Q. I was looking at the schematic diagram of an amplifier and I noticed that there was a 0.01 µF capacitor connected in parallel with the filter capacitor, which was 5,000µF.
The 0.01 µF capacitors value is so much smaller than the filter capacitors that it doesn't make any difference in the capacitance there.
Do you think this is a mistake?
A. No. The smaller value capacitor is there for a good reason. The 5,000µF capacitor is big, physically. It is basically two foil plates and an insulator rolled up into a tube. As a result, the two plates have some inductance.
At higher audio frequencies, and above the audible range, this inductance has a significant amount of reactance. This counteracts the capacitive reactance of the filter capacitor at these frequencies.
This would lead to oscillations in the amplifier, as some of the output signal would get coupled back to the input stages by way of the common power supply line.
The 0.01 pµF capacitor is much smaller, physically, than the 5,000µF one. It has far less inductance, so it is still effective at the higher frequencies. So it has a low capacitive reactance at the higher frequencies, and short-circuits the filter capacitor for them. This prevents oscillations.