Mullard Circuits For Vacuum Tube Audio Amplifiers Book

CIRCUITS FOR AUDIO AMPLIFIERS POWER AMPLIFIERS AND PRE-AMPLIFIERS FOR MONAURAL AND STEREOPHONIC REPRODUCTION FROM MICROPHONE, RADIO, TAPE AND PICK-UP SIGNALS

First published-April 1959 Second edition-August 1960 (Reprinted with minor corrections-March 1962) Reprinted - November 1962 Reprinted -October 1963 © Mullard Limited, 1963 Mullard is the trade mark of Mullard Limited and is registered in most o the principal countries in the world. PREPARED BY TECHNICAL INFORMATION DEPARTMENT AND ISSUED BY MULLARD LIMITED, MULLARD HOUSE, TORRINGTON PLACE, LONDON, W.C.1.(TP413)

CONTENTS CHAPTER PAGE Preface v Mullard Audio Publications vi 1 Amplifying Systems 1 2 Sources of Distortion in Recorded Sound 4 3 High-quality Amplification 16 4 General Notes on Construction and Assembly 26 5 Twenty-watt Amplifier 29 6 Ten-watt Amplifier 39 7 Three-watt Amplifier 53 8 Seven-watt, D.C./A.C. Amplifier 59 9 Two- and Three-valve Pre-amplifiers 67 10 Four-channel Input-mixing Pre-amplifier 82 11 Three-watt Tape Amplifier 88 12 Tape Pre-amplifier 102 13 Seven-watt Stereophonic Amplifier 113 14 Three-valve Stereophonic Amplifier 121 15 Stereophonic Pre-amplifier 127

PREFACE The first notable contribution of Mullard design engineers to the specialist world of high-quality sound reproduction was the now-famous `Five-Ten' amplifier circuit which was introduced in the summer of 1954. In subsequent years, a succession of designs for high-quality circuits has issued from the Mullard Applications Research Laboratory, and these designs include circuits for power amplifiers and preamplifiers, circuits suitable for tape-recording equipment and, most recently of all, circuits for stereophonic reproduction. Many of these circuits have been described in articles in various magazines or have appeared in the form of booklets or leaflets. The others have only recently been released by the laboratory, and have not yet been published. The object of this book is to present the most up-to-date versions of the published circuits together with the new stereophonic circuits in a way which will be useful and convenient to equipment manufacturers, service engineers and home constructors. A list is given on page vi of the earlier Mullard publications which have been incorporated in this book. These publications may be useful for purposes of comparison, but it should be appreciated that the information given here is the latest available and consequently, supersedes that issued elsewhere. Most of the material contained in this book is based on work undertaken at the Mullard Applications Research Laboratory. The largest contributions have been made by R. S. Babbs, D. H. W. Busby, P. F. Dallosso, C. Hardcastle, J. C. Latham and the late W. A. Ferguson. While the circuits described in this book have been designed by Mullard engineers. it should be noted that Mullard Limited do not manufacture or market equipment or kits of equipment based on the circuits. Mullard Limited will not consent to the use of the 'Mullard' trademark in relation to equipment based on the circuits described, but will not object to appropriate references to Mullard circuits, specifications or designs. Also, the information contained in this book does not imply any authority or licence for the utilisation of any patented feature.

M ULLARD AUDIO PUBLICATIONS The dates given below are the dates of the first appearance of each publication. The reference numbers are for the latest reprints of the publications. 5-valve, 10-watt, High Quality Amplifier Circuit August 1954 (out of print) High Quality Sound Reproduction* August 1955 (out of print) `3-3' Quality Amplifier Circuit August 1955(TP352) Circuits for Tape Recorders August 1956 (TP430)(out of print) Supplement to Circuits for Tape Recorders August 1957 (TP322) 7-watt D.C./A.C. High Quality Amplifier August 1957 (TP325) Four-channel Input-mixing Amplifier August 1957 (TP319) Two-valve Pre-amplifier August 1957 (TP339) Circuit for self-contained Tape Amplifier August 1958 (TP356) *The section of this book headed `Design for a 20-Watt High Quality Amplifier' was reprinted from articles which were first published in Wireless World, May and June, 1955.

