Patric Miller, Sugden Audio

IA: The predecessor of your present A21 Series two amplifier, the original "A21" was introduced in 1967, and was designed by Jim Sugden himself. Was it the first commercial product from J.E. Sugden Audio, and what is exactly the relationship between Sugden Audio Co. and Jim Sugden's Research Electronics ltd?

PM: Research Electronics was Jim's original company and this manufactured Scientific Instruments for collages and Universities, as well as Audio measuring equipment. The Original A21 was designed and manufactured by Jim Sugden through Research Elec. but sold under the Brand name Richard Allan, a well-known loudspeaker company, also based in west Yourkshire. The J E Sugden A21 was then released a year later under Jim's own J E Sugden brand name.

IA: I’m not an expert in the history of semiconductors, but I have a recollection that that the first-generation of transistor amplifiers, already on the market when the original A21 was introduced, almost exclusively applied class B NPN silicon devices, with some rare earlier exceptions with germanium transistors. On the other hand, dedicated audiophiles at the time were used to a different type of sound of the valve amplifiers, and were disappointed with the sound of most solid-state devices, including, I guess, Mrs. Allen and Sugden. Or what was the reason why they opted for their basic Class A topology of a voltage and current amplifying device? That was a pretty original solution compared to almost all other companies then. I can’t remember any other commercial solid-state class A -amplifiers released before A21. Linsley Hood, Nelson-Jones, Stan Curtis, all launched their amplifiers after A21, I think. Was A21 really the first class A SS-amp?

PM: The A21 was the first production Pure Class A amplifier. It was designed to over come the poor characteristics of the transistors that were used in Class B amplifiers at that time, and the problems of crossover distortion. The original transistor used in the A21 was the latest technology NPN Silicon BD121 with a maximum of 35 volts and 5A current rating. This restricted the power output to 10 Watts into 6 Ohms but the full bias class A design achieved negligible distortion at low power, rising in a linear manner with increasing power. The BD121 suffered from a similar problem to other transistors and this was thermal drift resulting in eventual failure in the Class B designs. Jim solved this problem by using a pair of carefully matched diodes to control this, together with the associated bias resistors.

IA: I believe we can all agree that Jim’s solution to the original problem has proven to be very successful. The recently introduced A21 Series two uses exactly the same configuration in its output stage, and it has now a production history of over 40 years. As far as I know, only very few products with a similar background exist. But my question is, is the absence of crossover distortion and negligible overall distortion – given that the speaker load is fair - the only technical advantage of Class A -amplifiers? And sonically, does it matter that the output stage of an amplifier is solid state or based on vacuum tubes, as long as it is class A? Or do all good amplifiers sound the same, as someone once claimed?

PM: There are three down sides to Pure Class A amplifiers; cost, efficiency and power output. However, looking at these three points it is also easy turn them round to positives. Although Class A amplifier are more expensive than an equally powered Class B design, longevity is improved. The use of better quality components improves sound, reliability and life of the product. Because only a percentage of the power produced by Class A amplifier is transferred into sound they are relatively inefficient, however, because they are quite low in power, amplifier such as the A21a only uses the same electricity as a 200 Watt light bulb.

As any Sugden Class A owner well knows, 25 Watts of Class A is like 50 Watts of AB power as the bass extension and transient speed is so great. We remain faithful to the original design principles but the introduction of better quality components allows us to develop the circuit further. We have improved the efficiency of the original design allowing more power delivery into low impedance loudspeakers, without having to use heatsinks the size of a fridge. The output devices we now use are multi-emitter types with high gain, and a cascode input stage is employed at the front end of the power amplifier. The A21 circuit has influenced a number of high profile amplifier designs by past, and also current competitors; we find this testament to a truly remarkable piece of audio engineering by Jim Sugden.

A well-designed Pure Class A amplifier should always offer a more natural and fatigue free listening experience that a switching amplifier, be it Class AB, digital or switch mode. The reduced effect of odd harmonic distortion will always provide a truer pitch. As we spend many months carefully selecting components for our amplifier and listening to different ones, it is impossible for all Class A amplifier to sound the same. By changing components in our own designs we can alter the sound significantly.

IA: There’s one important question that needs to be discussed in more detail. What does a relatively low power amplifier demand from the next link in the chain, the loudspeaker? Loudspeaker manufacturers may consider this question trivial, because there are loads of high power amplifiers out there that can deliver the required current to drive their speakers. My experience is, however, that low power amplifiers can sound actually more dynamic, fast and powerful than high power solutions, as long as the load is fair. What’s your view on this? And more specifically, what requirements the 20 Watt A21 set for loudspeakers? Or for a listener? How important variable the loudspeaker’s sensitivity is?

