Sugden Heritage

Sugden Si402 Class ‘A’ all silicon transistor stereo power amplifier.  Special features of the Si402 include two completely independent amplifier sections, common transformer, a choice of high or medium damping factors for high quality loudspeakers including electrostatics. (Mid 1960s)


J E Sugden & Co Ltd has been designing and manufacturing specialist hi-fi products since the 1960s.

James Edward Sugden was born in Cleckheaton, West Yorkshire and was brought up there except for a short stay on the coast during the second world war. He completed his education with E.M.I Electronics in London obtaining his degree and City and Guilds certificates and became a project engineer for Studio Sound with E.M.I broadcast equipment division. After leaving E.M.I he spent a short time with Granada TV and then in 1960 formed his own company.

The original company founded by James Sugden was Research Electronics Ltd, during 1967 he also formed J E Sugden & Co Ltd to produce electronics exclusively under the Sugden brand.

Research Electronics initially specialised in laboratory and test equipment and later produced audio instrumentation such as the Si 451 Millivoltmeter, Si 452 Distortion Measuring Unit and Si 453 Low Distortion Audio Oscillator.  James Sugden was very interested in audio electronics, in particular amplifier design.  During this period of time the first solid state Class ‘B’ amplifiers were available as an option to valve amplifiers that dominated the market place.  The new Class ‘B’ amplifiers were considered harsh sounding with poor fidelity, attributed to distortion at low signal levels.

For this reason James Sugden developed the Si 402 solid state Class ‘A’ power amplifier producing 15 watts into a 15 Ohm load (this being the standard of the time).  To compare his Class ‘A’ design Jim Sugden produced an identical 15 watt amplifier that operated in Class ‘B’, known as the Si 403.  On test both amplifiers had the same level of distortion at 15 watts, however, the Class ‘A’ design showed a significant drop in distortion as the power output reduced compared to the Class ‘B’ design.  This test showed that distortion in Class ‘B’ designs could also rise at low output.  The subsequent production models of the Si 402 were rated at 20 watts.

At that time James Sugden was the only one making pure class ‘A’ transistor power amplifiers in contrast with the rest of the industry who were class B circuits with attendant high cross over distortion. The absence of listening fatigue and JES’s  insistence on low volume manufacture of high quality specialist hand made products caused the amplifier section to grow  to such an extent that what had been a side line to the instrument business became an equal partner.

James Sugden is regarded to have designed and developed the first commercial solid state Class ‘A’ amplifier to appear on the market.  Whilst the design was unconventional, the sound performance won wide acclaim and established the company at the forefront of domestic hi-fi.

In the mid 1960s James Sugden formed a commercial relationship with Richard Allan a well established manufacturer of quality loudspeakers.  Jim Sugden arranged to market his Class ‘A’ design amplifiers under the Richard Allan brand.  The initial products were the A21 Class ‘A’ Integrated Stereo Amplifier and the C41 Pre-Amplifier and A41 Power Amplifier.

The Richard Allan A21 was rated at 10 watts into 6 Ohms and the A41 20 watts into 15 Ohms.

Jim Sugden also released the Original A21 under the J E Sugden & Co name.

Not long afterwards, the Series Two version of the A21 was released.  It could be distinguished by the following inscription on the front panel: J. E. Sugden class A stereo amplifier type A21 series two.

The main specification change of the Series Two was the increase of the maximum power output to 12 watts into 8 Ohm.

The A21 was further revised to Series Three.  At this stage it was significantly improved and was rated at 20 watts into 8 Ohm.  This version had the same power amplifier circuit as the Sugden P51.

Today the A21 is recognised as one of the classics of specialist consumer hi-fi products.


Photograph of James Edward Sugden, the founder of Sugden Audio. (1960s)


Early 1960s – The original factory for Research Electronics Ltd and J E Sugden & Co Ltd was located in Bradford Road and in 1972 moved to a modernised and extended old West Riding Mill in Carr Street, Cleckheaton. Research Electronics initially produced specialised scientific instrumentation and later precision audio test instrumentation. Sugden still use its own test equipment to calibrate all Sugden products.

The J E Sugden & Co Ltd factory is now located in Station Lane, Heckmondwike, West Yorkshire.

