Jul 2003 OnSemi released a new range of transistors, designed specifically for audio applications. These new transistors have been tested in the P68, and give excellent results. As a result, all previous recommendations for output transistors are superseded, and the new transistors should be used.
The transistors are MJL4281A (NPN) and MJL4302A (PNP), and feature high bandwidth, excellent safe operating area, high linearity and high gain. Driver transistors are MJE15034 (NPN) and MJE15035 (PNP). All devices are rated at 350V, with the power transistors having a 230W dissipation and the drivers are 50W.
Note:
- The total DC is over 110V and can kill you.
- The power dissipated is such that great care is needed with transistor mounting..
- The amplifier circuit is NO SHORT CIRCUIT PROTECTION. The amp is designed to be used within a subwoofer or other speaker enclosure, so this has not been included. A short on the output will destroy the amplifier.
- Transistor MJL4281A and MJL4302A are new most constructors will find that these are not as easy to get as they should be. The alternatives are MJL3281/ MJL1302 or MJL21193/ MJL21194.
All three driver transistors (Q4, 5 & 6)must be on a heatsink, and D2 and D3 should be in good thermal contact with the driver heatsink. Neglect to do this and the result will be thermal runaway, and the amp will fail. For some reason, the last statement seems to cause some people confusion - look at the photo below, and you will see the small heatsink, 3 driver transistors, and a white "blob" (just to the left of the electrolytic capacitor), which is the two diodes pressed against the heatsink with thermal grease.
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