This is a description of a Capacitance Multiplier Power Supply for audio power amplifiers with plus and minus supply. It is desirable to have a low ripple power supply for power amplifiers. A regulated supply is of course an option, but the solution is seldom seen. Because of the large current draw, a regulated supply may become more complicated than the amplifier itself, so the vast majority of power amplifiers are using an unregulated supply. This implies use of very large capacitors. Here is where the capacitance multiplier comes into play. This description shows the necessary components on a PCB to build a power supply for power amplifiers with reasonable current draw. In addition to the PCB you will at least need a fuse, transformer and rectifier.
If the power amplifier is a 25 W class A amplifier, the bias current is a minimum of 1.25 A. For 8 ohms the maximum load current then is 2.5 A. For our capacitance multiplier, it is thus important to keep the power loss to a minimum. As can be seen from the schematic below, we are using a Sziklai Darlington both to reduce the power dissipation and to give the necessary high current amplification. The components in the schematic are are all placed on the same PCB. J0-J5 are the connectors on the PCB. The values shown for the components in the schematic are suitable for a 10-30 W amplifier.
C1 and C2 are the main capacitors following the rectifiers. These will
present a DC voltage with a ripple of about 0.8 V to the capacitance
multiplier for our 25 W amplifier. It is used second
order low pass filters feeding
the medium power transistors Q15/Q16. The capacitance appearing at the
bases of these transistors are multiplied by the current gain in the
Sziklai Darlington pair, giving the name to the capacitance multiplier.
So a 330µF capacitor on the base of Q15/Q16 appears to be at least
an 1 F capacitor between the output and ground.
In the figure it is shown a bare minimum, probably you would add at least a mains switch before the mandatory fuse (F1) followed by the transformer (T1) and rectifier (D1), supplying the capacitance multiplier.
The BOM is shown below. It is of course possible to use other components than shown. One possibility is to use other transistors than proposed, but be sure that the size of the components and pinning is correct. This is important to notice also for other components, when mounting them on the circuit board.
0.6 W metal film resistors with 1 % tolerance was used, but other types are of cause possible. The medium power transistors Q15/Q16 may be changed to the cheaper pair BD139/BD140 without problem, as an example. When it comes to the output power transistors Q17/Q18, nor these are critical, but again, be aware of the housing and pinning.R3-R4, R7-R8, R13-R14 220 ohm