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

- John Atwood

    Schematics of amplifier circuits tend to show the source of power as an arrow pointing off-page, presumably to a mythical, perfect power supply. This supply provides a rock-stable voltage that doesn't change with varying current demand, with changes in the power line voltage, with changes in temperature, or with changes in time. This concept of the perfect power supply lulls the designer into complacency, with circuits often depending on the perfect behavior of the power supply. The reality is that there is no perfect power supply -- all supplies have imperfections of varying magnitudes. One technique to get around imperfect supplies is to design the circuit to be immune to power supply flaws. Monolithic Op Amps are good examples of this. This technique can work well, but can only be used on certain circuit topologies. Another philosophy is to realize that the power source and its filtering stages are part of the circuit being designed, and thus design the circuit to work with the vagaries of the power supply distribution system. This holistic philosophy takes the most realistic view of the situation but the power supply and amplifying circuits tend to get intertwined. A third approach is to treat the amplifying circuits as still separate from the power supply but to do a good job at making the supply behave well. In this approach voltage regulators are often used.
    All three approaches to power supply/circuit design described above should be appreciated by the designer. However, as a matter of expediency or lack of other possible circuits, the designer will often have to fall back on designing a "perfect" power supply. Before looking into the techniques to achieve such a supply, let's look at what effects such a supply has to deal with, from the point of view of tube audio design:

• Elimination of power line hum
  
• Changes in power line voltage (either spikes or long-  term changes)
  
• Changes in circuit loading (at audio frequencies)
  
• Changes in circuit loading (slow, below audio frequencies)
  
• Disturbances injected from other circuits
  
• Disturbances injected into the power supply that could disturb other circuits
  
• Changes with time and temperature
  

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