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Small pre-amplifier K8084

Discussion in 'General Electronics Discussion' started by kitao, Sep 30, 2012.

  1. kitao

    kitao

    10
    0
    Sep 23, 2012
    That's my project for october :)

    I built a K4003 (click to enlarge) last month:
    [​IMG]
    [​IMG]
    As you can see the volume control for the cd-player is fixed on a breadboard.
    I don't like that so this week I am going to build the K8084 to make a proper volumecontrol.
    [​IMG]
    http://www.velleman.eu/products/view/?id=369464

    I want to put them on the same transformer, so I don't have to buy a second one.
    The transformer is a 212048c
     
  2. BobK

    BobK

    7,599
    1,641
    Jan 5, 2010
    Well, the transformer was already marginal, having exactly the same current capability as the amplifier's requirement. But, if you are not going to be using the amp at full volume you should be fine.

    Bob
     
  3. kitao

    kitao

    10
    0
    Sep 23, 2012
    Tnx,
    The soldering of the components is halfway.
    I hope to finish it this week so I will let you know if it's working or not :)
     
  4. kitao

    kitao

    10
    0
    Sep 23, 2012
    Ok, this is the end-result :


    [​IMG]
    (click)
    [​IMG]

    And not to my surprise there was a big hum audible, so I was thinking, ****, I really am going to need a metal casing. Then I picked up the pre-amp and just by luck the hum disappeared so I figured the device needed some grounding.
    In the second picture at the right upper corner you can see I attached a clipwire to the mounting hole and the other side of the clipwire is now connected with the metal table-leg and everything is working fine. I am glad with that because I don't want to spent money for casings and such, this is only to experiment before I start with bigger amplifiers.

    There is another thing that came as a little surprise; in the manual it is advised to first put power on the pre-amp and then on the amp. That caused some trouble because I am using one trafo for both.
    In lack of a proper switch I took another clipwire and connected it between Va and the trafo so I just needed to unclip it and then the power would be disconnected.
    Well, that didn't happen, even with Va disconnected the music was still playing ??!!!
    :confused:

    It must have been running on one phase or something.
    Here's a picture that shows Va of the amplifier:
    [​IMG]
     
  5. Rleo6965

    Rleo6965

    585
    9
    Jan 22, 2012
    VB wire and center-tapped wire from transformer was still connected to the board bridge rectifier power supply circuit. Causing to power supply to work only in half wave rectifier thus you still have dc voltage supply of amplifier. But at lower voltage and poor filtering.
     
  6. kitao

    kitao

    10
    0
    Sep 23, 2012
    Ok, to be able to off the amplifier with a switch I will need at least a switch that can disconnect two leads from the trafo at once.
    Tnx
     
  7. Rleo6965

    Rleo6965

    585
    9
    Jan 22, 2012
    Normally, we place spst switch in the primary of transformer. If you place switch in secondary. You need dpst switch with current rating higher than your power supply drawn.
     
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