Elliott Sound Products

Valves vs. Transistors (Part
I)

Valves vs. Transistors (Part I) - What are the
differences?
Copyright © 2009 - Rod Elliott (ESP)
Page Created
10 Dec 2009

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Valves Index
Main
Index

Contents

Introduction

2 - Valve
Amps

2.1 -
Output Impedance

2.2 -
Frequency Response

2.3 -
Distortion

2.3.1
- Guitar Amps

3 -
Transistor Amps

3.1 -
Output Impedance

3.2 -
Frequency Response

3.3 -
Distortion

3.3.1
- Guitar Amps

4 -
Efficiency

5 - Heat,
Noise & Vibration

Conclusion

Introduction

The valve vs. transistor debate has been going ever since transistor amps
first became available, and it shows no sign of abating any time soon. Guitar
amp makers have been using hybrids - valves, transistors (and/or opamps) in
various configurations for many years, with varied success. For those who prefer
at least some valves in their amplifier, the choice is either valve preamp and
transistor output or vice versa. This arrangement is certainly cheaper and more
reliable, but if someone really wants a full valve amp, then that's what
they'll buy.

Hi-fi systems are more complex, because the relatively high distortion levels
of guitar amps are obviously not tolerable. This makes the design of a valve
system far more complex, because obtaining the vanishingly low distortion and
high power that are common now is not easy (or cheap) with valves. It can be
done - AM transmitter modulators have used the largest audio amplifiers ever
made, and valves are the only sensible choice for a single amp that can deliver
perhaps 50kW or more. That it would completely fill the average listening room
and cost as much as the house is something of a disincentive though.

A great many of the claimed differences are simply imagined, but there most
certainly are many very real differences that are clearly audible in a blind
test. The overall perception is that valves are 'better', but this is generally
not the case. It was a different matter in the early days of transistor amps
though - the semiconductor devices themselves had some serious limitations. They
were slow by modern standards, linearity was pretty poor (large gain changes
with variations in collector current for example), and crossover distortion was
not uncommon in low priced consumer audio products.

Despite these limitations, many of the high-end manufacturers managed to get
results that were so good that they not only beat their previous valve models,
but still stand up to scrutiny today. One of the biggest problems is the
reliance of the buying public on hi-fi reviewers. Almost all reviews now are
based solely on the results of a listening test - and I use the word 'test' in
its loosest possible sense. Technical tests used to be common, but are now
passé, and the subjective review generally consists of hyperbole, opinion, and
flowery language that conveys no actual usable information whatsoever.

The following short section is duplicated in the Preamps article that examines the design of valve preamp
stages. There is also some information in the Valve Myths article that shows the results of a direct
comparison between valve and transistor stages, and shows conclusively that even
a simple one-transistor stage can beat a valve stage for both noise and
distortion.

It's worth noting that there have been several 'studies' published by
electronics professional organisations, and while the results might appear to
show that valves are 'superior', the results have to be taken with a very large
pinch of salt. For example, and article on the IEEE
website 'The Cool Sound of Tubes' is seriously flawed
and simply shows the bias of the author. The article was published in 1998, and
many of the claims made regarding noise and distortion simply don't stand up to
scrutiny. In some cases, noise is measured with resistance in series with the
input, so the thermal noise of the resistor itself is a major limiting factor.
Comparing a triode with a single transistor circuit is somewhat unfair, because
other than a few very early designs, this is not how transistors are
generally used. It is notable that several opinions are provided, but they are
nearly all from designers of valve equipment, with nary a word from respected
designers who work primarily with semiconductor devices.

Another study from 1972 and published by the AES
(see Tubes vs Transistors: Is there an audible difference?) is
hardly worthwhile today, because so much has changed in the 40+ years since the
article was written. It doesn't help that the writer's bias is clearly obvious,
and that some of the claims made are best described as alarming! Of even more
concern is the fact that there is zero reference anywhere to proper double-blind
subjective testing, and that means that the published 'results' are worse than
useless.

A comparison table of advantages and disadvantages of valves and transistors
is laughable. Many of the comparisons fail to mention valve failings that are
highlighted as 'disadvantages' of transistors do not include a similar
disadvantage (which exists as a matter of physical principles) of valves. For
example, the stored change of transistors is listed as a disadvantage, but the
equivalent problem of electron transit time through a valve escapes mention. Ok,
so it's not a major problem, but limited high frequency response due to
anode-grid capacitance wasn't mentioned, nor was the inductance of the internal
lead wires (although this is not an issue with audio frequencies). Some of the
other comparisons simply defy reason, such as transistors have "Usually more
physical ruggedness than tubes (depends on chassis construction)". What
unmitigated drivel - the chassis has nothing to do with anything, and
needless to say wasn't mentioned for valve circuits where it's far more
important.

It can be taken as read that these articles (and many more like them) are
treated as gospel by those who imagine that valves are better, more linear, more
musical (etc., etc.) than transistor or IC designs. They will regard anything
that supports their view as 'proof' that they are right. There's no need to be
objective and do your own double-blind tests when the proof you are looking for
is all over the Net. Having your beliefs crushed is painful, and it's much
better all round to disregard any evidence that you find confronting.

