Fig. 4.
Intermodulation distortion, using several test frequencies with a
constant ratio of 4:1.

at least up to 20 watts output. This factor possibly
accounts for the superlative listening quality of the
amplifier.
Undistorted power, less than 2 percent
IM, is in excess of 20 watts. This power is delivered undistorted
within 1 db over the range from 20 to 20,000 cps. This power curve
(Fig. 5) is not a
response curve run at high power level. Instead
it represents clean power available at these frequencies. This is
particularly important with today's program sources. The dynamic
range of some of the best LP's is reputed to be in excess
of 100
db. It is necessary to have power to handle this range, and this
power is required over a wide frequency band. New standards of audio
fidelity are rapidly making obsolete the five or ten watt amplifier
which cannot even deliver its rated power at frequency extremes.

Another factor of considerable importance in evaluating amplifier
performance cannot be seen from the curves.
This is overload
characteristic. The amplifier has been given listening tests under
overload conditions with a pad on the output so as not to deafen the
participants. Peaks which would require a 40 watt amplifier are
transmitted without irritation even though the output can be
seen
to clip on the 'scope. The overload recovery is rapid and has no
noticeable hangover, so a clipped peak has no time to

penetrate the ear. Some amplifiers break up on a peak, and
for seconds thereafter the sound is distorted badly because of poor
recovery. In the ultra-linear amplifier transient instability has
been eliminated changes in amplifier characteristics caused by
overload do not make the circuit unstable; and, therefore. recovery
is almost instantaneous. Most feedback amplifiers fail miserably
under overload listening tests.

Figure 6 shows the voltage
gain versus frequency. Obviously, most present day amplifiers are
flat through the audio band. However, it is the band outside of the
audible region which makes some of the difference between one
amplifier and another. In this circuit it is evident that smooth
flat band pass extends more than two octaves on each side of the
traditional 20 to 20,000 band. This enormous band width is necessary
to eliminate phase shift over the customary region and to provide
good transient response.
The square wave performance of Fig. 7
testifies to the transient response. Evidently, a circuit with
response fiat within 2 db for a decade on each side of the audio
band should show a presentable square wave at most frequencies. How
ever, the low phase shift, fast rise time, and insignificance of
ringing in this circuit as indicated by the square waves shows

pg.
5

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