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-
  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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