11.13 Do tube amps sound better than transistor amps? FETs?
Description
This article is from the rec.audio.* FAQ, by with numerous contributions by Bob Neidorff others.
11.13 Do tube amps sound better than transistor amps? FETs?
Lets first list some commonly used active electronic
components and their good and bad attributes. What follows
are some generalizations. There may be exceptions to these
generalizations, but they are based on solid facts.
TUBE: (Valve, Vacuum Tube, Triode, Pentode, etc.)
Tubes operate by thermionic emission of electrons from a
hot filament or cathode, gating from a grid, and collection
on a plate. Some tubes have more than one grid. Some tubes
contain two separate amplifying elements in one glass
envelope. These dual tubes tend to match poorly.
The characteristics of tubes varies widely depending on the
model selected. In general, tubes are large, fragile, pretty,
run hot, and take many seconds to warm up before they operate
at all. Tubes have relatively low gain, high input resistance,
low input capacitance, and the ability to withstand momentary
abuse. Tubes overload (clip) gently and recover from overload
quickly and gracefully.
Circuits that DO NOT use tubes are called solid state, because
they do not use devices containing gas (or liquid).
Tubes tend to change in characteristic with use (age). Tubes
are more susceptible to vibration (called "microphonics") than
solid state devices. Tubes also suffer from hum when used with
AC filaments.
Tubes are capable of higher voltage operation than any other
device, but high-current tubes are rare and expensive. This
means that most tube amp use an output transformer. Although
not specifically a tube characteristic, output transformers
add second harmonic distortion and give gradual high-frequency
roll-off hard to duplicate with solid state circuits.
TRANSISTOR: (BJT, Bipolar Transistor, PNP, NPN, Darlington, etc.)
Transistors operate by minority carriers injected from emitter
to the base that are swept across the base into the collector,
under control of base current. Transistors are available as PNP
and NPN devices, allowing one to "push" and the other to "pull".
Transistors are also available packaged as matched pairs,
emitter follower pairs, multiple transistor arrays, and even
as complex "integrated circuits", where they are combined with
resistors and capacitors to achieve complex circuit functions.
Like tubes, many kinds of BJTs are available. Some have high
current gain, while others have lower gain. Some are fast,
while others are slow. Some handle high current while others
have lower input capacitances. Some have lower noise than
others. In general, transistors are stable, last nearly
indefinitely, have high gain, require some input current, have
low input resistance, have higher input capacitance, clip
sharply, and are slow to recover from overdrive (saturation).
Transistors also have wide swing before saturation.
Transistors are subject to a failure mode called second
breakdown, which occurs when the device is operated at both
high voltage and high current. Second breakdown can be avoided
by conservative design, but gave early transistor amps a bad
reputation for reliability. Transistors are also uniquely
susceptible to thermal runaway when used incorrectly. However,
careful design avoids second breakdown and thermal runaway.
MOSFET: (VMOS, TMOS, DMOS, NMOS, PMOS, IGFET, etc.)
Metal-Oxide Semiconductor Field Effect Transistors use an
insulated gate to modulate the flow of majority carrier current
from drain to source with the electric field created by a gate.
Like bipolar transistors, MOSFETs are available in both P and N
devices. Also like transistors, MOSFETs are available as pairs
and integrated circuits. MOSFET matched pairs do not match as
well as bipolar transistor pairs, but match better than tubes.
MOSFETs are also available in many types. However, all have
virtually zero input current. MOSFETs have lower gain than
bipolar transistors, clip moderately, and are fast to recover
from clipping. Although power MOSFETs have no DC gate current,
finite input capacitance means that power MOSFETs have finite
AC gate current. MOSFETs are stable and rugged. They are not as
susceptible to thermal runaway or second breakdown when
compared to bipolar transistors, although a badly designed
MOSFET circuit can still self-destruct. MOSFETs can't
withstand abuse as well as tubes.
JFET:
Junction Field Effect Transistors operate exactly the same
way that MOSFETs do, but have a non-insulated gate. JFETs
share most of the characteristics of MOSFETs, including
available pairs, P and N types, and integrated circuits.
JFETs are not commonly available as power devices. They make
excellent low-noise preamps. The gate junction gives JFETs
higher input capacitance than MOSFETs and also prevents them
from being used in enhancement mode. JFETs are only available
as depletion devices. JFETs are also available as matched
pairs and match almost as well as bipolar transistors.
