Power Supplies
Power amplifiers
A power amplifier is basically nothing else than a modulated
power supply. Any weaknesses in the power supply will inevitably
degrade the sound. The power supply must be rock stable to
preserve imaging and dynamics when the amp works hard close to
clipping. I cannot stress this enough, a 20W design with a
stiff power supply will outperform a 100W design if the latter
has a poor power supply.
My rule of thumb is to pick a transformer with at least 8 times the power of the amp. 20W -> 160VA, 50W -> 400VA. This is per channel.
As for the size of the capacitance after the rectifiers you can never use too much, I squeeze in as much as I can on the available PCB (in the PCB Design section it is explained further why PCB mounted caps are vital). It is not a good idea to place some 100.000 uF of capacitance far away from the output devices.
Another important thing is to have a stable and quiet supply for the input and intermediate stages, the improvement in smoothness and resolution is significant. The simplest way is to use and RC-link connected to the power supply feeding the output devices. 12 ohms / 10.000 uF gives a further 37dB of ripple reduction at 100 Hz. More complex solutions are possible but I prefer to keep the complexity low to increase the reliability of the amp.
Use two full-wave rectifiers to derive the symmetrical supply voltage. Connect the (-) of the upper recifier with the (+) of the lower rectifier. This point is the system ground. By doing this you avoid the large charging current spikes for the power supply caps being routed through the system ground. This concept is pretty much standard in high-end equipment. Note that the transformer must have two separate secondary windings for this topology to work, a centre tap is no good.
Low level circuitry
Also here the scheme with two full-wave rectifiers should be
used. Charge current spikes in the system ground of nA circuitry
is no good (nA in terms of what defines the low level resolution
of an amp).
For regulating the voltage the LM317/337 is pretty much standard in the audio industry today and often works fine.
For even better performance shunt regulation is recommended. A ripple rejection of 80 dB from DC to over 1 MHz can be achieved with shunt regulation. I will expand more on this topic at a later date. Stay tuned...
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