~ Valves versus Transistors ~

It is generally agreed
by those that care about such things that there is a difference in 'sound'
between a Transistor Amplifier and a similar Valve Amplifier ~ As you have
migrated to this page you probably also care and already have an opinion
on the subject

The argument is often about power amplifiers but
there are also noted differences with pre–amplifiers and especially RIAA
stages using Valves or Transistors or hybrid combinations of both or
ICs

When
Transistors were first let loose on the public their circuits used very
similar topologies to Valve circuits ~ initially using Germanium PNP
Transistors then later Germanium and Silicon NPN Transistors ~ In most
designs there was only a single negative or positive supply and Hi–Fi
power amps with push–pull outputs to obtain more power from the single
supply used output transformers similar to Valve power amplifier designs

In 1956 H.C. Lin of RCA 'removed' the output
transformer and introduced quasi–complimentary push–pull ~ Later
variations of his amplifier used silicon Transistors with their 'delayed'
and 'steeper' 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 and between amplifiers in general began in earnest ~
See Evolutionary
Audio by John Linsley Hood

As years past by Transistors and Transistor
amplifier designs improved but the Hi–Fi press continued to promote their
argument to the extent that nowadays people are even discussing the
difference in sound between new and old Valves ~ probably due to the fact
that Valves are very easy to swap ~ I wonder how different things would be
if Valves were always hard wired and Transistors came in 'transparent'
plugin packages ?

A significant sound affecting factor is bad
design like using excessive and mixed time constants for coupling and
de–coupling and using high speed circuit elements such as cascode stages
that have to be 'tamed' by the addition of capacitors and ~ since the
elimination of coupling transformers ~ lots of negative feedback to give
impressive bandwidth and distortion and damping figures required by the
Hi-Fi press for the 'hard of listening'

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 made with Valves or Transistors or both
also requires many other components 'in the signal path'
either as couplers or de–couplers ~ Passive and active components are also
required to power and bias the amplifying devices for optimum (we hope) performance but do they also affect
the amplifier 'sound' ?

TRANSFORMERS

When Transistors first
appeared they had to be operated with low voltage supplies or the
germanium Transistors would break down ~ This was not actually 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 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 identical Transistors to get sufficient
power output from the low voltage battery supply ~ Like this all British
BUSH
A177 with a Celestion speaker

Early
Transistor radios like the A177 and many Transistor amplifiers up to the
1970s were constructed similar to their Valve predecessors and were built
on metal chassis using point to point wiring laid out very similar to the
schematic which made repairing them easy ~ Early transistor radios even
kept the Transistors 'above' the chassis along with the Transformers
including the radio intermediate frequency transformers similar to a valve
radio

It was not long before radios and amplifiers took
full advantage of the reduced size and power requirement of transistors
but it was not only the Transistors that led to a reduction in size ~
other low voltage components were made smaller and the introduction of
printed circuit boards changed the format of amplifiers such that nowadays
some are claiming point to point wiring and over sized components sound
better !

A transformer performs the role of 'transforming'
or raising the low impedance of the speaker to a higher optimum value
required to extract maximum power from the output Transistor(s) while they
also provide useful voltage gain in their high output impedance common
emitter mode ~ unlike complimentary emitter or source follower output
stages which have a naturally low output impedance but no voltage gain so
require more driver stages

With complimentary ~ emitter follower ~ output
stages operating in class B ~ cross–over distortion can be measured and
detected by ear ~ Transformer coupled push–pull (PP) output stages using 2
devices of the same type whether Valves or Transistors generally get good
reviews ~ Probably because they are often biased nearer to Class A but
even when biased to Class B PP transformer coupled output stages appear to
handle the crossover region better and good performance in the crossover
region tends to make a better 'sounding' amplifier

The d.c. current required by the Valves
or Transistors flows through the primary of an output transformer and when
the output devices are biased closer to Class A this current becomes the
dominant factor determining the size of the transformer due to the
resistance of the wire and the point at which the core saturates due to
d.c. current magnetisation

In a PP output stage the transformer core
magnetising current gets cancelled out along with the even 'nice sounding'
harmonics and as power output increases the core produces relatively
higher levels of odd or 'bad sounding' harmonics giving a distinct PP
sound irrespective of the the fact that the amplifier is a QUAD 50
Transistor amplifier or a LEAK TL25 with EL34 Valves ~ which may explain
why Peter Walker claimed 'all well designed
amplifiers will sound the same' ~ see
this 1978
comparison of 3 amplifiers ~ All made by QUAD

Transformers used in single ended (SE) output
amplifiers ~ again using either Valves or Transistors ~ cannot cancel the
current of the output device which has to be biased in Class A ~ The
output transformer needs to be much larger than a similar power PP design
and should have an 'air gap' in its iron core to prevent magnetic flux
saturation which would produce odd harmonic distortion

SE transformer output amplifiers clearly have the
appeal of 'well engineered' ~ The output device naturally has to work
Class A so at worst produces nice even harmonics which unlike a PP output
are not cancelled in the transformer ~ The output transformer has to be
very large to avoid saturating and so we have an over engineered Class A
amplifier ~ Surely it has to sound good so why are there not many
Transistor SE amplifiers with transformer output ?

