~ Valves versus Transistors ~

 

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 topology to Valve 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 incorrect time constants for coupling and de–coupling and using circuit elements like cascode stages that need to be 'tamed' by the addition of capacitors and since the elimination of coupling Transformers excessive negative feedback [NFB] to give the impressive bandwidth and distortion and damping figures required by the Hi–Fi press and 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

TRANSFORMERS

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 another 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 made BUSH 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 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 NFB that can be applied but how it is derived ~ Most commercial Valve amplifiers take parallel derived feedback voltage from across the secondary winding or like the QUAD II 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

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 NFB 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 nostalgic romanticism

Capacitor effects like Leakage ~ Dissipation Factor ~ ESR ~ Dielectric Polarisation ~ Dielectric Constant change ~ Voltage Coefficient and 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/off if the time constants are not correct

 

RESISTORS

Resistors cannot and will not affect the sound of a Valve or Transistor amplifier unless they are the wrong value for the circuit or they change value within a short time due to temperature or humidity or voltage across them or they add excess noise due to their materials ~ all of which occur with older carbon resistors as pictured here ~ Try changing bypass capacitors before cathode resistors

It is likely that a vintage Valve amplifier has a particular sound because it has many out of value original carbon composition resistors which have leaked wax or have been run close to their power rating ~ Changing these old resistors for 0.01% holistic unobtainium flimflam resistors may give a change in sound but any good modern correctly rated resistor will give a similar or same effect while leaving you money to waste on gold fuses or . . .

 

NOISE

Transistors and Valves both add noise to the output of amplifiers made with them ~ Vinyl records and magnetic tape add their own particular noise which over the years has become less and less ~ Amplifier noise alone was higher in the past because the expensive active devices were still being developed and were naturally noisy and were used at maximum gain to reduce their numbers to keep costs down

Anecdotally we humans like a bit of noise rather than absolute silence as anyone who has tried to relax in an anechoic chamber will confirm ~ Many amplifiers that have an audible but not annoying background noise have gained a good reputation although this mainly applies to Valve or Transistor phono stages or high gain Valve systems like the LEAK range with varislope pre–amp

Early Transistor amplifiers from UK makers like Boothroyd–Stuart and Townshend Audio and Kelvin Labs had wide bandwidth with low but audible noise ~ Some of the best Valve amplifiers like Croft were also 'noisy' ~ In my younger days in studios and monitor rooms I would expect to hear a low level hiss when passing speakers like the BBC LS5/8 but the 15.625kHz whistle of the monitors was often louder

Phono stages may sound nice and warm using Valves or Transistors due to the background noise but the signal source needs to be a record disc which brings its own welcome nostalgic noise ~ The argument then turns to passive or active replay equalisation [EQ] with most passive EQ amplifiers described as sounding natural or clinical but if this means accurate then clinical is good

Active EQ phono amplifiers often preferred for Transistor and Op-amp designs fall into 2 further categories where the negative feedback [NFB] is either series or shunt [parallel] applied ~ Shunt NFB requires a resistor in series with the op–amp inverting input which can make these amplifiers noisier than series designs but series NFB cannot reduce gain less than 1x so noise rises with frequency

Modern Valve designs use passive EQ that is often split in 2 sections between 3 valve stages so the noise can be optimised and the final passive EQ section which must contain a pole ensures both gain and noise fall as required by the phono EQ spec ~ Valve or Transistor or Op–amp designs with active EQ and series feedback often have a passive pole to complete the EQ fall in gain and noise

Without the final pole section the output noise of a series NFB amplifier [active EQ or flat response] will rise as frequency increases and more so once the gain is 1x ~ Most early Transistor series NFB pre–amps did not have a final pole and produced high frequency noise beyond the audio range which probably was not passed by the power amplifier or measurable at the time

Around 1995 Allen Wright suggested the RIAA EQ for phono amplifiers should not continue to fall beyond the audio frequency range as specified because in practice the recording process signal did not continue to rise with frequency but there is nothing to record beyond 20kHz except possibly amplifier or tape noise and any audio signal at 20kHz is about -30dB lower than mid band

Around 1996 my favourite magazine Wireless World had a resurgence of Valve Hi-Fi articles and the letters to the editor became interesting although the articles themselves became mundane and full of errors ~ One letter in May 1996 from Allen Wright suggested that altering the EQ of an otherwise correct phono amplifier to exaggerate the ultra sonic noise would give an unexpected sonic improvement

Noise from Valves and Transistors should be similar but how it is distributed throughout the audio range and beyond may have an influence on the sonic difference between amplifiers made with them ~ I have been listening to some 1960s/1970s 'digitally remastered' tracks on Apple music which claim to be lossless encoded at 24bit but clearly have something missing ~ Noise

STILL Being Tinkered with



" I hear you are singing a song of the past ~ I see no tears "