[SOLVED] Audio amplifier distorted output

[Herni]

Hey!

I'm building a synthesizer for my school project and for that I am using Maxim Integrated MAX98307 audio amplifier.
The output for my amplifier is a small 8 Ohm speaker, where I am sending sinus waves of different frequencies.

When looking at the output however, there seems to be some kind of distortion that makes the voltage go up and down.
This happens even if there is no input.
As the voltage is frequently jumping, I don't get a very loud volume from the speaker.

The input for my audio amplifier is a DAC and I'm changing the values according to sine wave tables.

One of the pictures below shows the distortion with no input and the other one is a distorted sinus wave.
The scheme is soldered according to MAX98307 Typical Application Circuit diagram.
What could be the cause of such interference?

Thanks in advance!

 

[srizbf]

Did you test the circuit in breadboard wiring ?

 

[KlausST]

Hi,

It is a class D amplifier, no linear one.
Therefore the scope picture looks distorted.....but does it sound distorted also?

Klaus

 

[Herni]

Hi,

It is a class D amplifier, no linear one.
Therefore the scope picture looks distorted.....but does it sound distorted also?

Klaus

Hey,
The sound doesn't actually sound distorted, it's just not very loud and I thought that's because of the voltage going up and down.
I am not very familiar with amplifier types, but obviously should be.

 

[Herni]

No, unfortunately I didn't have time and components to do that.

 

[schmitt trigger]

As Klaus mentions, this is a class-D amplifier.
It is not distortion; it is the carrier frequency

Without a LC post filter (you don't show anything on your schematic) the switching carrier will show up on your scope.
Of course being very high in frequency, you cant' listen it.

 

[barry]

Even if it is class D, that scope waveform looks bad-there are some serious gaps there. It should still have a sinusoidal envelope, which it obviously does not.

But when you say it's 'not loud' that's pretty subjective. How loud is it SUPPOSED to be? And I would definitely put the required inductor on the output before you make any judgements.

 

[Herni]

Thank you all for your help. The problem was in the missing LC filter.

 

[Audioguru]

The MAX98307 is filterless. Each of its bridged outputs cancels the switching artifacts of the other output so the bridged output is smooth. The datasheet shows the awful-looking waveforms on each output wire.