Controlled Audio Valve Amplifier

Associação Portuguesa de Engenharia de Áudio
Secção Portuguesa da Audio Engineering Society
Artigo Apresentado no 13 Encontro da APEA
7 e 8 de Outubro de 2011 ESMAE
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Controlled Audio Valve Amplifier
Tiago Campos1 , Vı́tor Tavares1 , Ricardo Carvalho1
1
DEEC - Faculdade de Engenharia da Universidade do Porto, 4200-465 Porto, Portugal
A correspondência deverá ser endereçada para: Tiago José da Silva Campos ([email protected])
ABSTRACT
Valve amplifiers are well known for their typical problems. Valves tend to age and deteriorate much faster
than solid-state devices, making their characteristics to drift quicker with time of use. Consequently, these
amplifiers need particular care on this issue, being necessary a regular calibration of bias currents at the output
stage, usually adjusted by the user or technicians. It should be also noted that these circuits are known
for having typically higher levels of distortion when compared to transistorized amplifiers. Because valves
have relatively low amplification factor and also because of large phase shifts, mainly caused by the output
transformers, high amounts of (global) negative feedback are not admissible. In opposition, with solid-state
amplifiers huge amounts of feedback are generally employed, making transistor amplifiers to present superior
linear characteristics. Cathode-poisoning is another problem associated to valves with its consequent reduction
of electron emission, significantly increasing the noise resistance of the valve. These features make the study of
alternative automatic control methods, of valve amplifiers, a pertinent subject, which together with the design
of a Hi-Fi amplifier, is the main focus of this dissertation.
RESUMO
Os amplificadores a válvulas são bem conhecidos pelos seus problemas tı́picos. As válvulas tendem a envelhecer
e deteriorar-se muito mais rapidamente do que os dispositivos de estado sólido, fazendo com que as suas caracterı́sticas se alterem com o tempo de uso. Consequentemente, necessitam de cuidados especiais, sendo necessária
uma calibração regular das correntes de polarização nos andares de saı́da, geralmente ajustado pelo utilizador
ou por técnicos. Estes circuitos apresentam também nı́veis superiores de distorção quando comparados com
amplificadores a transistores. Devido ao facto de as válvulas terem um factor de amplificação relativamente
baixo e também devido ao desvio de fase causado principalmente pelos transformadores de saı́da, grandes quantidades de feedback negativo (global) não são admissı́veis. Em oposição, nos amplificadores de estado sólido são
empregadas enormes quantidades de feedback, apresentando assim superior linearidade. O envenenamento do
cátodo é outro problema associado a válvulas com a sua consequente redução da emissão de electrões, aumentando significativamente a resistência de ruı́do da válvula. Estas caracterı́sticas tornam o estudo de métodos
alternativos de controlo automático, de amplificadores de válvulas, um assunto pertinente, que, juntamente com
o projeto de um amplificador Hi-Fi, é o foco principal desta dissertação.
1. INTRODUCTION
Electron valves fell in disuse from most electronic applications since the appearance of transistors. Still, many au-
diophiles remain faithful to this type of amplifiers. It is
true that valve amplifiers, associated with their filaments
glow, make truly unique designs which may marvel most
Tiago Campos et al.
Controlled Valve Amp
Figure 1: Valve amplifier schematic
of the people. However, elegance is not the real essence
of a good valve Hi-Fi system, at least for people who like
to sit on a couch hearing and appreciating a good sound.
Some years ago, when starting playing guitar, the author
heard some players speaking about this vintage technology
and its sound qualities. As soon as he had the opportunity
to hear a guitar valve amplifier for the first time, he quickly
perceived a really good and natural feeling, the higher
notes seemed like real bell tolls, with really “punchy” mids
and lower notes. Quickly, his interest in these antique devices was triggered. When the author heard some modern
Hi-Fi valve systems, he could tell they all sounded very
three dimensional, each instrument within the music was
really distinct. This totally convinced about valves against
transistors that sound more sterile. It is important to state
the fact that musical instrument amplifiers and Hi-Fi amplifiers are designed in totally distinct manners.