If you happen to be a 'believer' (one way or the other), then don't bother
reading any further.

2.0 - Valve Amps

There are noticeably larger audible differences between low quality and high
quality valve amps. However there are negligible audible differences between the
various high quality valve amps.

One of the most popular beliefs is that valve amplifiers have
'nice' second harmonic distortion. The fact is that no distortion is nice
because even (predominantly) second harmonic distortion also creates very
'not nice' intermodulation distortion. The best examples of high quality
valve amplifiers have extremely low distortion - below 0.1% in some
cases.

If you haven't done so already, it's worthwhile to have a read of preamps, Design Part 1 and Design Part 2 articles. While these are all quite technical,
the three articles explain the basic principles that are needed to get a really
good design working well.

There are some characteristics of valve amps that are very obvious -
particularly those with little or no global negative feedback. Ignoring
distortion for the time being, the first thing that you hear when a zero
feedback valve amp is connected to most loudspeaker systems is that the music
seems to come 'alive'. This happens because the valve amp has a relatively high
output impedance, so the speaker behaves very differently from the way it
performs with a transistor amp. This is usually a problem in reality, not a
benefit. With no exceptions I can think of, loudspeaker systems are designed to
be driven from a constant voltage amplifier (ie. an amp having an output
impedance close to zero ohms). Many valve amps have an output impedance that may
be several times the speaker impedance, so they approach constant current (ie.
the output impedance is high - a perfect constant current source has an infinite
output impedance).

2.1 - Output Impedance

The high output impedance allows the amp to behave somewhere between constant
voltage and constant current. This may result in roughly equal power to the
drivers, despite impedance changes. If impedance is made higher still, the power
will increase at higher than normal impedance.

At bass resonance, the impedance of the woofer increases, and a transistor
amp delivers less power. A valve amp will deliver roughly the same, or in some
cases more power, so bass is accentuated. The same thing happens at higher
frequencies where normally, less power is delivered. If a measurement is taken
of the loudspeaker with both amps, it may measure flat response with a
transistor amp, but will be anything but flat with the valve amp connected.

This is a test I've done many, many times, with many different people. Almost
everyone thinks that the high impedance drive sounds better! It's only after
listening to the system for a while that it becomes obvious that it's not right.
Midrange might be 'subdued', and low impedances around crossover frequencies can
reduce the level dramatically, causing frequency response notches (sometimes
referred to as a 'suck-out' by reviewers). In general, the sound is rarely
unpleasant though.

Figure 1 - Output Response of High Impedance Amp Into Speaker
Load

As you can see from the above, the frequency response (red trace) tends to
follow the impedance curve (green trace). You can see that the output voltage
increases with increasing impedance, and falls with decreasing impedance. The
simulated speaker for the graph is a sealed box two-way system, with a nominal
impedance of 4 ohms. Source impedance was also 4 ohms. When an actual
loudspeaker is driven with the same impedance, the response anomalies are very
audible, but as noted above, are rarely unpleasant. It is certainly a sound that
you can get used to hearing, and after a couple of weeks will sound completely
normal. The voltage to the speaker varies by 6dB, but if the amp's output
impedance were higher the voltage change would be higher too.

There is absolutely no reason that a loudspeaker system can't be designed to
give a flat frequency response curve when powered by a high impedance amp. The
problem here is that the loudspeaker will only work properly when
connected to an amp with exactly the same impedance that the system was
designed for. Since there are often wide variations with different valve amps
(and it changes as the valves age), this approach is not economical - it can
only be sold as a complete system.

This is one of the reasons that low output impedance was determined to be the
best arrangement - and that decision was made long before transistors were even
invented. Engineers had figured out that high impedance amplifiers were a
nightmare for just this reason. They could just as easily have decided that
perhaps 10 ohms output impedance was to be 'standard', but the problems of
woofer damping were already well known in the 1940s and 1950s. Only by driving a
woofer with a low impedance could the bass be made accurate, without 'overhang'
- the bass note from the speaker extending for longer than the applied signal.

The specification for so-called damping factor has been with us for well over
50 years, although in the early days it was found that anything greater than 20
served no useful purpose (and this still applies even today). A damping factor
of 20 means that an 8 ohm speaker should have a source impedance (from the
amplifier) of 0.4 ohm. With careful design, this figure was achieved easily with
many high quality valve amps. All but a very few transistor amps have output
impedances that are measured in milliohms - much lower than really needed, but
it's very easy to do.

The traditional way to reduce output impedance is to apply negative feedback.
Valve amplifiers have significant phase shift at both high and low frequencies,
largely because of the output transformer and coupling capacitors. By making
very good transformers, valve amp designers have been able to apply enough
feedback to reduce the output impedance to under 0.5 ohm. In order to be able to
achieve low impedance, the valve output stage must be configured
appropriately.

Figure 2 - Pentode or Tetrode Connection

The pentode or tetrode connection has the highest output impedance of all the
valve output stage configurations. Distortion is also comparatively high. It is
difficult to get output impedance low enough to be classified as voltage drive.
This is the most common connection for guitar amps, because it has high gain,
high output power (compared to the alternatives shown below) and predictable
distortion characteristics. Very easy to drive to full power, requiring the
lowest input signal to the grids.

