Another serious issue is known as „hysteresis effect“ due to energy needed to influence the magnetic microparticles to move. In the case of very low signals there is a „dead zone“ since the energy of the signal is not sufficient to produce the coupling effect through the transformer. What's not pleasing is the fact that human ear can hear this anomaly in the sound reproduction..., thus a good amplifier should be able to handle the low resolution signals, well.
The toroidal design of the transformer can provide a good low-level resolution, but unfortunatelly it cannot be used as an output transformer in amplifiers, as the slightest D.C. offset in the output stage would magnetically saturate the core and produce the audible distortion. With such limitations, tube amplifiers with output transformers cary the compromised low-level resolution and less feedback capability.
Knowing all a.m. Futterman decided to go for new design that would:
· eliminate the need for output transformer
· carry the tubes which can handle larger current signals
· have enough such tubes connected in parallel, to reduce the effective output impedance of
the amplifier
However, at that time (1950's) many factors did not lend themselves favourably to the OTL amplifier design:
·there wasn't a good choice of tubes for this particular operation
·Futterman himself was at the beginning of the learning curve
·early design suffered from unstability, operation reliability, had to be large, expensive
and produced excessive amount of heat
So, it is no wonder why the transformer design remained firmly as the primary choice for the vast majority of the amplifier designers.