A high level of distortion is generally an undesirable effect in Hi-Fi equipments, while the opposite is commonly
desired for electric guitars. Besides their warmth feeling,
most people report that it is perceptible a higher volume
in valve systems, when compared to transistor amplifiers
with the same output power rating. It should be noted,
however that in general valve amplifiers show poorer electrical figure-of-merits than solid-state amplifiers. This indicates that the reason for such impression must reside on
psychic-acoustic models, but there are no known studies
done on this matter so far.
As a student of the Master in Electrical and Computer Engineering, at Faculdade de Engenharia da Universidade do
Porto, the author did not have the opportunity to put a
strong effort on the study of valve circuitries and theory,
however, allied to today’s technology, he strongly believes
that the valve amplifier market will have a growing tendency. Introducing the use of microcontrollers to monitor
and assist with some of the issues particular to valve amplifiers, such as quick aging, is surprisingly a recent idea
[1] [2] [3] [4] [5] [6] that is worthy of further investigation,
being one of the subjects under study in this dissertation.
2. VALVE AMPLIFIER
An analog platform consisting of an integrated Hi-Fi amplifier was designed, being the test platform for the digital
control devices. The elected topology for the preamplifier was a µ-follower followed by a long tailed pair phaseinverter. The output stage comprises a circlotron (or parallel push-pull) stage preceded by a grounded cathode driver,
with the first having an unitary gain. The driver anode resistors are bootstrapped with the corresponding in-phase
signal on the anode of the output valves, increasing the capability of the driver valves to drive the input signals and
also increasing gain and linearity of the stage. A small portion of global negative feedback (approximately 12.3dB) is
applied to increase the linearity, bandwidth and to reduce
the output resistance of the amplifier.
Figure 1 represents the topology of the designed amplifier.
3. OUTPUT TRANSFORMER
The operating point and output transformer primary
impedance was set and optimized by simulation. The bias
current was set at 60mA with a anode voltage of 400V
and a grid voltage of -43V. The following plots were taken
from values obtained from simulations using SPICE, at the
specified bias conditions. This was made isolating this circuit from the rest of the amplifier stages.
The non-dashed trace presented in the upper plot shows the
THD (%) as the impedance at the output varies, fixing the
output power value at a mean value of 20W by changing
the input signal amplitude. This shows that at a relatively
low output power, the distortion decreases as the output
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Controlled Valve Amp
impedance increases. The dashed trace represents the obtained THD (%) as the impedance at the output changes,
this time setting the maximum peak of the Vgk voltage to 20V. This graphic clearly shows an optimal minimal point
for the distortion level. The plot represented on the bottom, shows the output power, when varying the impedance
at the output of the circuit, while maintaining constant the
maximum peak of Vgk voltage at -20V.
5. DIGITAL CONTROL HARDWARE
These graphics indicate an optimal primary impedance for
the output transformer of 1200⌦, minimizing the distortion and maximizing the output power.
Figure 3: Control circuit schematic
As shown in figure 3, the currents of the output stage are
acquired from series sense resistors at the cathodes of the
stage. A High Common-Mode Voltage Difference Amplifier IC, with high linearity, is used to protect the microcontroller from the high common-mode voltages that
are present at the sense resistor terminals. A microcontroller communicates with a DAC, which output is driven
by an inverter level shifter circuit, guaranteeing the conditions to digitally control the bias currents in the output
valves. It also controls the state of some press buttons and
relays, being capable of switching on/off the high voltages
of the amplifier and also the mode of operation of the output valves, between pentode/triode.
Figure 2: Study for the minimal distortion/maximum
power optimization of the output transformer
4. POWER SUPPLY UNIT
The design of a suitable power supply unit for the amplifier was one of the requirements for implementing the proposed system. This part of the design required an extraordinary care since it would influence the overall behaviour
of the amplifier. A poorly designed PSU may lead to an
amplifier with unwanted characteristics such as high noise
floor or parasitic oscillations. All the power supplies were
designed using full-wave solid state rectifiers since maximum efficiency was needed, but also because half-wave
rectification causes small portions of DC currents to flow
through the transformer, which may cause core saturation
[7].
Since the pre-amplifier and phase-inverter circuits are critical, due to their low-signal levels, all the PSUs concerning these sections, including the filament heaters, were designed as regulated supplies. At the driver and the output
stages, the signal has a significant amplitude, being more
rubust to noise and because of it, these supplies were not
regulated.
