[illustrations and graphs to come; printed copies available from VAC]
Introduction
VAC Technical Monographs are provided to help anyone interested in vacuum
tube electronics to better understand the issues involved in the design of
truly "high end" amplifiers. They are a direct response to the
(unintentionally) inaccurate impressions created by the marketing arms of
manufacturers, as well as other writings fraught with misunderstanding and
outright inaccuracies regarding basic concepts of tube electronics, laws
of physics, operation of circuits, measurement standards, and historical
attribution. It is our intention to create an unambiguous and accurate
reference for many of these issues. As such, these Monographs should prove
valuable not only to individuals who are just becoming aware of these
issues, but also to many experienced audiophiles who are awash in the
competing claims.
To ensure accuracy and provide the reader with a source for even more
information on these topics, extensive reference will be made of
authoritative works in the electronics field. Thus, the thoughts presented
herein are not merely the random musings and recollections of one
individual, but a condensation of the accumulated wisdom of a great many
authorities.
Naturally, no one work is ever exhaustive, so the reader may encounter an
omission, or even spot an error (hopefully only typographical in nature).
We are anxious to clarify any fuzzy points and correct any inaccuracies.
As such, we encourage all interested readers to correspond with us on such
points. After all, the goal here is to enlighten, not to confuse!
Readers with little electronics background will probably wish to check
into some of the introductory sources contained in the "Recommended
Readings" at the end of this Monograph.
Finally, remember that in audio electronics there is never one uniquely
and absolutely correct way to design an amplifier. Design always entails
compromise. The real question is which parameters are compromised and to
what degree. The best way to judge audio equipment remains familiarity
with live acoustic music. Listen, and let the sound be your guide.
STARTING AT THE BEGINNING
In a sense the power supply is the heart of an audio amplifier. It
supplies the raw material that the tubes, capacitors, resistors, and other
components will use to amplify the input signal. The quality of
amplification is directly dependent upon the quality of the power source.
The British and Australians refer to the vacuum tube as a valve, which is
a descriptively satisfactory word. In essence, a high voltage is applied
across the tube (between the plate, also called the anode, and cathode). A
small voltage applied to the grid of the tube then can control the passage
of electrons between the cathode and the plate. To use a fluid analogy,
the plate voltage is like water under pressure at a faucet, and the action
of the grid signal controls the passage of electrons just as a faucet
controls the passage of water. An input signal applied to the grid can
thus impress its character on the large current flowing through the tube,
creating an amplified replica of itself.
In most cases, the purpose of the power supply is to provide a ready
reservoir of DC energy at a voltage that remains essentially constant at
all times. However, the demand for energy placed upon the power supply by
an amplifier is not constant, but changes with input signal. In our fluid
analogy, this is similar to needing the water pressure to remain constant
even when the faucet is opened widely.
A power supply that has poor regulation will not deliver a steady voltage
to the amplifier as the load varies, and thus will superimpose its
variation on the amplifier and the musical signal. It is important that
the DC energy provided to the amplifier circuit be steady, since the tube
will essentially modulate that steady DC with the varying AC musical
signal. Any variation in the DC energy will combine with the musical
signal, resulting in some forms of distortion.
A simple unregulated power supply consists of a transformer, rectifier,
and filter (Figure 1). The transformer converts the 120 (or 220) volt AC
line voltage to a higher or lower voltage, as appropriate for the
associated amplifier circuit. The rectifier converts the AC to DC, but a
great deal of ripple (fluctuation) present in this DC. The ripple is
smoothed out by a filter system consisting of capacitors along with
resistors and/or inductors.
A regulated power supply incorporates additional tube and/or
semiconductor devices. They are arranged such that they sense the
difference between a desired voltage and the voltage actually being
delivered to the amplifier. The regulator then increases or decreases its
conductance so that the delivered voltage is close to the desired voltage.
When implemented correctly, the action of the regulator causes the
amplifier circuit to see an essentially steady and clean source of DC
power.
POWER SUPPLY PERFORMANCE
There are two major measures of power supply performance.
1) Regulation refers to how much the supply output voltage drops
as the amplifier presents a more difficult load to the supply, such
as when the amplifier is called upon to produce high output power and/or
reproduce very low frequencies.
2) Voltage stabilization refers to changes in the power supply
output as the AC line voltage varies.
While it may at first seem a contradiction in terms, but even an
unregulated supply possesses a degree of regulation. This is primarily a
function of the size and quality of capacitance in the power supply
filter. Regulation is also affected by the resistances of its components
and the leakage inductance of the transformer. In essence, a power supply
that has low impedance at all audio frequencies will have good regulation.
