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