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.

  • \$\begingroup\$ how do you know for sure that those are outputs? ... please provide a datasheet that supports that fact \$\endgroup\$
    – jsotola
    Jul 20, 2020 at 22:52
  • \$\begingroup\$ I can't say that I know this for sure. Just the odd behavior I observed and the comment from the car electrician. Unfortunately I have no data sheet or manual or anything. \$\endgroup\$ Jul 20, 2020 at 22:55
  • \$\begingroup\$ bidirectional pre-amp out audio port. good find \$\endgroup\$ Jul 20, 2020 at 23:01
  • \$\begingroup\$ A long time ago, in the 20th century, I saw a Car Stereo whose RCA jacks were connected to the output of the internal pre-amplifier, so, those jacks could be used both ways: as an INPUT for the Stereo's power amplifier, or as an OUTPUT for the device's CompactCassete player audio signal. \$\endgroup\$
    – mguima
    Jul 20, 2020 at 23:01
  • 1
    \$\begingroup\$ @Tony I followed your comment only 7 seconds after... ;-) \$\endgroup\$
    – mguima
    Jul 20, 2020 at 23:02

1 Answer 1


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.

  • \$\begingroup\$ I forgot to mention that the Aux Input on the Front Panel is broken. I've updated my question. \$\endgroup\$ Jul 21, 2020 at 18:24
  • \$\begingroup\$ The risk your Talking about, is it to the Car Radio or to my cellphone? \$\endgroup\$ Jul 21, 2020 at 18:27
  • \$\begingroup\$ Considering the low level of signals in 'Line in', 'Line out' and 'Earphone out' and with 'DC blocking' capacitors present in those ports, no damage should be caused either to the radio or the phone. My point was, why take the chance when 'Aux in' is available. \$\endgroup\$
    – vu2nan
    Jul 22, 2020 at 4:46

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