It is generally agreed by those that care about such things or earn their living writing about Hi–Fi in the What News World Choice rags that there is a 'difference' in 'sound' between Transistor and a Valve amplifiers ~ but what does this throwaway statement mean?
The argument is often about power amplifiers but there are also noted differences with pre–amplifiers and standalone RIAA stages using Valves or Transistors or hybrid combinations of both or even can we go there ~ Integrated Circuits
When Transistors first appeared in public their circuits were based on existing Valve radios and amplifiers ~ initially they used Germanium PNP Transistors then later Germanium and Silicon NPN and PNP Transistors ~ Early Transistor amplifiers used only a single negative or positive supply and Hi–Fi power amps had push–pull [PP] output Transformers to obtain power from this single low voltage supply and were very similar to Valve power amplifier designs
In 1956 H.C. Lin of RCA removed the need for an 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 a 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 an excessive number of time constants for coupling and de–coupling and using high speed circuit elements such as cascode stages that then 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'
Another consideration when comparing Transistor and Valve amplifiers or just amplifiers is the age difference between them ~ The first commercial Transistor designs were compared to Valve amplifiers that could have been more than 10 years old and used every day ~ Comparing a 1990s made Valve amplifier with the best 1980s Transistor design is not valid ~ If an hour warming up changes the sound of an amplifier what is the effect of 10 years use?
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 ~ Other passive and active components are required to power and bias the amplifying devices for optimum we hope performance and these may also affect the amplifier sound
Transistors in the mid 1950s had to be operated with low voltage supplies or the germanium could be damaged ~ This was not a problem for portable devices 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 ~ 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 [PP] using two identical Transistors to get sufficient power from the low voltage battery supply like this British madeBUSH A177 with a good size 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 [IF] 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 rated 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 followers which have a naturally low output impedance but no voltage gain and so require more driver stages
With complimentary emitter follower output stages operating in Class B cross–over distortion can be measured and even 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 amplifiers like the QUAD 405 306 . . .
The d.c. current required to operate Valves or Transistors flows through the primary of the 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 the core size is increased
In a PP output stage the effect of Transformer core magnetising is cancelled along with the even or 'nice sounding' harmonics and as output level increases the core produces relatively higher 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 but both with an output Transformer ~ This may explain why Peter Walker claimed all well designed amplifiers will sound the same ~ see Wireless World 1978 article Valves versus Transistors
The article above compared 3 amplifiers made by the Acoustical Manufacturing Company ~ The QUAD II 15W Valve amp with output 15W into 8Ω (assumed set to match NS1000 speakers) made between 1953–1970 ~ The QUAD 303 Transistor amp 45W into 8Ω (30x3 = 90W or 45W per channel) made 1967–1985 and the QUAD 405 Transistor amp 100W into 8Ω (40x5 = 200W or 100W per channel) made 1975–1982 ~ Maybe a comparison between 1970 manufactured QUAD II and 303 and 50 would have been a better 'shoot out'
Transformers used in single ended [SE] output amplifiers using either Valves or Transistors cannot cancel the magnetising current of the output single 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 large enough 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 ~ The Transistor QUAD 50 or the Heathkit amplifier 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 like the QUAD II only part of it ~ The QUAD 50 germanium Transistor amplifier and the Valve McIntosh MC series and in the UK many BBC and GPO and other professional amplifiers like Vortexion used a separate isolated winding on the OPT specifically for NFB
The BBC and many telecoms companies like the GPO used a separate tertiary feedback winding so the output of their amplifiers could be truly floating and perfectly balanced for driving long 'telephone' lines ~ These amplifiers achieved very high specifications using Valves then germanium and later silicon Transistors and many such amplifiers could be connected in series with little degradation of link performance despite having input and output Transformers ~ The input and output impedances were also well controlled across the audio frequency band by such feedback
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 ~ see BBC patent
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 Transformers or inductors ~ They are easier to make and can now be made technically excellent at a reasonable price ~ The choice of dielectric material and the construction may have an effect on amplifier reproduction of sound ~ A problem discussing Capacitors in an amplifier as opposed to Transformers is that amplifiers use so many of them in different circuit positions from large value high voltage electrolytic types in PSUs to low value ceramics used for supply decoupling of op-amps and of course paper in oil and Teflon PTFE
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 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 any amplifier although many British designs of Valve pre and power amplifiers 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 coupling with a –3dB turnover frequency of about 1.6Hz which can be considered low enough
Good Transistor amps from the mid 1970s like the QUAD 33 used 'polycaps' for coupling to give a –3dB @ 20Hz high pass response at the input and lower frequencies between internal stages ~ Other amplifiers at the time used cheaper wide tolerance electrolytic Capacitors for coupling and this may explain why there was a period between the 60s and 80s when many Transistor amplifiers got a bad press as they were often compared with good Valve amplifiers which by then were very good
Due to the high voltages and high impedances between their stages Valve amplifiers tend not to use electrolytic Capacitors for coupling because the leakage of the best is too high ~ The bulk of Valve amplifiers up to 1970s used low value paper in oil [PIO] Capacitors because they were cheap at the time and had high voltage and low leakage for their size ~ PIO Capacitors have a good reputation for sound quality even in guitar amplifiers but is this nostalig romanticism
Leakage ~ Dissipation Factor ~ ESR ~ Dielectric Polarisation ~ Dielectric Constant change ~ Voltage Coefficient and chemical effects found in electrolytic Capacitors and possibly dielectric compression may affect the sound of an amplifier but if the correct type of Capacitors are chosen for their position in the circuit then Capacitors should not have a significant effect on sound but may cause thumps at switch on or off if the time contants are not correct
STILL Being Tinkered with 2023 and converted to wrap on small screens?