Associac¸ao˜ Portuguesa de Engenharia de Audio´ Secc¸ao˜ 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,V´ıtor Tavares1, Ricardo Carvalho1 1DEEC - Faculdade de Engenharia da Universidade do Porto, 4200-465 Porto, Portugal A correspondenciaˆ devera´ ser enderec¸ada para: Tiago Jose´ 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 v´alvulas s˜aobem conhecidos pelos seus problemas t´ıpicos. As v´alvulas tendem a envelhecer e deteriorar-se muito mais rapidamente do que os dispositivos de estado s´olido, fazendo com que as suas carac- ter´ısticas se alterem com o tempo de uso. Consequentemente, necessitam de cuidados especiais, sendo necess´aria uma calibra¸c˜aoregular das correntes de polariza¸c˜aonos andares de sa´ıda, geralmente ajustado pelo utilizador ou por t´ecnicos. Estes circuitos apresentam tamb´em n´ıveis superiores de distor¸c˜aoquando comparados com amplificadores a transistores. Devido ao facto de as v´alvulas terem um factor de amplifica¸c˜aorelativamente baixo e tamb´em devido ao desvio de fase causado principalmente pelos transformadores de sa´ıda, grandes quan- tidades de feedback negativo (global) n˜aos˜aoadmiss´ıveis. Em oposi¸c˜ao,nos amplificadores de estado s´olido s˜ao empregadas enormes quantidades de feedback, apresentando assim superior linearidade. O envenenamento do c´atodo ´eoutro problema associado a v´alvulas com a sua consequente redu¸c˜aoda emiss˜aode electr˜oes, aumen- tando significativamente a resistˆencia de ru´ıdo da v´alvula. Estas caracter´ısticas tornam o estudo de m´etodos alternativos de controlo autom´atico, de amplificadores de v´alvulas, um assunto pertinente, que, juntamente com o projeto de um amplificador Hi-Fi, ´eo foco principal desta disserta¸c˜ao.

1. INTRODUCTION diophiles remain faithful to this type of amplifiers. It is Electron valves fell in disuse from most electronic appli- true that valve amplifiers, associated with their filaments cations since the appearance of transistors. Still, many au- 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 and assist with some of the issues particular to valve am- of a good valve Hi-Fi system, at least for people who like plifiers, such as quick aging, is surprisingly a recent idea to sit on a couch hearing and appreciating a good sound. [1] [2] [3] [4] [5] [6] that is worthy of further investigation, Some years ago, when starting playing guitar, the author being one of the subjects under study in this dissertation. heard some players speaking about this vintage technology and its sound qualities. As soon as he had the opportunity 2. VALVE AMPLIFIER to hear a guitar valve amplifier for the first time, he quickly An analog platform consisting of an integrated Hi-Fi am- perceived a really good and natural feeling, the higher plifier was designed, being the test platform for the digital notes seemed like real bell tolls, with really “punchy” mids control devices. The elected topology for the preampli- and lower notes. Quickly, his interest in these antique de- fier was a µ-follower followed by a long tailed pair phase- vices was triggered. When the author heard some modern inverter. The output stage comprises a circlotron (or paral- Hi-Fi valve systems, he could tell they all sounded very lel push-pull) stage preceded by a grounded cathode driver, three dimensional, each instrument within the music was with the first having an unitary gain. The driver anode re- really distinct. This totally convinced about valves against sistors are bootstrapped with the corresponding in-phase transistors that sound more sterile. It is important to state signal on the anode of the output valves, increasing the ca- the fact that musical instrument amplifiers and Hi-Fi am- pability of the driver valves to drive the input signals and plifiers are designed in totally distinct manners. also increasing gain and linearity of the stage. A small por- tion of global negative feedback (approximately 12.3dB) is A high level of distortion is generally an undesirable ef- applied to increase the linearity, bandwidth and to reduce fect in Hi-Fi equipments, while the opposite is commonly the output resistance of the amplifier. desired for electric guitars. Besides their warmth feeling, most people report that it is perceptible a higher volume Figure 1 represents the topology of the designed amplifier. in valve systems, when compared to transistor amplifiers with the same output power rating. It should be noted, 3. OUTPUT TRANSFORMER however that in general valve amplifiers show poorer elec- The operating point and output transformer primary trical figure-of-merits than solid-state amplifiers. This in- impedance was set and optimized by simulation. The bias dicates that the reason for such impression must reside on current was set at 60mA with a anode voltage of 400V psychic-acoustic models, but there are no known studies and a grid voltage of -43V. The following plots were taken done on this matter so far. from values obtained from simulations using SPICE, at the specified bias conditions. This was made isolating this cir- As a student of the Master in Electrical and Computer En- cuit from the rest of the amplifier stages. gineering, at Faculdade de Engenharia da Universidade do Porto, the author did not have the opportunity to put a The non-dashed trace presented in the upper plot shows the strong effort on the study of valve circuitries and theory, THD (%) as the impedance at the output varies, fixing the however, allied to today’s technology, he strongly believes output power value at a mean value of 20W by changing that the valve amplifier market will have a growing ten- the input signal amplitude. This shows that at a relatively dency. Introducing the use of microcontrollers to monitor low output power, the distortion decreases as the output

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impedance increases. The dashed trace represents the ob- 5. DIGITAL CONTROL HARDWARE tained 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 bot- tom, 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. These graphics indicate an optimal primary impedance for the output transformer of 1200⌦, minimizing the distor- tion 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 Am- plifier IC, with high linearity, is used to protect the mi- crocontroller from the high common-mode voltages that are present at the sense resistor terminals. A microcon- troller communicates with a DAC, which output is driven by an inverter level shifter circuit, guaranteeing the con- ditions 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 out- put valves, between pentode/triode.