Works, Station Lane, Heckmondwike, West Yorkshire, England.



Sugden A21 Original Pure Class ‘A’ Stereo Amplifier – 10 watts per channel (1967).


Sugden A21 Series Two Pure Class ‘A’ Stereo Amplifier – 12 watts per channel (1969).


Sugden A21 Series Three Pure Class ‘A’ Stereo Amplifier – 20 watts per channel (1972).

The A21 Series 3 was significantly improved and was rated at 20 watts into 8 Ohm.  This version had the same power amplifier circuit as the Sugden P51.


Sugden A51 Pure Class ‘A’ Stereo Power Amplifier – 25 watts per channel (1967).


Sugden R21 Stereo FM Tuner & Sugden R51 Stereo FM Tuner (1967-1981)

Sugden R51 Stereo FM Tuner

Sugden tuners were designed to match the Sugden A21, A48 and C51/P51. Sugden FM tuners are regarded the finest available on the market and used for reference standard.


Sugden C51 Control Unit (Pre-Amplifier) (1969-1983)

The C51 is a fully comprehensive, highly sophisticated control unit of advanced specification using silicon transistors throughout. Designed initially for use in conjunction with the A51 stereophonic power amplifier and then the P51. Important features included, low distortion, wide frequency range, excellent high signal to noise ratio, special input facilities and the elimination of the mains transformer, enabling the control unit to be mounted in close proximity to a low output magnetic cartridge, power being derived from the main amplifier.

Sugden C51 Control Unit – rear panel view


Sugden P51 Mono/Stereo Power Amplifier (1972-1983)

The original P51 in stereo set up has power output of 45 watts RMS per channel into 8 ohms both channels operating. The amplifier is of dual mono construction with a common power transformer. In mono configuration (i.e. two amplifiers in bridge mode) the power output increases to 100 watts RMS per channel.

Today the C51/P51 combination are regarded as reference and are almost impossible to purchase.


Sugden A48 Stereo Amplifier – 40 watts per channel. (1973-1983)

From the original 1973 Sugden brochure: ‘The A48 amplifier is a ‘luxury’ development of the A21 amplifier. It is in many ways similar, almost identical in size and appearance, except for the 10 button push button bank instead of 6.’


Sugden A48II Stereo Amplifier – 45 watts per channel. (1976-1985)

The Sugden A4II amplifiers smooth fatigue free listening owes much to the Class A circuitry of the power stages. The configuration has been chosen to give the maximum benefits of the better reproduction qualities of Class A and the economic output of Class B.

During the reproduction of both music and speech full output is required only during very short periods. Sugden had chosen a level of Class A operation which ensured the majority of your listening would be in Class A with short excursions into AB.

The amplifier was dynamic, with precise imaging, sound stage and separation normally found in more expensive “separates” combinations. The colour combination of light brown, orange and white was novel at the time and much talked about. Today the A48II would blend in as an art piece as well as a finely tuned music reproducer!

Later versions of the Sugden A48II Integrated Stereo Amplifier were available in brown/fawn or black/silver (pictured below).



Sugden T48 Stereo FM Tuner. (1976) Match for Sugden A48II Stereo Amplifier.


Sugden C128 Pre-Amplifier & C128 SL Pre-Amplifier (1980-1990)

The Sugden C128 SL (for Straight Line) Pre-Amplifier is housed in an unusual console style case with a sloping fascia. The sides and front panels are of solid Rosewood with three rotary top controls for on/off, volume and input selection, that includes moving magnet and moving coil cartridges.


The Sugden C128 SL as shown in the original brochure.


The Sugden C128 SL after minor revison of controls.


The Sugden C128 Pre-Amplifier is housed in a conventional case with numerous inputs and outputs. It featured DIN standard and RCA sockets and has two disc inputs for turntable connection.


Sugden P128 Mono Block Power Amplifiers (1980-1990)


The Sugden P128 mono power amplifiers are a MOSFET configuration. The power amplifier cases are handed left and right for siting next to each loudspeaker, if desired, and have massive heat sinks on the front and back panels.  Power output is 150 watts RMS per channel into 8 ohms both channels operating, 240 watts RMS per channel into 4 ohms and 460 watts RMS per channel into 2 ohms.