With no feedback, pentode circuits can have an output impedance of over 100
ohms (referred to the 8 ohm output tapping). Beam power tetrodes are generally a
little lower, but the difference is largely academic - the impedance is still
rather high. Typical feedback ratios of perhaps 12dB or so will generally get
the impedance down to about 8 ohms or so.

Figure 3 - Ultralinear Connection

Ultralinear operation applies some of the plate voltage directly to the
screen grids. This lowers output impedance dramatically compared to
pentode/tetrode operation, and reduces distortion. At high power levels,
intermodulation distortion is actually lower than the triode connection. While
power output is a little less than pentode/tetrode stages, it's usually not
noticeable. This is one of the easiest circuits to drive, (second only to
pentode), so drive requirements are not difficult to meet.

Figure 4 - Triode Connection

Triodes have a low output impedance, so after feedback is applied it's
relatively easy to obtain an output impedance that meets the requirements for
voltage drive. Intermodulation distortion is often quite high at close to full
power. A very high drive voltage is usually needed, making preceding stages more
difficult to design for low distortion.

Figure 5 - Split-Load Connection

Not many manufacturers used this arrangement, but Quad, McIntosh and a few
others found that this (or other similar) scheme has very low output impedance,
and is generally better than the ultralinear connection. The disadvantage is
that because part of the load is in the cathode, the transformer is more
difficult to wind, and therefore more expensive than other arrangements. Because
of the winding in the cathode circuit, the grid drive voltage is also higher
than the others shown.

There are also other arrangements - one Audio Research amp that I saw on the
Net connects the cathodes of the output valves to the secondary of the output
transformer, as well as using an ultralinear transformer. Whether the cathode
connection to the secondary actually achieves anything useful is unknown.

While it's certainly not impossible to get very low output impedance from
valve amps, it is far more difficult than with a transistor amp. High output
impedance is generally preferred by guitarists. Although it's easy to do with
transistor amps, there are many designs that use voltage drive - this is a
mistake IMO, especially since it's simple to implement.

2.2 - Frequency Response

With no feedback, most well designed valve amps have a perfectly acceptable
frequency response into resistive loads. At low power, it's fairly easy to get
flat response from 20Hz to 20kHz, but when the amps were made cheaply for
consumer items, the low end usually suffered - often badly. Even many properly
designed amps would only have full power response down to about 30Hz, but at
lower frequencies the amount of energy in typical programme material is very
low, so provided response is flat it will perform fine.

By applying feedback, response could be made flat within less than 0.5dB
across the audio band. However, feedback will not change the low frequency full
power. If the transformer saturates at 30Hz at full power, all the feedback in
the world won't change that. As shown above though, if the output impedance is
too high, the varying impedance of the loudspeaker will cause the frequency
response to change.

In general, there is very little difference in resistive load frequency
response between a good valve or transistor amp, but a transistor amp will
certainly go a lot lower at full power, and many get to 50kHz or more - again at
full power. While neither extreme is generally useful in a full-range amplifier,
it's easy to do.

2.3 - Distortion

As noted in other valve articles, the most common claim about valve amps is
that they have 'nice' second (or even) harmonic distortion, whereas transistor
amps have 'nasty' third (or odd) harmonic distortion. This is true in only the
most limited number of cases, and is especially noticeable with single-ended
amplifiers (which includes preamps). The fact is that it's not the order of the
harmonics (odd or even), but how far they extend beyond the original frequency.
High order harmonics (typically from the seventh and above) do sound relatively
nasty, and that applies for both odd and even harmonics.

What this claim fails to understand (or recognise) is that harmonic
distortion is accompanied by decidedly unpleasant intermodulation distortion.
You can't build any amp that has second harmonic distortion but no
intermodulation distortion (IMD). The two go hand-in-hand.

Well designed valve gear (of old) will have distortion levels of below 1%,
and some below 0.1%. Once distortion is below the audibility threshold - which
varies depending on the type of distortion and programme material - it doesn't
matter if the amp uses valves or transistors. Inaudible distortion is, as its
name suggests, inaudible. The technology that got you there is immaterial.

2.3.1 - Guitar Amps

The situation is different for guitar amplifiers. Some degree of distortion
is generally preferred, although not by all players. A valve amp will generally
have a reasonably smooth transition from 'clean' - only a bit of distortion, to
'dirty' - lots of it. Depending on how well the preamp's gain structure has been
tailored to suit the power stage, it is possible to get an almost seamless
transition from clean to dirty. As the level is increased, the sound gets
progressively more distorted, and as a result is also compressed. The
compression effect is an important consideration for many players, because it
increases sustain.

A normal transistor amp will have almost no distortion at all from the
quietest playing right up to the onset of clipping. Distortion becomes audible
suddenly, and the smooth transition of the valve amp is missing. The gain
remains constant regardless of level until the amp clips, so there is much less
sustain. This can be addressed in a number of ways, one being to include a valve
in the preamp (the technique used in the Marshall 'Valve State' amplifiers, for
example). The same thing can be done without adding valves, but getting the
smooth transition and progressive gain reduction is not especially easy.

