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From Triodes to Transistors in Hi-Fi

Mullard Outlook, November, 1963.
    
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Designs for transistor amplifiers, which have been shown to equal the performance of the best valve equipment, are the latest of many Mullard contributions to high-quality sound reproduction. This article sketches briefly the development of hi-fi from its early days to the present.
At least equal in performance to its valve equivalent (left, above) the transistor Class AB 10 + 10 Watt Hi-Fi amplifier on the right is also much smaller and more compact.

Designs for transistor amplifiers evolved by the Mullard Applications Research Laboratory are the latest, and probably the most significant, advance in high-quality sound reproduction. In recent listening tests held at Mullard House, lay, technical and Press audiences confirmed that these experimental models provide sound reproduction of at least as high quality as that from comparable valve amplifiers - which have, of course, hitherto held sway in this field.

This notable advance has been made possible by the recent development of suitable transistors and the evolution of circuit techniques that enable them to be used successfully in high-quality sound equipment. Nonetheless, important as it is, this achievement does not over- shadow the succession of improvements in valves and valve circuit designs which Mullard have contributed to high-quality sound reproduction.

Hi-fi was first coined as a descriptive label for this purpose in 1927. But it is in the last decade or so that the most remarkable improvements have been made, both technically and also in bringing hi-fi equipment within reach of the man in the street.

There have been improvements in recording techniques, giving increased frequency range, lower distortion and lower background noise, and, concomitantly, improvements in reproducing amplifiers, in loudspeakers and their enclosures, and in pick-up heads and turntables.

In 1950, the 33ࡩ rev/min microgroove record arrived to give longer playing time, a further reduction in distortion, and because of its plastic vinyl-based composition, virtual freedom from needle scratch.

A few years later, there came magnetic tape records, and, finally, stereophonic recordings on tape and disc.

During this period of progress the Mullard contributions have been of benefit to the manufacturer of hi-fi equipment and to the home constructor, for Mullard circuits have been made freely available to both - and many dealers, of course, offer kits of parts for these circuits.

Triodes to Pentodes

In the early stages of the post-war history of hi-fi, triode output valves were used in amplifiers because of their comparative freedom from distortion. But they are relatively inefficient as audio power amplifiers, so to provide the required high output power, they had to be very large and they needed high levels of input. Size for size, pentodes gave a much higher output power, enabling more negative feedback to be used, thus further reducing distortion, while giving adequate output power from low drive voltages.

From the early pentodes used in certain types of Service equipment such as naval loudhailers, a 25 W output pentode, the Mullard EL37, was developed, and found application in 20 to 30 W amplifiers.

Development of pentodes for input stages was also under way, the most notable of the earlier types being, perhaps, the EF37, which was designed to reduce hum and microphony troubles. Hum was further reduced by the use of a bifilar heater in the EF37A.

The next significant development was the all-glass valve, the Mullard E40 series. Circuits were evolved for hi-fi amplifiers incorporating the EF40 voltage amplifying pentode, the ECC40 double triode, and the EL41 output pentode. The new valves brought about a reduction in the size of equipment and gave better performance.

Ten and Twenty Watt Circuits

Further Mullard developments included the EF86, a noval version of the EF40, a double triode, the ECC83, and a new 12 W high-slope pentode, the EL84. These provided a good line-up which led to the now-famous Mullard 5-10, the five-valve, ten-watt circuit first published in August 1954. This was the first of the series of Mullard circuit designs which were to influence the design of hi-fi equipment and bring high-quality sound reproduction to the wider public. The 5-10 amplifier uses two EL84 output pentodes in the push-pull output stage, an ECC83 double triode in the intermediate stage, consisting of a cathode-coupled phase-splitting amplifier, and the low-noise pentode EF86 in the high-gain voltage amplifier preceding it.

In earlier hi-fi amplifier circuits, the need for an output transformer of adequate performance made the sets large in size. But along with the new output valves came transformers that were much smaller, though showing little loss of performance. They were also much cheaper than the transformers which had been used earlier.