6. CONTROLLER OPERATION
The microcontroller delays the high-voltage power supplies, giving time for the valves to heat. At this stage it
is possible to switch the working mode of the output stage
between pentode/triode. After this process, it sets a test
current through the output valves, verifying if they are in
good working conditions. The controller then turns the
grid voltages to a lesser negative value, setting the bias
currents to the desired value. Automatic bias only occurs
when the output stage is working as class A or without signal, guaranteeing that the mean current value is always the
bias component, as shown on figure 4:
Figure 4: Automatic bias range
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Controlled Valve Amp
Transconductance of the output valves is determined and
reported on a display. The same measurements are used
to determine a FFT of the signal. This allows for an estimative of the THD applying at the input an 1kHz sinusoidal wave with 400mV of amplitude. The user needs to
manually adjust a potentiometer at the phase-splitter stage
(P2 represented in figure 1), and the microcontroller sends
the determined THD value to the display. This allows to
vary the THD from 0.3% to 0.8% (approximately), varying
mainly even order harmonic distortion which, to a certain
level, is relatively benign to human ear [7].
given to available electric simulators that allowed a much
easier optimization procedure.
7. RESULTS
The measured slew-rate was 7V/µs. The rest of the main
characteristics, measured on a built prototype of the designed amplifier, are summarized in the following table.
LFRes and HFRes are the low frequency and high frequency response at -3dB, respectively. All of these measurements were made with an 1kHz sinusoidal wave.
Manual calibration of harmonic distortion, with the aid
of a measured THD reported by the microprocessor, was
implemented. Nonetheless, since the measurements were
made at the primary and not at the transformer secondary,
this value is not matching the one measured, at the output,
by a Network Signal Analyzer. The output transformer
imposes a certain degree of distortion that is compensated
at its secondary by means of the global negative feedback
applied in the circuit. The next obvious step would then be
to measure THD at the output.
Table 1: Amplifier measured specifications
P(Wmean ) THD(%) S/N(dB) LFRes HFRes
1
0.007
102
3.5Hz 65kHz
10
0.122
107
41
0.321
110
8. PROTOTYPE OF THE AMPLIFIER
Figure 5 represents the built prototype of the valve amplifier.
The digital part was delineated and implemented with success, bringing a very comfortable mean of adjustment to
an excellent sounding amplifier, while the user is listening
and appreciating a good sound experience. The user does
not need to worry about biasing the valves as they age with
use. Moreover, it may serve as an alert system for possible
destructive damages, preventing major harms to the amplifier by warning the user for the need of substituting the
defective devices.
Some gaps were detected in the area of valve amplifiers
that should be further investigated. Judging by the experience obtained from this work, the existent triode valve
SPICE models have a good degree of accuracy, but pentode models can probably be improved. Still, the biggest
deficit in simulation models, in the context of valves circuits, is the output transformer. In fact, after quite a
searching, simulation with the use of output transformer
models for audio is very unusual. Very few models are
available and probably, if research was made around this
subject, good models for these devices would become popular both to transformer and amplifier designers.
10. REFERENCES
[1] R. F. Carvalho, “Amplificador de áudio integrado
a válvulas controlado por microprocessador,” Final
Rep., FEUP, Porto, July 2006.
[2] G.
Anderson,
“Minitron
[online],”
October
16,
2007,
available
in
http://www.circuitcellar.com/microchip2007.
Figure 5: Prototype
9. CONCLUSION
Considering the multiple variables that may influence the
performance of the analog audio-amplifier, all major goals
were accomplished. An integrated valve amplifier with excellent characteristics has resulted. In part credit should be
[3] KBO-Dynamics, “Tubesynch [online],” available in
http://www.tubesync.co.uk.
[4] C. Arrowsmith and A. Fallon, “Controlling the performance of a thermionic tube,” February 11, 2010, pub.
No. US 2010/0033245 A1.
[5] ——, “Microprocessor-controlled bias adjustment in a
thermionic valve audio amplifier,” February 10, 2010,
pub. No. GB 2462445.
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[6] ——, “Adjustment of quiescent cathode current in a
thermionic valve audio amplifier,” February 11, 2010,
pub. No. GB 2462368.
[7] M. Jones, Valve Amplifiers, 3rd ed.
Kingdom: Newnes, 2003.
Oxford: United
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