PM: The combination of loudspeaker and amplifier depends on a number of factors. The most important is the size of the listening area. If the listener is sat within 3.5m of the speakers, most amplifier speaker combinations will be able to reproduce satisfactory sound pressure levels for normal listening. If the listener is sat a much further distance from the loudspeakers, the amplifier power and loudspeaker sensitivity need to be considered. The other important factor is how loud the listener wishes to listen to music. Reproducing a full orchestra faithfully in a large room requires many thousands of watts and this is not possible from a quality domestic audio system. Certainly using 25 Watts with a loudspeaker of above 90dB will provide more than enough headroom in an average living room. At the factory we have approximately twenty pairs of different loudspeakers and easy access to many more. The efficient designs we use have a fast transient response, and help us tailor the sound in the high frequencies more easily. For careful listening tests we prefer to use loudspeakers of medium or low efficiency allowing the amplifier to work harder. We find this gives the best results for bass control and an even frequency response. Full range loudspeakers and small monitors will have an effect on the perceived power of an amplifier. Trying to get extra bass from a small loudspeaker will probably cause the amplifier to run out of power. A full range design will be able to produce the correct bass without the need for extra power. The most recent A21a amplifier has a new power supply and this provides more power into lower impedance loudspeakers.

IA: A fellow audiophile just described his visit in the Heathrow 2006 show in this way: "I didn't see too many smiling and happy faces in the show rooms”. Perhaps this was because of most of the visitors realized that their recently purchased xxyyzzSE27mkII is now considered as passé, and that they therefore are in the urgent need of upgrading to the next model, probably costing a small mortgage. If I understood correctly, you have not decided to transfer your production to China, and to start making money by introducing "revolutionary" new models at a regular pace. Why is that?

PM: Sugden had the opportunity to expand considerably a few years ago when approached by a well know manufacturer in the Far East. It took less that 30 seconds for us to say “thank you, but no thank you”. The main reason we are in business is to satisfy our own appetite for music and reproducing it in a way we enjoy. Although we do not bring a new model out every six months, our current product range is in constant development. Recently we upgraded the power supplies in every product, many manufacturers would take this chance to release a new product range but this is just our normal process of evolution. I think we are very lucky to be a small specialist manufacturer. This avoids us having to be over involved with magazines and shows, and become dependant on new products to sustain a high commercial turnover. Our biggest challenge as a manufacture at the moment is to produce equipment in a climate that appears to put companies like ours at a disadvantage in the market place. It is our job to clearly show customers that there is an extra dimension of pleasure in owning a handmade quality British product. The fact that many Sugden customers are using equipment we manufactured forty years ago, and can still get them serviced is something we are proud of. The fact that we spend many hours listening to our products, and even the components we use to build them, should be reassuring.

IA: On behalf of our readers and my self, I’d liked to thank you for finding time to answer our questions. And I wish you success in your work!

James Sugden on Class A ("Wireless World", 6/1970):

"By definition Class A is a method of operation in which all the output devices - transistors or valves - operate on the linear portions of their transfer characteristics all the time, drawing constant mean current from the power supply irrespective of the signal. A Class B amplifi er, on the other hand, is one in which the output devices usually split positive and negative portions of the signal waveforms between them, each operating from a point of low initial standing cur- rent. Thus, the current drawn by the stage, from the power supply, rises with the signal drive."

Toinen, ehkä modernimpi määritelmä käyttää luokkien määrittelyyn signaalin vaihetta. Sen mukaan A-luokan vahvistin vahvistaa 360 astetta signaalista. Tämän mukaan ahtaasti tulkittuna ainoastaan niin sanotut single end -vahvistusasteet tekevät vahvistimesta A-luokan vahvistimen. Vuorovaihevahvistimille vain luokat AB1 ja AB2 ovat mah- dollisia. Kolmas, puolijohdeorientoitunut määritelmä on, että A-luokan vahvistimen antoasteen toimipiste on säädetty niin, että ohjaussignaali pysyy kaikissa tilanteissa sulkeutumisen ja kyllästymisen välisellä aktiivialueella, jolla transistorin toiminta on lineaarisinta. Tällöin ei myöskään synny kytkentä- eikä ylimenosäröä. Tällä tavoin rakennettu pääteaste vahvistaa koko signaalin, eikä vain osaa siitä. Hintana on yletön lämmöntuotto, so. erittäin huono hyötysuhde.