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PAPERS
HiFi Issues
Tubes or Transistors - Science or Art
Hugh R. Dean
I offer a few musings about hi-fi, particularly amplifiers and speakers.
For many of us the hi-fi journey seems effortlessly to span a lifetime
- a type of febrile, nervous affliction. But like so many other
absorbing pursuits, it could benefit from a little objective analysis,
based on a few practical observations and some enlightened tinkering.
What follows is not an engineering treatise; it is a considered
opinion, presented here for interest but without substantive evidence
aside from experience.
In 1972, the American Russell O. Hamm wrote a seminal paper comparing
tubes with transistors. The paper is long, but in essence, Hamm,
who came from a pedigreed recording background, argued that tubes sound
better than transistors as full overload is approached. The reason, he
said, is that tubes overload gradually, taking almost 15 decibels of 'grace'
before they produce objectionable distortion, whereas transistors are
lucky to give more than two decibels of 'grace' before the onset of hard
clipping. While amplifiers using both devices certainly sound terrible
at full overload, the nature of music is intrinsically peaky, and the
minor overload condition is very common in domestic settings. Thus for
music, Hamm argues, tubes will always sound better.
This is probably simplistic and arguable. In fact, Peter Walker
of Quad fame once said that, kept within overload limits, tubes and transistors
sound just thesame. Daniel D'Agostini, the owner of Krell, surely
an impressive name in solid-state amplification, recently derided the
tube aficionados, saying that hi-fi is about headroom and only solid-state
amplifiers can meet this demand. He builds amplifiers of outrageous
power and ruthless, surgical accuracy; the latest reportedly delivers
peaks of 24,500 Watts into 1 ohm for short periods. Notwithstanding,
the jury is still out. However, it should be said that the cost
of owning a tube amplifier can be quite high since tubes burn out just
like light bulbs. For many audiophiles this aspect is a deterrent.
Yet there is a large and growing group of music lovers who swear by tubes,
and who take pains to point out that in terms of tone, soundstage, stage
presence and sheer ease of listening the tube is superior. After
some decades in the solid-state camp, I am now not so sure. To me
it appears to be a matter of taste; and the truth is, I all of us change
my tastes according to circumstance. So whom do I believe, and why?
Many years ago when transistors were new, the design of solid-state amplifiers
was dominated by the engineering requirements – and frequently also
by a long tradition of tube amplification. At a time when tubes
had reached the pinnacle of their development with the high power beam
tetrode, the power pentode and highly refined transformers, the first
readily available transistors were constructed of germanium. They
soon proved to be thermally vulnerable. A great deal of engineering
was required to ensure the output stage survived all operating conditions,
and in meeting this requirement sonic considerations were necessarily
pushed to second rank. Furthermore, the early devices, even in silicon,
exhibited inferior secondary breakdown characteristics, and many pairs
of devices were required to ensure reliability - a design constraint still
in evidence today.
The active element of a transistor is so small – less than 0.2 gram
in even the larger devices – that efficient heat dissipation is crucial
to device survival. This has major repercussions for processing
music signals because the electrical characteristics of the semiconductor
materials vary considerably with temperature, which largely depends on
how hard the device is driven at any instant in time. In marked
contrast, tube elements are large, metallic, and weigh up to fifty grams
each. Furthermore, the parasitic capacitances and leakages of transistors
are much more adverse than tubes because of the nature of charge aggregation
in a solid-state crystal. Thus electrical parameters are far more
stable in tubes with temperature and voltage effects, and instantaneous
effects which might affect the music are vastly less intrusive. And
while transistor amplifiers can be made with vanishingly low distortion
by generous use of global negative feedback, it may be that temperature,
voltage and associated bias effects, one of the significant differences
between these devices and tubes, could influence the way in which they
process music.
Over the last twenty years recording technology has advanced to the point
where differences in domestic hi-fi amplification can be easily discerned.
While many would hold that modern digital recording is inferior
to the best vinyl engineering of yesteryear, it is still true to say that
there are some very competently recorded CD’s in today's marketplace
which can tax any hi-fi system to its limits. Interestingly, the best
recordings are rarely classical, but often jazz and other contemporary
popular music.
One of the issues of modern recording techniques has been gradually increasing
flexibility - and complexity, particularly since the advent of digital
technology. These developments have enabled the sound engineer to
process and mix down multiple tracks days and even years after the recording
event. While this flexibility has conferred greater control over
the final recording it has led to a diversity of methodologies - some
of them approaching the weird - particularly in sound stage composition.
For this reason, many criticisms of artificial sound staging and
improper instrumental and vocal emphasis can usually be attributed to
inappropriate sound engineering. But today's distortion performance
and bandwidth are unquestionably superior to those that prevailed twenty
years ago.
The differences between tube and transistor sound do not merely concern
the devices themselves. I have found the topology of the amplifier
to be very important. Gordon Rankin of Wavelength Audio, (a successful
manufacturer of single-ended tube amplifiers in the US), argues that an
amplifier's quality comprises equal parts of componentry, layout, and
topology. If the same components are connected in a different configuration,
it is logical to presume the sound will be different. Thus, all
other factors being equal, (and they seldom are!), the push-pull amplifier
sounds different to the single-ended (SE) amplifier, and for some reason
not fully understood the SE amplifier always features a more prominent
and soulful midrange. I can certainly verify this; I prefer the
tube SE or MOSFET SE sound while a number of longstanding audiophile friends
prefer the push-pull, solid-state sound.
