The power supply in a high-fidelity amplifier needs to have fairly ``stiff'' lines in the face of dynamic loading. In the specific case of our class A amplifier, the power supply also needs to be able to widthstand a high constant power draw, used in biasing the output stage. In addition, I'd like each component to be rated at twice the average current, voltage or power it will normally see.
A class A source-follower topology approaches 25% efficiency (see  p.652 for a description.) Assuming that our 20W amplifier will have 20% efficiency, we can expect an idling power of 80W per channel! Our single supply must be able to handle a continuous 160W draw. For this reason, we'd like at least a 320VA transformer, and preferably a toroid at that, due to the magnetic field handling.
The dynamic headroom of an amplifier is defined as the ratio of its power at clipping to the ratio of its continuous average power output, in dB. Our 20W amplifier needs approximately 18V rails to supply its rated power into 8 ohms, per the simple relationship:
Our rated voltage is actually goverened simply by the commonly available power toroids. A quick glance at the various manufacturers shows that the lowest voltage center-tapped 330VA toroid has 25VAC secondaries, yielding supply rails of about 35V. Our ideal dynamic headroom with these rails is about 12dB.
The bypass capacitors are going to be of the large aluminum electrolytic breed, and need to be the highest grade we can afford, so as to minimize the typical shoddy performance of electrolytics. A quick calculation based on the rated voltage and the average power delivered to the amplifier shows that each side of the supply is delivering current into a 7.7 load impedance. The time constant on the rectified supply line is goverened by the load capacitance times the load impedance ( ). We'd like as little ripple as possible, but even with of capacitance, our time constant is 0.15 seconds, yielding a ripple voltage of 0.6V!
We'll just do what we can.