ATI Audio L1000-1 - Manual (Page 2)

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DESCRIPTION
The IC’s that were developed primarily for the instrumentation market had many
shortcomings when used in high quality audio applications. The lack of audio
related specifications, crossover distortion, high noise for low source
impedances, limited output capability and limited gain bandwidth product forced
many compromises when used in high gain audio systems. High ratio input
transformers, output boost stages and multiple stage designs were all used to
compensate for op amp deficiencies and in turn added additional response and
distortion problems of their own.
Walter Jung, in a definitive series of articles, analyzed and defined slew rate
induced distortion mechanisms, tested many commonly available ICs and
correlated various distortion tests with subjective (listening) criteria. A very
significant result of his efforts was the identification of an IC originally developed
for the European professional audio market which has almost ideal
characteristics for audio use and in particular provides a symmetrical high slew
rate capability of 13 volts/microsecond, virtually eliminating slew induced
Transient Intermodulation Distortion. By contrast the old standard 741-op amp
has a slew rate of only .6V/microsecond.
This op amp is now available from several U.S. Manufacturers in single and dual
versions and forms the basis for our designs. This chip incorporates a low noise
input stage designed for excellent noise performance even with low source
impedances and eliminates the necessity for touchy high ratio input step up
transformers in the M1000. High output, capable of driving 600 ohm loads
directly to +22dBm, with total freedom from crossover distortion, high inherent
linearity, 100dB open loop gain and 50 MHz gain bandwidth product make this an
ideal device for highest quality audio. The absence of Transient Intermodulation
Distortion may be detected by the smooth effortless high frequency output
capability, the absence of the harsh, raspy sound typical of IC amplifiers driven to
full output at high frequencies and the freedom from increasing harmonic
distortion vs. frequency. As used in the amplifiers, a minimum of 40dB of loop
gain is available for 100: 1 distortion reduction even at 20kHz.
The high output swing capability allows the amplifier to use a simple, low
distortion, flat response, 1 - 1 output transformer. ATI provides two models of
each; Transformer balanced outputs (with faraday interwinding shields) for high
RF field environments, and an active balanced output version (bridge circuit) for
low RF environments where balanced output lines are required for hum rejection.
Inaudible crosstalk characteristics allow use for either stereo or dual monaural
applications. Dual gain controls are provided to facilitate either mode of
operation.