Woodside MA50 Renaissance monoblock power amplifier
The late Arthur Radford (footnote 1) one of the better-known figures in audio, was involved in radio and electronics for half a century. He held patents for speaker, amplifier, and transformer designs. In 1946, he developed a successful electronics business, and went on to produce amplifiers based on a design by D.T.N. Williamson. The audio transformer in this amplifier was particularly good, which led Arthur to produce transformers for OEM users like Daystrom and Rank. In the early 1960s his company manufactured the MA and STA series of tube amplifiers—robust, wide-band devices that became popular, particularly the stereo STA-25 Series III. Radford sold over 12,000 units (footnote 2). Radford also worked with Arthur Bailey on transmission-line loudspeakers.
In 1986, Radford's health was not good, and he went into retirement. At that time, the US distributors of Grant amplifiers were looking for a more reliable British tube amplifier. The Radford amplifier was chosen, but had to be supplied by John Widgery, Radford's former plant manager, who had been responsible for the design of the output transformers in the STA-25 series. Widgery bought all rights to the Radford name, and remanufactured the STA-25 Series 3 with up-to-date components, cosmetics, KT77 power tetrodes, and LED biasing. He dubbed it the Mark IV, an amplifier by "Woodside, formerly Radford." Only 100 units were produced, complete with Arthur Radford's signature. Widgery went on to produce in 1987 a slightly revised STA-25 Renaissance. This product utilized a dual-triode ECC88/6DJ8 in a cascode configuration, and used American-made 6550A tubes in the output stage.
In his 1987 review of the STA-25 Renaissance, Dick Olsher compared it with an earlier Radford STA-25 Series III; he found both amplifiers lacking. The older Radford, despite its "sweet and liquid mids" and three-dimensionality, suffered from euphonic colorations. The newer Renaissance model excelled in soundstage width and depth, with consistently sweet string tone, but the rest of the sonic picture was troubled. The amplifier had a colored sound and suffered from a loss of focus and resolution. DO was very specific: the kickdrum sounds were "anemic," and female vocals were "phasey," and stretched across the soundstage. There was only one moral to this review: Back to the drawing board!
The new design
The latest Woodside amplifiers (footnote 3) have redesigned printed circuit boards (pcbs) and new power supplies. American designer Arnold Weisenberg is responsible for this new, fully regulated and decoupled high-voltage power supply. Key to his new approach is a solid-state regulator to prevent interaction between stages. In addition, the redesign lowers the impedance between stages. The amplifier is equipped with a toroidal power transformer with a current capacity five times the quiescent requirement, and three separate filament windings: one for each pair of output tubes, and a third for the driver stages. The power supply uses 750µF (at 450V) of capacitance. Weisenberg developed these approaches doing modification redesigns on the Marantz 8B with Sid Smith.
The MA50's front end is said to be stable. It uses an ECC811 direct-coupled input stage with an active-load twin-triode 12AX7, followed by a second tube, a 12AU7, used as a cathode follower to avoid Miller effect. (These tubes are Golden Dragons, produced by the Shuguang tube factory in China.) The first stage in the amplifier is coupled to a phase splitter with a polypropylene capacitor. The output stage uses two matched pairs of Czech Tesla EL34 power pentodes in a push-pull parallel configuration, run in class-A. The MA50 is autobiasing, so there's no need for customers to use meters or LEDs when the tubes are replaced.
The Woodside employs a multi-section audio output transformer. This proprietary design of Widgery's features only one pair of secondary taps rather than the commonly found taps for 4, 8, and 16 ohm loudspeakers. Speaker designs from 20—30 years ago typically ranged from 3 to 20 ohms. Within the past decade, however, the impedance modulus of loudspeakers has narrowed to 4–8 ohms, and is often a nominal 5 ohms. The modern tube amplifier can have its entire secondary wound to drive a single narrow impedance range. The manufacturer claims that the combination of solid-state rectification and very low internal impedance (high damping factor) allows the MA50s to exert better control over woofers or bass ribbons.
High-end tube amplifier cosmetics go in a number of directions: exotic (Wavestream V8), art deco (Cary CAD-805), extravagant (Carver Silver Seven), and gilded (Jadis). The MA50 Renaissance exudes the gilded look, with its gold side-plating and Radford's traditional rod handles at each end of the chassis. These clever handles allow one to lift the amplifier with ease, and can also serve as a frame to rest the amplifier on if a technician has to service the under-chassis while the tube cage is removed. Between the handles, a black perforated metal screen serves as a cover for the tubes. The flashes and handles are gold-plated, and the Woodside manual warns the owner not to use abrasive cleaners, which will remove the gold plating.
Footnote 1: According to Wikipedia, Arthur died on November 21, 1993. It had been my privilege to have met Arthur at hi-fi shows in the 1970s and I was for a while an enthusiastic user of his Tristar 90 loudspeaker.—John Atkinson
Footnote 2: Radford amplifiers enjoyed much the same popularity in Britain that Dynaco did here, albeit at a higher price level.
Footnote 3: The product's name was simplified to "Woodside" to signify the modernization of former Radford designs.