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Thread: Musical Fidelity X-A50 monoblock amp help

  1. #1
    Join Date: Apr 2009

    Location: Ayrshire, Scotland

    Posts: 231
    I'm Ian.

    Default Musical Fidelity X-A50 monoblock amp help

    Trying to bring a pair of these back to life.
    Does anyone know the correct procedure for setting the quiescent current for the output transistors and what this current should be ?
    Technics 1200 tweaked,Ianmac Bearing,DACT DIY Phono Pre, Belcanto CD-2, Engineered Switzerland DIY Renderer, Benchmark DAC2, Hypex NCore, ATC SCM40
    2nd system --Engineered Switzerland DIY streamer, Sony CDP11,Topping D50 DAC, Hypex NCore. Kef 1point2 speakers. Ianmac Filters

  2. #2
    Join Date: Oct 2012

    Location: The Black Country

    Posts: 6,089
    I'm Alan.

    Default

    I have found a picture of a clone board, this has only one adjustable resistor on it.
    Assuming the real thing is similar then this will be the adjustment for the quiescent current. For a 50W amp I would suggest setting the current between 50 to 100mA.

    If you get any distortion at very low levels with 50mA then set it higher.
    You need to measure the current consumption of the pcb, preferably just the current in the output transistors. If you can only measure the total current of the pcb then add another 10mA to the above figures.

  3. #3
    Join Date: Apr 2009

    Location: Ayrshire, Scotland

    Posts: 231
    I'm Ian.

    Default

    I have managed to get a cct of the original MF design and the equivalent for the output transistors.
    There is one pot on the board which will set the bias on the output transistors. I want to change to a multi turn pot
    Ill go for 50mA as you suggest and work upwards if needed.
    Many thanks for your help.
    Technics 1200 tweaked,Ianmac Bearing,DACT DIY Phono Pre, Belcanto CD-2, Engineered Switzerland DIY Renderer, Benchmark DAC2, Hypex NCore, ATC SCM40
    2nd system --Engineered Switzerland DIY streamer, Sony CDP11,Topping D50 DAC, Hypex NCore. Kef 1point2 speakers. Ianmac Filters

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