Class-D (or A) amplifier from a SMPS possible?

Discussion in 'The Projects Forum' started by RogueRose, May 16, 2015.

  1. RogueRose

    Thread Starter Member

    Oct 10, 2014
    I was reading about class D amps and it said that they are similar to SMPS (switch mode power supplies) and in a few articles I read some people commenting that it would be "easy" (relative term I know...) to make a nice class D amp out of some of the supplies they had. I've read about each of these devices and it seems they do have a lot in common and was wondering how difficult it would be to base an amplifier off of one of these supplies?

    IDK if using 2 identical supplies might be better, running out of phase, one doing the "push" the other "pull" on the speaker.

    Another idea, which is just as interesting to me, would be a Class A amp powered by one of these SMPS. It seems that one of the largest components of large receivers/amps are the transformers in 120/240v systems. I was told that these supplies could be used in this application as well.

    For the class D I'm mainly looking at making an amp that produces frequencies between 20-200 Hz (or anything workable).

    If anyone has any knowledge in this area and could point me in the direction of what would need to be done to do this (if even possible) and what to research I would greatly appreciate it!
  2. crutschow


    Mar 14, 2008
    Why do that rather then just build a Class-D amp with parts made for that purpose which will likely have better performance?

    I have seen subwoofer amps that use a SMPS power supply whose voltage tracks a few volts above the peak output signal voltage (sort of a preregulator) to a Class AB amp called TD (Tracking class D). That significantly increases the efficiency of the Class AB amp will still giving the low distortion of a linear amp.
    Last edited: May 19, 2015
  3. atferrari

    AAC Fanatic!

    Jan 6, 2004
    Sorry for the digression Carl, and what about the switching noise then? Quite high to be actually heard?
  4. R!f@@

    AAC Fanatic!

    Apr 2, 2009
    Google Cold Amp
  5. crutschow


    Mar 14, 2008
    It's made to be higher than 20kHz so is inaudible.
    atferrari likes this.
  6. AnalogKid

    AAC Fanatic!

    Aug 1, 2013
    A switching power supply basically is a series pass element between a high voltage and a low voltage. As such, the output is great at sourcing current but lousy at sinking it. This is different from the requirements for an audio amplifier driving a resistive/inductive load. A quality audio amp must have a symmetrical output impedance when sourcing current to *and* sinking current from the load.

    Also, the control loop for a power supply is optimized for the nominal output voltage range, typically +/-10%. This is way smaller than the output voltage range needed for an audio power amp. Both the magnetics and the control loop might fall apart at the low signal peaks.

    Finally, most industrial switchers run around 100 kHz, while most class D amps run around 250 kHz or higher. Both are ultrasonic, but the main noise contributions are subharmonics and quantizing and both of these increase with the lower switching frequency.

  7. crutschow


    Mar 14, 2008
    If you only need to go to 200Hz I assume the speaker is some sort of subwoofer and shouldn't have much response to frequencies in the high audio range.