01-01-2012, 12:45 PM
In the early '70s there was a lot of enthusiasm for transistor power amplifiers. I seem to recall that the phrase 'the transistor sound' was often banded about: typically - and vaguely - described as a certain 'sharpness and harshness', especially at higher volume levels. Obviously, this subjective assessment was made in relation to valve power amplifiers, but few seemed to appreciate (at the time) that this 'characteristic' was indeed relative to such valved equipment. (What the ear had got used to hearing: all physiological sensations are relative). Now if my understanding of what I have read about the active devices in power amplifiers is correct, valves have a transfer characteristic that tends towards a square law, whereas transistors follow a curve that is far more 'curved'. As a consequence, valve amplifiers produce significant amounts of third harmonic distortion (especially when driven hard), whereas transistor amplifiers tend to produce even-order harmonics, predominately second-order. Since third harmonic products are a lot more pleasing to the ear than even-order products, this may go quite some way to explaining why valve amplifiers have a more 'mellow sound' compared to transistor amps.
Its also worth noting that transistor amplifiers generally dispense with an output transformer - and this helps to produce a higher power-bandwidth compared to valve amps. Consequently, a given design of negative feedback circuit will respond differently to high-frequency transients developed in a transistor power amplifier compared with that of a valve amp. This adds to the alleged 'harshness' of transistor amps. This difference in transient response in amplifiers with N.F.B. is difficult to measure and quantify in the laboratory.
Anyway, that little lot is what I have read about over the years, and to some extent, agrees with my experiences.
Al.
Its also worth noting that transistor amplifiers generally dispense with an output transformer - and this helps to produce a higher power-bandwidth compared to valve amps. Consequently, a given design of negative feedback circuit will respond differently to high-frequency transients developed in a transistor power amplifier compared with that of a valve amp. This adds to the alleged 'harshness' of transistor amps. This difference in transient response in amplifiers with N.F.B. is difficult to measure and quantify in the laboratory.
Anyway, that little lot is what I have read about over the years, and to some extent, agrees with my experiences.
Al.