Whether PP or SE ~ All output transformers become
bigger and heavier and more expensive as the power requirement goes up ~
Ultra linear PP or the QUAD
II style PP output transformers can be made relatively small but all
audio transformers are difficult to design due to the number of octaves
they have to cover and all have undesirable phase changes at either end of
the 10 octave audio range

Series and parallel resonance due to leakage and
primary inductance acting with the winding and external capacitance limit
the amount of negative feedback (NFB) that can be applied to a power
amplifier with OPT and this leaves the output susceptible to changes of
loudspeaker load that are not experienced with emitter follower output
stages that are naturally low output impedance and can be made lower still
with lots of NFB

It is obviously difficult to find comparable
Valve and Transistor based transformer coupled power amplifiers with
similar topologies let alone similar amounts of NFB ~ A Transistor QUAD 50
or the Heathkit I had in 1970s versus the Valve LEAK TL25 or Radford STA25
will not give much insight into the differences between the active devices
they use ~ Any differences in the way they 'sound' will mainly be due to
the output Transformer limitations

A subtle and often overlooked effect
using output transformers is not only the limited amount of negative
feedback that can be applied but how it is derived ~ Most commercial valve
amplifiers take their parallel derived feedback as the voltage across the
secondary winding or part of it ~ The Transistor QUAD50 and the McIntosh
MC series plus many BBC and other professional valve amplifiers used a
separate winding specifically for deriving the NFB signal

The BBC and many telecomms companies used a
separate feedback winding so the output of their 'line' amplifiers could
be truly floating and thus perfectly balanced for driving long 'telephone'
lines ~ These amplifiers achieved very high specifications when using
Valves then germanium and later silicon Transistors and many could be
connected in series with little degradation of link performance ~ The
input and output impedances were also well controlled by the feedback ~
usually at 600Ω ~ across the audio frequency band

Using a separate feedback winding for an audio
power amplifier would appear to have advantages but the complexity of
calculating and adding another winding just for feedback means that many
commercial designs do not use them ~ A separate NFB winding can have its
own optimum loading and need not be referenced to signal ground which
allows more scope to apply the feedback where and how it will work best in
circuit

The McIntosh MC275 sounds as good as it does
partly due to the feedback path and not just the output transformer
cathode coupling ~ Amplifiers with coupling transformers or inductors
cannot be used to compare Transistors with 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 very similar

CAPACITORS

Capacitors generally have less effect on
amplifier 'sound' than signal transformers or inductors ~ They are easier
to make and can be made technically excellent at a reasonable price ~ The
choice of dielectric material and the construction may have an effect on
the reproduction of sound ~ A problem discussing capacitors in an
amplifier as opposed to transformers is that amplifiers use so many of
them for many different reasons from large value high voltage types in
PSUs to low value ceramics used for supply decoupling of op-amps to
'prevent them oscillating' ??

As with
other 'sound affecting' components capacitors do not 'work alone' ~ Valve
and early Transistor amplifier designs used coupling capacitors at their
input and output and often between stages ~ The coupling capacitor feeds
the audio signal into the impedance of an amplifier stage without passing
d.c. from the previous stage which would upset the bias ~ For Valve
amplifiers the d.c. voltage across interstage coupling capacitors can be
several hundred volts or put another way 1000s of times more than the
signal

Each series coupling capacitor and shunt input
impedance of the next stage form a high pass filter which should have a
sufficient low frequency response such that the overall bass response is
unaffected ~ For amplifiers with overall negative feedback the low
frequency response and phase change due to the coupling of several stages
can cause instability especially with the additional phase changes of an
output transformer

It is good practice to provide a coupling
capacitor at the input to an amplifier although many old British designs
of Valve pre and power amps like the QUAD and LEAK and Radford did not
have them ~ probably because the power amps were intended to be used with
the makers own pre–amps ~ American power amp designs like McIntosh had
input CR with –3dB turnover frequency of about 1.6Hz which can be
considered low enough

Many transistor amps from the 1970s onward had CR coupled inputs that
were -3dB @ 20Hz and used polycaps ~ before that electrolytics
were used as they are nowadays but this time to achieve very low and often
not required low frequency responses

~ Leakage ~ Dissipation Factor ~ ESR ~ Dielectric Polarisation ~
Dielectric Constant change ~ Voltage Coefficient and chemical effects
found in electrolytic capacitors and possibly dielectric
compression can all affect the sound of an amplifier

STILL Being Tinkered with OCT
2014

RESISTORS

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