Nelson Pass, a prominent US audio designer, believes that the SE amplifier
deals with the positive half cycle in a different way to the negative;
this mimics differences in the compressive and rarefactive behaviour of
air and thus sounds more 'natural'. I have some difficulties with
this statement; by way of a test, what happens if I reverse the speaker
leads? Having swapped compressions for rarefactions, the sound should
be very different. However, when this is done, the SE amplifier
remains easily distinguishable from the push-pull. It is also true
that a transformer output stage adds a degree of 'rounding' to the sound;
Trace Elliott made a MOSFET guitar amplifier with a transformer output
stage and the sound was very similar to an ultra-linear pentode amplifier.
Following on from topology issues is the great feedback debate. Feedback
is highly relevant to this discussion; it marks a fundamental difference
in the way tubes and transistors are used. Feedback is much more
complicated than first appears, and in few other areas of amplifier design
has the debate been more dominated by measured performance. I have
tinkered with the feedback loop of an amplifier and carefully listened
to music at varying gains between 5 and 50. You soon appreciate
that feedback is intimately related to sound quality; in general, (and
with some caveats), the higher gain, lower feedback configuration sounds
better.
Tubes are much more linear than transistors, and can be used in zero feedback
designs without difficulty. In fact, too much feedback in a tube
amplifier induces instability because of phase shifts in the output transformer.
But transistor amplifiers require lashes of negative voltage feedback
- around 30dB, which represents a reduction in gain of more than thirty
times. While this keeps distortion and output impedance down I believe
that in almost every instance a sonic price is paid. Feedback, whilst
commendably improving measured performance, somehow drains the vitality
from the music, conferring a flat, uninspiring sound. The reasons
may relate to dislocated phase relationships in the music that are caused
by the group delay of the feedback process. I concluded that a transistor
amplifier with less feedback would sound more lifelike, and verified this
in my observations. Measured performance degraded, however - you
can't have your cake and eat it too, even in audio!
While amplifier design has clearly been driven until now by measurement
philosophies, one can understand this because all electrical phenomena
should be measurable, and the approach is founded in rationalist, scientific
methodology. However, using conventional Lin topologies a zero feedback
transistor amplifier is almost impossible to construct. This is
because transistors suffer forbidding non-linearities when configured
for voltage amplification. And only in recent years have alternative
low-feedback topologies begun to emerge.
The unnerving truth is that almost everything in a hi-fi system, even
the hookup wire, makes a difference to the sound, and when this surprising
but reasonable observation sinks in, these effects become truly bothersome
to the hobbyist and committed designer alike. I recall once replacing
a teflon-insulated, silvered copper earthing wire with a PVC-insulated
'regular' copper wire; the splashiness disappeared and the high frequencies
lost their jagged, bright edge. No other changes were made; a small
alteration produced quite astonishing changes in the sound, and none of
this was visible on an oscilloscope with any waveform. I have found
similar differences with capacitors - which frankly left me amazed - and
now have quite definite prejudices about what to use and even where to
use it. But infuriating as it may seem, different amplifiers respond
to different approaches; a successful tweak on a SE amplifier may not
work so well on a push-pull amplifier.
A critical difference between engineering and art lies in the final stages
of amplifier preparation for the market. Once an amplifier has been
successfully designed and tested, the business of 'voicing' the amp must
begin. Design and testing fulfils the engineering requirements of
unconditional stability, reliability, bandwidth, distortion and sensitivity.
But a 'blameless' amplifier may sound quite unappetising; it may
be bland, and lifeless. This applies as much to tube as to transistor
amplifiers, and often means the difference between commercial success
and failure. Voicing the amp relates to component choice in particular,
with some emphasis on layout, particularly earthing, which can easily
destroy an otherwise competent sonic presentation.
Voicing is not measurable, but bears testimony to the extraordinary discriminating
powers of the ear. Tricks used on one design may not work on another;
this also demonstrates quite clearly that distortion and bandwidth measurements,
in all their quantifiable glory, are not telling us the whole story. This
is all the more remarkable when I consider that the SE triode amplifier,
widely acknowledged as the zenith of sonic realism, boasts very high total
distortion of typically 3%, an inferior output impedance characteristic
and a somewhat constrained bandwidth.
A promising avenue remains somewhat unexplored in the area of hi-fi amplification;
the hybrid amplifier. The tube is the voltage amplifier par excellence.
The transistor is a wonderful current amplifier, a role in which
it happily serves with almost zero feedback. I remain convinced
that the careful combination of the two devices leads to superior amplification.
Tube amplifiers are renown for a deep, layered soundstage and subtle,
low level detail; transistor amplifiers are admired for their pin-sharp
lateral imaging and high-powered transient performance. It is possible
to build such an amplifier using tubes solely for voltage amplification,
and transistors for current amplification. Yet another interesting
possibility exists; the integration of a tube into a conventional transistor
amplifier, which somehow preserves the powerful advantages of the solid-state
amplifier with the tonal subtlety and low level detail of the tube.
I have designed and tested an amplifier to the first topology (the Glass
Harmony), and recently completed a solid-state amplifier of 80W per channel
using the second topology (the Lifeforce). The Glass Harmony, a
30W SE amplifier, has matured since the original prototype; and now I
believe it well combines the dynamics and stunning resolution of solid-state
with the tonal subtlety and warmth of the tube. The Lifeforce amplifier
exhibits low-level detail and a deep sound stage consistent with a tube
amplifier and yet preserves the transient response, lateral imaging, efficiency
and power of the solid-state amplifier. It certainly lives up to
its name. There is even evidence of a more prominent, musical mid-range
which confers some of the sonic qualities of the SE tube amplifier.
Hugh Dean
© Copyright Hugh R. Dean 1999
All rights reserved
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