IGBT: (or IGT)
Insulated-Gate Bipolar Transistors are a combination of a MOSFET
and a bipolar transistor. The MOSFET part of the device serves
as the input device and the bipolar as the output. IGBTs are
now available as P and N-type devices. IGBTs are slower than
other devices but offer the low cost, high current capacity of
bipolar transistors with the low input current and low input
capacitance of MOSFETs. IGBTs suffer from saturation as much
as, if not more than bipolar transistors, and also suffer from
second breakdown. IGBTs are rarely used in high-end audio, but
are sometimes used for extremely high power amps.
Now to the real question. You might assume that if these
various devices are so different from each other, one must be
best. In practice, each has strengths and weaknesses. Also,
because each type of device is available in so many different
forms, most types can be successfully used in most places.
Tubes are prohibitively expensive for very high power amps.
Most tube amps deliver less than 50 watts per channel.
JFETs are sometimes an ideal input device because they have
low noise, low input capacitance, and good matching. However,
bipolar transistors have even better matching and higher gain,
so for low-impedance sources, bipolar devices are even better.
Yet tubes and MOSFETs have even lower input capacitance, so
for very high source resistance, they can be better.
Bipolar transistors have the lowest output resistance, so
they make great output devices. However, second breakdown
and high stored charge weigh against them when compared to
MOSFETs. A good BJT design needs to take the weaknesses of
BJTs into account while a good MOSFET design needs to
address the weaknesses of MOSFETs.
Bipolar output transistors require protection from second
breakdown and thermal runaway and this protection requires
additional circuitry and design effort. In some amps, the
sound quality is hurt by the protection.
All said, there is much more difference between individual
designs, whether tube or transistor, than there is between tube
and transistor designs generically. You can make a fine amp
from either, and you can also make a lousy amp from either.
Although tubes and transistors clip differently, clipping
will be rare to nonexistant with a good amp, so this
difference should be moot.
Some people claim that tubes require less or no feedback
while transistor amps require significant feedback. In
practice, all amps require some feedback, be it overall,
local, or just "degeneration". Feedback is essential in
amps because it makes the amp stable with temperature
variations and manufacturable despite component variations.
Feedback has a bad reputation because a badly designed
feedback system can dramatically overshoot or oscillate.
Some older designs used excessive feedback to compensate
for the nonlinearities of lousy circuits. Well designed
feedback amps are stable and have minimal overshoot.
When transistor amps were first produced, they were inferior to
the better tube amps of the day. Designers made lots of mistakes
with the new technologies as they learned. Today, designers
are far more sophisticated and experienced than those of 1960.
Because of low internal capacitances, tube amps have very
linear input characteristics. This makes tube amps easy to
drive and tolerant of higher output-impedance sources, such
as other tube circuits and high-impedance volume controls.
Transistor amps may have higher coupling from input to output
and may have lower input impedance. However, some circuit
techniques reduce these effects. Also, some transistor
amps avoid these problems completely by using good JFET
input circuits.
There is lots of hype out on the subject as well as folklore
and misconceptions. In fact, a good FET designer can make a
great FET amp. A good tube designer can make a great tube amp,
and a good transistor designer can make a great transistor amp.
Many designers mix components to use them as they are best.
As with any other engineering discipline, good amp design
requires a deep understanding of the characteristics of
components, the pitfalls of amp design, the characteristics
of the signal source, the characteristics of the loads, and
the characteristics of the signal itself.
As a side issue, we lack a perfect set of measurements to
grade the quality of an amp. Frequency response, distortion,
and signal-to-noise ratio give hints, but by themselves are
insufficient to rate sound.
Many swear that tubes sound more "tube like" and transistors
sound more "transistor like". Some people add a tube circuit
to their transistor circuits to give some "tube" sound.
Some claim that they have measured a distinct difference between
the distortion characteristics of tube amps and transistor amps.
This may be caused by the output transformer, the transfer
function of the tubes, or the choice of amp topology. Tube amps
rarely have frequency response as flat as the flattest
transistor amps, due to the output transformer. However, the
frequency response of good tube amps is amazingly good.
For more information on tubes, get one of the following old
reference books, or check out audioXpress Magazine (see the
magazine section of the FAQ for more info on audioXpress).
The Receiving Tube Manual (annual up to 1970)
The Radiotron Designers Handbook
Fundamentals of Vacuum Tubes" by Eastman 1937, McGraw-Hill
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