I drive an older car before the days of 3.5mm line-in jacks. A couple years ago, I found an interesting idea online where some owners of the same car patched in a line-in directly into the L/R channels of the cassette player deck (since no one really uses cassette players anymore).


I did the same thing, and it seemed to work great for a couple Nokia phones that I had and also an old iPod shuffle.

Recently, however, I got a new phone - the Samsung Galaxy S III and it seems that it doesn't like the line-in at all. When the tape deck is powered off, the phone detects the line-in properly (e.g. it shows that headphones are connected, and any audio is routed to the headphone jack; of course there is no sound played in the car because the tape deck is powered off).

However, once I power on the tape deck, the phone detects that there is nothing connected to the headphone jack.

Oddly though, if I start playing a song before powering on the tape deck, then the phone maintains the connection when powering on the system and the music plays fine through the car's audio system. However, the moment the song ends or play back is stopped/paused, the phone thinks its no longer plugged into anything again.

Curious about this, I assumed that perhaps there's something odd with the grounding of how I hooked up the cable's wires the first time. The PCB I patched into had a labeled terminal for Left and Right, but none for Ground - so I had just grounded the cable to the chassis (for better or worse).

In opening up the stereo system again and checking with a volt meter, I noticed the following:

  • When the tape deck is powered on, there is a 2.8 V potential between the signal lines (e.g. L or R) and ground
  • When the tape deck is powered off, there is a 0 V potential between the signal lines and ground (I suppose that would make sense; but what about the first bullet point?)


  • The potential between the signal lines and ground of the 12 V outlets in the car also show the same 2.8 V potential
  • There is a 10 V motor in the tape deck - its ground also shows a 2.8 V potential from the audio signal lines

When I play music through the system using the method described above (where I "trick" the phone into keep the connection), the potential between ground and signal is something less than 1 V (close to 0 V when measured with a basic multi-meter).

I thought perhaps I needed to add some pull-down resistors to the signal lines, but I'm not sure if this is true? I tried this with a couple different resistor values (680 Ohms and 10 kOhms) but neither seemed to have any noticeable benefit (the difference was still greater than 2 V).

I was wondering if anyone had any ideas about this?

  • \$\begingroup\$ If I read it well, you connected your line in where the magnetic head usually feeds the signal, that is not exactly a good idea. The 2.8V you are reading might be a polarization voltage needed to let the head work. A bit of reverse engineering to understand where to connect the input properly would be better I think. \$\endgroup\$ Jan 9, 2013 at 7:43
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    \$\begingroup\$ @Vladimir : there in no such polarisation voltage (it would magnetize the head and probably partially erase the tape!). Instead if he HAD tapped into the tape head inputs he would find massive gain and a very odd frequency response - totally unusable. \$\endgroup\$
    – user16324
    Jan 9, 2013 at 11:09
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    \$\begingroup\$ Have you considered just buying an adapter? aliexpress.com/promotion/electronic_car-cd-deck-promotion.html \$\endgroup\$ Mar 10, 2013 at 15:43
  • \$\begingroup\$ @ScottSeidman: I would think a noise-free connection would likely require using some form of ground isolation--transformers being the simplest. A cassette adapter is basically a couple of half-transformers, with the other halves being built into the player. The biggest caveat with adapters is that some players try to sense the movement of the tape and will perform a safety-shutoff if it's not detected (minimizing the amount of tape that can get jammed in the machine if there's a malfunction). Some adapters include mechanics to fake out such mechanisms. \$\endgroup\$
    – supercat
    Jul 9, 2013 at 15:31

3 Answers 3


Sounds like you should consider placing some good quality capacitors in series with the audio signal lines from the jack that you added and to where you connected the inputs to the guts of the old cassette player's amplifier input. I would start with a value of 10uF with a voltage rating of 16V or more. It would be best to utilize non-polarized capacitors but if you cannot find those then consider one of two options:

a) Use two 22uF capacitors in series wires + to + and then one - lead to the jack and the other - lead to the cassette input.

b) Try to see how a polarized cap would work. Put the + lead toward the cassette player amplifier and the - to the jack.

These capacitor ideas block the DC bias that is apparently present at the connection point where you attached the signal wires into the cassette amplifier board. Some of the earlier devices that you connected to the cassette jack may very well have had capacitor coupled outputs. On the other hand your newest device may not have an output configured like this.

  • \$\begingroup\$ Thanks for the notes; I will try to get a few capacitors to give it a try. I did manage to get the phone to work in the end - upon closer examination of the daughter board I patched into, the L/R channel pins actually are connected to what look like potentiometers on the other side of the board (which I couldn't see until removing the tape deck from the stereo system). I reversed the pins I soldered onto and used the third pin as ground and at least the phone now works. \$\endgroup\$
    – acee
    Jan 9, 2013 at 22:08
  • \$\begingroup\$ That said though, a bluetooth receiver device I am trying to patch in still doesn't. There is still around a 1.2 V potential or so - I will try out the capacitors once I can get a few to test. \$\endgroup\$
    – acee
    Jan 9, 2013 at 22:10

the Galaxy s3, and really, any smartphone post iphone2, have finicky 4 conductor (TRRS) 3.5mm jack (L/R/G/Mic, or a variation of that). When you use a regular 3 conductor (TRS) 3.5mm plug, the 3rd and 4th conductor are shorted together. This causes problems, sometimes. Especially with a higher than 0 potential. You are essentially triggering the mic/remote sensor.

You will most likely need to get a TRRS to TRS adapter. These have a 4 conductor plug connected properly to a 3 conductor jack, without shorting any of the conductors.

Also, Did you connect the ground to the actual chassis ground? That might be the problem. Most radios are going to isolate audio ground, digital ground, and chassis ground as best they can. You would want to look for a ground point close to the cassette audio points. Just as a proper setup.


I cannot add a comment but make sure to use a capacitor at least on the ground/common line between the phone and the car. A lot of internal audio input stages are floating at half the supply voltage (so it might be 6V or more). You will not see any problems until you connect a charger which closes the car ground to the audio input middle point, through your phone. Source: own experience, I fried an iPod doing this.


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