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Audio Power Amplifier -- "Electronics" magazine (McGraw-Hill

Posted: Thu Oct 09, 2014 7:17 am
by Dean Huster
There was one of these "Engineer Design Idea" type of articles in an issue of Electronics magazine, a McGraw-Hill publication that was 90% advertizing, thick, clay-coated glossy paper, etc. A professional magazine for engineers.

The magazine article appeared somewhere around 1971-1972 and the circuit used a 741 op amp, four more driver transistors (or was it two) and a pair of 2N3055 output transistors. The thing was DC-coupled and had a frequency response of DC to over 100KHz with pretty low distortion. It could output something like 125 watts. The circuit was odd in that the output of the 741 was not used, just dropped to ground through a low-value resistor. The power supply leads had 10-ohm resistors in series, then hit capacitive decoupling before going to the power supply. The output from the op amp circuit was taken off the POWER SUPPLY CONNECTIONS which provided the phase-splitting output to the remaining circuitry, hence why the decoupling was separated from the op amp by 10 ohm resistors. A Navy Senior Chief in our shop in Puerto Rico built one and I tested it for him -- danged impressive and totally wrecked a speaker in the process shoving 1 Hz through it at full power out. I copied the schematic and built one for myself, and never could get it to work. I'd like to get the original article, schematic and text of the article so I can see what's wrong. I built the thing 35 years ago and never messed with it since! My original intention was to build four of them for a quadraphonic power amp back in the days when quad was the thing. I had already bought two hog transformers for running a pair of amps each (60v at 10a or so) and managed to lose those in a move. Devastating!

Can anyone point to an online Electronics download collection of the era or have access to paper copies? I think that if I were to go back to the University of Missouri - Rolla (their engineering school) library, they would probably have the mag in bound format in the basement. But last time I was there was in 1969.

Re: Audio Power Amplifier -- "Electronics" magazine (McGraw-

Posted: Fri Oct 10, 2014 12:57 pm
by Robert Reed
Can't help you with the magazine Dean, but I have come across several published designs in the past few years ( EDN mag I think) that use that same OP-Amp design. They used 20-30 ohms as their load resistors and I have never heard a good theoretical discussion about using that "twist". Point being, if that is your snag, you may still get more info on it via the web or EDN website-"Ideas for Design"