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This post features a project from 1981, my second to last attempt at design and construction of a low distortion audio power amplifier. It took several months on the bench, tweaking constant current sources \u00a0before etching the PCB and a few weeks cutting out the aluminium panels and wiring it all together.<\/p>\n
Hitachi’s\u00a0HMA7500 offering would have been a easier option but not as much of a challenge although it was quite expensive back in the 80s. Designed around a complementary pair of 2SK134 and 2SJ49 power Mosfets, I wanted to focus on peak current delivery, slew rate and of course avoiding the cross-over distortion that can ruin class A\/B output stages.<\/p>\n
A clue to how this was achieved to some extent is shown in the under the hood\u00a0photos below. The wiring loom uses thick silver plated PTFE covered wire for all the high current paths with a star earth point to minimise earth loops.<\/p>\n
One early limitation was the single 31-0-31v toroidal transformer I had available, so there’s a pair of heavy duty low ESR Gould 10,000\u00b5F reservoir capacitors hiding under the regulated control power supply board. The capacitors smooth the main \u00b142v DC to each power amplifier. An improved design would have separate toroids for left and right channels but would not fit in the box! This single toroid and caps provide 6% regulation at full output power, an acceptable compromise.<\/p>\n
A regulated +-30v DC supply feeds a rear socket to power a separate preamplifier. This design avoids potential earth loops that could arise from a mains powered preamplifier.<\/p>\n\n\t\t