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I am looking at a BMW's 3.5mm AUX cable diagram, BMW e53 AUX Cable diagaram

the left and right wires are connected together through a resistor and there is a capacitor on each left and right audio. and the third is a ground.

so I have two questions,

  1. Why there is a capacitor on each left and right ? why we need to store energy, is it gonna delay the audio ?
  2. Why the right and left audios are connected together with a resistor? wouldn't that make my audio mono as opposed to stereo ?

P.S: more detail, this is used to add AUX female socket to BMW Model E39/53's Audio system, the X13598 will be connected to the back of the audio system and the B1 (3.5mm female aux socket), will take input from phone (or ipod) through a male AUX cable .

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  • \$\begingroup\$ Pretty sure the capacitor is to reduce unwanted electric noise \$\endgroup\$ – htownclyde Mar 7 '16 at 17:49
  • \$\begingroup\$ how about the resistor ? \$\endgroup\$ – Medya Mar 7 '16 at 17:53
  • \$\begingroup\$ I don't know, but someone answered with the right information \$\endgroup\$ – htownclyde Mar 7 '16 at 17:53
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Audio signals do not have a DC components, but some devices add DC to the output, either as artifact, or because the circuit is more simple if you allow some residual DC. On the other hand, the amplifier in your car might expect a DC-free signal at its input. The capacitors serve as protection against bad devices outputting DC.

Typically, the output impedance of an AUX connector is around 5 to 10 kiloohms, and the output impedance of headphone jacks is between 20 and 200 ohms. Both of these are way below the 300 kiloohm resistor, so it appears electrically as if the resistor was not there if both the left and right channel are properly driven. You likely never notice the small amount of stereo mixing that is caused by this resistor except with special test signals. On the other hand, if a mono source is connected to only one of the input channels, and the other channel is left open, the resistor transfers the audio signal to the unconnected channel. If the input impedance of the amplifier is high (several mega ohm are likely if they use a FET buffer amplifier in the input stage), the 300 kilo ohm resistor will not reduce the volume a lot. The R-C combination will cause a slight frequency dependent phase shift, which is not a completely bad idea, because it makes the sound less mono-like and more appealing to the casual listener.

So the capacitors are a fix for bad devices that output DC on their jack, and the resistor is a fix for (bad) devices that do not output a stereo signal. I can't explain why they put these fixes into the cable and not the input circuit if the car amplifier.

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  • \$\begingroup\$ Thanks a lot for the detailed excellent response, I would like to also ask you this, I bought a non-oem cable ($35 cheaper) and I believe it doesn't have resistor and capacitors (as the diagram above). it works with my phone but it doesn't work with my (bluetooth to aux) devices. could that problem be solved if I add the resistor ? (in other words could my problem be, the phone makes enough power to send audio but the -bluetooth to aux- device doesn't make much power and because there is no resistor it wont match the impedance for highest power ? ) \$\endgroup\$ – Medya Mar 7 '16 at 18:53
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    \$\begingroup\$ I doubt the resistor is any help if it doesn't work at all. As I explained, its main function is to fake one missing channel from a working one. If none of the stereo channels work, your problem has to be different. Possibly your Bluetooth to aux converter has some DC offset that overdrives the input stage so its unable to see or amplify the AC part. The capacitors will help if this is the cause. \$\endgroup\$ – Michael Karcher Mar 7 '16 at 20:54
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The capacitors AC-couple B1 to X13598, effectively removing any DC bias on those audio lines. The resistor is probably in place for impedance matching the source.

To further elaborate, capacitors have a lower impedance (or reactance) at higher frequencies. To AC signals, they look almost like a short circuit. To DC they look almost like an open circuit. It is possible that they are not using a bipolar output, and thus the capacitors could remove the DC offset, effectively making the signal bipolar.

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  • \$\begingroup\$ is that gonna make my audio not stereo anymore ? since they left and right channels are connected? \$\endgroup\$ – Medya Mar 7 '16 at 17:54
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    \$\begingroup\$ They aren't connected, there is a resistor between them. There is a difference. The resistor is just another load as seen from the source, in the same way that a speaker is just a load, as seen from the source. \$\endgroup\$ – Brendan Simpson Mar 7 '16 at 17:55
  • \$\begingroup\$ could you please make it more clear for me what is the advantage of using the resistor, you said its for impedance matching the source, if we don't have this resistor what happens ? \$\endgroup\$ – Medya Mar 7 '16 at 17:58
  • \$\begingroup\$ @Medya without more of the circuit, it's hard to elaborate further. Like I said, the resistor is probably there to match the impedance of the source to the load, because this allows for maximum power transfer. I agree that it seems strange to connect audio-L to audio-R as is done here, but I imagine that if we knew what the source's output structure looked like, we could do a deeper analysis. Can you elaborate more on what your final goal is, once you have your question answered? \$\endgroup\$ – Brendan Simpson Mar 7 '16 at 18:01
  • \$\begingroup\$ Thanks @bss36504 I just added more detail : this is used to add AUX female socket to BMW Model E39/53's Audio system, the X13598 will be connected to the back of the audio system and the B1 (3.5mm female aux socket), will take input from phone (or ipod) through a male AUX cable \$\endgroup\$ – Medya Mar 7 '16 at 18:05
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BMW radio uses the resistance between Left and Right to determine whether to make aux available. To ensure all devices work correctly, you should have it. The effect of no resistor might be desirable to you, or not: if you remove the cable from your device (e.g. iPhone) then the radio jumps out of Aux input back to Radio (e.g. FM). Drives me nuts, am going to add the resistor.

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  • \$\begingroup\$ And I assume the caps are for one of 2 reasons: 1. things plugged into aux might have some odd DC on their L and R, you'll see reports on other forums of loss of audio from phones when connected to 12v charger in the car. 2. To block the DC from the test signal that the radio send down the L or R line (I don't know polarity of that, I have heard it detects mono vs stereo connected device). Note the resistor is on radio side of the caps. \$\endgroup\$ – Chalky Nov 7 '16 at 8:57

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