How to feed audio signal to car stereo volume controller (LC75421M)?


New member
Username: Faceu

Post Number: 1
Registered: Nov-16
I'm trying to hack AUX input into car stereo with tape mechanic. I managed to identify the most important circuits. The idea was to use tape mode to amplify custom audio input.

The original wiring was something like this.

tape head -> preamp -> volume controller -> amplifier

So I removed the preamp and soldered my signal wires to pre-amps output. I also added a switch which makes stereo to think the tape is inserted. So I can also use a radio receiver.

And it works. But the there are to things which bother me. The signal level of my input is lower then output of radio receiver. So I have to adapt the volume when switching between radio and AUX.

The next thing is that sound is actually a little bit distorted. Maybe some frequencies are missing or something. So I guess my wiring is not right.

I also noticed that there are some empty holes and unused labels on PCB. Probably for actual AUX input. I identified the circuit and it should look something like this (if it was there).
Indeed I don't know values since the parts are not actually present on the PCB.

So I thought I should use something similar to feed my signal to the tape input of the volume controller.

Can someone help me to understand what is the purpose of parts in the circuit? And what values to use?

I guess C1 should filter DC component? There are some capacitors in similar positions and have values of 2.2u so could I use the same?

And the R1, R2 acts just as voltage divider? So it's for adjustment of signal level? So something in kOhms?

And the C2 is mystery to me.

link to the datasheet of volume controller:

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