A great many guitar amp makers have simply not bothered, and the amps may use
opamps or transistors (in earlier types) that don't attempt to emulate a valve
amplifier. Some of these are from major manufacturers, and their sales don't
seem to be too badly affected - they are still making the amps with very similar
circuitry to that used 10-20 years ago and are still in business. This leads me
to think that some of the effects are too subtle for the majority of guitarists
to hear. Others are much more discerning, and this is perhaps one reason why
valve guitar amps have never gone away. Peer pressure and brand worship have
helped to keep the technology alive, but it should have been allowed to die a
natural death.

While some players are happy to use a sustain pedal and fuzz-box to get the
sound they want, others expect it from the amp. Can it be done well in a
transistor (or opamp based) guitar amp? Of course it can - it simply becomes a
matter of working out what arrangement sounds the best (and making it variable).
For some, it probably wouldn't matter if the end result was far better than
anything heard in a valve amp, but they'd still want the valve amp anyway.

3.0 - Transistor Amps

In this context, transistor can mean bipolar, junction FET (JFET) or MOSFET.
While there are certainly differences between amps that use one form of
semiconductor or another, they are usually inaudible. It will require a blind
A-B test to convince many people that if well designed they do sound the
same if they have similar specifications.

Virtually all major manufacturers stopped using valves once usable
transistors became available. Several tried very early devices and they were
commonly found wanting. Although they appeared to measure better than
'equivalent' valve amps, they sounded worse. Things have come a long way since
then, and although it's still possible to make a really bad transistor amp,
no-one seems to want them. Strangely, really bad valve amps are popular in some
quarters.

It is traditional that transistor amps have negative feedback - usually lots
of it. There are several reasons for this, although some are not obvious. Valve
amps have one endearing feature (which is also their downfall), and that's the
output transformer. The entire amplifier can go up in flames (literally), but
the speakers are protected from any DC fault that might occur. Because the
transformer cannot pass DC, no DC can get to the speakers.

Modern transistor amps connect directly to the speaker, and a component
failure can (and does) often cause the demise of an expensive loudspeaker.
Because the amp is direct coupled, part of the job of the feedback loop is to
maintain (close to) a zero volt output with no signal. With no transformer to
buffer the load from the power supplies, this is an important function. It's
generally desirable to maintain the quiescent (no signal) output voltage at less
than ±100mV - the lower the better.

Transistor amps can be designed that will operate reasonably well without
feedback, but there's really no point. Because there's no transformer,
maintaining a low DC offset can be a problem. Many of the early transistor amps
used a single supply rail, so the speaker was isolated by an electrolytic
capacitor. While this solves the problem of DC offset, it's far more complex to
design a transistor amp that will give good performance with no feedback, but
relatively easy to design one that gives excellent performance with
feedback.

3.1 - Output Impedance

Most transistor amps have output impedances that are so close to zero that
even a short length of speaker lead can double the impedance. While there is no
real point, in most cases it comes free - you don't actually have to do anything
special to get a low impedance. Once feedback is applied, the already low
impedance of the output stage is reduced to typically less than 100 milliohms
(0.1 ohm) - most are much lower than that.

Since all commercial loudspeakers are designed to be driven by a voltage
source - a very low impedance - it makes sense that they should be driven that
way when you get them home. However, it's an easy matter to convert the
impedance to something else. I've experimented with current drive (high output
impedance) for many years, and have even tried negative impedance. Yes, such a
thing exists, but it makes nearly all loudspeakers sound dreadful. For further
information on this, see Effects Of Source Impedance on Loudspeakers.

When the output impedance of an amplifier is increased, it changes the
loudspeaker's behaviour in exactly the same way as a valve amp might do.
The advantage with a transistor amp is that you can select the impedance that
suits you - it can even be made variable with a knob on the front panel if you
wanted to go that far. It's pretty easy to make the impedance variable from
about 0.1 ohm up to about 25 ohms, and I have a test amp that I use in my
workshop system that allows me to do just that.

The basic arrangement is shown below. A current sensing resistor provides
feedback to the amplifier that is based on current rather than voltage, although
it's generally better to combine the two - voltage and current feedback. By
doing so, you have the ability to determine how much of each type is used, so
the output impedance can be set to a specific value. Working out the ratio of
voltage to current is somewhat irksome, and will not be covered here. The
voltage feedback path is shown inside the amplifier - this can't be removed or
the amplifier would have no DC reference, so quiescent output voltage would not
be zero. C1 is used to separate the AC and DC feedback paths - 100% feedback is
used at DC.

Figure 6 - Transistor Amp With 4 Ohm Output Impedance

R1 is the current sensing resistor. Loudspeaker current flows through this
resistor and generates a voltage across R1 that depends only on the current
through the load. R2 and R3 are used to determine the mix of AC voltage and
current feedback, allowing the amp's output impedance to be set for a specific
value. For low frequencies, the output impedance can be 200 ohms or more if
desired.

