Logitech speakers channel out

Discussion in 'General Electronics Chat' started by kmclaren, May 19, 2009.

  1. kmclaren

    Thread Starter New Member

    May 19, 2009
    5
    0
    I have a set of Logitech Z-2300 speakers, one channel has been slowly dying over the last year and recently it has stopped all together. I have already done some troubleshooting and figured out what is causing the problem. I tore the speakers apart and found that the input signal hits a rheostat/potentiometer (the volume knob) then goes out to the amp. The problem is that one of the channels on the knob has been shorted/opened inside the rheostat. This thing is too small for me to take apart and examine further so my question is:

    Where can I find a 2 channel rheostat that I can replace this with?
    and
    What measurements should I take off of the still working channel so that I buy the right one? If it helps, the only writing on the rheostat is "A103" but I'm going to throw a guess that's just a code they use in their factory...

    P.S. I was dumb and bought these "refurbished" so I can't just send them back to logitech for a replacement (besides where's the fun in that?!)
     
  2. kmclaren

    Thread Starter New Member

    May 19, 2009
    5
    0
    I just drew up a rough pic of the rheostat, here it is
     
  3. bertus

    Administrator

    Apr 5, 2008
    15,650
    2,348
    Hello,

    That look like a stereo potentiometer.
    Is there a value noted on this potentiometer?
    This can be noted in diffrent ways like 100K or 104.

    Greetings,
    Bertus
     
  4. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    A103 is a logarithmic (audio taper) 10k ohms pot.
     
  5. kmclaren

    Thread Starter New Member

    May 19, 2009
    5
    0
    Awesome, thanks a lot guys, but where can I buy something like that?
     
  6. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    Buy the stereo volume control whererever stereo volume controls are still sold.
    You probably will not find one that physically fits.
     
  7. kmclaren

    Thread Starter New Member

    May 19, 2009
    5
    0
    I'm wondering if maybe Logitech is weird and put a linear tapered pot in there...
    Because I got an audio tapered pot and put it in, fixed the problem, speakers sound perfect again, but now I am presented with a new "problem". It get's too loud too fast, I can only turn the knob about 30 degrees before it is past my comfortable listening zone. If I swapped it for a linear tapered pot the resistance would decrease slower right? So at low volumes I would have more control?
     
  8. bertus

    Administrator

    Apr 5, 2008
    15,650
    2,348
  9. flat5

    Active Member

    Nov 13, 2008
    403
    17
    If you have to, you can play with some resistors and get the effect you want.
     
  10. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    The problem of the volume being too loud is caused by using linear pots intstead of audio-taper pots.

    When a linear pot is turned down to half-way then the volume is still almost at maximum.
    when an audio-taper pot is turned down to half-way then the volume is turned down pretty low.

    Maybe you connected the pins on an audio-taper pot backwards. Clockwise with the knob should increase the volume.
     
  11. kmclaren

    Thread Starter New Member

    May 19, 2009
    5
    0
    I know that I hooked it up correctly. I made sure that, like the original, the resistance was at it's highest when the knob is fully counter clockwise and the resistance is at its lowest when the knob is fully clockwise. The volume increases to fast and I have little control over the volume when it is at "low" levels. It's behaving like Audioguru is suggesting a linear tapered pot would, but the packaging it came in clearly says "10k dual audio taper"

    It seems that this circuit takes the signal, runs it through the pot, then out to the amp. So lower volume at higher resistance because less electricity is allowed through.

    I found a graph in one of bertus's links (attached). If it is correct, you can see that when they are near their highest values the resistance in a logarithmic pot changes faster than that of a linear pot and much faster than that of a revers log pot.

    So I was thinking that it should have been a revers log pot. And seeing how those are so hard to find the next best thing would be a linear pot, that way it would change slower at low volumes. I never have any need to turn these all the way up, or anywhere close for that matter, they're a bit of overkill for computer speakers. Because of that I don't mind losing the control at higher volumes.

    Does it sound correct that it needs a reverse log pot. Also, that a linear pot, while not the best, would be better than a normal log pot.?
     
  12. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    Then you connected it as a two-terminals rheostat instead of as a three-terminals volume control.

    A volume control has one terminal connected to the circuit's ground so it can be a voltage divider.

    The "audio tapers" form a divide-by-10 voltage divider when connected with all three terminals and are set to half-way.

    A "reverse-taper" pot has the entire pot turned with the knob and the shaft does not move.

    Here is a schematic of a "rheostat" and of a "volume control".
     
Loading...