Cd-player Frankenstein

Discussion in 'The Projects Forum' started by Psycho0124, Dec 1, 2008.

  1. Psycho0124

    Thread Starter New Member

    Sep 23, 2008
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    0
    I put together a computer for my 4-year old to play kids games or watch movies over the network. The machine uses an old LCD screen with tiny built-in speakers (connected by separate audio patch cable) which just aren't cutting it (little or no amplification for low-volume movies).

    I want to set him up with some amplified speakers without going out and buying some so I thought up an alternative. He already has a small table-top stereo/cd-player in his room which seems to have pretty good speakers and amplifier. Only problem: no audio input of any kind.

    I have an audio jack that I want to install in the back of the thing to play sound from the computer. The stereo is using the KA2206B Amplifier IC (sheet here) and I was thinking of just splicing my inputs directly onto it:

    Pre Gnd : ground of audio
    In1: Right channel
    In2: Left channel

    The amp seems to be powered in CD mode even when a CD is not playing so it could be left in CD mode for computer input operation without any background noise.

    Are there any foreseeable problems with my plan? Any risk of damaging either the radio or (more importantly) the computers audio chip (assuming I get the wiring correct)? Also would there be a more ideal location to splice into the circuit?

    Thanks for reading and for any tips ya might have! :D
     
  2. Wendy

    Moderator

    Mar 24, 2008
    20,766
    2,536
    Use a resistor and a capacitor per channel. There is always the risk it won't work after surgery, but you need a way of attinuating the signal (a variable pot) and a capacitor, non polorized, per channel. This can be outside the case between the computer if you choose.
     
  3. leftyretro

    Active Member

    Nov 25, 2008
    394
    2
    Well a little expermintation will probably be called for before actually drilling holes for a new audio input jack. If you can locate the volume control pot ( 2 gangs for stereo right?) the movable wiper routes the signal on to the amp. One end of a pot will go to circuit common. The other end of the pot would be a good place to wire a capacitor to inject your signal from your PC sound output. Try a .5mfd. You will need two one for each pot gang. Also the common from the PC sound input has to be wired to the common for the CD/radio player.

    Lefty
     
  4. Psycho0124

    Thread Starter New Member

    Sep 23, 2008
    3
    0
    Ahh. Inject the signal just before the volume control pot! Doh!

    I've already mounted the jack (drilled a hole by carefully spinning a pocket-knife) and mounted it with some (non-conductive) epoxy. I temporarily soldered the audio jacks conductors onto the IC for testing and the quality is wonderful but only the bottom 10% of the PC software volume control is within reasonable volume (blasting Alladin at insane volume :p ). Swapping the connection over to the pot will solve that little detail though. Thanks!

    Hey, (time for the requisite noobish question I guess) what is the purpose of the capacitor? It seems like having a cap wired between an audio channel and its ground would filter out the audio frequencies relevant to the caps charge/discharge rate? Or are we talking about connecting the audio channel to one side of a cap and using the other side to tie into the existing circuit to prevent current flow while still allowing signal to pass? Sorry, I'm (hopefully temporarily) kinda out of my element with this stuff.
     
  5. Wendy

    Moderator

    Mar 24, 2008
    20,766
    2,536
    You don't want any DC stuff screwing up your circuits. Caps are cheap insurance.
     
  6. leftyretro

    Active Member

    Nov 25, 2008
    394
    2
    "Or are we talking about connecting the audio channel to one side of a cap and using the other side to tie into the existing circuit to prevent current flow while still allowing signal to pass? Sorry, I'm (hopefully temporarily) kinda out of my element with this stuff."

    Yes, the latter is the case, Caps goes in series with audio input and the top of the pot, one for each audio channel. It would allow the AC audio voltage to pass but not mess up any DC bias on either the PC or the Radio. More then likely your PC audio output may already be coupled via a cap inside the PC and therefore you might not even need the caps, but why risk screwing up your PC sound output when a cheap cap can eliminate the risk so easily.

    Good luck
     
  7. Psycho0124

    Thread Starter New Member

    Sep 23, 2008
    3
    0
    Hehehe.. Well I fried the entire motherboard. The largest non-polarized caps I had just werent cutting it, causing loads of signal distortion. So, in the hopes the sound board had a dc filter, I left em out :rolleyes:. The machine worked fine (with wonderful audio) without the DC filter for a few days. Then I borrowed the machines power strip and It never came back on. I'm guessing the stereo (which was plugged into another outlet and was still on) bled current into the pc while it was off and toasted something critical on the board. No biggie, the board was a junker anyway (found it sitting by the road in the rain).

    Oh well..
    You learn more from defeat than you do from victory.
    Also, toasted equipment makes a nice upgrade oppertunity!

    Luckily, I had a (far superior) mobo in the 'pile'. It had some blown caps and wasn't working but the 'recently deceased' board had a dozen perfect size and rating replacements (looked and tested good). A little solder-slinging, a quick mobo-swap, and it's up and running like a champ (he's playing Battlefield 2 on it now :p)!

    I scrapped an old car amp and found some non-polarized caps:
    2x .22mf
    4x .1mf
    2x .047mf

    Could I run a .22 and a .1 in paralell for each channel? Should the common also have a DC blocking cap? Also what is the intended purpose of the resistor? About what value should I use?

    Thanks for all the help! This has been a very educational little project!
     
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