The net result of the impedance increase is exactly the same as any valve amp
with the same output impedance. Response is no longer flat, and there is
generally a lot more bass energy, although it is not fidelity. As noted above,
the common reaction is that it sounds 'better', but of course it's no longer
accurate.

3.2 - Frequency Response

This is generally not an issue with transistor amps. With no transformer to
restrict the low frequencies, it's entirely possible to amplify DC if you want
to. It's completely pointless to do so, but there is no real lower limit for
transistor amps.

High frequencies are sometimes a little more troublesome, but it's fairly
easy to get full power at 20kHz or more. Like amplifying DC, this is also
pointless, because the energy content of music is very low above ~8kHz - it
starts tapering off from about 2kHz. Most transistor amps can produce ruler flat
response from 20Hz to 20kHz, ±0.1dB at the most. They can do this into any load
impedance within their ratings, and loads that vary with frequency have no
effect.

Naturally, if the amp is made to have a defined output impedance, it will
behave just like a valve amp with that same output impedance. This will cause
the response to vary with the load impedance as described above. Doing so is
rare with hi-fi equipment, but common in guitar amps. For dedicated systems, a
defined impedance can be helpful to modify the behaviour of a loudspeaker.

3.3 - Distortion

In the early days of transistors, there would naturally be any number of
people who would poo-poo the idea that these 'new fangled' little things could
possibly replace the valves that they grew up with. Not just replace, but
render completely obsolete within a few years. Just as there are those today who
insist that vinyl sounds better than any of the (new-fangled) digital
audio formats, they will be ever vigilant to point out the smallest flaw - real
or imagined. Strangely, no-one seems to have claimed that 78 RPM shellac discs
are better than vinyl, or perhaps they are keeping very quiet.

Unfortunately, the nay-sayers had plenty of real ammunition. While there were
obviously some very good early transistor amps, there were some real shockers
too. One of the biggest issues was crossover distortion - the point where
control of the output signal is passed from one device to the other. Because of
the nature of this distortion, it may barely register on a distortion meter, so
the figures looked excellent. Unfortunately, despite the meters saying
that the distortion was low, listeners could hear the distortion - it was
plainly audible, and sounded dreadful.

"Game over" said the valve enthusiasts. "Instruments can't be trusted, the
figures are nonsense, therefore all measurements can't be trusted, so
we'll just use our ears. No double-blind testing for us thank you - that's too
stressful, and we want to be able to make assessments based on our feelings (at
the time) and describe them using a thesaurus." Thus it has been for many years,
with little chance of any change that might, possibly, resolve some of the
arguments that have never gone away.

How could a meter get the distortion measurement wrong? It's a bloody meter -
it measures what's there and ignores what's not. Unfortunately, not really. See
Figure 7 for a waveform that shows the crossover distortion residual ... that
signal which remains after the input signal has been filtered out. The
distortion meter measures either the average or RMS value, which reads
low because the spikes on the residual are often very narrow. In this case, the
RMS value is 6.8mV, but the peaks are much higher at 25mV. The peak
amplitude isn't normally measured, and many people to this day do not connect an
oscilloscope to the output of the distortion meter. This allows you to see the
exact nature of the distortion, and a little experience will allow the user to
just look at the waveform and decide that it will either be audible or not. The
meter isn't lying - the user is faulty! Many measurements have to be interpreted
in electronics, and prior to the easy and cheap access to simulators, Fast
Fourier Transforms (PC programs can do them easily) and digital capture and
analysis, it wasn't always easy to ensure that the correct interpretation was
made. As measurements become more sophisticated, the user has to be more
vigilant than ever because it can be too easy to miss something important. There
is a very big difference between knowing how to operate a piece of test
gear and knowing how to use it.

The trace is the distortion residual - ie. the waveform after the
fundamental frequency has been removed with a notch filter. When the left-over
signal looks like that shown, you know that you have a problem, even if it's not
audible due to background noise or a poor resolution monitor speaker. Crossover
distortion gets worse as the signal level is reduced. It can often be heard
quite plainly if a low-level low frequency tone is used (around 300-400Hz) - the
amplified signal will have a discordant low-level buzz that's usually quite easy
to hear.

Figure 7 - Transistor Amp Crossover Distortion

The above graph gives you some idea of how audible crossover distortion can
be - the sharp transitions mean that the distortion has significant high
frequency content. Although the level is low (less than 0.2% THD), if
compared against a valve amp with similar or even worse distortion performance,
an amp with this much crossover distortion will sound a lot worse. Part of the
reason is shown below in the spectrum analysis. There were a few people in the
1960s who looked at the distortion residual on an oscilloscope, but far too few.
Unless you look (or listen to the residual on an amplifier), you can be unaware
that the 0.2% distortion measured is 'nice' (low order harmonics only) or
otherwise. While there has been a lot of study into the audibility of distortion
based on the harmonic structure, for some reason it was ignored by many in the
early days of transistor amps. A lot people would never notice of course - the
general populace listens to music as a background activity, and any distortion
may easily be missed.

