After the success detailed in the last post, I wanted to do the same for our car. Unfortunately, it was not quite as easy. The cost of the last one was $0, since I had all the parts. This project is not finished, due to a line-level pre-amp on order.
After taking out the radio (which requires some disassembly of the dash) here is the circuit board of the stereo. The CD player connects via a small ribbon cable (which has to be undone to take out the circuit board). Notice the solder points labeled L-CH and R-CH, for Left Channel and Right Channel. These are out outputs from the CD player to the stereo, that I want to hack into.
However, the CD player outputs signals in the 5 Volt range, not in the 2 Volt range (normal headphone output). While music does play, the volume has to be turned up so high, that the static and noise floor overpowers the music (it sounds really bad).
After some head scratching, I remembered I had a small amplifier circuit I used to drive a speaker from the headphone output of Aubri’s old Apple computer (its speakers could not be heard over the loud fan). Here is that circuit:
This simple op-amp circuit worked beautifully with a 9V battery in the Camry. With the potentiometer (the variable resistor with a knob) maxed out, the sound was great. Aubri can verify that. However, hooking this circuit up to the car stereo’s Vcc (12V) proved its doom. It would only have been mono sound, since a pre-amp would be needed for both channels.
I ordered this pre-amp off e-bay: Pre-Amp RCA Input / Output Line Driver Signal Amplifier through the seller uneeksupply at a cost of $13.97 including shipping.
Here’s the rear of the stereo with the wires coming out the back:
The CD player is on bottom. The red wire is 12V Vcc. The black wire is the signal ground. Of the 4-strand wire, only the red and green wires are used, for left and right channel inputs from the headphone cord.
Hopefully, the pre-amp will bring the 2V line level up to 5V CD line level, so the static noise will not be heard. I burned a CD of digital silence to play while sound came through the headphone input. In my simple test, with the test amp, it sounded like CD quality to me. I am quite impressed, even though it’s not a finished project yet. With the hard work done, it should be a simple matter of hooking up the wires and mounting the pre-amp securely. Hopefully.
The plan is to listen to mp3’s and podcasts from my mp3 player as I drive around on visits. One can only take so much sports radio or political commentary or modern hit music.
