OK, first big thing to note on this car:
It seems different than some 2005's discussed on this board. I'm assuming that a production date of 6/23/2005 is a 2006 model. I don't mean to contradict anyone else's findings with another car, but this was a big enough surprise to me that we tested and re-tested, so I'm confident that this car was not what I expected after reading other forum member's comments.
This car has a fixed 2-channel line audio output between the factory CD player and the factory amplifier.
Not only was it fixed in level, it was huge in level. With a 1K sine wave test disc, I measured 2.45 volts RMS and 7 volts peak-to-peak with a Flike Scopemeter. The factory deck in an Acura sends maybe 1.5 volts peak to peak and good aftermarket decks usually send 4 to 5 volts tops. I had to change the input sensitivity of my NTI audio analyzer to measure frequency response on this preamp line, and it was flat as a pancake.
But the amplitude was not the big deal. The big deal was that it did not change with the volume setting. At all. Not a darn bit. We tested it with the Fluke Scopemeter and the shape of the sine wave didn't budge (nor did the numeric display)! To make sure the setup was good, I tagged a speaker output from the amp and IT changed hugely with volume, no problem. This seems to be a new development.
So we will be using speaker level converters and summing their output together and then running the signal into an EQ to correct all this factory equalization folderal.
With this car I never had the chance to solve the Uconnect/Nav audio problem (which is pretty weird). When you disconnect the CD player's audio output from the factory amp, you lose Uconnect and Nav audio too... but the wires from the factory CD player/nav unit show NO audio coming out the factory CD player at those times. No signal at all. And research shows that the audio from the nav brain and the Uconnect box go into the CD player and nowhere else, so when things are working normally, the audio signal MUST be on those lines between the Factory CD player and the factory amp. There's no other way for them to get there (assuming thay haven't started digitizing this signal and putting it on the CAN bus : ). So the CAN bus must be foiling attempts to retain those features and replace the factory amplifier, by killing the audio from them when the factory amp isn't working correctly (or something like that).
We will take the F 6x9 output (which is NOT high-passed at all - these door speakers get all the bass there is) and the front dash speaker output (which is high-passed at about 120, and possibly low-pass filtered at about 18K too, hard to say) and we will step them down and then sum them together. We will EQ the 18k rolloff and eq the dips in the midrange response. (The EQ we use here is a 4-band parametric.)
If you'd like to see the response curves of the signal sent to the factory speakers by the factory amp, see this:
I did not post the preamp out signal, because I ran out of memory, and because it's flat as a darn pancake anyway. Great signal to tap a Clean Sweep into - and with this car, you WOULD lose your volume control from the HU and the steering wheel.
Remember that pink noise, the test signal used here, is a randomized noise that is equal in energy at every octave, so it corresponds to how humans hear. It should measure perfectly flat on these speaker wires, and any deviations from perfectly flat are either crossover filters or equalization in the factory amp.
We are not implementing fade in this car - it does have R speakers, but we're not using a fader. The nav and Uconnect voice will come out of the L side speakers. If we were we would do it by adding a knob, since I don't want to mess with the rear-speaker equalization. If we wanted to ONLY have the nav and Uconnect voice to come out the L FRONT speaker, we would need two equalizers. Not in the budget, I'm afraid. We could run the rear speakers unequalized if we have to, but at present, we'll see how this works out.
I tried not to use any three-letter acronyms : )