miklyd@ihlpb.att.com (07/26/90)
Greetings: I recently received a paper I ordered from the AES entitled "A Current Feedback Audio Power Amplifier" by Mark Alexander at PMI, Inc. This paper is a more thorough discussion of the power amplifier design on page 7.86 of the 1990 PMI Databook (which is probably a worthwhile acquisition, IMHO). If you are interested in "rolling your own" amps, I recommend getting a copy of the paper and reading it. I just wanted to share a few highlights. Spec Summary of Tested Design (using supplies current limited to 1A and a {most likely resistive} 8 ohm load ): Output Power: 50 W THD at 1KHz: 0.0017% at 50 W THD at 20KHz: 0.01% at 50 W DIM-100: 0.0015% at 46 W Freq Response: DC to 1MHz Risetime (WITH input filter!): 400nS Slew Rate: > 200V/uS A quick glance at the schematic yields the following. The major gain stage is an SSM-2131 op amp (basically an OP-42 - BiFET, unity gain stable, slew rate about 50v/uS). The power rails of the op amp provide the drive signals to the output stage via complimentary current mirrors. The output stage consists of double darlington pairs driving a pair of power MOSFETs (not Hitachi this time; International Rectifier). An OP-97 op amp (low power, low speed, high precision, low offset) acts as a servo to reduce offset voltage for the whole amplifier. The AES paper give a lot of info on the advantages of current feedback and the topology of the amplifier. One of these days when I get some free time (ha, ha, ha), I may build a version of this to see how it sounds. Again, for people who like to tinker with building power amps, this paper is worth taking a look at. To get your own copy, send a check for $5.00 to: The Audio Engineering Society 60 East 42nd Street New York, New York 10165 ASK FOR: Preprint #2902 If anybody has already built one of these let us know what they thought?? Enjoy, Michael LLoyd