μblog: engineering from the trenches

The other week, the tape deck part of the head unit in my 1999 Jeep broke down. It turned out to be a belt drive and the belt snapped in its old age. The initial consequence was that the tape reels would not spin and cause the tape deck to think that it had reached the end of the tape, it would then change direction and find its self in the same position, and then it would finally conclude that there was some error and bypass playing the tape. The secondary consequence was that I could no longer listen to XM radio or my MP3 player using the tape adaptor. The drive from Washington, D.C. to State College, PA became long and tedious.

The first solution would be to replace the broken belt, but that would take a long time and would not be very much fun. Using an FM tuner would also be out of the question since the FM frequency space in most places is fairly saturated. I figured that since I haven’t listend to FM radio in a few years, there was no need to start now. I decided to add an audio line input to the stock head unit at the expense of FM radio.

Most modern radio receivers use a demodulator chip for the FM radio part, mine used a Sanyo LA1862, which has left and right audio output channels. The chip ran on a +9V supply and the audio outputs were centered around +4.5V, so my audio output signal would also have to be biased to be around +4.5V. The circuit I designed (schematic) takes the inputs from a 1/8 inch audio jack and loads the right and left channel with a 50 Ohm resistor. This is to ensure that the circuit gets a correct audio signal regardless of the drive circuitry, so even open collector outputs would work. Next, the left and right channels are taken differentially using a pair of INA105 diff-amps. This is to give a little bit of isolation and prevent any possibly ground loops caused by listening to something that is powered by the car. The reference pins on these chips are driven by a +4.5V reference which is generated by using a voltage buffer and a resistor bridge. Since the op amps and diff amps require 10V to operate, they run off the +12V power while the resistor bridge for the +4.5V reference is between the ground and +9V power line. The radio board had a LM7809 voltage regulator so it was easy to pull off the +12V, +9V and GND.

After putting everything together and testing it, I cut the traces from the LA1862 FM demodulator and hot-glued the designed circuit board to the original radio board and replaced the metal cover. Since I used SOIC components, everything fit together nicely. The end result is that the signal on the 1/8 inch audio jack is played through the sound system whenever FM radio is selected on the head unit regardless of the channel it is tuned to. The sound quality seems to be a bit better than the tape adaptor.

I am sure that there are more efficient ways to add this line input, so if you know, please feel free to share.

Update: While sleeping on this, I think that adding a low pass filter on the output of the additional board would not hurt. Even a single pole passive filter.  There was a low pass filter implemented on the original radio board so that is why not having it on my board didn’t have much of a negative effect on circuit noise performance.

( la1862.pdf )

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