The Tiny, Awesome Class D Amp


In one of [Hans Peter]’s many idle browsing sessions at a manufacturer’s website, he came across a very cool chip – a 10 Watt class D amplifier chip. After the sample order arrived, he quickly put this chip in a box and forgot about it. A year or so later, he was asked to construct a portable boom box kit for a festival. Time to break out that chip and make a small amplifier, it seems.

The chip in question – a Maxim MAX9768 – is a tiny chip, a 24-pin TQFP with 1mm pitch. Hard to solder freehand, but this chip does have a few cool features. It’s a filterless design, very easy to implement, and perfect for the mono boombox project he was working on. A simple, seven component circuit was laid out on a breadboard and [Hans] got this chip up and running.

For the festival, a breadboarded circuit wouldn’t do. He needed a better solution, something built on a PCB that would work well as a kit. The requirements included the MAX9768 chip, a guitar preamp, stereo to mono summing, and through-hole parts for easy soldering. The completed board ended up being extremely small – 33.6mm by 22.5mm – and works really great.

After the festival, [Hans] found a 20 Watt chip and designed an all-SMD version of the board. Just the thing if you ever want to stuff a tiny amplifier into a crevice of a project.

21 thoughts on “The Tiny, Awesome Class D Amp

  1. I remember when Class D amps were just theoretically possible. ;-)

    Generate a high amplitude, high frequency square wave. Modulate it with the incoming analog audio then convert the modulated square wave to a sinusiodal analog output, keeping the higher amplitude. The output waveform has lots of “fuzz” along it but it’s low amplitude relative to the audio signal and way above 20Khz.

    Benefits are very low power use compared to the best push/pull Class A transistor amps and hellaciously less heat output.

    1. Elektor magazine published a 555 based class D amp design just recently.

      I had a play with one of those Maxim filterless class D amps but my soldering wasn’t nearly so neat. After reading the datasheet I was paranoid about keeping the current carrying wires nice and thick to minimize instability. Does it work ok all the way up to 10W in a DIP header like that?

      1. Not at all.
        The DIP header was a quick way to prototype the design. Breadboarding was no good. the output is filled with 300kHz switching noise and the breadboard is not a good place for power traces. I ended up doing a birds-nest construction on top of the DIP socket, directly at the solder joints. the result was the heat creeping up on the chip desoldering the tiny wires. I got them back in place and the amp was working okay but still with noise issues. They were all solved with a proper PCB.

    2. @Galane: I’ve never heard of such a class D scheme.

      The typical method is to compare a triangle wave against the audio signal and use the resulting PWM to drive a full or half H bridge. With an LC or LCR low pass filter before the speaker.

      1. The output is sort-of a modulated square wave, since that’s what PWM is. Tho you couldn’t take an actual square wave and modulate it to do that. Maybe Galane’s thinking of some kind of oversampled 1-bit DAC method (and I’m too hungry to check if that’s what I’m thinking of).

    1. One could also try the Class-D amps from Texas Instruments. Those are fairly popular among chinese class-D amp modules (see Sure electronics). Some even do up to 300 – or was it 600? – watts output with a bit of active cooling.

    2. The only downside to these chips is the terrible THD+N performance.
      The advertised 15/30W are not all that useable, a good rule of thumb is to halv the value to get the useable power output.

      1. This. If my memory serves me, the TA20xx series will typically run at 0.1% THD just shy of their midpoint, but by the time you’re up to the max output, you’re passing 10% THD with ease.

        For sake of headroom, once you’ve determined the max power (Watts) you’ll *actually* need (probably lower than you think, thankfully… no you really don’t need 100w so long as your speakers don’t suck), go for a class D amp with a stated output of 2.5x – 3x greater. You’ll stay good and low on the distortion, but still leave yourself plenty of room for the occasional peaks. Beyond that, be aware that the power supply is typically the choke point for Class D amps. If it says it needs 2 amps, give it a 6 or 8 amp supply (under heavy use a 2a is going to briefly draw at peaks near 5a, if not more, if you give it the chance to do so)… you might not ever need that much, but you might. For minimal extra cost, your amp will thank you. Throw some larger caps on the amp’s power input while you’re at it, and you’ll have a very happy, healthy class D amp that will serve you very well.

        1. Little known fact – Hi-end Logitech computer speakers (for example Z-5500) are power rated at 10% THD, with sub rated at 100Hz (normally you would rate subwoofer at 10Hz, but Logitech one cant handle 10Hz :D)

          for comparison ~5x cheaper average home theatre system (dvd/amp/speakers combo) is rated at 1%.
          Computer speakers is a special niche for the deaf I guess :)

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