Extensive listening evaluations reveal that the behavior of a high fidelity amplifier several octaves above and below the normal range of human hearing distinctly affects reproduced sound in the areas we do hear. It has been determined that an amplifier having a wide frequency response at useable power levels below 5 cycles has a tight and clearly defined low end, particularly in the 4-100 cycles region.
A similar condition applies to the performance of an amplifier in the high frequency spectrum. If an amplifier limits its high frequency response to slightly above the limit of audibility, it may have a tendency toward strident reproduction and poor differentiation of instruments in the highest audible overtones. Conversely, an amplifier which has a frequency response beyond 100,000 cycles without evidence of ringing or instability with reactive loads will offer clean, transparent tone qualities in the higher frequencies with excellent instrument separation.
It is for this reason as well as other considerations that conventional power amplifier design has been by-passed in the general design concept of this basic amplifier. Current power amplifier design is based upon “single loop” feedback techniques, and linearization is obtained by overall feedback from the speaker output terminal to the cathode of the input tube. Stability problems limit this application of adequate feedback which represents a 10/1 to 20/1 reduction in distortion.
Careful listening evaluation of amplifier performance proves conclusively that extremely stable amplifiers with a minimal degree of feedback, or none at all, provide a noticeable improvement in sound quality and a definite reduction in listening fatigue. This improvement can be attributed to lower harmonic and intermodulation distortion products, more linear phase characteristics, and improved transient response.
A “multiple loop” approach to achieve the desired degree of useable feedback is the most logical approach to lower distortion without sacrificing stability. These “multiple loops” become additive if their ratio is adjusted to the relative degree of distortion produced.
It is essential to have a well regulated power supply in order to maintain clear transient response. The power supply of this amplifier incorporates four Silicon Rectifier Diodes, which together with extremely low copper loss in the power transformer provides regulation equivalent to that of a regulated supply. Leakage inductance in the output transformers has been kept to an absolute minimum, and the distributed capacitance of the primary halves has been carefully balanced against each other to maintain the natural resonances of the output transformers well above 200,000 cycles. The combined use of these special output transformers in conjunction with video pulse amplifier techniques and multiple feedback loops has enabled us to achieve the exceptionally wide frequency response of two octaves above and below the normal range of human hearing.
This design employs 2-6550 beam power pentodes in each channel, driven by video power pentodes used conventionally in pulse amplifiers and wide band industrial equipment such as computers. All low frequency coupling networks have been inserted into internal feedback loops, thereby reducing phase distortion to an absolute minimum in the subsonic region. At the same time the high frequency response of the amplifier, exclusive of the output transformer, is flat to the megacycle region.