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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. The third is a ground.

I have two questions,

  1. Why is there a capacitor on each left and right? Why do we need to store energy, is it going to delay the audio?
  2. Why are the right and left audios 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
    Commented Mar 7, 2016 at 17:49
  • \$\begingroup\$ how about the resistor ? \$\endgroup\$
    – Medya
    Commented Mar 7, 2016 at 17:53
  • \$\begingroup\$ I don't know, but someone answered with the right information \$\endgroup\$
    – htownclyde
    Commented Mar 7, 2016 at 17:53

5 Answers 5

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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
    Commented Mar 7, 2016 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
    Commented Mar 7, 2016 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
    Commented Mar 7, 2016 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
    Commented Mar 7, 2016 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
    Commented Mar 7, 2016 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
    Commented Mar 7, 2016 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
    Commented Mar 7, 2016 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
    Commented Nov 7, 2016 at 8:57
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The two capacitors are there because surprisingly the audio out ground on the radio is not shared with the chassis ground of the car. Maybe they were trying to keep out alternator whine. I don't know why but I do know they aren't shared (beep it out yourself.) In fact if you use an alligator clip and short the ground from aux to auto ground several time very quickly you will observe some interesting (but not harmful) effects. The capacitors are there to prevent those grounds from coming together (DC wise.)

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    \$\begingroup\$ The ground is not capacitively coupled though, it's only the left and right signals which are. \$\endgroup\$
    – Colin
    Commented Jan 21, 2021 at 9:37
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Why are there Capacitors in the AUX cable?

Not for most of the reasons I see listed. There are caps in the cable because BMW wanted to avoid a current path from unknown Audio Sources into their DC direct input circuit maintain (differing quiescent points) and also isolates Audio system ground. If you don't believe they did that Use your ohm meter and beep out the radio box to the cigarette lighter ring, no continuity.

Why is there a Resistor in the AUX cable? The resistor is there for the radio to test for its presence. The radio does that to determine if an aux cable is actually connected. If it detects that flow, it presents AUX as a choice. without it AUX will only be a choice when your source is plugged in and unreliable at that (I'll skip the reasons).

Some AUX cable makers don't know this.

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