CHAPTER 1 Amplifying Systems 1n the last ten to fifteen years, remarkable progress has been made in the field of sound recording and reproduction. The introduction of high-fidelity disc and tape recordings and the improvement in the design of playback equipment have set new standards for the discerning listener and have consequently created the need for systems providing amplification of the highest quality. Because of the variety of signal sources in existence, the range of application of these systems has had to be wide. Therefore, preamplifying circuits which cater for pick-up and tapeplayback heads, f.m. tuner units and microphones have assumed great importance. Furthermore, the emergence of stereophonic recordings and pick-up heads has created a demand for dual-channel amplification and this has been fulfilled in equipment consisting of either self-contained stereophonic amplifiers or specially-designed stereophonic pre-amplifiers coupled to pairs of conventional power amplifiers. This book contains descriptions and details of high quality amplifying and pre-amplifying circuits for monaural and stereophonic applications which can be classified as shown in Table 1. Most of these circuits have been designed to be used with each other, and examples of various arrangements of the items of equipment are discussed in subsequent paragraphs of this chapter. MONAURAL SYSTEMS The choice of line-up is dictated to a large extent by the strength of the signal to be reproduced. For example, the version of the 10W circuit incorporating volume and tone controls can be used satisfactorily without a pre-amplifier if input signals greater than 500mV are available. (Such signals can be obtained from high-output crystal pick-up heads or f.m. radio tuner units.*) The 3W circuit with volume and tone controls can be used directly with sources which give signal voltages greater than 100mV. However, signal voltages from low-output crystal pick-ups, magnetic pick-ups and tape-recording heads (which can have values of up to 500mV but are usually between about 2 and 100mV) are normally too low for direct use with the power amplifiers, and a preamplifier is necessary to increase the sensitivity of the system. In addition, discs and tapes are deliberately recorded in such a manner that the strength of signals derived from them will be a function of frequency (see Chapter 2). Consequently, the strength of these input signals will usually vary in some way not related • The circuits described in this book can be used with the better quality f.m. tuner units available commercially. No tuner unit is described here because it is felt that the inherent difficulties of alignment cannot be overcome with the test equipment currently available to the home constructor. TABLE 1 Classification of Mullard Circuits Power Amplifiers of General Application Five-valve, twenty-watt circuit (`5-20'); Five-valve, ten-watt circuit * (`5-10'); Three-valve, three-watt circuit * (`3-3'). Pre-amplifiers of General Application Three-valve circuit; Two-valve circuit; Input-mixing circuit. Power Amplifiers of Special Application Seven-watt, d.c./a.c. circuit; Three-watt tape circuit (Type A revised); Seven-watt stereophonic circuit; Two-watt stereophonic circuit. Pre-amplifiers of Special Application Tape-equalising circuit (Type C revised); Twin-channel stereophonic circuit. * Two versions: one with, one without tone and volume controls. to the original sound patterns and, to obtain realism from such recordings, some degree of compensation is required in reproducing these sounds. This equalisation is best incorporated in a pre-amplifier, so that the scope of application of power amplifiers can be increased without any resultant fall in the standard of performance. The 2- and 3-valve pre-amplifier circuits described in Chapter 9 have been designed to precede either the 3W, 10W or 20W amplifier. The simple arrangement for the 10W and 3W amplifiers connected directly to the input source is shown in Fig. 1. The use of pre-amplifiers is indicated in Fig. 2.

