HIGH-END POWER AMP-High-class watts （5）

Published:2011/8/21 21:29:00 Author:Phyllis From:SeekIC

By Ton Giesbets

Power supply

A separate printed circuit board has been designed for the power supply (Figure 4). It just fits into the enclosure between the transformers. The power supply design is quite conventional, with a standard 80-VA toroidal transformer, rectifier, and electrolytic smoothing capacitors for each channel. To make it easier to connect the transformers, we also designed a separate circuit board with three terminal blocks and two fuses (Figure 5) to allow the primary leads of the two transformers for a stereo amplifier to be properly connected and fitted. The secondary leads of the transformers are connected to the printed circuit board holding the smoothing capacitors and rectifier diodes. The electrolytic smoothing capacitors are quite ordinary types (6800 uF, 35 V) with radial leads and a diameter of 25 mm (there’s no room in the enclosure for anything larger). The type used here has a quite low profile (only 30 mm), so there’s more than enough room in height.







Due to the relatively low supply voltage, we used Schottky rectifiers made by ON Semiconductor. They are rated at 100 V / 20 A peak. The forward voltage drop per diode is only 0.5 V at 3.5 A.

Three-way circuit-board terminal blocks are used for all external leads to simplify fitting the wiring and any necessary maintenance. There are also two LEDs on the circuit board (one per channel) to serve as supply voltage indicators. If the mains switch does not have a pilot lamp (on the front panel), they should be used instead.

Construction

The compact, double-sided printed circuit board for the amplifier is shown in Figure 6. We don’t often use double-sided boards for Elektor Electronics final amplifiers, but here this provides definite advantages and yields shorter signal paths between the components. Fitting the components to the board should not pose any particular difficulties (no SMDs this time), but you do have to work accurately. All of the components are very close together, which means you have to fit them tidily, use components with the proper dimensions, and take care when fitting them (correct polarity and value).





There are a few details that deserve special mention. LEDs D9 and D10 must be fitted with their flat sides in contact with T3 and T4 for good thermal coupling, in order to make the current sources temperature-independent. Transistors T7-T10 are screwed to a common heat sink. Use a piece of aluminium for this, with dimensions of 85 x 38 mm (the same as the circuit board) and a thickness of 1.5-2 mm. The height is such that the top panel of the enclosure can just be fitted in place. Use ceramic insulators (Fischer type AOS 220) for all four transistors, and screw them in place in facing pairs.

Use the specified low-inductance resistors for R34 and R35, since otherwise there’s a very good chance that the amplifier will be unstable.

Coil LI can be made by winding 1.5-mm diameter enamelled copper wire on a 10-mm drill bit (the smooth part, of course). Leave fairly generous leads and scrape them nicely clean with a small knife. Then insert R36 in the coil, bend over its leads and fit the assembly to the circuit board. Then solder the coil and resistor in place. Cut off the free ends of the coil leads after they are soldered, but not too close to the circuit board.

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