WARNING! High AC & DC voltages are present on the power supply and power amplifier modules when power is applied. In particular, make sure you don’t get across the two 40V AC input terminals on the power supply. The 40V AC transformer windings that connect to these terminals are wired in series, so there’s 80V AC between them. Similarly, note that there is 110V DC between the +55V and -55V supply rails, both on the power supply module and the amplifier module. Do not touch any of this AC or DC supply wiring (including the fuseholders) when the amplifier is operating, otherwise you could get a very nasty shock which could even prove fatal.

This new 200W audio amplifier module gives superlative performance – better than any of our previous class-AB amplifiers. That’s been made possible by the use of On Semiconductor’s new ThermalTrak power transistors, a circuit based largely on our high-performance class-A amplifier (published in 2007) and a new double-sided PC board with plated-through holes.

As mentioned last month, the double-sided PC board is critical to the performance of this amplifier module. It not only simplifies the supply wiring but has also been designed to largely cancel the magnetic fields produced by the asymmetric currents drawn by each half of the class-B output stage.

In addition, the double-sided board eliminates the need for wire links, the exception being a couple of 0W resistors.

The assembly is really quite straightforward although there’s a fair bit of work involved to do the job properly. When building a high-power amplifier module like this, it’s important to take your time, do a neat job and check your work carefully at each assembly stage. After all, blowing up expensive output transistors can be a real pain.

Transistor quality

We’ll begin the assembly details shortly but first a word about the transistors used in this module.

To ensure published performance, the NJL3281D & NJL1302D power transistors must be On Semiconductor branded parts, while the 2SA970 low-noise devices must be from Toshiba. Be wary of counterfeit parts (although it’s probably too early for counterfeit versions of the power output devices).

We recommend that all other transistors used in this project be from reputable manufacturers, such as Philips (NXP Semiconductors), On Semiconductor and ST Microelectronics. This applies particularly to the MJE15030 & MJE15031 output driver transistors.

PC board changes

Fig.9 shows the parts layout on the double-sided PC board. This board is coded 01108081 and measures 135 x 115mm. The orange tracks and pads show the copper on the top of the board, while the blue-grey tracks are on the underside of the board.

The first thing to note is that the PC pattern differs slightly from that used for the prototype module. That’s because we subsequently decided to increase the number of vias used to link the top and bottom supply rail tracks. Up to 4.5A peak can flow through each output transistor when the module is operated into a 4W load, so it’s important to ensure sufficient current-carrying capability.

However, the main reason for increasing the number of vias was to make sure that a fault in the output stage would not cause the vias to fuse, instead of the 5A fuses blowing. If that happened, the board would be difficult to repair, as the solder mask goes right up to the edges of the vias.