Figure 8 - Crossover Distortion Harmonic Content

Figure 8 shows the harmonic content, which rather than doing the honourable
thing and producing distortion components that diminish rapidly with increasing
frequency, those from crossover distortion are odd-order, and tend to remain
almost constant. They do roll off, but far too slowly, so there is still over
1mV of harmonic signal at 10kHz - and this is from a 318Hz source signal. To
make matters worse, the crossover distortion residual signal does not diminish
significantly as the level is reduced. While you might measure 0.2% distortion
at full power, it will be a great deal higher at 100mW because the crossover
component remains almost constant.

Why did many of the early amps show significant crossover distortion? That
much is easy. Thermal stability of early transistor amps was often pretty poor,
and if the amp were allowed to get hot it could get into a state called thermal
runaway. As it get hotter, more current is drawn, so it gets hotter, until ...
bang. To prevent this from happening, amps were often biased too low -
they didn't get into thermal runaway, but had crossover distortion instead. This
problem has been solved for many years, and modern power transistors also have
much higher gain and are far more linear then the early devices.

3.3.1 - Guitar Amps

It's considered by many that transistor guitar amps suck. While this can be
true of many cheaper brands (in particular those made in China), many guitarists
will happily use an amp that is fully transistorised (or uses opamps) right up
to the output valves. Others have a valve front-end (either in whole or in
part), and then use a transistor power amp. Neither qualifies as a valve
amplifier in my books. If transistor front-ends and transistor power amps are Ok
to use, then why not just combine all the transistorised bits and be done with
it?

There are DSP (digital signal processor) systems that claim to be able to
emulate the sound of any amplifier - valve or transistor, and you won't find
many valves in the DSP. By one means or another, the dynamic distortion and
signal compression abilities of a valve amp can be duplicated very well, and
without resorting to a token valve in the circuit. Valve amp distortion comes
with the valves, but with opamps and transistors (or DSP) you can adjust the
circuitry to be as clean or dirty as you like - user selectable.

I remain at a loss as to why some guitarists won't even plug into a
transistor amp to try it, yet will spend far more than necessary to buy a valve
amp that is guaranteed to break down at the worst possible moment, and requires
regular maintenance to keep going.

The distortion characteristics of a valve amp can be duplicated. It's not
especially easy, and requires a lot more effort than may initially be apparent,
but it can be done. Even the traditional hard clipping of a transistor power amp
can be smoothed out to some degree, but this remains the final frontier.
Ultimately, the speaker is expected to roll off at around 6-7kHz to reduce the
upper harmonics. It that's not done, even a valve amp can sound dreadful when
driven hard.

4 - Efficiency

One of those things that isn't often looked at is the efficiency of the
different amp types. In this context, it's not the conventional 'watts in vs.
watts out' efficiency that we're interested in, but how close to the supply
rails the amplifier can get before it clips. Many people have found (or claimed)
that a brand 'X' valve amp provides a rated power of (say) 30W, but will tell
you "It sounds just as loud as a 100W transistor amp". That's a 5.2dB
difference, so the valve amp would indeed sound louder ... if it were true.
Essentially, this is rubbish - it can do no such thing, although a more modest
difference is very common.

This hypothetical 30W amp might operate from a 400V supply, which in turn
might be provided via a valve rectifier. Power valves will rarely be able to
pull the plate voltage below around 50-100V (depending on many design factors)
at rated load, so the supply voltage is effectively reduced by this amount - the
valve can't actually use it, so it's gone. This is the valve's 'saturation'
voltage, and it varies depending on many factors. Rectifier and transformer
efficiency and regulation might mean that at full power, the supply voltage
might fall by 10-15%, from 400V to perhaps 340V.

At full power into a resistive load, the valve plates are now supplied with
340V, cannot swing below (say) 80V, so the plate peak-to-peak voltage is reduced
from the maximum possible of 452V RMS to 368V RMS. The full power measurement
will be done with the 368V signal, because that's the continuous power rating.
This would give you about 16V RMS across an 8 ohm load - close enough. A high
impedance load, such as a speaker at resonance, may be 30 ohms or more. With
this light loading the supply might only fall by 5V, and the output valve can
pull the plate voltage down to maybe 15V. The RMS plate voltage is now 537V.

VP = ( Vsupply -
Vreg - Vsat ) * 2 / 1.414

(Where VP is RMS voltage plate-to-plate)

VP = ( 400 - 60 - 80 ) * 2 / 1.414

= 386V RMS (Rated impedance)

VP = ( 400 - 5 - 15 ) * 2 / 1.414

= 537V RMS (High Impedance)

Converting that to secondary voltage, the amp delivers 16V into the rated
load, but can deliver 22V into the speaker at resonance. 22V can easily be
misinterpreted as being the voltage across the rated load - it's very common to
measure a signal and work out the power that would be developed if the
impedance were 8 ohms as expected. The difference is 3dB - the amp
appears to deliver up to 60W into the higher impedance. With music
playing through a loudspeaker, look at the oscilloscope to see the voltage peaks
- they are usually noticeably larger that you measured when the same test was
done using a dummy load.

The inefficient valve stage and poorly regulated supply make it appear that
you can get an extra 3dB of amp headroom. Since most valve amps have a
comparatively soft clipping characteristic, some peak clipping will be inaudible
because they don't create a cascade of high order harmonics. If it's only
transient peaks that clip, it may go completely unnoticed, regardless of the
type of amp.

