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Jim Sugden ran a company that specialised in laboratory and test equipment. This operated under the name, Research Electronics. During the 1960’s many companies like this made test equipment for use by schools and technical colleges. The UK government were keen that the country should produce more scientists, engineers, and technicians. So they’d introduced a new curriculum and provided funds for the necessary lab equipment. As a result Research Electronics during this period was selling instruments for ‘Nucleonics’ – e.g. counters and amplifiers for use with Geiger-Muller tubes to detect and measure radiation for teaching lab experiments. However Jim had a keen interest in audio electronics, in particular amplifier design.

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J. E. Sugden Si 402

At that time the first solid state amplifiers were starting to appear, but in many cases their audio performance left much to be desired. The phrase “transistor sound” started to appear in audio circles, and mean a harsh sound with poorer fidelity than the valve amps that had dominated the market. Jim was one of those who soon identified as a cause was what we now know as “crossover distortion”. This was a common problem with early solid state Class B power amplifier designs. The amplifiers measured well at their full rated power output. But – unlike the then-familiar valve amplifiers – they produced a nasty form of distortion when the signal levels were low. Most engineers were used to working with the valve designs. The crossover distortion wasn’t something they’d anticipated when they developed the early solid-state designs. Also, the test equipment most audio amplifier makers used simply wasn’t sensitive enough to detect the tell-tale signs of crossover distortion.

As a result, one of the things Jim Sugden had to do in order to compare Class B and Class A amplifiers at low output levels was to produce improved test and measurement equipment. This allowed him to investigate the problems and develop his new amplifiers. He then also sold equipment like the Si 453 Low Distortion Audio Oscillator and Si 452 Distortion Kit which were advertised in Hi Fi News under the JES Audio Instrumentation name. Although an initial reaction these days to the model names might be to guess that the “Si” meant “Silicon”, referring to the use of Silicon transistors, it actually stood for “Scientific instrument”.

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The first fruit of his work on improving solid state power amplifiers by employing Class A arrangements was the Si 402. The details of its performance were reported in an article in Hi Fi News magazine in the November 1967 issue. This compared the Si 402 with another experimental design he’d produced, the Class B Si 403.

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The figure above re-plots the results shown in the Hi Fi News article. The two designs have the same level of distortion at 15 Watts output. The obvious advantage of the Class B design is that it could reach higher maximum power levels. However as Jim Sugden pointed out, the Class A design showed amounts of distortion that fell smoothly as the required output power level was reduced, but the Class B amplifier did not. In those days reviews (and tests made by developers) often only checked distortion in the range from a Watt or so up to full power. People had tended to assume that the distortion would fall away at lower powers. But with many early Class B solid state designs it could actually rise again at low output power.

At the end of the Hi Fi News article, Jim Sugden wrote,

“If you desire something to compare with a first-rate Class A valve amplifier costing £50 or more for a stereo unit you must compare it with an equivalent standard of transistor amplifier which unfortunately is not at present available. As is obvious from this article my own company is working on one, and some recent advertisements and discussions and, in some cases, obvious silences, make it clear that other companies are doing similarly.”

Jim’s original plan was to market the Si 402. Specifications for the amplifier were given in the Hi Fi News article.

Specs for HFN version of Si 402

Rated o/p 15W THD at Rated o/p 0·15% THD at 1W 0·01%
Feedback 30dB Freq. resp. +/-0·25dB 20Hz-20kHz Rise time 5 microsec
Hum+Noise -85dB Sensitivity for 15W 0·5V rms Input Imp. 47 k
Output Imp. 0·6 Ohm Power Transistor diss. 50W (no sig) Stability Unconditional

(The power ratings are with the 15 Ohm loads that were standard at the time.)

Shortly afterwards J. E. Sugden and Co Ltd issued an announcement that they had started production of the Si 402, giving slightly different specifications.

Announced Si 402 specifications.

