A40 Part Substitutions — Nelson Pass / 2000

After 22 years, it appears that quite a few DIY enthusiasts are still eyeing the A40 Class A amplifier design, but are discovering that not a single transistor from this design is still available. The following information is for those scouting out substitute parts: The Lambda output devices are no longer available, and they weren’t that common to begin with. Lambda appears to still be in business, but not in the business of power transistors. Common everyday complementary power darlington transistors will substitute for these. They need to be rated at 75 watts or higher, 5 amps for higher, and… More...

A75 Part 2 — Norman Thagard & Nelson Pass / 1992

THE STORY BEHIND the A75 power amplifier continues. In Part I, we discussed its origins and direct predecessor, the Pass A40, as well as some general guidelines to follow before initiating such a project. We covered such topics as MOSFETs and differential pairs. In this concluding part, we will analyze the amplifier's component parts in depth, beginning with the source. The Power Supply Figure 11 shows the amplifier's power supply circuitry. AC line power enters through the power cord and passes through fuse F1. To reduce the effects of inrush current, thermistor TH1 is employed. At room temperature, this will… More...