Internal view of (right channel) Sugden P128 Monaural Power Amplifier.


Sugden HAMC Head Amplifier Moving Coil (1982-1985)

The Sugden HAMC is a low noise head amplifier, powered from internal batteries or from a suitable external power source. Suitable for all low output phono cartridges without the need for resistive or capacitive matching.


Sugden A28 Pure Class ‘A’ Integrated Amplifier. (1983-1984)


The original Sugden A28 Amplifier had a special SQC circuit, short for “Switched Quiescent Current”.


Sugden A88 Integrated Stereo Amplifier. (1983)

The Sugden A88 Integrated Amplifier was designed from the Sugden A48 being its flagship amplifier giving pleasure to over 10,000 Sugden devotees. The A88 produces 80 watts per channel.


Sugden A28 Class ‘A’ Integrated Amplifier, Sugden T28 & Sugden DT28 FM Tuner. (1984-1986)


Brochure cover produced by Sugden.


Sugden A25 Mosfet Stereo Amplifier. (1985-1990)



Sugden A28II Integrated Stereo Amplifier. (1986-1990)


Sugden A48III Integrated Stereo Amplifier. (1985-1990)


Sugden Au51 (Original) Stereo Power Amplifier. (1986-1990)

Sugden Au51 (Original) Stereo Power Amplifier – internal view, dual mono configuration.


Sugden Au51c Pre-Amplifier with Sugden Au51cs Power Supply & Sugden Au51p Power Amplifier. (1990-1999)


Sugden C28 Pre-Amplifier (1987-1994) and Sugden P28 Class ‘A’ Power Amplifier. (1986-1993)



Sugden A21a Original Pure Class ‘A’ Integrated Amplifier and Power Amplifier. (1989-2000)

The Sugden A21a Pure Class ‘A’ Integrated Amplifier is the continuing evolution of the 1964 Sugden A21.


Sugden A28B Integrated Stereo Amplifier. (1990-1995)


The Sugden A28B Integrated Amplifier was the final (fourth) amplifier in the A28 Amplifier Series.  It has cascode circuitry, available as a line amplifier or fully integrated with a plug-in phono board for use with MM or MC cartridges.  The amplifier developed 40 watts per channel.


Sugden A48B Integrated Stereo Amplifier. (1990-1995)


Sugden A25B Mosfet Integrated Amplifier. (1990-1995)



Sugden Au41 Pre-Amplifier & Sugden Au41 Power Amplifier. (1991-1996)


Sugden SDT-1 Compact Disc Player. (1993-1997)


Sugden Optima 80 Passive Line Amplifier. (1995-1999)


Specifications: 40 watts per channel. Frequency Response: 8Hz – 100KHz. Noise: 95dB. Distortion: 0.015%. Separation: 80dB.


Sugden Optima 140 Passive Line Amplifier. (1995-1999)

Specifications: 70 watts per channel. Frequency Response: 6Hz – 105KHz. Noise: 95dB. Distortion: 0.015%. Separation: 80dB.


Sugden Optima 200 Passive Line Amplifier. (1995-1999)


Specifications: 100 watts per channel. Frequency Response: 6Hz – 105KHz. Noise: 95dB. Distortion: 0.018%. Separation: 90dB.

The Sugden Optima was recognised for its realistic, involving and natural music reproduction.


Sugden Optima Compact Disc Player. (1995-1999)



Sugden Symetra Monaural Pure Class ‘A’ Power Amplifiers. (1995-1999)

The Sugden Symetra outstanding 40 watts per channel (into 8 Ω) pure Class ‘A’ monaural power amplifiers.


Sugden Signature 41C Pre-Amplifier & Sugden Signature 41P Power Amplifier. (1996-2004)


The Sugden Signature 41c Pre-Amplifier has five line level inputs including tape, with the option of a plug-in dedicated phono boards for either moving magnet or low output moving coil cartridges. The matching Sugden Signature 41p Power Amplifier is rated at 100 watts per channel.


Sugden Audition-C Integrated Amplifier. (1999-2004)


The Sugden Audition-C Integrated Amplifier is a sophisticated integrated amplifier featuring current feedback, DC coupled and very wide bandwidth. Gold plated relays were used in the switching circuit. Complete musical experience.


