Accidental success with LM388K

Discussion in 'The Projects Forum' started by 2015, Feb 19, 2013.

  1. 2015

    Thread Starter New Member

    Feb 19, 2013
    I made a 30 volt power supply with the LM388k and a 24vac 12 amp transformer to eventually use on a 4 x 100 watt Class D amp.
    The circuit initailly put out only 20 volts until the resistor connectiong the input voltage to the input of the LM388k and the base of the 2N3907 opened. Now the power supply puts out 30 volts @ 68 watts. All the heat is across the 2 resisitor and everything else is cool.

    My question; is this circuit an accident or a usable design?
  2. crutschow


    Mar 14, 2008
    In your circuit diagram there is no resistor between the input of the LM338k and the base of the 2N3907. Did you remove it to show how the circuit is now? What was that resistor value?

    Why do you have such a large capacitor at the regulator output? You only need a large capacitor at the rectifier output.
  3. #12


    Nov 30, 2010
    Posting an asc file eliminates several helpers from seeing the circuit. Try a png or a pdf.
  4. tubeguy

    Well-Known Member

    Nov 3, 2012
    If you are supplying 100 watt audio amplifiers the output voltage for 8 ohms would be 28.28 v RMS. I'm afraid you selected the power supply voltage based on RMS voltage. You need rails which supply a Peak to Peak voltage of 2.8 times that, or about 80 VDC. (unless this amp has a built in DC-DC converter)

    Edit: The peak to peak for 100 W at 4 ohms would need to be about 60 V
    Last edited: Feb 19, 2013