ark (09/02/82)
I am building the Heathkit AA-1800 power amplifier (I THINK I have the model number right). It's a HULK -- 250 watts/channel and weighs about 50 pounds. I finally decided to buy that one because Heathkit is having a sale (30% off!): at that price I couldn't find anything with anywhere near as much power. The amplifier has three circuit boards: a fairly elaborate protection circuit and one board for each amplifier channel. Although the protection circuit is quite complicated, the amplifier boards are still more complicated. Each amplifier board has about 28 transistors, including eight power transistors in TO-3 cases. Said power transistors mount to a huge heat sink, one for each channel. Mounting is with the component side toward the heat sink, so testing will be easy with the board on the heat sink but replacing components will be a pain. I have finished the protection board and one of the amplifier boards and am now about halfway through the second amplifier board. After that comes the chassis wiring -- probably about 15 hours left to go, at about halfway through the kit. There are a reasonable number of circuit checks along the way -- mostly checking for bad transistors and shorts in the power transistor connections. You get to build a simple VOM to check things. Final checkout has you first run the amplifier with a 100-watt light bulb in series with the AC supply as a current limiter, to make it harder to fry things if you screw up. The amplifier has several nice features. First, it is designed to be put somewhere out of the way and forgotten. Thus, the front panel has no controls, not even a power switch. Because most preamps do not have switched outlets that will handle the 1200 watts the amp draws at full power, the amp comes with two (!) AC cords. The thick one supplies power to the amplifier itself; the thin one drives a relay that turns the thick one on and off. This keeps you from melting the power switch in your preamp. There are input level controls, but they are on the back panel. I think this is where they should be -- use them to get the overall gain of your system right and then forget about them. The only things on the front panel are carrying handles and four LEDs. The LEDs are: power, protection, L and R full power. The latter two come on whenever a channel is getting close to clipping. The protection LED is on whenever an internal relay has disconnected the speakers from the amplifier output. This happens whenever: (a) the amp has been turned on for less than about 8 seconds, (b) either heat sink gets too warm (about 200 degrees F), or (c) excessive DC is present at the output. This latter thing is determined cleverly: whenever the output exceeds about 2.5 volts magnitude, it starts a timer. If the voltage has not found its way below 2.5 volts within 50 ms, the speakers are disconnected. The 50 ms figure means that even a full-power signal at 15 Hz will not trip it, but even a moderate amount of DC will. When I finish the amp, I will report the results of my measurements and listening tests. If it works as well as it (and its manual) appears to be designed, I will be very pleased.