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I added and AUX connection for my car radio by soldering straight into the audio lines between the CD player and the car radio. I connected my phone to this AUX connection and added an external mic. So the 4 poles (CTIA) connect to LEFT, RIGHT, GROUND, MIC where LEFT, RIGHT and GROUND go to the car radio (connection between the radio and the CD player) and the GROUND and MIC go to an external mic.

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The problem is that my phone and the CD player share the same lines so basically the cd player "sends" its audio into my phone through the LEFT and RIGHT lines. Of course, this has no effect on sound and also, I play a silent CD, but, I am having problems with my phone (android) where it automatically turns down the volume all the way to zero. I think this is happening because of a feature that turns down the volume when high audio is detected on the lines. What I thought was maybe there is a way to prevent the cd player from "entering" my phone lines? For example a diode or so..? I know that audio is achieved from change in voltage, but there must also be current I guess..? Also, could you recommend what I can do to avoid noise from the radio? I guess that it is coming from the GROUND line, so maybe some sort of capacitor on the ground line?

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What you've done is a very, very bad idea. You're shorting outputs from different sources. You can mess it up big time if the two sources become active at the same time. One of them (or both) can be easily damaged.

The phone is probably detecting this condition and turning its audio output off in order to prevent that from happening. A clever little gadget, isn't it?

So I'd recommend you to consider using a switch instead. There are commercially available solutions, like this one. You'll have to look for something that's right for your intended used (you might want a switch with an additional MIC input).

If you are in a DIY mood, you may want to build it yourself. Turn to google for schematics, and have some fun with that project!

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  • \$\begingroup\$ Thanks @enricblanco, could you elaborate on why that might damage them? The link you sent would be the solution for me if I had an option to split the connections, but unfortunately I only had the chance to solder straight on the left and right channels. I could try to cut the connection between the CD and the radio altogether though.. I don't really mind not having audio from the CD ever again. \$\endgroup\$ – SatA Feb 19 '17 at 19:10
  • \$\begingroup\$ At a given instant of time one output may the trying to go +Vpeak and the other one -Vpeak. Because you're shorting them with wire (low impedance), a destructive overcurrent condition may develop in the output transistor of one or both of the sources. Even if the outputs are designed to tolerate brief short circuits to ground, they may not tolerate being short-circuited to -Vpeak. \$\endgroup\$ – Enric Blanco Feb 19 '17 at 19:55
  • \$\begingroup\$ Ok got it thanks. Question that comes to mind though - if I play silence in the cd, the risk is minimal, right? Would it be better to put a resistor between the aux and the radio on each channel? \$\endgroup\$ – SatA Feb 19 '17 at 20:15
  • \$\begingroup\$ And another question, would it be possible to somehow electronically prevent the CD output from entering the AUX? like a diode for current \$\endgroup\$ – SatA Feb 19 '17 at 20:22
  • \$\begingroup\$ No, a diode won't work because it'll rectify the audio signal. Also, line level is too close to the forward voltage of diodes. Maybe a relay instead? \$\endgroup\$ – Enric Blanco Feb 19 '17 at 20:36
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I agree with Enric, but you might also actually build a mixer with little effort:

have a look (e.g. Wikipedia) at "summing amplifier" Opamp configurations. The trick here is that you connect the audio sources to a common point through high-valued resistors. If you want to do it really properly, I'd even go a bit further:

Get four op-amps. Use these

  1. Opamp as an inverting amplifier for the CD player (left channel). Use a trimmer, poti or fixed resistors to set the gain.
  2. Opamp as an inverting amplifier for the CD player (right channel). Use a trimmer, poti or fixed resistors to set the gain. There's plenty of dual potnentiometers, i.e. actually two potentiometers in one case, so you can adjust left and right of a stereo signal at once.
  3. Opamp as an inverting amplifier for the phone (left channel). Again, you can make the gain adjustable
  4. Opamp as an inverting amplifier for the phone (right channel).
  5. Opamp as an inverting amplifier for the Mic. Here, you definitely want to make the gain adjustable, I'd guess! Use output of this amplifier to drive both channels of the summing amplifiers below:
  6. Opamp as summing amplifier (left channel). Since the standard summing amplifier is inverting, too, you don't even get a phase-shifted signal! Aim for low gain (e.g. 1 or 2 or 0.5 or so)
  7. Opamp as summing amplifier (right channel).

Luckily, Opamps can be bought in sets of 1, 2, or 4 of them in one IC. It's often a good idea to simply click through on one of the major semiconductor sited (here: TI) and look for usable parts.

There's a lot to be said about the design of such a system (e.g. input AC coupling, output coupling, supply concerns, using the unused 8. opamp as voltage stabilizer for a virtual ground), but I think that might exceed the scope of an answer here.

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  • \$\begingroup\$ Thanks @Marcus, would that be possible even without having separate connections of: CD, radio, aux? The thing is that I only have the option of soldering these together or to cut the connection between the CD and the radio (only the right and left channels). Then I would never have CD audio again, but that's fine. Of course I could try to carefully solder back the connection to the CD separately after cutting, but that is a very delicate job to do. What you suggest is mixing between the CD and aux right? Would there be other benefits of such a configuration? \$\endgroup\$ – SatA Feb 19 '17 at 19:14

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