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question about ALPS potentiometer.

Discussion in 'General Electronics Discussion' started by HellasTechn, Sep 6, 2015.

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  1. HellasTechn

    HellasTechn

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    Apr 14, 2013
    Hello everybody.

    I have an old audio amplifier what has a bad volume control potentiometer. Bad thing is that it is an old ALPS potentiometer 4 pin (3+ center tap for loudness control). It is impossible to find a replacement. If i use normal 3 pin potentiometers i can not use the loudless controls.

    It is a 100K potentiometer taped at 50%

    Here is a picture fo what it looks like.

    If i use 50K external resistors to make the 4th pin will it work ?
     

    Attached Files:

  2. Colin Mitchell

    Colin Mitchell

    1,416
    313
    Aug 31, 2014
    Just get RP7 or CRC and with a long screwed shaft, spray into the pot via the shaft.
     
  3. HellasTechn

    HellasTechn

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    215
    Apr 14, 2013
    I wish it was that simple. Unfortunately the resistive surface is damaged and resistance readings are wrong. I do need a replacement
     
  4. Alec_t

    Alec_t

    2,980
    806
    Jul 7, 2015
    Providing your amp doesn't mind seeing an apparent 50k pot (100k in parallel with 100k) you should be ok.
     
  5. AnalogKid

    AnalogKid

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    Jun 10, 2015
    "Will it work?" No.

    A loudness control works by changing the frequency response of the amplifier to compensate for frequency response changes in human hearing at low volume levels. Patents go back to 1939. That 4th terminal probably connects to a capacitor or some other frequency shaping network. As such, it is dependent on the position of the wiper to "know' when to kick in. The simple voltage divider in your 2nd photo will emulate the signal level at 50% rotation but does not change as the pot is rotated, a critical requirement of a loudness tap.

    https://en.wikipedia.org/wiki/Loudness_compensation
    http://www.eleccircuit.com/audio-loudness-control/

    Loudness pots used to be fairly common. Check the surplus market for pots with loudness taps. Also, there are many manufacturers other than Alps.

    ak
     
  6. HellasTechn

    HellasTechn

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    Apr 14, 2013
    It is not in parallel because the signal comeing to the potentiometer goes to the power amplifier while signal from the 4th pin goes to the loudness control circuit.
     
  7. HellasTechn

    HellasTechn

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    Apr 14, 2013
    All that are true.

    the divider i made indeed emulates 50% potentiometer rotation bus still that gives the user a "basic" control over loudness (better than not at all). I already installed it and it works, it is same as original but it works.

    I can find similar potentiometers on-line but it takes for ever to ship here so i think i will just stick with my modification.

    Thank you for your usefull information !
     
  8. HellasTechn

    HellasTechn

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    Apr 14, 2013
    Thank you all for your input !
     
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