Well, earth something! (But only one thing)
it can be the transformer centretap, the ground of the input socket, the 0V of the smoothing caps etc.
Then see what happens. Try covering your cardboard case with cooking foil and earth that.

I’d say that a few multilayer ceramics close to the switching FETs would be more important the low ESR electrolytics, but low ESR electrolytics are better than ordinary ones in most cases. (in some situations the addition of the ESR makes it work like a snubber)

 

Well, earth something! (But only one thing)
it can be the transformer centretap, the ground of the input socket, the 0V of the smoothing caps etc.
Then see what happens. Try covering your cardboard case with cooking foil and earth that.

I’d say that a few multilayer ceramics close to the switching FETs would be more important the low ESR electrolytics, but low ESR electrolytics are better than ordinary ones in most cases. (in some situations the addition of the ESR makes it work like a snubber)

Thanks for the tips! The aluminium foil seems to be a good idea anyway, because when I put my phone or a working RF transmitter near the cardboard box I can hear high frequency noise coming from the speaker. Some kind of coupling, I think.

 

Does the humming change in amplitude when you change the volume control?

 

Thanks for the tips! The aluminium foil seems to be a good idea anyway, because when I put my phone or a working RF transmitter near the cardboard box I can hear high frequency noise coming from the speaker. Some kind of coupling, I think.

What kind of inductor did you use in the output filter?

 

Does the humming change in amplitude when you change the volume control?

I have tried to play with the volume control while no sound was fed into the amplifier. The humming stays same no matter where I turn the volume control.

 

What kind of inductor did you use in the output filter?

I have used a 47 uH ferrite core inductor with a peak current of 5A.

 

I have used a 47 uH ferrite core inductor with a peak current of 5A.

What shape?

 

I have tried to play with the volume control while no sound was fed into the amplifier. The humming stays same no matter where I turn the volume control.

Then it appears that the hum is coming from the power supplies. What you can hear is probably 100 Hz from the full wave rectified supply. 50 Hz is below most people's hearing threshold. I am assuming that you used twisted pairs or shielded cable from the input connectors to the circuit board. Does the noise diminish if you short out the input connectors? If so, it is being picked up by the input wiring.
If not, try using more smoothing on the amplifier power supply.

 

What shape?

Radial, through-hole ones.
This kind:

 

Then it appears that the hum is coming from the power supplies. What you can hear is probably 100 Hz from the full wave rectified supply. 50 Hz is below most people's hearing threshold. I am assuming that you used twisted pairs or shielded cable from the input connectors to the circuit board. Does the noise diminish if you short out the input connectors? If so, it is being picked up by the input wiring.
If not, try using more smoothing on the amplifier power supply.

You might be right Keith. I'm using a shielded, twisted input cable. Shorting the inputs to ground didn't help. Guess I will have to include an inductor in between my two parallel power supply capacitors in a pi filter fashion.

 

Radial, through-hole ones.
This kind:
View attachment 259669





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That's not the greatest shape to avoid radiating magnetic flux all over the place, but I don't think that's your problem.
I agree with @KeithWalker
Does the hum change if you pick up the transformer and move it about? (especially tilting it)

 

That's not the greatest shape to avoid radiating magnetic flux all over the place, but I don't think that's your problem.
I agree with @KeithWalker
Does the hum change if you pick up the transformer and move it about? (especially tilting it)

Well, I tried to tilt the transformer and a move it a bit (as much as the wires let me), but the humming remained the same. Ian, would an axial inductor be better for the output filter or I should use a toroid one?