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Computer front panel audio

Discussion in 'Audio' started by jedikalimero, Sep 27, 2011.

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  1. jedikalimero


    Sep 27, 2011

    I have a problem with my computer. the motherboard has HD-Audio but the front panel is the usual AC97 standard. You can change the BIOS config so the front panel audio connector acts as an AC97 one, but then the problem is the speakers connected to the back keep playing even if you connect a set of headphones to the front panel.

    The reason is the audio jacks used for HD-audio are different from regular jack connectors (and very rare to find!). While the insertion of the plug in a regular jack connector results in breaking a circuit (and so, physically muting the loudspeakers), in an HD Audio jack, the insertion of the plug results in closing a switch (sending a signal to the driver ordering to mute the speakers by software).

    Since I can't find this kind of jacks anywhere, I managed to create a sort of HD audio Jack by combining a regular jack connector and a zero pressure micro-switch so when the plug enters the hole, a tip inside the hole closes the micro-switch.

    I wanted to make a good quality front panel so instead of just wiring the jack connectors (and micro-switches) to the cable, I decided to add an EMI filter.

    I took the circuit from this doc:
    and more precisely from fig 6 at page 25.

    The circuit uses two 220 pF capacitors and two inductors for each jack connector. In this aspect the circuit is identical to that proposed for AC97. The problem is I have no idea what inductors to use or how to ask for them at the shop.
    All the diagram says in a footnote is: ZL should be 600 Ohms or greater @ 100MHz with a low Q (broad Impedance curve over frequency)
    but not even the guys at the shop knew what piece to give me.

    So what I did was to cannibalize the inductors from two cheap promotional radio receivers with jack connectors for the headphones and use them for my circuit. After all, they perform the same function: filter the audio signal at the headphone connector so I supposed they should have the correct values I needed.

    But my circuit is very noisy! headphones have a constant electronic background noise (that seems to react to things like mouse movements, hard disk activity or whiteness of the screen) and microphone has a lot of noise (of course I'm using very good quality mic+headphone). The cables are audio cables for front panel audio so they are supposed to be correctly shielded so the problem is in the circuit at my custom board.

    Any ideas?
  2. davelectronic


    Dec 13, 2010
    PC speaker audio

    That sounds like a real problem, i dont know of a circuit that would be a complete fix, diy on pc boards even periferals should be done carefully do avoid MB damage. I dont know how old your bios revision is, but a bios flash is something i would look into. This needs care as incompatibility problems can arise. Dave.
  3. TBennettcc


    Dec 4, 2010
    I can tell you this: I have noticed the same noises you are talking about (mouse movements, screen color, etc.) I have an Intel D975XBX2 motherboard, in a case with typical AC'97 front panel audio. There are also two USB ports right next to the front panel audio jacks. You think moving the mouse is noisy?! Try having an iPhone or other active USB device plugged in! Whoo!

    I can't tell you whether or not my motherboard has HD Audio onboard or not. But I can tell you that my front panel audio connections are noisy when I have a pair of headphones plugged right in. So, unfortunately, I don't think it's your circuit. It might just be a victim of AC'97. I don't know.

    By the way, Mouser has a good filtering system for choosing metrics of components. (Z is impedance; I'm guessing ZL is the impedance of an inductor).

    This should narrow down your search some:|0

    Just go to Mouser, select 'Passive Components', 'Inductors', then 'Common Mode Inductors (Chokes)'. From there, you can select the metrics you want to apply. In your case, click on '600 ohms' under the heading 'Impedance', scroll to the end of the list, hold the Shift key down, and click on '10 MOhms' to select everything between 600 ohms and 10 MOhms. Then select 'Apply filters'. You should be left with 16 choices. Should be able to pick a good one from there.

    I'm not an EE or an audio guy, so I can't tell you how that circuit is supposed to work, or how well it might or might not actually work.

    Good luck.
  4. jackorocko


    Apr 4, 2010
    I have noticed the same thing with my mouse and audio. I have commercial gear to, so you are fighting an issue that isn't easily fixable. Or at least one that hasn't found a solution over the years in the computer industry.

    I always noticed it was way worse with a bigger amplified signal, so that leads me to believe the noise is always there, just sometimes it is inaudible.
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