~ Valves versus Transistors ~

Marconi Osram wire leaded EF86 - VX13503
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

OC71 Germanium Transistor with paint removed OC71P
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 Transistors and Transistor amplifier designs improved but the Hi–Fi press continued to promote their arguments 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 ~ excessive negative feedback to give impressive bandwidth and distortion and damping figures promoted 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 required to power and bias the amplifying devices for optimum (we hope) performance and may also affect the amplifier 'sound' ?


LEAK TL50 Output transformer

When Transistors first appeared in the mid 1950s they had to be operated with low voltage supplies or the germanium Transistors would break down ~ 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 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 stage was push–pull using two identical Transistors to get sufficient power 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 using similar topologies to their Valve predecessors and were built on metal chassis using point to point wiring laid out to match 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 just like valve radios

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 size reduction ~ other low voltage components were made smaller and the introduction of printed circuit boards changed the format of amplifiers such that nowadays some audiophooles are claiming point to point wiring and greatly over sized components sound better !

A transformer performs the role of transforming or raising the low impedance of speaker to a higher optimum value required to extract maximum power from the output Transistor(s) while they also provide useful voltage gain in the high output impedance common emitter mode ~ unlike complimentary emitter or source follower output stages which have a naturally low output impedance and no voltage gain and 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 to operate Valves or Transistors flows through the primary of an output transformer ~ When the output devices are biased toward 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 would saturate due to d.c. current magnetisation unless its size increases

In a PP output stage the transformer core magnetising current is 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 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 using either Valves or Transistors cannot cancel the current of the output device which now 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 (SET) output amplifiers clearly have the appeal of being 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 resonances due to leakage and primary inductance acting with the winding and external capacitances limit the amount of negative feedback (NFB) that can be applied to a power amplifier with an 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 which 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 characteristic

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 a parallel derived feedback voltage from 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 OPT winding specifically for 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 such amplifiers could be connected in series with little degradation of link performance ~ The input and output impedances were also well controlled by 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 its feedback path and not just the output transformer cathode coupling ~ Amplifiers with coupling transformers or inductors cannot be used to compare Transistors with Valves ~ you may like one better than the other but when both are producing similar amounts of transformer related distortion at the same output level you may find you have a preference for any amplifier with an output transformer


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

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 100s times more than the signal voltage

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 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 coupled with –3dB turnover frequency of about 1.6Hz which can be considered low enough

Many transistor amps from the mid 1970s onward had CR coupled inputs using 'polycaps' that were -3dB @ 20Hz but before that wide tolerance electrolytic capacitors had been used for all interstage coupling and this may explain why there was a period between the 60s and 80s when many transistor amplifiers got a bad press both at the time and subsequently ~ Due to the high voltages and higher impedances between stages valve amplifiers can use non electrolytic capacitors which tend to sound . . .

~ 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 2017


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