Class D switching?

[bjmsam]

I recently installed a subwoofer (JBL CS1214) powered by a cheap class D amplifier (Domanki SDT1000.1D) beneath the cargo deck of my SUV.

While setting the gain, my basic digital oscilloscope (FNIRSI DSO-510) displayed a clean wave on the line-level input from the head unit to the amp but a more complex wave on the high-level output from the amp to the sub. My oscilloscope does not support FFT but I detect no harmonics. Here is what I see when "zooming in" on the wave:

Is that the 100kHz triangle wave described here?

This is a quick overview of how class D amplifiers operate. Note the “D” does not mean “digital”; it is simply the next letter in order as standards bodies enumerated amplifier types (A, B, C, D, E, F, G, H, I, etc.) It takes an analog, not digital, input and produces a pulse width modulated (PWM) output.
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Here is a picture showing some of the waveforms inside the amplifier. The input at the top is a ~10 kHz sine wave. The triangle generator is producing a 100 kHz signal as shown in the middle. The bottom plot shows the comparator’s output. It creates a pulse train similar to that shown with a DC reference, but now the pulse train produces much higher-frequency pulses. Notice the high pulses are still generally longer as the input signal is higher, and shorter as the input signal drops below 0 V, but the pulses occur and a much higher rate. Now a simple filter can remove the high-frequency switching noise to create a smooth analog output.

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Is this "normal" for a budget class D amp, and is the speaker safe as Texas Instruments seems to suggest in their Application Report, Measuring Class-D Amplifiers for Audio Speaker Overstress Testing?

At first glance, the amplifier may appear to violate the speaker specifications by measuring the unfiltered output voltage. However, the output voltage may actually meet the specifications, because only the audio frequency band needs to be considered.

The sub sounds clean and undistorted, so I'm hoping it's good to go, but I'd rather be safe than sorry. Thanks for any insight!

 

[DVDdoug]

It doesn't look pretty but It seems to be low-level and way above the audio range. Certainly above what the woofer can reproduce. The speaker coil is likely acting like an inductor (higher impedance at higher frequencies) so that means less current & power too.

It should be fine but you never should have looked at it!

 

[mhardy6647]

It should be fine but you never should have looked at it!

Yup.

 

[wwenze]

Yup, 100kHz is definitely the switching frequency. The class D output filter does not remove it completely (nothing mathematically can), but rather reduces it to, for example, -40dB relative to 5W into 4ohm. Doing the math in the below example, 0dB is 2 * 6.3V peak voltage so -40dB would be 2 * 63mV. So that's right around the same order of magnitude that you got.

I don't know what is the industry's acceptable value for residual switching noise tho i reckon the FCC would likely say something. (Since they don't let me be or let me be me so let me see, dang I'm old)