Now, compare the above with a 30W transistor amp. To obtain 30W into 8 ohms,
the power supply will be about ±25V at full load. The losses in a transistor
output stage are generally very low, typically a couple of volts (peak). Supply
regulation is somewhat better because the valve rectifier is gone, but is
otherwise similar. and the supply voltage at no load might be ±28V or so. The
maximum voltage this amplifier can produce with no (or light) loading will be
52V peak-to-peak (18.4V RMS), or an 'apparent' 42W if the voltage is assumed to
be across an 8 ohm load.

With programme material and a loudspeaker load connected, the peak amplitude
you will see is unlikely to exceed 25V (~17.7V RMS), or an apparent 39W.
Compared to the valve amp, this is well down - 0.3dB instead of 3dB. Neither amp
can possibly achieve even 6dB, let alone 10dB - the latter is an apparent 300W
output, requiring a voltage swing of ±50V across the speaker. Quite clearly,
while there is certainly a difference between the two amps, it's not massive
(2.7dB in fact). It is readily noticeable though, and some valve amps
seem 'louder' than they should be based solely on their power rating into a
dummy load.

When a transistor amp clips, provided it's only on the occasional
transient, it will generally be inaudible, just as was the case with the valve
amp. At one stage, the term 'dynamic headroom' was specified to try to prevent
silly power specifications like music power, peak (music) power and peak-to-peak
(music) power. These were grossly inflated, bore absolutely no relationship to
reality, and generally gave the industry a bad name.

Dynamic headroom was to be tested with a specified tone-burst signal, and a
few specifications even included it for a while. It was realised by many that
higher dynamic headroom numbers simply meant that the power supply was
undersized, and was unable to provide enough continuous current to be useful.
The term was eventually dropped, only to be replaced eventually with 'PMPO'
(peak music power output), a term that has no defined meaning at all. Suddenly
5W computer speakers were rated at 450W PMPO. The term is still around, and
means no more now than it did 10 years ago. I recently saw a piddlingly small
'home theatre' system rated at 6,000W PMPO - it would be unlikely to deliver
more than 30W in total.

In general, well designed valve and transistor amps of the same rated power
and similar specifications will sound just as loud as each other. There may be a
very slight advantage towards the valve amp, but depending on the quality of the
power supply, the overall difference is unlikely to exceed 2dB in most cases.
Many claims that valve amps sound louder are generally wishful thinking.

5 - Heat, Noise & Vibration

Valve amps generate heat, whether they are making music or just sitting idle.
Whether you consider this to be a problem or not probably depends to some extent
on your views on global warming (aka climate change) and whether or not your
listening room is air-conditioned. None of this is relevant in the middle of
winter. A typical valve amp may dissipate around 100W or more whenever it's
turned on, and somewhat more if it's played loudly. Most transistor amps
(Class-A excepted) dissipate perhaps 10-20W when idle, depending on output
power.

While the heat itself may not be an issue (especially in winter unless you
live in the tropics), everything in a valve amplifier tends to get hot - or at
least very warm - when the amp is on for any length of time. This may reduce the
life of components like electrolytic capacitors. Valves themselves have a
limited life, which may vary from 1,000 hours to 50,000 hours, depending on
valve type and how heavily it's stressed. Output valves certainly require
regular replacement, and getting valves that work well is something of a lottery
these days. Bias current also needs to be checked and adjusted periodically.

Transistor amps require no maintenance when used for home systems. Some amps
might have fans, so cleaning filters is needed every so often, but most don't
and there is nothing to do. In theory, there is no reason that a well engineered
transistor amp can't operate for 30 years or more without the user doing
anything at all to it other then turn it on and off as required. If it normally
runs fairly cool, then no components are stressed in the slightest.

It's not at all uncommon for valve amplifiers (especially preamps) to be
comparatively noisy. No valve can ever approach the noise levels that are
commonplace with a well designed transistor (or opamp) stage. Typical noises
from valve amps may include hum (possibly heater to cathode leakage), and a hiss
level that may be audible from the listening position in some cases. Crackles,
pops and farts can also be a problem - high voltages and high impedances make a
circuit very susceptible to the smallest amount of leakage. This may occur in
any insulating material, including capacitor dielectrics. Resistors make noise
too, and the amount of noise is proportional to the resistance and the applied
voltage.

By comparison, most transistor amps are (or should be) completely silent. You
may be just able to detect some hiss from the tweeter if you place your ear very
close. Otherwise, noise levels should be undetectable from the listening
position. In reality, both valve and transistor systems will normally have noise
levels below the ambient background noise, so it should not be a problem with
typical loudspeakers.

Valves are often microphonic, and pick up vibration only to re-amplify it.
You should be able to tap any valve in the system with a pencil, and not hear
anything, but this is uncommon. Most valve preamps will show microphony,
particularly the first valve when the gain is high. Whether this is a problem or
not can only be determined by testing and experimenting - for home systems it's
unlikely to be an issue unless very high gain preamps are used (phono preamps
for example).

