In 1956 H.C. Lin of RCA removed the output transformer and introduced complimentary push—pull — Later variations of his amplifier used silicon transistors with their delayed and sharper transfer characteristics which introduced us to cross over distortion and then other audible effects were given titles and the flood of opinion about the difference in sound reproduction between Valve and Transistor amplifiers or between amplifiers in general began in earnest

Many years later transistors and transistor amplifier designs have improved but the Hi-Fi press continued to promote the argument to the extent that nowadays people are even discussing the difference in sound between new and old valves — but this is mainly due to bad circuit design and or out of spec valves and the fact that valves are easy to change — I wonder how different things would be if valves were always hard wired and transistors came in "transparent" plugin packages ?

Is the perceived difference in sound due to the nature of the Valves or transistors 'alone' or are there other factors to consider ? — An audio amplifier may use Valves or Transistors or both but it also requires many other components "in the signal path" either as couplers or de-couplers — More passive and active components are required to power and bias the active devices for optimum (we hope) performance but do they affect the amplifier "sound" ?

A significant sound affecting factor is bad design such as excessive and mixed time constants for coupling and de-coupling and using high speed circuit elements such as a cascode stage that has to be "tamed" by the addition of extra capacitors and of course — since the elimination of coupling transformers — lots of negative feedback to give the impressive bandwidth and distortion and damping figures required by the Hi-Fi press to publish for the "hard of listening" — See also this 1978 comparison of 3 amplifiers all made by QUAD

When transistors first appeared they had to be operated with low voltage supplies or the germanium transistors would break down — But this was not a problem and the fact that you could take a small battery powered radio to the beach and play it all day was a major selling feature of the small transistor radio — The radio most likely had a transformer in the audio output stage and maybe also a transformer driving the output transistors if the output was a push—pull stage using two similar transistors to get sufficient power output from the battery supply

With "modern" complimentary emitter follower output stages low level cross-over distortion can be measured and apparently detected by those with golden ears — Transformer coupled push-pull (PP) output stages whether using valves or transistors generally get good reviews — Probably because they are often biased closer to class A but even when biased class AB (like the emitter follower stage) transformer coupled output stages appear to handle the crossover region better and good performance in the crossover region makes for a good amplifier

Transformers do a good job "transparently" combining two (or more) high impedance sources like the collectors of transistors or the anodes of valves — In a PP output stage the transformer core magnetising current gets cancelled out along with the even or nice sounding harmonics and as the power output increases the core produces higher level odd or bad sounding harmonics giving a distinct PP sound irrespective of the the fact that the amplifier is a QUAD 50 transformer output transistor amplifier or a valve output LEAK TL25 which may explain why Peter Walker claimed "all well designed amplifiers will sound the same"

Transformers used for single ended (SE) output amplifiers — again using either Valves or Transistors — do not cancel the steady state current of the class A biased output device — The Transformer should have an air gap in the core to reduce the effect of core saturation but this cannot be eliminated and the non linear effect of the transformer produces a harmonically rich distorted output — Unlike a PP output the even harmonics are not cancelled and the high harmonic distortion is thus acceptable and even preferable by some listeners and always ignored by the Hi—Fi press due to the pay back from SE manufacturers and letters from the golden eared who buy their magazines

Whether PP or SE — All output transformers need to be much bigger and heavier and expensive as the power requirement goes up — Ultra linear PP or the QUAD II style PP output transformer can be relatively small but all audio transformers are difficult to design due to the number of octaves required and all have severe phase changes at either end of the audio range — Series and parallel resonant modes due to leakage and primary inductance acting with winding and external capacitance severely limit the amount of negative feedback that can be applied leaving the amplifier susceptible to changes of the speaker load — A dynamic effect which is difficult to measure and quantify

It is obviously difficult to find comparable valve and transistor based transformer coupled power amplifiers with similar topologies let alone simlar feedback — A Transistor QUAD 50 or the Heathkit I had in 1970s versus the valve LEAK TL25 or Radford STA25 will not give any insight into the audible difference between the active devices — The "sound" differences will be mainly due to the design including feedback and the output transformer limitations

Amplifiers with coupling TRANSFORMERS or INDUCTORS cannot be used to compare TRANSISTORS and VALVES — Sure you still prefer one amplifier over the other but when both produce similar amounts of transformer related distortion under the same conditions transistor and valve amplifiers may well sound the same — To evaluate the "sound" of the active devices alone a test circuit without transformers is required and this is not likely to be a power amp or any amplifier with a signal path transformer

CAPACITORS

Capacitors generally have less effect on amplifier "sound" than transformers or inductors — They are easier to make so can be made better — The choice of dielectric material and the construction may have an effect on the reproduction of sound — The problem with capacitors as opposed to transformers is that so many get used and for many different reasons from large values in PSUs to supply decoupling of op-amps to prevent them oscillating