(Note for those reading the older references: the capacitance in the
power supply filter was often referred to as by-pass capacitance. The
modern use of this term refers to the use of high quality film capacitors
across electrolytic capacitors in the power supply filter. This is done to
help prevent audio frequencies from flowing through the electrolytic
capacitors, which generally resonate within the audio frequency range.)
Voltage stabilization becomes an issue because the power available in
your home is far from constant. In the United States the goal of power
companies is to hold the voltage delivered at 120 volts AC (VAC) +/- 6%,
which is a range from 113 VAC to 127 VAC. Some authors have reported
measuring brief variations to as low as 90 VAC and as high as 135 VAC.
To illustrate how serious the problem of voltage stabilization can be,
consider an unregulated high voltage supply for the output stage of a tube
power amplifier. Suppose that the designer targets 480 volts DC (VDC) as
the appropriate voltage for the output plates. Given the power companies'
goal, the actual voltage delivered to the tubes can range all the way from
451 VDC to 509 VDC (Figure 2). This will be true even if the power supply
has perfect regulation. Such wide swings of supply voltage with affect by
the performance and reliability of the tubes, altering the amplifier's
power output, distortion, and frequency of catastrophic tube failure.
Recognizing the risk of tube failure implied by an unregulated power
supply, some designers will ensure that the highest voltages likely to be
encountered are well within the maximum limits for the tube and circuit
type used. In many cases this means that the tubes can not be used to
their full potential in terms of distortion or power. This is usually
preferable to frequent tube failures.
Unlike regulation, voltage stabilization can not be improved by
additional filter capacitance. In addition, one or two of the current crop
of devices sold as power line conditioners can help with voltage
stabilization, but can not cure poor regulation.
THE NEED FOR REGULATION
The more an amplifier circuit's demands for power vary, the greater the
impact of poor power supply regulation. Accordingly, different types of
amplifier circuits display different tolerances for unregulated power
supplies. Class B power amplifiers are perhaps the least tolerant of all,
since the tubes are constantly being switched off and on (cut-off),
constantly varying the load on the power supply, even at low power levels.
(See VAC Technical Monograph 90-8.) Class AB power amplifiers are somewhat
less sensitive, as cut-off is not always being hit. A push-pull Class A
amplifier is least sensitive to poor regulation, as the lack of cut-off
here results in a more consistent load on the power supply.
Surprisingly, single-ended (not push-pull) Class A operation, as found in
virtually all unbalanced home preamplifiers, is also sensitive to power
supply regulation. The low frequency limit of this configuration is a
function of the capacitance in the filter of an unregulated power supply.
Filter capacitance in most modern designs is sufficient for robust low
frequency performance.
Pentode amplifiers tend to be more sensitive to voltage variation than
triode amplifiers. A change of only 1% in a pentode's plate and screen
voltage will cause a reduction of approximately 2.5% in available power
output. The screen circuit is the more sensitive of the two. When the same
supply is used for both plate and screen, regulation can again have a
profound impact on low frequency performance. Beam power tubes may be
slightly less sensitive than pentodes in this regard.
Poor supply regulation can also allow for an increase in unwanted
coupling between the various stages of a complete amplifier. This is an
often undiagnosed problem in many contemporary amplifiers that use only
one large capacitor reservoir common to all stages, and can lead to an
increase in dynamic distortions or, in extreme cases, cause parasitic
oscillations. In a stereo amplifier, this coupling will affect the channel
separation and imaging. Power supply interaction is probably the single
most important reason why audiophiles tend to prefer two separate
monophonic ("mono-block") amplifiers over stereophonic
amplifiers.
FORMS OF REGULATION
As previously noted, a regulated power supply utilizes an electronic
feedback or control circuit to ensure both good regulation and good
voltage stabilization. This can be accomplished with either vacuum tubes
or solid state devices. The following discussion will refer to tubes.
The most primitive form of voltage regulation uses a gaseous voltage
regulator (VR) tube in shunt with the power supply (across the output of
the supply and in parallel to the amplifier circuit) as shown in Figure 3.
As the output voltage ER increases, the regulator tube takes more current,
and thus increases the voltage drop across resistor R1. As a result, the
voltage across the load (shown here as resistor RL) is held near the
desired value. Of course, there are limits beyond which this circuit can
not maintain the desired voltage, and beyond which damage to the regulator
tube will occur.
Better regulation may be obtained through series regulation, such as the
cathode follower series regulator. This circuit converts the high
impedance of the basic power supply to a low impedance, with a
corresponding improvement in regulation. Figure 4 shows a basic cathode
follower regulator, in which the regulator tube is referred to as the
series pass tube.
