Using car radio output jacks for input

On the backside of my Sondpex RUSX903 car radio I found two RCA jacks: a red and a white one. With an adapter cable I connected these to my cellphone headphone jack. So far so good, I noticed two odd things though:

  • The volume control of the car radio does not affect the volume of the music coming from my cellphone. I gets pretty loud though when I increase the volume on the cellphone.

  • The music from the cellphone plays independently of the selected input. If the selected input has some audio as well (e. g. the radio,) I can hear them both mixed together.

Recently a car electrician saw my setup and told me the RCA jacks I’m using for input are in fact output jacks.


  1. Why is this even working? Why is it even possible to use the output jacks for input?

  2. Is it safe,or should I expect to run into problems or even damage something?


The Aux Input Jack on the Front Panel of the Car Radio is broken.

Electrical Engineering Asked by user1785730 on November 11, 2021

1 Answers

One Answer

The RCA jacks on the rear of Sondpex RUSX903 Car Radio are intended for 'Line Out'.

You are indeed inputting music from your cellphone into the 'Line Out' jacks instead of the Aux Input jack which is on the front panel!

It works, though not as it should, because the 'Line Out' may be tapped after the preamplifier and the following stages can amplify the music you are inputting.

Even assuming no damage would be caused, it's not worth taking the risk when the appropriate input is available.

Answered by vu2nan on November 11, 2021

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