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I have built this little kit for a class AB Amplifier. This is the schematic if you do not want to follow the link

enter image description here

It works fine, I can hear decent sound.

I then tried to check the output with an oscilloscope to learn a bit more. I am using a bitscope micro USB oscilloscope. I have replaced the speaker with 5 47 Ohms resistors in parallel to cope with the load. The voltage peaks match what I get in LTSpice, but I was a bit surprised to see the straight lines on the descending and raising part of the wave. enter image description here

This is for a 50mV peak signal @ 1kHz (the maximum allowed by the design). The clipping is obvious when I increase the signal, and the strange behaviour is still there even if I halve the signal to 25mV. enter image description here

The device is also a function generator. This is what the input signal looks like (100mV sine wave).

enter image description here

Do you think it is because of the oscilloscope limited sampling or some kind of distortion?

Solution (I think) The answer from bobflux made me think that problem was in the quality of the signal. If you look at the signal trace you can see that there is a lot of noise.

The solution was to introduce a 10/1 voltage divider (10K/1K) and increase the input signal to 1.1 V. This creates a beautiful sine wave (although it is seems a bit attenuated).

enter image description here

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  • \$\begingroup\$ Limited sampling speed almost certainly. \$\endgroup\$
    – Andy aka
    Dec 30 '20 at 15:37
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    \$\begingroup\$ This looks really weird. Can you probe the input signal? Is the input stgnal generated by the bitscope? \$\endgroup\$
    – bobflux
    Dec 30 '20 at 15:37
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    \$\begingroup\$ Hard to tell without expierence with that scope. Perhaps try measuring a known good sinewave with it as a test. \$\endgroup\$ Dec 30 '20 at 15:37
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    \$\begingroup\$ Passing audio though the amp, how does it sound? Is it obviously distorted-sounding? \$\endgroup\$
    – rdtsc
    Dec 30 '20 at 15:51
  • \$\begingroup\$ @bobflux Done. Edited my question \$\endgroup\$
    – Rojj
    Dec 30 '20 at 16:10
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enter image description here

Quantization steps are visible in the waveform. The amplifier can't generate those, so they either come from the (presumably digital) signal generator, or from the scope analog to digital converter.

However... If the quantization steps were from the scope ADC not having enough bits, then the sloped top would be flat and horizontal: it would be a single quantization level. If we're seeing ADC quantization, then the signal as displayed has values between quantization steps, which is not possible.

So, the sloped top happened after the quantization that creates the visible steps. This means either:

1- The quantization steps come from the signal generator (probe signal generator output to investigate)

2- The quantization steps come from the ADC, but some sort of digital processing is applied to the data after conversion, which gives this abnormal result.

In both cases, this is a measurement problem, and your amp is most likely not to blame.

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    \$\begingroup\$ If anything, I'd say that the presence of the "stairs" show enough bandwidth for the amplifier. \$\endgroup\$ Dec 30 '20 at 16:01
  • \$\begingroup\$ Poor LF response, either in the amplifier or the measurement setup. Replace C3 with 1000uF for better bass. \$\endgroup\$ Dec 30 '20 at 16:30

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