Figure 2: Study for the minimal distortion/maximum 6. CONTROLLER OPERATION power optimization of the output transformer The microcontroller delays the high-voltage power sup- plies, giving time for the valves to heat. At this stage it is possible to switch the working mode of the output stage 4. POWER SUPPLY UNIT between pentode/triode. After this process, it sets a test The design of a suitable power supply unit for the ampli- current through the output valves, verifying if they are in fier was one of the requirements for implementing the pro- good working conditions. The controller then turns the posed system. This part of the design required an extraor- grid voltages to a lesser negative value, setting the bias dinary care since it would influence the overall behaviour currents to the desired value. Automatic bias only occurs of the amplifier. A poorly designed PSU may lead to an when the output stage is working as class A or without sig- amplifier with unwanted characteristics such as high noise nal, guaranteeing that the mean current value is always the floor or parasitic oscillations. All the power supplies were bias component, as shown on figure 4: designed using full-wave solid state rectifiers since max- imum 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 crit- ical, due to their low-signal levels, all the PSUs concern- ing these sections, including the filament heaters, were de- signed 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. Figure 4: Automatic bias range

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Transconductance of the output valves is determined and given to available electric simulators that allowed a much reported on a display. The same measurements are used easier optimization procedure. to determine a FFT of the signal. This allows for an es- The digital part was delineated and implemented with suc- timative of the THD applying at the input an 1kHz sinu- cess, bringing a very comfortable mean of adjustment to soidal wave with 400mV of amplitude. The user needs to an excellent sounding amplifier, while the user is listening manually adjust a potentiometer at the phase-splitter stage and appreciating a good sound experience. The user does (P represented in figure 1), and the microcontroller sends 2 not need to worry about biasing the valves as they age with the determined THD value to the display. This allows to use. Moreover, it may serve as an alert system for possible vary the THD from 0.3% to 0.8% (approximately), varying destructive damages, preventing major harms to the am- mainly even order harmonic distortion which, to a certain plifier by warning the user for the need of substituting the level, is relatively benign to human ear [7]. defective devices. 7. RESULTS Manual calibration of harmonic distortion, with the aid The measured slew-rate was 7V/µs. The rest of the main of a measured THD reported by the microprocessor, was characteristics, measured on a built prototype of the de- implemented. Nonetheless, since the measurements were signed amplifier, are summarized in the following table. made at the primary and not at the transformer secondary, LFRes and HFRes are the low frequency and high fre- this value is not matching the one measured, at the output, quency response at -3dB, respectively. All of these mea- by a Network Signal Analyzer. The output transformer surements were made with an 1kHz sinusoidal wave. 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 Table 1: Amplifier measured specifications to measure THD at the output. P(Wmean) THD(%) S/N(dB) LFRes HFRes 1 0.007 102 3.5Hz 65kHz Some gaps were detected in the area of valve amplifiers 10 0.122 107 that should be further investigated. Judging by the expe- 41 0.321 110 rience obtained from this work, the existent triode valve SPICE models have a good degree of accuracy, but pen- tode models can probably be improved. Still, the biggest 8. PROTOTYPE OF THE AMPLIFIER deficit in simulation models, in the context of valves cir- Figure 5 represents the built prototype of the valve ampli- cuits, is the output transformer. In fact, after quite a fier. 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 pop- ular both to transformer and amplifier designers.

10. REFERENCES [1] R. F. Carvalho, “Amplificador de audio´ integrado avalvulas´ controlado por microprocessador,” Final Rep., FEUP, Porto, July 2006. [2] G. Anderson, “Minitron [online],” October 16, 2007, available in http://www.circuitcellar.com/microchip2007. [3] KBO-Dynamics, “Tubesynch [online],” available in http://www.tubesync.co.uk. Figure 5: Prototype [4] C. Arrowsmith and A. Fallon, “Controlling the perfor- mance of a thermionic tube,” February 11, 2010, pub. 9. CONCLUSION No. US 2010/0033245 A1. Considering the multiple variables that may influence the performance of the analog audio-amplifier, all major goals [5] ——, “Microprocessor-controlled bias adjustment in a were accomplished. An integrated valve amplifier with ex- thermionic valve audio amplifier,” February 10, 2010, cellent characteristics has resulted. In part credit should be 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. Oxford: United Kingdom: Newnes, 2003.

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