Capacitance Multiplier Power Supply

Introduction

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.

Schematic

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.

This capacitance multiplier reduces our 0.8 V ripple to less than 1 mV, and the power loss for each output power transistor Q17/Q18 is about 4 W for our 25 W amplifier example. However, to have such low level of the ripple, the finished build must address the wiring and layout. The resistors R11/R12 ensure that there is sufficient voltage across the capacitance multiplier to allow for short term variations. These resistors can be used to set the output voltage, but be aware that a reduced value will increase the power dissipation in addition to lower the output voltage.
 

Transformer and rectifier

A full power supply in addition to the capacitance multiplier typically would consist of a fuse, transformer and rectifier as shown below. For a 25 W amplifier, a minimum of 2x18 V transformer is needed. The VPOS, VNEG and GND outputs from this circuit are connected to the inputs on the capacitance multiplier PCB.

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.

Capacitance multiplier layout

It is necessary to have heatsinks for the power transistors Q17/Q18 as shown in the layout below. In addition, it is necessary to take into account the size and pinning of the capacitors, please see the BOM below. The PCB measures 99x76 mm.
 


The inputs are at the top of the PCB, while the outputs are at the bottom. Please use bleeding resistors at the output if you are testing the capacitance multiplier without a load.

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.

BOM

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
R11-R12 12 kohm
R19-20 100 ohm

C1-C2 22000 µF 35 V Electrolytic D 30.0 mm P 10.0mm
C5-C6, C9-C10 330 µF 35 V Electrolytic D 10.0 mm P 5.0mm
C21-22 1000 µF 35 V (50 V) Electrolytic D 16.0 mm P 7.5 mm

Q15 KSC3503 TO-126
Q16 KSA1381 TO-126
Q17 KSA1943 TO-247
Q18 KSC5200 TO-247

J0-J5 PCB Tab P 5.0 mm

Heatsink SK129

 
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Knut Harald Nygaard