 75%. 'LowV' is a lower volume setting, around 50% of MaxV. All are with tone controls at neutral.)

So, based on frequency response, the Bose is a much better signal source.


However, I also found that the signal out of my non-Bose CDM is perfectly clean, while the signal from my Bose CDM is an ugly mofo. I see both periodic amplitude variance (with low frequency input tones it looks like an AM signal) and also a settling of the amplitude over time.


Based on this, I have to assume that my Bose CDM is bad, but on the other hand, since people have reported since the beginning of time that the signal out of the Bose CDM is crap, and since it clearly isn't the frequency response, maybe what I'm seeing is the problem.

If anybody has a Bose CDM they don't need I'd be interested in checking another one to see if it performs the same as mine.


There are some interesting control quirks with the Bose CDM. When you turn the bass control up, it cuts the mid frequencies. With the knob up all the way it boosts 30Hz by 30% and cuts 1kHz by 30%. The non-Bose behaves in a normal fashion, just boosting low frequencies.

The other quirk is with the fader. With the Bose CDM, fading 100% to one direction only reduces the opposing volume about 50% (a WAG from listening - didn't actually measure this). With the non-Bose CDM you get the more normal behavior of turning off one set of speakers when the control is in an extreme position.