AC to DC Power Supply for an Amplifier

Discussion in 'The Projects Forum' started by logans-electronics, Sep 8, 2009.

  1. logans-electronics

    Thread Starter Member

    Sep 1, 2009
    I am in the basic building stage (just trying to understand the concept)....
    I have not made any purchases yet.
    I plan on building a Class A, Class B, or Class AB amplifier in the next few months. I have many steps to take before that….. This is step one form me.
    From what I have read so far...I am having trouble finding formulas to prove the this basic design. ....see attachment...:confused:

    1. Should I use the bridge rectifier? I can't seem to understand what the max spec values really mean.
    2. What size in uf capacitors should I use and why for filtering?
    3. Will the capacitor values affect anything other than filtering?
    4. What is the calculated current available for the output?
    5. Once loaded down, should the +20V rail have the same load value as the -20V rail?...I plan on using the positive rail for some other 12v and 5 v regulator to drive small sub circuits.
    6. Any other design considerations that I should research, modify, or evaluate?
  2. beenthere

    Retired Moderator

    Apr 20, 2004
    1. Practically speaking, bridges are really cheap. The one that is rated 50 volts @ 25 amps should be more than adequate. Due to greater sales of that size, the 400 volt 35 amp bridge may be cheaper. The load (amp) will need some amount of current. But that will depend on the design of the amp, and be limited by the capacity of the transformer. At this point, this a cart before the horse.

    2. The filter caps store energy to source current when the transformer voltage is too low to keep the rails up. Again, like bridges, capacitors are cheap. You want to select ones with at least a 50 volt rating as they should charge to around 28 volts. 4700 uF should be plenty, but 10,000 uF is not overly large. This is another calculation that depends on the type of amp - class A will need more filtering than AB.

    3. If you place too much capacitance on the bridge, the room lights will dim briefly as they take an initial charge.

    4. Beats me - what is the rating of the transformer? That is the limiting device.

    5. Yes, loads on the rails should be symmetrical. If you are going to add stuff to run on the +28 rail, consider another transformer to handle that power.

    At this point, concern about the power supply is getting ahead of yourself. The actual amplifier design needs to be settled on first. Class B is a disaster for audio, so your choices really get to be class A or AB.

    Class A is impressive as to cleanliness of the output, but not efficient at all. My class A headphone amp dissipates 45 watts on a side for an output of maybe 2 watts. I would focus on AB designs for anything but headphone usage.

    Once you know what the amp's power requirement is, then you can design the power supply.