Sugden Bijou CDMaster Compact Disc Player, Sugden HeadMaster Class ‘A’ Headphone Amplifier & Pre-Amplifier & Sugden AmpMaster Power Amplifier (1999-2008).



Sugden Masterclass AA Remote Volume Pre-Amplifier, Sugden Masterclass Compact Disc Player & Sugden Masterclass AA Class ‘A’ Stereo Power Amplifier. (1999-2007)



Sugden Masterclass AA Class ‘A’ Stereo Power Amplifier in Black finish.


Sugden Masterclass AA Class ‘A’ Remote Volume Pre-Amplifier. (1999-2007)



Sugden Masterclass AA Class ‘A’ Remote Volume Pre-Amplifier, internal circuit layout. Separate power transformers and circuits for right and left channels. Note gold plated circuit boards.


Sugden A21a Pure Class ‘A’ Integrated Stereo Amplifier and matching Sugden CD21 Compact Disc Player. (2000-2007)


SUGDEN A21a & CD21

Evolution of the 1989 Sugden A21a Original: no change to model code, still Sugden A21a Pure Class ‘A’ Integrated Stereo Amplifier and now a matching Sugden CD21 Compact Disc Player.


Sugden Masterclass Compact Disc Player. (2000-2007)



Sugden Masterclass Phono Amplifier. (2001-2007)



Sugden Bijou HeadMaster Headphone Pre-Amplifier. (2000-2009)



Sugden Masterclass Integrated Amplifier. (2002-2006)

The Sugden pure Class ‘A” A21 has been the most popular amplifier Sugden Audio has ever produced. Originally released in 1967 the A21’s longevity can be put down to a simple formula of superb sound quality and reliability. The Masterclass Integrated Amplifier offers the same benefits but with a wealth of upgrades and improvements and considerably higher power output.


Sugden Bijou CDMaster Compact Disc Player. (2004-2008)



Sugden Bijou PhonoMaster Phono Amplifier. (2004-2008).



Sugden Bijou MusicMaster Pure Class ‘A’ Power Amplifier. – 16 watts per channel (2004-2008).



Sugden CD21SE Cascode Current Compact Disc Player. (2004-2011)


The CD21SE uses a dedicated compact disc loader/laser mechanism that is specifically designed to replay red book specification audio and CDR Discs.  Digital conversion is by DAC 7 hybrid integrated circuit with four times over sampling.  All digital filtering, noise shaping and digital-to-analogue stages are Sugden designed and manufactured circuit boards.  The analogue output stage is a low-impedance cascode current circuit with zero feedback.  A substantial audio grade mains transformer provides power for seven separate regulated power supplies to minimise interference.

The use of a highly modified compact disc transport system delivers the best possible performance from the Philips/Sony red book compatible audio discs.


Sugden A21SE Stage One Phono Amplifier. (2005-2012)


Housed in a heavy weight compact case, the single input will accept both Moving Magnet and Moving Coil cartridges.  There are internal adjustments for gain and the circuit is able to recognise input impedance and adjust accordingly.

The circuit is totally discrete enabling Sugden to tailor the output transistors for the best possible sound, combining it with a high gain front-end.  The circuit is single ended like the A21SE amplifier.

The Sugden A21SE now has an LTVS low tension voltage supply facility on the rear panel of the amplifier.  The three pin socket provides a high current power supply for the Stage One.

The Sugden A21SE Stage One can only be used with the LTVS supply from the A21SE Amplifier.


Sugden A21SE Special Edition Pure Class ‘A’ Integrated Amplifier (Line). (2005-2015)


Sugden CD21 Series 2 Precision Compact Disc Player. (2007-2011)



Sugden Masterclass PDT-4 Precision Digital Transcriptor. (2007-2011)


Sugden A21a Series 2 Pure Class ‘A’ Integrated Amplifier. (2007-2015)

Its origins are the A21 (1967) followed by the A21a series which started production in 1989.  The original A21a model was in production between 1989 to 2000, a facelift version between 2000 to 2007, and the A21a Series 2 between 2007 and 2015.  The A21a Series 2 featured the following upgrades: heavier chassis and strengthened case, 10mm thick aluminium front panel, remote volume control and beefier power supply. At the same time the mono and tape loop switching and the balance control were deleted enabling improvements to the pre-amplifier circuit by shortening signal paths.