Rated o/p 20W THD 1kHz 15W 0·1% THD at 1W 0·01%
Rise Time 5 microsec Freq. resp. +/-0·25dB 20Hz-20kHz Power Consumed 110 W
Hum+Noise -85dB Sensitivity for 15W 0·45V rms Input Imp. 470 k
Norm. o/p Imp. 0·25 Ohm ELS o/p Imp. 1·6 Ohm Stability Unconditional
(Power ratings into 15 Ohm load.)

The obvious changes were that the maximum output power had been increased to 20W, the ‘normal’ output impedance reduced to 0·25 Ohms, and the input impedance increased. In addition, there were now two sets of output terminals. One the ‘normal’ set for use with most loudspeakers. The other the ‘ELS’ terminals for use with Quad Electrostatic loudspeakers. (What these days we tend to call the Quad ‘ESL-57’ to distinguish their initial production model from later Quad electrostatic designs.)

The ELS terminals seem to simply have had series resistors fitted so as to avoid the amplifier from being directly connected to the loudspeaker load. The original Quad speakers were regarded as a ‘difficult’ load and these series resistors were presumably to ensure amplifier stability and safety.

The wording of the announcement sheet for the Si402 was as follows:


MODEL Si 402

J. E. Sugden & Co Ltd announce that production of the only commercially available all transistor Class A stereo amplifier has now started. The amplifier formed part of the subject matter of a recent article in one of the technical journals and has provoked much interest. The problems of cross over distortion, with its attendant production of the more objectionable high order harmonics, especially at lower power levels, are now well known. Only recent developments in output power transistors have permitted true Class A operation to be achieved. Class A operation offers a distortion content which is negligible at low power and rises in a linear manner with increasing output. The use of Silicon transistors offers almost absolute reliability and also due to the absence of valve heaters the quiescent heat dissipation is considerably lower than that from a valve amp of equivalent rating.

Special features of the Si402 include two completely independent amplifier sections, the only common part being the mains transformer primary, current and voltage limiting in the output stages, a choice of high or medium damping factors and the designed for load impedance in the 15 to 20 Ohm makes the amplifier eminently suitable for the high quality loudspeakers including electrostatics. Comprehensive facilities for pre-amplifier power, remote switching, circuit protection, and hum loop avoidance are included. The construction is to professional rather than commercial standards and all aspects mechanical and electrical are rigorously inspected at various stages of manufacture. In addition as proof of maintenance of inspection each individual amplifier is provided with a copy of the final inspector’s test sheet and includes frequency response figures, noise figures, and distortion graph for each half of each individual amplifier.

The recommended retail price is expected to be about 100 GNS. A matching high quality versatile pre-amplifier control unit will also shortly be available.

In addition to the specs listed in the table earlier, the sheet also stated that the amplifier size was 7 x 9 x 15 inches and its weight was 25 pounds.

The above announcement and specifications indicated an amplifier which was designed for serious or professional use as a high quality instrument. This showed Sugden’s background as an instrumentation engineer and someone who produced items engineered to high standards. However the price was probably a bit of a shock to some. “100 GNS” was 100 Guineas. For those who aren’t old enough to recall the ‘Guinea’ and its use in the UK, 1 Guinea used to mean 21 shillings whereas one Pound Sterling was 20 shillings. The Guinea was often used in shops to price items, although its use evaporated during the latter half of the 20th century following decimalisation of the UK currency. In part its use was intended to indicate ‘quality and exclusivity’ but it also made the price look lower!

The price was therefore “about 105 Pounds”. This was at a time when you could have bought a pair of Quad II valve power amplifiers for around 50 Pounds, or an Armstrong 421 stereo integrated transistor amp for about 40 Pounds. So the announced price for an Si 402 was high for the period.

It isn’t now clear how many (if any!) Si 402’s were made and sold at the announced price. However the amplifier was rapidly overtaken by events – perhaps in part prompted by the price level which Sugden had arrived at for the Si 402. Despite this, the Si 402 can have some claim to being the first Class A solid state power amplifier advertised and made by J. E. Sugden. If any were sold, then it may well have been the first commerically available solid state Class A audio power amplifier.

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