The Brick Amplifier (June 1999):
At around half range for the volume control RV1, the impedance at the input of each input stage is a maximum and they can pick up a significant amount of high frequency noise. Although this is mainly too high in frequency to be audible, constructors may wish to reduce the noise level by adding a 56pF ceramic capacitor in parallel with each of the input bias resistors R3 and R4. The extra capacitors can be added to the copper side of the PCB and have no significant effect on the amplifier's high frequency response.

Also when The Brick is connected into a hifi system where none of the existing components (main amplifier, CD/DVD player, etc) has its signal ground tied to mains earth, audible mains hum may appear at its outputs. In such cases a 100k resistor may be connected between the Brick's signal ground and its transformer earthing lug, to remove the hum.