Power Supplies for Audio
Powering an electronic circuit is a complex issue. Signal processing circuits vary in their ability to reject interference from supply voltage fluctuations. This ability is measurable and is called the power supply rejection ratio (PSRR). Any interference on the power supply output will be passed into the electronic circuit, at a reduced level, governed by the PSRR of the circuit. For example, if the PSRR is 20 dB, the amplitude of the interference will be 10 times lower than the power supply fluctuation itself (40dB is 100 times lower, 60dB is 1,000 times lower, 80dB is 10,000 times lower and 100dB is 100,000 times lower). Different signal circuit topologies have different PSRR's which can be quite low particularly at high frequencies. When power supply interference mixes with the signal, it masks low-level information and causes inter-modulation with the signal. This destroys the integrity of the music signal. For the highest performance from audio electronics, great care is required in the design of the power supplies to minimise power supply induced problems.
To ensure no interaction with the powered circuit, an ideal power supply, should have no output voltage fluctuations under any load conditions, which means that the output impedance should be zero at all frequencies of operation. You can't generate a voltage across zero impedance. Also, it should not allow interference to break through from other sources like the supply line, such as rectifier diode switching, digital clocking and radio frequency interference (RFI) etc via the mains supply. This implies that the power supply should have infinite PSRR of its own power source.
Unfortunately, due to the limits of the various power supply design options available, power supply interaction will occur. The level of interaction is governed by the ability of the power supply to approach the ideal performance of zero output impedance, and infinite supply rejection of it's power source, at all frequencies of operation. It is logical to conclude that the better the power supply performance in these areas, the better the audio performance. Practical application proves this to be true.
Any regulator design worthy of consideration in high performance audio systems must offer a low output impedance over a very wide bandwidth (much wider than the audio bandwidth to deal with digital and RF interference). It should also have a high slew rate, fast rise-time and fast settling time to control the regulator output voltage during fast varying load current transients. It should be noticeably faster than the circuit to be powered to avoid load induced output fluctuations. It should also be quiet enough not to compromise the noise performance of the audio circuits it powers.