Smartphone works nicely with Blaupunkt SC 202 car radio DIY AUX but hiss is pronounced after 50+ volume level. It’s not big deal, because I don’t listen louder than 50% in car. Outside one don’t hear any hiss at all. Some people recommend to change capacitors on amplifiers but I decided to leave as it is. According to LCR-T4 old capacitors have high ESR but acceptable. To get less hiss, turn high volume on phone and low volume on car radio. Old cheap phone produces more hiss comparing with modern one.
When phone is charging in cigarette lighter, sound becomes distorted. Is it so called GND looping? Tried additional GND cable from car body to car radio, but it made no difference. Ideally I need to GND cigarette lighter same way as car radio. It’s pain...
Phone usually is charged 100% and it won’t discharge much in 4 hours, so I can live with this. As alternative I can use mp3 player with AA battery. Anyway task is accomplished – preserved original radio and added new functionality.