Capacitors in Series Effect on Audio Quality

Discussion in 'General Electronics Chat' started by aberrant0ne, Sep 26, 2013.

  1. aberrant0ne

    Thread Starter New Member

    Sep 26, 2013
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    I have a question that I'm not sure belongs here or not but I figured I would at least get an explanation as to why I'm experiencing this and possibly a workaround. So let me give a brief overview of what's going on.

    I have a BMW 330i and the radio supports AUX however you need to purchase (or make) an AUX cable that connects to the back of the stock radio. I opted to make my own cable. I used the diagram from the BMW manual to wire it, here's the diagram in case anyone wants to see it: http://www.ge39.com/files/EBADoc11512-auxin.pdf . Anyways in order to use a device and charge it at the same time you need to wire a capacitor to the left and right channel wire in series to prevent some type of power surge or something? This is what I'm not clear about. There's also a resistor that goes on the side of the capacitors closest to the head unit this is to provide resistance to let the head unit know that it should enable AUX. Now when I add the capacitors to the right and left channel wires the bass is more or less non-existent and I've been told the reason is that the capacitors, when added in series, act as a sort of filter and filter out the lower frequencies.

    Given my lack of electronic knowledge, I've come to you intelligent people to ask if anyone has an ideas why the capacitors act like this and see if there's anything I can do to keep the circuit how it is but not mess with the lower frequencies? Perhaps a larger capacitor? The capacitor in the diagram lists 0.22 F but everyone that makes these cables on the forums says that it should be a 0.22uF capacitor however people who use either one seem to still have an issue with the bass so I'm not sure if the size of the capacitor is causing this behavior.

    Does anyone have any insight, or can tell me why the capacitors are needed to charge my phone and listen to aux at the same time?

    Thank you!
     
  2. BobTPH

    Active Member

    Jun 5, 2013
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    You are putting a .22uF capacitor in series with the speakers? This would pretty much eliminate all the bass. Who told you you needed to do this?

    Is it possible they were supposed to be in the input wires to the aux input instead?

    Bob
     
  3. wayneh

    Expert

    Sep 9, 2010
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    The capacitors are used to prevent any DC circuit between the source and the input. A capacitor couples an AC (audio) signal but does not pass DC.

    The lower the frequency of an audio wave, the more it starts to look like DC. A bigger capacitor can pass a lower frequency.

    It seems odd to me that they would recommend coupling with a capacitor that filters the audible range. Are you trying to feed it from your headphone port? Perhaps it's meant for line level input.

    FWIW, automotive modifications are not allowed for discussion here. A moderator may decide this qualifies for the ban. Kinda grey in this case.
     
  4. aberrant0ne

    Thread Starter New Member

    Sep 26, 2013
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    Yes, the cable that comes from BMW has two capacitors that run in series with the two wires that connect to the aux female connector (left and right channel). If I remove them I can't power my phone from the cigarette lighter while the aux cable is plugged in. The last page in the PDF that I linked to shows the simple schematic.
     
  5. BobTPH

    Active Member

    Jun 5, 2013
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    A larger capacitor would allow more base. Try 2.2uF, that would make more sense.

    Bob
     
  6. aberrant0ne

    Thread Starter New Member

    Sep 26, 2013
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    I appreciate the advice that would explain why. Just to clarify I'm trying to understand the behavior of capacitors as it pertains to audio waves not trying to get help hooking up my stereo or run by big 200" subwoofers. So increasing the size of the capacitor would allow for lower frequencies to pass through. How would a larger capacity affect this circuit in particular? Is there a downside to the size of a capacitor?
     
  7. aberrant0ne

    Thread Starter New Member

    Sep 26, 2013
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    OK thanks Bob, I will give that a shot, thank you!
     
  8. wayneh

    Expert

    Sep 9, 2010
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    The down side is that a big capacitor will allow a relatively bigger pulse to pass through. It will block true DC, but if it passes, say, a big 2Hz wave, that might damage sensitive components. It would "look like" DC for long enough to do damage. That said, I would have no fear of increasing the capacitor by 2-10X if that makes an audible improvement.

    But it still begs the question of what is going wrong. BMW engineers aren't known for making mistakes. I wonder if the difference between headphone output and line output is the issue. What is the intended AUX input that they designed this for?
     
  9. Veracohr

    Well-Known Member

    Jan 3, 2011
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    Coupling capacitors form a high-pass filter with the input impedance. The larger the capacitor, the lower the cutoff frequency. A lot of audio circuits (not necessarily car audio) use something in the range of 10-100uF for coupling.
     
  10. aberrant0ne

    Thread Starter New Member

    Sep 26, 2013
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    Perhaps the intended use is different than they intended but I'm not sure what else they would think people would use the AUX for. I'm plugging the AUX cable from my headphone jack in my Android phone to play music through the car speakers.
     
  11. wayneh

    Expert

    Sep 9, 2010
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    In most applications, an AUX input would be a line-in port. I agree that in the modern world, in a car, a headphone port makes more sense. A line-level port expects lower voltages than what's delivered by a speaker driver.

    I'm just speculating that's the problem. It'd be easy to test, if you want. You could load the headphone port with, say, a 1K and 4.7K in series and then feed your aux port with connections to both sides of the 1K resistor. This would drop the voltage ~6X and possibly allow a better sound. Or not.
     
  12. crutschow

    Expert

    Mar 14, 2008
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    Try a larger capacitor, it can't hurt. But check if there is any DC voltage to ground on the inputs from the car and from the Android phone outputs to ground. That will determine the direction to put the capacitor if you use a large electrolytic polarized capacitor.
     
  13. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    A 1uF cap would give a -3dB drop-off at ~15Hz if the input impedance of the Aux input of the amp were 10k. I'd be surprised if the input impedance were as low as 10k, but I have no knowledge of BMW amps.
     
  14. MrChips

    Moderator

    Oct 2, 2009
    12,421
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    Quit messing around and put a 10μF cap in series. Wouldn't hurt a thing.
     
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