The 3 W tape circuit comprises a self-contained record-playback system for microphone and radio input sources. The circuit is also suitable for replaying pre-recorded tapes. A programme-monitoring signal is available when recording and, during playback, this source will provide a low-level signal suitable for driving a power amplifier if an output power higher than 3 W is required. STEREOPHONIC SYSTEMS For stereophonic reproduction, two separate amplifying channels are required, one for each of the twin signals obtained from the stereophonic signal source. In the simplest arrangement, the 2W or 7W stereophonic amplifier is connected to the source as shown in Fig. 6. If the stereophonic pre-amplifier is used, it should be coupled to two power amplifiers and loudspeaker systems as in Fig. 7. The tape pre-amplifier provides recording facilities comparable with the self-contained unit but, on playback, the circuit only provides an equalised signal for driving a power amplifier. A simple arrangement for the combination of tape pre-amplifier and power amplifier is shown in Fig. 3. This line-up is suitable also for the 3W tape amplifier when the low-level playback output is used. For a complete record/monitor-playback system using pick-up input sources, the tape pre-amplifier should be used in conjunction with the line-up of Fig. 2. The new arrangement is shown in Fig. 4*. When recording, the input signal is fed to the appropriate input channel of the pre-amplifier and then from the programme-recording output of the pre-amplifier to the input of the tape circuit. The programme can be monitored by using the normal output of the preamplifier to drive the power amplifier. For playback, the equalised output from the tape pre-amplifier is fed via the Auxiliary input channel to the main amplifying chain. Stereophonic systems should be suitable for reproduction from monaural sources, and `compatibility' switches are usually incorporated for this purpose. For instance, the switching indicated in the preamplifier stage of Fig. 7 selects the input channels* and also sets the equipment for normal or reversed stereophonic reproduction or for monaural reproduction using both power amplifiers and loudspeaker systems. * It is important that the tape deck in both these arrangements should always be switched to the playback position before the connection from the tape output to the pre-amplifier input is made. 1f the tape recorder is in the record position when the connection is made, oscillation will occur. To feed several inputs into an amplifying system at the same time, the main pre-amplifier in Fig. 2 should be replaced by the input-mixing pre-amplifier. Recording, monitoring and playback are possible with the new arrangement shown in Fig. 5.* The output impedance of the mixer is low so that the inputs of the tape pre-amplifier, and the power amplifier can be connected simultaneously to the mixer without affecting the quality of reproduction. * Facilities are provided for stereophonic radio signals in anticipation of stereophonic transmissions. POWER SUPPLIES Facilities are provided in the 10W and 20W amplifier circuits for obtaining power supplies for a singlechannel pre-amplifier or f.m. radio tuner unit. For stereophonic applications, the h.t. supply for each channel of the pre-amplifier can be taken from the power amplifier used with 'that channel.

The h.t. supply facilities for pre-amplifiers are also available in the 3W circuit. Those for f.m. tuner units are not immediately so, but the modifications required are not extensive. The current drain when an f.m. tuner is coupled to the amplifier is increased considerably and it will be necessary to choose a rectifier and mains transformer which will supply this current. Extra stages of decoupling will be required to prevent excessive hum voltage being picked up in the sensitive early stages of the tuner. A separate power stage is described for the tape pre-amplifier, and this can be retained when the preamplifier is used with any of the power amplifiers. Physically, however, the extra unit may prove an encumbrance and consequently it may be preferable to use the supply stage of the main amplifier to provide the needs of the tape pre-amplifier. NOTE: The word monaural is at present commonly used to depict conventional 'sound reproduction in which a single-track recording and a single pick-up head (or tape playback head) are used. The word monophonic may often be encountered as a synonym of monaural and may eventually supersede it. Monaural means, literally, of one ear and a monaural effect is that of listening with one ear, when no impressions of space or displacement are conveyed to the listener. Monophonic means of one sound source so that monophonic reproductions again lack any impressions of spread or position. Monaural has been adopted in this book simply because it is currently more popular: its use should not be taken to imply any etymological superiority of the word over monophonic

