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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
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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
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