the system's hum. This is the least difficult with circuits that cancel power supply noise. The traditional approach to power supply hum is to add sufficient inductance and capacitance to the power supply. A voltage regulator can act as a "capacitor multiplier" and make hum filtering easier.
Changes in power line voltage
(either spikes or long-term changes)
The ability to handle this is commonly called "source reg-ulation." Slow changes in the supply output voltage will change the operating point of circuits, and can change the sonics of an amplifier, especially with single-ended designs. Spikes or other brief disturbances can leak through and inject thumps or clicks into the signal path. Voltage regu-lators can be quite effective in stabilizing the power supply output voltage, but have a distinct "drop-out" voltage. If the power source falls below this voltage, the stabilization is suddenly lost and other benefits of regulation, such as hum reduction and low output impedance, are lost as well.
Changes in circuit loading
(at audio frequencies)
Many amplifier circuits rely on the power supply to absorb rapid current changes at audio frequencies and not change their voltage, much the same way as a brick wall stays firm despite pushing and pulling on it. A perfect voltage supply has an output impedance (that is looking back into the supply) of zero. This source impedance parameter is what is important here. This should be low and constant over (and in many cases beyond) the audio frequency range. A large capacitor is a common way of getting low source impedance, but large capacitors tend to be electrolytic, with sonic disadvantages. Regulators can work well here, but not all work well at all frequencies. Also, certain circuits, such as power amps driving very reactive loads, actually will try to push current back into the supply. Many regulators can't handle this.
Changes in circuit loading
(slow, below audio frequencies)
The ability to handle this is commonly called "load regulation." Slow changes can come from circuits being turned on or off, drift in tube currents, etc. The main problem in tube amps is current change due to Class AB or Class B operation of push-pull output stages. The supply current will vary at a rate that depends on the average intensity of the signal. Depending on what's playing, this can vary at frequencies from the low audio range down to essentially zero Hertz. The common solution to this back in the 1970's and 80's was the add-on capacitor box full of computer-grade electrolytic capacitors. Aside from the tremendous current surges these capacitors had on the power supply, they never effectively regulated at very low