distorted & hiss sound after "add aux-in" to my old car stereo

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distorted & hiss sound after "add aux-in" to my old car stereo

I'm very sorry if there's someone else
already posted the same topic like mine.

but i've searched back to 2004 (car-audio),
with only got one result which similar like mine :

i've tried my old car stereo SONY RX-1700
which has no cd changer option
by cutting the wire from cassette's
head unit (which goes to the board).
there are 3 wires, for LEFT-RIGHT&GROUND line.

so i put a dummy cassette, connect the cut wire
to my nokia cellphone with stereo male cable,
then i play the mp3 from the cellphone.

it worked, but my problem is : distorted sound.
1. if i turn-down at lowest volume on the cell,
and turn-up my car stereo volume a bit, i got
a very noticable hiss sound noise, and
the distorted music is there but not much.

2. if i put 1/3 volume on my cell,
then the hiss reduced, but more music distorted.

3. if i put more than 1/2 volume on my cell,
then i got a completely distorted music,
even in a low volume on my car-deck.

4. if i feed the dummy cassette, but not plug the
male cable to my cell, turn-up volume on car stereo
and then tap the body cable, i hear "dep"
from the speaker. what did i do wrong ?

i just wonder, is there anyway i can do
to normalize all this hiss and distorted sound ?
is cutting the cable which goes from the head point
to the headpgone jack of my cell and put in between
a stereo potentiometer on the cable will solve ?
or putting a capacitor/resistance maybe ? how many ohms ?

as suggested on the similar topic before,
i've been thinking to try find the line after the pre-amp,
but i wonder, will tapping all the soldered point is safe ?
(I mean to get the "dep" sound, so i know it;s the audio in)

another suggestion on the similar topic before,
is to rip the chip which is glued to the iron-body of the deck,
then find the line in....etc..etc, which i don't quite understand.
honestly, i don't dare enough to do this,
because i don't have enough skill on electronic.

anykind of respond will be greatly appreciated,
and i'm sorry for my broken english as i'm from
non-english speaking country.

thank you in advanced.

The cassette preamp has a very high gain - so you can try a resister (47k, i think) in series with the left and right input. that way you will be able to turn the cell phone volume up a bit and might help with the hiss. Another thing i would try is unplug the motor that runs the cassette - the motor might also be putting noise into the power supply line.

the best way is to bypass the cassette deck's first gain stage, but it's not so easy.
Exactly /\. You are probably hearing hiss that is coming from the nokia even when turned way down. You could try using a low power "step down", the type used to convert hi level speaker signal to low level rca signal when installing amplifiers into oem systems. They have four wires going in for speaker signal and one left and right rca coming out with low preamp signal. Many also have adjustable gain pots too.

Like one of these:

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