Needless to say, there's a veritable army of people on the Net and in your
local hi-fi shop who can sell you the latest magic stone, anti-vibration rubber
rings for preamp valves, isolation bases with real granite or marble and rubber
feet made lovingly by hand by spiritually complete monks from the outer reaches
of Mongolia. Add to this various gizmos that (allegedly) use quantum physics to
magically separate noise from signal and remove the bits you don't want to hear.
Some will have an effect (good, bad and indifferent are equally possible),
others will do nothing at all except make you look like a goose for wasting
money on something that can't possibly work (and doesn't).

Other than eventual mechanical failure, most transistor amps can handle
intolerable amounts of vibration, and not make any unwanted noise whatsoever.
It's preferable that excessive vibration be avoided, but no special precautions
are needed. Amps and preamps can be placed where they suit ... avoiding induced
hum from power transformers of course, and that applies to valve gear too.
Microphony is almost unheard of in transistor equipment. Most can be tapped with
a pencil or smacked with a shovel¹, without any noise from the speakers. Magic
rocks and all the other 'magic' paraphernalia are simply wasted on transistor
amps, and are best avoided. (Actually, they are best avoided with any
audio gear because they are nonsense at best.)

1 - I don't wish to imply that smacking your audio
equipment with a shovel is a good idea, and in general it's a practice that I
strongly discourage.

In some cases, people like to spend money on handsome stands or racks for
their audio equipment, and if they happen to have vibration isolation it
certainly won't do a transistor amp the least bit of harm. Problems are created
when people are told that they must have this or that accessory when it
will do nothing useful.

Conclusion

The one thing that is missing from almost all reviews and subjective
comparisons of valve and transistor (including opamp) equipment is double blind
tests. Without properly conducted blind testing, a review isn't worth a sausage.
The experimenter expectancy effect guarantees that differences will be heard
when there are none, and tests that don't attempt to reconcile subjective vs.
objective results are worthless. Silly statements like "some things we can
hear just can't be measured" are just nonsense. Our sense of hearing is
easily fooled - look up the 'McGurk Effect' and take the test yourself if you don't
believe me. The effect is just as real when you can see which piece of equipment
you are listening to, and you will hear things that don't exist.

Other than overdriven guitar amps, there should be no audible difference
between equivalently good quality and properly engineered equipment, regardless
of whether it uses valves, Nuvistors, transistors, opamps, MOSFETs or any other
amplifying device. The general requirements are the same, and differences should
be below the limits of audibility.

Even for guitar amps, most of the things the valve amp does can be emulated
fairly successfully, although the (often) limited bandwidth of the output
transformer is difficult to duplicate easily. Transistors have a major advantage
when driven to hard clipping, because power dissipation is very low. Valve amps
dissipate considerable power when clipping, with pentodes (such as the EL34)
suffering the most - primarily due to high screen grid current. Valve and
transistor amps do have different clipping behaviour, but although the two are
very different from each other, many players will not be able to tell which is
which in a blind test. It has even been suggested that some players like the
100/120Hz hum modulation that is typical of amps that are driven into clipping
but have inadequate supply filtering (this gem came from a Marshall service
note). I don't see it as being very likely, but transistor amps can do that too.

Whenever possible, you need to do a blind A-B test to decide if there is any
difference between any two amps of similar specifications. If so, then it's up
to you to decide which is better. Without the blind test, you are too
susceptible to suggestion to make an informed judgement. Unfortunately, reading
reviews generally doesn't help at all, because no magazine or website that has
paid advertising will jeopardise their ad revenue by saying that the company's
products are a pile of junk.

So, which is 'better'? Neither - both valve and transistor equipment is often
capable of as good or better performance than the equipment used in many
studios. Should your material be 'blessed' by a mastering studio it's probably
been compressed to within an inch of its life, often with equipment that's
chosen for its sex appeal or status, rather than anything tangible (such as
performance).

Ultimately, if you do hear a difference between a valve and transistor
hi-fi amplifier, one of them is outside the normal parameters that we expect of
good quality audio gear. It could be faulty, or is deliberately designed to
heighten something that is claimed to be 'good' - typically much higher levels
of low, even order harmonics than we are normally used to hearing, as well as a
higher output impedance than is considered acceptable. While such systems may
well sound 'nice' under some conditions, it's not hi-fi, and will usually be
incapable of the overall clarity that is defined by the term 'high fidelity'.

Naturally, the choice is the buyer's to make, but reviewers of SET amplifiers
in particular wax lyrical about the wondrous sounds they heard. Apparently,
anyone who doesn't use a SET amp is 'missing out' on the pleasure these systems
allegedly bring. No, people who don't use SET amps are missing out on higher
than normal output impedance, colouration, harmonic and intermodulation
distortion and regular maintenance of expensive underpowered amplifiers, and all
this for prices that in many cases are just plain silly.

Personally, I don't see that as missing out on anything - to not have
these issues is a huge benefit.

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article, including but not limited to all text and diagrams, is the
intellectual property of Rod Elliott, and is Copyright © 2009.
Reproduction or re-publication by any means whatsoever, whether
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Page published and copyright © 09 Nov
2009