What Is Happening Here?

[gibon0695]

I've built an amplifier circuit.

I hooked it up to an oscilloscope and I get this response at the lower bass frequencies, at 100% volume.

I am only worried about this for analysis purposes. The amplifier is Class D and distortion is expected at 100% volume.

Is this pure distortion, clipping, or is something else going on?

Cheers

 

[(*steve*)]

There's something wrong there. What does it look like at lower volume settings?

 

[gibon0695]

There's something wrong there. What does it look like at lower volume settings?

At around 75% volume it looks like this... it works. That image above in the OP occurs at 100% volume. What could cause this?

As the frequency gets higher the sine wave gets better.

I think it is caused by a lack of power at the lower bass frequencies. It does not do this at higher frequencies. Could I be right?

 

[(*steve*)]

Does it happen suddenly or does it progressively get worse as the volume is increased?

 

[shrtrnd]

What part numbers are the components you're using for your amp?
21V at 20Hz makes me wonder if this is a circuit problem, over-driving the circuit components, or maybe a limitation
on the scope value parameters for the functions you've selected. Measurement instruments are precise, but you
have to be aware of their limitations. Triggers/filters/ automatic functions can kick-in so that what you see on the scope isn't always what's actually there.

 

[Sunnysky]

check your Vdc supply rails
I looks like insufficient storage caps for low frequency power.

 

[(*steve*)]

Curiously, only on one rail.

The waveform looks like a discharging capacitor.

I can see this happening if the demanded output voltage exceeds the supply rail and the power supply is incapable of supplying sufficient current to maintain even that lower voltage.

However, the small class D amplifiers I've seen use a single ended power supply and what is effectively an H bridge to drive the load.