In Figure 5 we see amplified series regulation. Here the sensitivity of
the regulator is improved by an additional tube (T1) which amplifies the
difference between the reference voltage and the actual output voltage.
The reference voltage is usually provided by a VR tube, as in the complete
circuit shown in Figure 6.
The solid state equivalent of a VR tube is the zener diode, and
transistors may be used as series pass and amplifier elements.
CAVEATS IN THE USE OF REGULATION
It is easy to create problems with regulators, to the point that they
degrade sound rather than improving it. One important factor is the
location of the regulator. If the regulator is placed directly after the
rectifier, it will be subjected to the switching transients created by the
rectifier. These can be significant, particularly in the case of solid
state rectifiers, and can momentarily overload the regulator, causing an
erroneous output both during and after the transient.
Similarly, if adequate bypass capacitance is not placed between the
amplifier circuit and the regulator, a significant portion of the audio
signal will flow through the regulator itself. This is undesirable as the
regulator may not be a particularly linear device. If the regulator is a
semiconductor, the inherent charge storage and poor dielectric
characteristics of the device can color the sound.
The problem of adequately isolating the regulator from the audio signal
can be particularly severe when regulation is used to provide an "automatic
bias" circuit for the output stage. In many such circuits a regulator
device, often a semiconductor such as the type 7805, is placed in the
cathode circuit of the output stage in such a way as to allow the audio
signal to flow through it. This can cause degradation of the audio signal.
In spite of the above problems, it is important that the regulator or
final capacitive reservoir be physically close to the circuit which will
use the power. This is important both to keep the overall power supply
resistance low and its impedance relatively low at high frequencies. In
additional, the self inductance of the wire connecting the supply to the
amplifier circuit can significantly lower the self resonant frequency of
the power supply capacitance, thus effectively lowering the frequency at
which the capacitor stops behaving like a capacitor, often well into the
audio frequency range.
It is also important to note that active regulation dissipates power
during normal use. As such, a highly regulated power supply may run quite
warm as compared with an unregulated supply. Also, power transformers with
good inherent regulation tend to be quite a bit larger and heavier than
their lesser counterparts.
SUMMARY
The load presented by an amplifier to its power supply changes rapidly as
the musical signal varies. This is particularly severe when a tube is
driven to cutoff or draws grid current, as in the case of Class AB1, AB2,
and B power amplifiers. (See VAC Technical Monograph 90-8.)
A power supply that has poor regulation will not deliver a steady voltage
to the amplifier as the load varies, and thus will superimpose its
variation on the amplifier and the musical signal. This will result in
increased harmonic and intermodulation distortion, lower power output, and
distorted dynamics.
An unregulated power supply also can not deliver a consistent
(stabilized) voltage to the amplifier circuit as power line conditions
vary. In the case of a typical tube output stage, the plate voltage may
change by upwards of 60 volts even when the house current stays "within
spec". This means that the circuit is often functioning at a point
other than that intended by the designer, with significant possible
degradation to the sound.
Thus, while less costly to construct, lighter weight, and cooler in
operation, the unregulated supply may be inadequate for two reasons. The
first by definition is poor regulation; even with the best precautions the
output voltage is far from constant as the load varies. The second is no
voltage stabilization. A power supply with active regulation can address
both of these problems.
To prevent active regulation from damaging the sound in its own ways, it
is important to locate the regulator correctly and provide adequate
filtration.
BIBLIOGRAPHY
Recommended Background Reading
McIntyre, Bob, Vacuum Tube Fundamentals, Part I. The Audio Amateur, 2/86,
pages 26-36. (contains an excellent reference list)
McIntyre, Bob, Vacuum Tube Fundamentals, Part II. The Audio Amateur,
2/87, pages 25-29.
Moir, James, High Quality Sound Reproduction. Macmillan, 1958.
RCA Staff, RCA Receiving Tube Manual (RC-26). RCA, 1968. Pages 3-10,
13-14, 25-37.
References Used In Preparation of Monograph 90-10
Langford-Smith, F. (Editor), Radiotron Designer's Handbook. RCA, 1953
(Fourth Edition). Chapters 13, 33.
Millman, Jacob, Vacuum-tube and Semiconductor Electronics. McGraw-Hill,
1958. Pages 520-526.
Pressman, Abraham I., Switching and Linear Power Supply, Power Converter
Design. Hayden. Chapter 6.
Tremaine, Howard M., Audio Cyclopedia. Howard W.Sams, 1969. Pages
1206-1212.
Graph Credits
Figure 3 after Langford-Smith, page 1215.
Figure 4 after Millman, page 521.
Figure 5 after Millman, page 522.
Figure 6 after Millman, page 524.
