If a.m. Is True, Why Then Output Transformer Designs Dominate Today's Market of Tube Amplifiers ?
It is a very tough question and to provide the correct and objective answer, a little of physics and electronics theory is needed...
First, what is the amplifier's role in the audio system ?; it actually must take the delicate audio signal derived from the source (vinyl record, CD, radio, tape,...) and accurately transform it to a powerful signal - strong enough to drive the loudspeaker. This task may sound simple, but it is far from that...
The electron flow between the amplifier and the loudspeaker should be controlled by the speaker's impedance and not the amplifier's, that is - the output impedance of the tube amplifier should be only a fraction to that of the speaker's if a maximum sound output must be obtained.
Yet, there's the resistance in tubes present, referred to as a plate resistance. This resistance is
typically between several hundreds to several thousands of Ohms, and is much higher than the typical resistance found in the majority of loudspeakers (2 - 8 Ohms). Thus, the most convenient (certainly, not the best) solution to this technical issue is to use the output transformer which would transform the high resistance of the plate to the even lower impedance than the average impedance of the loudspeaker. However, opposed to the „ELEGANCY“ of this solution, transformers suffer from the specific problems - leakage magnetic energy and capacitance between the wires and the core, resistance in the wires and
reluctance in the core, which all cause signal phase shifts (differences between the primary and secondary signals in relative phase and amplitude) Therefore, the phase shifts need to be
considered when the transformer is a part of the feedback loop, resulting in reduction of the amplifier's available stability phase margin. By not doing it that way, the amplifier would produce the „ringing“ effect in the output and oscillation.
