Not all xformers are the same

When we think of a vacuum tube power amplifier the first image is that of the classic open frame chassis. Glowing bottles atop a bed of metal with three large transformer-bricks at the rear. The classic McIntosh MC2102 pictured here represents that well. What you won't see is the donut-like toroidal transformer shapes, and there are likely several reasons for that. The first reason can be more easily understood if we consider why this amplifier has three transformers in the first place: one for AC power, two for signal output. Classic vacuum tube amplifiers, unlike a modern hybrid like the BHK Signature, cling to the tradition of coupling loudspeakers to tubes through a matching transformer. This is because in all but the rarest of designs (like OTL), tubes cannot connect directly to a loudspeaker load. Instead, they pass the audio through a transformer's coils of wire and sheets of iron. Transformers are not very good at the frequency extremes. They don't pass the lowest frequencies at all, and are limited on their high frequency content. Which is why traditional tube amplifiers often have "tube bass", rolled off on the low end, a problem solved with a proper hybrid approach like the BHK. There's nothing to be done about an audio transformer's lack of low frequency, but steps can be taken to extend the upper ranges: the construction of the transformer is key. Turns out EI transformers are more easily made to pass the highs than toroids, thus this style is often employed. And while a toroid remains a better choice for AC power, the look of this amp would be violated by two boxes and a donut shape at the rear.