My PC amplifier problem....?

Discussion in 'The Projects Forum' started by k41, Dec 14, 2006.

  1. k41

    Thread Starter New Member

    Dec 14, 2006
    2
    0
    Hi. I'm trying to put an internal stereo amplifier and speaker inside my computer case, since I'm tired with all the plugging and unplugging of my active speakers everytime the computer have to be moved...

    So I open my active speaker, take the amplifier components out of it, put those components inside an used floppy disk drive, and mount the speakers on one of the empty drive bays, after some soldering, all worked well, but here goes the problem:

    The original pot (variable resistor) used to control the volume of the amplifier is 10k OHM (the rotating version), and because of the space, I have to replace it with a sliding version, which I'm only able to find 50k OHM sliding pot in my city...

    So I tested the amp with the new 50k pot, but now it have humming noise out of the speakers, so, does anybody know how to modify this pot (perhaps by adding some resistors or something) to make it works like the original 10k pot?

    I have done:
    - Checking all the line in and speaker connections
    - Adding the LM7809 regulator circuit (I'm taking the power from
    the 12v rail of my computer PSU)
    - Testing it with another active speaker (which is worked fine)
    - Entering almost every electronic components shop in my city
    to find 10k OHM sliding pot!

    So, any suggestions...?
     
  2. Gadget

    Distinguished Member

    Jan 10, 2006
    613
    0
    Sounds like an earthing problem. Have you tried connecting the Metal case around the pot, to the audio ground (via the common/gnd leads going to the low side of the pot). Are you using shielded cable for the audio..? Are you using a "Star" earthing arrangement.? Is the Amp Shielded in some sort of cage from the RF ravages present inside the PC..?
    You could possibly change the resistance of the pot by fitting a 15k resistor from the wiper to the High side, and a 15k resistor from the wiper to the Low side, but this will also change the response from Log to ...... something else.......
     
  3. k41

    Thread Starter New Member

    Dec 14, 2006
    2
    0
    Thanks for the reply

    I put the amp entirely into an empty unused floppy disk drive cage, so I guess it shielded very well.

    I kept the floppy interface (the power and data connector) function as it originally should be, the power connector to get power from the pc psu, and 8 top left pins of the data connector for speaker out and audio in, using this configuration:

    l_spk - gnd - gnd - r_spk - l_in - gnd - gnd - r_in

    that means I have to keep the floppy pcb in place, but I use a cutter to cut the pcb lines from the connector so it doesn't interfere with the floppy components, and to make sure I'm using a multimeter to test it

    There's no metal case on the pot, it's a plastic pot, and for the grounding to the floppy case, I use two screws to mount the pcb into the floppy case that directly connected to the pcb ground

    I guess I'll try to fit those 15k resistors and see the result anyways...

    btw, the noise appears only when I turn up the volume about 1/3 of the max volume, and it's a small single IC amp (dunno about the IC, the writing on it is removed, but the speakers says 3 watts 4 ohms)...

    Yes, the cables are shielded...

    Any more suggestions anyone?
     
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