As a follow up to Tom's comments, I want to make some general ones. These apply to regulated or unregulated supplies, and higher-voltage supplies for tubes as well.
The power supply for an audio application really is half the sound of the amplifier, and thus it is important to get it right. Basically, the most important thing is having low output impedance with lots of energy storage for meeting the demands of the load. Often regulation is not needed, but it often provides a lower cost way of removing line-frequency ripple. It also can be very advantageous for circuits with high gain at low frequencies, especially ones with somewhat less than stellar power-supply rejection such as tube phono preamps. If active regulation is used, it should be isolated from the feed from the audio stage with a series impedance such as a resistor or inductor, and then one or more effectively large shunt capacitors to ground that feed the audio circuit directly. One large electrolytic capacitor is usually sufficient for tube preamps, and several in parallel for tube power amps.
On the subject of bypassing these electrolytics one needs to look at the high-frequency requirements of the particular load or circuit. For most tube audio circuits the impedances are high and bypassing is not needed because the impedance of the typical electrolytic is much lower and dominates. For low-voltage high-speed digital loads, absolute care must be taken in bypassing as the impedance if the power supply must be kept low into the hundreds of megahertz region. This is the domain of surface-mount ceramics on transmission-line configurations. Extending back down into audio applications, high-speed op amps require bypassing as well, although not as rigorous. Here, a leaded ceramic capacitor or pair of capacitors is generally effective without transmission line requirements, but leads should be as short as possible for best performance. Inadequate bypassing can lead to oscillations that make the op amp perform improperly, and while it may seem to have the right output it may not sound as good as it could because something is going on that shouldn't.
One more note on the type of capacitor best suited for bypassing. All capacitors have a self resonance where the impedance falls as the frequency rises at a 6-dB slope, then takes a sharp dip towards zero at a faster rate (self resonance), and then rises as the inductive properties take over. The higher the Q of the capacitor, the sharper the dip. A strong self resonance can actually cause problems in the circuit, and it is usually better to have a capacitor that can absorb energy and dissipate it as heat rather than exciting an undamped resonance. So ceramic capacitors are generally the best choice here because they are somewhat lossy. High Q film capacitors are best for coupling in audio circuits, but not appropriate for bypassing, although in a small number of cases film caps may be used in high-voltage applications.
Hope this helps-David
|