Sugden Mystro Integrated Amplifier.  (2008-2011)

The Mystro is the first Sugden amplifier to include a ‘VCV’ (voltage current voltage) pre amplifier stage.  The configuration of this pre-amplifier includes a buffered input, voltage to current converter (Howland Current Pump) and current to voltage output stage.  This produces a perfect current, independent of the volume control setting, in the HCP stage, resulting in perfect structure and extremely low signal to noise characteristics.  This current drives the virtual earth output stage achieving an exceptional tonal balance, even at very low listening levels.

The pre-amplifier is combined with an AB power amplifier which has been developed over many years to give the best compromise between Class ‘A’ and Class ‘B’ type operation.  The power amplifiers are configured as dual mono output stages with low impedance power supplies to ensure excellent bass timing.  The circuitry has been tuned to give extended high frequency clarity, with purpose and punch, to deal with some of the latest digital recordings techniques.  Tremendous headroom has also been included in the tone burst capabilities of the 125-watt, multi emitter, bi-polar output devices.


Sugden Masterclass FPA-4 Pure Class ‘A’ Power Amplifier. (2011-2019)

The Masterclass FPA-4 power amplifier is a Pure Class ‘A’ Power Amplifier of 30 watts per channel. The output stage is of the classic Sugden single ended design.

It has the characteristics of a very good Valve amplifier with soft clipping and low internal overloads. The phase shift at high frequencies is also extremely low resulting in a very open, uncompressed big and fatigue free sound stage.

The amplifier benefits from a comparator and correction circuit so that errors which would occur in the power stage, due to varying loads and distortion, are corrected to match the output of the high impedance input amplifier. Whilst it is maintaining the signal authenticity it is also increases the load stability and forces the output impedance to zero. The Amplifier with Sugden Error Correction has extremely low open loop output impedance with a resultant high damping factor at quite high frequencies.


Sugden Mystro Compact Disc Player.  (2011-2013)

The Mystro Compact Disc Player is based on the CD21 (as partnered with the A21a).  It differs only in it has no digital output, so will not be suitable for use with a separate DAC.


Sugden PDT-4F Precision Digital Transcriptor.  (2011-2017)

The PDT-4F is the fusion of various multitudinous DSP devices available for CD product development and Sugden have achieved the pinnacle of natural musical performance with a combination of mixed techniques and technologies.

A DVD disc spinner modified to become a dedicated CD engine. This unit includes an output buffer with storage and extremely accurate clocking facilities.

An S/PDIF digital interface transceiver with jitter attenuating PLL. A pair of stereo 16 bit audio DACs without over sampling and digital filtering. Current to voltage converter. A 55 kHz post conversion filter stage.

A ‘Class A’ discreet component output stage with optional phono/asymmetric or XLR balanced output options.


Sugden Fusion 21 Precision Compact Disc Player. (2013-2017)

The heart of the Fusion player is a precision ‘CD Engine’ comprising of a very accurate loader/laser assembly driven by custom software.  The interconnected front-end board (servo) is capable of short term data storage through its on board memory, made possible by the compact disc running at a higher speed than conventional compact disc mechanisms.

The buffered output from the servo is passed to a dedicated digital interface transceiver.  An external high quality master clock allows low jitter recovery of S/PDIF through the high performance internal PLL.  The transceiver is also able to clean up (de-jitter) the received digital audio signals.  The data signal is re-formatted and the output status bits are selected before being sent to the multi-bit digital to analogue converter.  This method of signal processing has made both over sampling and digital filtering redundant.

The analogue signal from the DAC is then put through a current to voltage converter (trans-impedance amplifier) using circuits chosen and developed specifically for their performance and sound quality.  To remove any serious audible noise produced in the digital domain, an analogue 55K post filter is incorporated prior to the signal reaching the final pure Class ‘A’ analogue output stage.

To expand the possibilities of digital playback, the Fusion 21 CD player is also capable of acting as a high quality stand-alone digital to analogue converter.  When in Standby mode the S/PDIF coaxial input is activated allowing replay of an externally connected digital source.


E. & O. E.