CHAPTER 2 Sources of Distortion in Recorded Sound Some distortion is always present in the sound reproduced from a gramophone record or magnetic tape. It is inevitable that part of this distortion will be distortion recorded on the disc or tape itself, but the loudspeaker, pick-up and recording head also make substantial contributions. A carefully constructed amplifier is far from being the weakest link in the chain: in fact, it is improbable that the limit to the quality of reproduction which can be obtained will be set by the performance of the amplifier. High-quality reproduction will only be achieved if careful attention is also paid to the sound input source and to the loudspeaker. Even if perfect associated equipment could be obtained, however, it would not be possible to eliminate distortion entirely. There would still be some distortion - a comparatively small amount it is true which is bound to occur when discs or tape are used. The function of this chapter is to give brief accounts of the main items of associated equipment needed for disc or tape reproduction and also to discuss some of the causes of distortion, indicating the methods by which imperfections are minimised. LOUDSPEAKERS The choice of a suitable loudspeaker is very important for both disc and tape reproduction. While it would be wrong to assume that price is the only guide available, normally -the higher grades of loudspeaker will justify their extra cost by giving correspondingly better reproduction. The choice should never be made hastily. It is not sufficient to listen; to one or two records because it is very easy to construct a system which will sound impressive at first hearing. Extended periods of listening to music are desirable to discover if mental fatigue is likely to be caused by the system. Furthermore, because it is easier to judge speaker quality by listening to comparatively simple sounds rather than to full orchestral recordings, the system should also be tested by assessing the fidelity with which good-quality broadcast speech is reproduced. A simple, direct-radiator loudspeaker - that is, a single-cone unit handling the entire range of frequencies- can give excellent results when mounted in a suitable enclosure. It is important to remember that this enclosure should always be that specified by the speaker manufacturer since it is not possible to design a suitable cabinet without knowing accurately the basic performance of the loudspeaker unit. One of the main disadvantages of all types of simple direct radiator is that the high-frequency response is very directional. This fact, however, can to some extent be overcome by using the loudspeaker in conjunction with some form of reflector. The reflector can be built into the cabinet, or the corner of a room can be used for the purpose. The effect of the reflector is to distribute the high-frequency components in a more random manner so that, with the adequate treble boost provided by the amplifier, good results will be obtained. There is no doubt, however, that even the simplest of dual systems is much better than a single unit. Such a dual system may consist of two direct radiators- for example, a 12 in, unit for the bass-end of the spectrum and a 5 in. unit for the middle and treble components- used in conjunction with a suitably designed cross-over network. It must be remembered that, while the effective frequency response may not be very much increased by this type of dual system, various forms of distortion will be reduced considerably, added to which there will be a better distribution of high frequencies from the smaller source. Dual systems using twin direct radiators are available in two forms: either with two completely separate units, each mounted in its own enclosure or, alternatively, with the high-frequency unit mounted on some form of modified frame, making it an integral part of the low-frequency unit. Greater improvement may be obtained by using three loudspeaker units together with a three-way cross-over network. This extends the frequency range and reduces the distortion still further, as well as providing a wider range of distribution patterns. Normally, suitable cross-over networks are recommended by the loudspeaker manufacturers, and it is usually best to adopt these recommendations. However, an example of a two-way network having a very compact design and unusually low losses is shown in Fig. 1. Two Mullard Ferroxcube pot-core inductors, type LA23, are used in conjunction with the special metallised-paper capacitors now available, and a maximum rate of attenuation of 12dB per octave can be obtained from the constant-resistance circuit. The values of L and C in the diagram are given in terms of the loudspeaker impedance R and the crossover frequency f by the following equations: L = R√2 2∏f C = √2 4∏fR The approximate number of turns required on the inductor for a given inductance L is 53√L, where L is in millihenries. In practice, the impedance R of the speaker will already be determined, and it will be convenient to obtain the value of capacitance to give the required cross-over frequency from the appropriate section of Table 1. The corresponding value of inductance, and the approximate number of turns to give this value, can then also be obtained from the table. One advantage of horn-loaded loudspeakers over direct-radiator types is that the effective damping is increased, and, consequently, there is improved reproduction of transients. Some units incorporate a

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