Fig. 5.
Undistorted power output vs. frequency

Fig. 6.
Frequency response, showing effects above and below the audible
frequency range

finding, even the less discriminating listeners could
identify the ultra linear amplifier on "blind" tests and could
recognize its superiority.  Listeners agree that the bass
region is more articulate, better defined, and better damped than in
other amplifiers. This damping is not a function of internal
impedance alone but also relates to stability under dynamic
conditions. For example, no low frequency cutoff is required in the
preamplifier as no ill effects are audible due to momentary
overloads from turntable rumble, switching ground thumps and similar
disturbances. Certain types of signals such as organ pedal tones
combined with rumble will cause other amplifiers to break up even at
levels as low as a few watts in the mid-
frequencies.

Another audible feature in the bass range is that the amplifier does
not have more bass, but it has lower bass. Other amplifiers, of good
quality in terms of measurements, by comparison were generating
harmonics and intermodulation products. This was also apparent on
scratchy "dirty" recordings which cleaned up on the ultra-linear
amplifier while remaining mushy and irritating on others.
In the
treble region the consensus of opinion is that the amplifier sounds
"smoother." The scratch level of shellac records is less irritating
while the high frequency sounds, particularly of a percussive type,
cut through the scratch and seem far more prominent. This seems due
to the fact that intermodulation between

Fig. 7. Square
wave performance of the

Ultra-Linear
amplifier

that more than just the frequency response is excellent. In
addition, square waves were checked on a speaker load with
practically identical results, thus demonstrating that performance
of the amplifier is unaffected by a load of varying impedance.

Other circuit configurations can be used with this ultra-linear
output stage. However, they should have-a phase characteristic
permitting substantial feedback, and they should have the lowest
possible distortion for the early stages. The popular Williamson
circuit has been converted to this output arrangement with
gratifying results. This conversion permits 30 watts of output plus
the other benefits inherent in the increased linearity of the output
stage.
Listening Tests

The majority of listeners agree
readily to the superiority of this circuit. None felt that other
equipment was better although some could not recognize differences
on the program sources used. However, during the course of the
tests, certain recordings were found which demonstrated differences
vividly; and after this

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