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I am using a MAX9744 Class-D amplifier and want to get a sine wave in the output. Someone suggests using an LC low-pass filter for this, and in the datasheet of the amplifier, the values for inductors and capacitors are provided. When I constructed the circuit (I attached the schematics), the output was a high-frequency triangular wave, not a sinusoid. The input is 100-1000 Hz sine wave.

Why is this happening? Is it not enough to build an LC filter circuit in the output of the class-D amplifier to get a sine output?

What else do I need to add/change to get a sine wave?

The circuit from the datasheet, I used the values of L and C for 4 Ω load. The inductors that I use are the color ring inductors 0307 1/4 W of 0.47 uH, the image is attached.

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    \$\begingroup\$ Can you confirm that you built the LC-RLC filter as depicted in Figure 8 and described in table 8, and not just an LC filter? No matter what you did: how did you load the filter, and how did you measure, i.e. at which point of your load did you attach your oscilloscope(?) or other measurement device? \$\endgroup\$
    – Marcus Müller
    Mar 27 at 15:42
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    \$\begingroup\$ How exactly did you connect the scope? You said 'probes', did you use two probes on on each side of the speaker, or did you connect one probe with the probe tip to one side of the speaker and probe ground to the other side? Is/are your probe/s compensated properly? \$\endgroup\$
    – GodJihyo
    Mar 27 at 15:52
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    \$\begingroup\$ @BroBro I specifically asked you to reduce the amplitude, not increase it, because, just as tobalt, I'd guess you're just overdriving the input. \$\endgroup\$
    – Marcus Müller
    Mar 27 at 15:57
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    \$\begingroup\$ Show us pics or give us specific part numbers for the inductors you used for L1 & L2. \$\endgroup\$
    – brhans
    Mar 27 at 20:16
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    \$\begingroup\$ _"The inductors that I use are the color ring inductors 0307 1/4Wt of 0.47uF, the image is attached." Several issues here. I assume the uF is just a typo of uH. What's the saturation current on that inductor? I would suspect 100 less than you need, if not 1000. What's "1/4 Wt"? \$\endgroup\$
    – winny
    Mar 28 at 6:57

2 Answers 2

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You are looking at the switching ripple at 300 kHz. This is inaudible, albeit not nice.

Due to the scope's persistence, you can also see that this triangular waveform has a few 100 mV vertical "shift" on different captures. This is due to the instantaneous sine wave component.

Zoom out to a 5 ms time division and you should see the sine wave.

It is possible to suppress the ripple much more with a second LC filter if desired.

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  • \$\begingroup\$ yes, this is what class D amps do by default. \$\endgroup\$
    – danmcb
    Mar 28 at 5:49
  • \$\begingroup\$ unfortunately, I played around with zooming in and out, the wave is still triangular. \$\endgroup\$
    – mangozu
    Mar 28 at 7:11
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    \$\begingroup\$ @BroBro I agree with tobalt. What you are seeing is 300kHz 40mV pk-pk, which is certainly class-D ripple, and not the low frequency analogue wave. Show us a 5ms/div & 1V/div scope trace. \$\endgroup\$
    – Simon Fitch
    Mar 28 at 9:47
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The normal for class-D 300kHz is already reduced to a very low level so what is the problem? I think the filter will work better if the circuit is built compactly on a pcb and if the inductors are larger. My very low power solar garden lights use a small inductor like that.

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