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

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kyson lin
How to feed audio signal to car stereo volume controller (LC75421M)?
on Apr 19, 2017 at 9:37:58 am

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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Ty Ford
Re: How to feed audio signal to car stereo volume controller (LC75421M)?
on Apr 20, 2017 at 9:00:43 pm

Hello Kyson and welcome to the Cow Audio Forum.

Richard Crowley may be the best person to address your questions. He'll probably be along soon.

If he can't, I suggest you try the goggle group Scott Dorsey or Mike Rivers there can probably provide answers.


Ty Ford
Cow Audio Forum Leader

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Richard Crowley
Re: How to feed audio signal to car stereo volume controller (LC75421M)?
on Apr 21, 2017 at 12:18:48 am

It would be much easier and faster to do this with the schematic diagram of the car stereo you are trying to hack. We don't really know where in the circuit you are injecting your aux input. According to the data sheet you cited, the LC75421M device actually has 10 inputs (5 Left and 5 Right channel).

If your signal level is too low, then maybe you removed too much of the tape preamp circuit. There could be several causes of distortion, but impossible to diagnose without better details of the circuit. It is quite likely that the different signal levels from different sources (radio, tape, etc.) are "normalized" in firmware in the car stereo. It is programmed in that the tape preamp is perhaps x.x dB higher than the radio, so when you switch inputs, it automatically adjusts the gain. This makes it easy for them to do in software, but nearly impossible to hack after the fact.

As I said before, there are actually five stereo inputs on that chip. But the ability to select any more than the ones already implemented are locked away in the firmware of the car stereo. You can see the pin numbers of the inputs on page 3 of the data sheet.

I don't see anything like your circuit fragment in the data sheet? C2 is a crude low-pass filter to keep noise out of the audio. R1/R2 form a fixed L-pad attenuator to reduce the signal level, and C1 is a simple DC-blocking (AC-coupling) capacitor to keep external DC from interfering with the chip's internal circuits, and likely to allow the chip internally to use a "virtual reference ground" of ("Vref") of VDD/2 (around 6.2V)

You will see in the data sheet that Sanyo recommends 1uF for "C1" as the DC-blocking input capacitor.

Note that this is really an electronics question and not really very appropriate for this forum. The best forum on the internet for electronics is I don't think there's much traffic anymore on Usenet (, for the last few decades.

Recording audio without metering and monitoring is exactly like framing and focusing without looking at the viewfinder.

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