At about the same time, greatly improved crystal pick-ups also came on to the market. These were ideal for use with the 5-10 amplifier. Some 30,000 copies of the circuit design were distributed, and, with the improved components, a new concept in hi-fi reproduction was born.

A five-valve, twenty-watt design, the Mullard 5-20, followed about a year later. Two EL34 bantal, all-glass pentodes, rated at 25 W anode dissipation, were used in the output stage. This circuit also set a new high standard in sound reproduction and made a widely favourable impression. It was used in manufactured equipment and by home constructors. While the 5-10 circuit could include volume and tone controls and thus be used directly with crystal pick-ups and other high-level signal sources, the 5-20 was designed for use with a pre-amplifier incorporating the controls.

Tape and Stereo Circuits

By this time, magnetic pick-up heads of wide response and high performance were meeting with general acceptance, but of course their basic output voltages were too low for direct use with power amplifiers. Pre-amplifier circuits were consequently developed, using Mullard EF86 low-noise pentodes and ECC83 double triodes.

Then tape recorders and record players to hi-fi standards began to appear and further circuits were developed for tape pre-amplifiers and recording/play-back amplifiers. The more complex problems of amplifying tape-recorded sound to high-quality standards were solved by Mullard amplifier and pre-amplifier circuits.

Meanwhile, the quality of sound recording had been steadily improved and stereophonic sound reproducers had made their appearance. Keen home constructors naturally became interested, so Mullard designed and published circuits for stereo hi-fi amplifiers and pre-amplifiers.

new Mullard Transistor Circuits

The next important step forward is also a step into the present. Earlier this year (1963) Mullard demonstrated designs for transistor stereophonic amplifiers. Objectively, and subjectively as judged by lay and technical audiences, the transistor designs perform at least as well as the equivalent valve circuits and are, moreover, much lighter and smaller, as the photograph shows. The transistor circuits had transformerless output stages which, besides contributing to the reductions in size and weight, eliminate the particular limitations on performance that may be imposed by an output transformer.

The new developments are a 10 + 10 W amplifier with class AB output stage and a 5 + 5 W amplifier with class A output stage. A stereo pre-amplifier suitable for both power amplifiers has also been designed.

The 10 + 10 W amplifier uses a total of sixteen transistors. Those in the output stages function in a new 'π-mode' of class AB operation in which the bias conditions change according to the level of the input signal so that the full output power of 10 W is obtained with minimum distortion. Each channel consists of a two-stage directly-coupled amplifier using an OC71 and an OC81, followed by two AF118 in a long-tailed-pair phase splitter. The phase splitter is in turn directly coupled to two OC81Z which drive a matched pair of AD140 output transistors working into a 15 Ω load.

In the 5 + 5 W three-stage amplifier, the class A single-ended output stage is choke-capacitance coupled to the load. Each channel uses an LRC2 package (consisting of an AD140 output transistor and an OC82DM driver) preceded by an OC44.

The pre-amplifier consists of a two-transistor, directly-coupled input stage with equalisation provided by overall negative feedback. The input stage is followed by the volume control and a further stage which incorporates the tone control circuits. It provides inputs for a crystal pick-up, a magnetic pick-up and a radio tuner unit. The transistor complement of each channel may be three OC75, but better noise figures may be obtained by using AC107 in the first and last stages.

Comparative Tests

Listener-participation tests held at Mullard House enabled the audiences to make individual assessments of the performance of the transistor amplifiers in comparison with a hi-fi valve amplifier of acknowledged high performance, and also to compare the transistor amplifiers with each other. (The transistor pre-amplifier was used throughout.) There were six test pieces, each on disc, which consisted of popular and classical instrumental, orchestral and vocal music. Marks were given for the performance of each amplifier, which was identified only by a number. The result of these tests was that the 10 + 10 W transistor was most widely acclaimed, followed by the valve amplifier and then the 5 + 5 W amplifier.

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