Audio T Verdict
The Sugden Sapphire FBA-800 is a formidable stereo power amplifier for listeners who want the company’s pure Class A engineering with greater authority, lower noise and improved control over demanding loudspeakers.
Its floating-bridge architecture combines four 20W amplifier sections to produce 40W per channel into 8 ohms. The amplifier operates in a fully balanced mode from input to output and provides both RCA and balanced XLR connections, with front-panel selection between them.
This is a substantial, specialist power amplifier that requires a suitable preamplifier and careful system matching. Partnered well, it provides the foundation for an uncompromising analogue or digital Hi-Fi system.
Sugden Sapphire FBA-800 Pure Class A Stereo Power Amplifier
The Sugden Sapphire FBA-800 is the company’s most advanced stereo power amplifier.
Designed and handmade in Heckmondwike, West Yorkshire, it combines pure Class A operation with a fully balanced floating-bridge circuit, a custom low-impedance power supply and substantial output devices.
The FBA-800 produces 40W per channel into 8 ohms with both channels operating and provides selectable unbalanced RCA and balanced XLR inputs.
Part of the Sugden Sapphire Series
The Sapphire Series represents Sugden’s most ambitious preamplifier and power-amplifier engineering.
The range was developed around redesigned pure Class A circuits and carefully selected modern components, while retaining the company’s long-established emphasis on musical involvement, signal purity and British manufacturing.
The FBA-800 can be partnered with a suitable standalone preamplifier, including a compatible balanced design, to create a dedicated pre and power amplifier system.
Pure Class A Power Amplification
The FBA-800’s output stages operate in pure Class A.
Class A operation maintains a fixed standing current through the amplifier’s output devices rather than switching them between different operating states as the music signal changes.
This approach is central to the amplifier’s low output impedance, load tolerance and ability to reproduce subtle musical information without crossover switching between output devices.
Floating-Bridge Architecture
The FBA-800 uses a floating-bridge power-amplifier architecture.
Four individual 20W amplifiers are combined within a single stereo chassis. Each channel contains two amplifier sections working together as voltage amplifiers and driving a bridged power-transistor output stage.
This configuration allows the FBA-800 to achieve greater power relative to its internal voltage rails while preserving pure Class A operation.
Four 20W Amplifiers in One Chassis
Rather than using one conventional amplifier circuit for each stereo channel, the FBA-800 contains four 20W amplifier sections.
Two sections work together for the left channel and two work together for the right channel, providing a combined stereo output of 40W per channel into 8 ohms.
The floating-bridge arrangement is designed to provide a lower noise floor, increased power and improved tolerance of changing loudspeaker loads.
40W Per Channel Into 8 Ohms
Sugden publishes an output of 40W per channel into 8 ohms with both channels operating.
The figure may appear modest compared with the headline power ratings of many Class AB or Class D amplifiers, but the FBA-800 is designed around continuous pure Class A operation and a substantial low-impedance power supply.
Available listening level will still depend on the loudspeaker’s sensitivity, impedance behaviour, room size and the distance between the speakers and listening position.
Loudspeaker Matching
The FBA-800’s low output impedance and floating-bridge circuit are designed to provide good control and load tolerance, but no amplifier is automatically suitable for every loudspeaker.
Very low sensitivity, unusually difficult impedance characteristics or extremely high listening levels should be considered carefully before purchase.
Contact your local Audio T store to discuss your loudspeakers or arrange a demonstration with your preferred music.
Fully Balanced From Input to Output
Sugden states that the FBA-800 operates in a fully balanced mode from its input through to the final power output.
Balanced operation uses two complementary signal paths, helping the amplifier reject electrical noise that is common to both sides of the connection.
The internal balanced architecture also forms an important part of the floating-bridge output design.
Balanced XLR Input
A stereo balanced XLR input is provided for connection to a compatible preamplifier or source component with a properly controlled variable output.
Balanced connections can offer improved rejection of electrical interference, particularly when longer interconnects are required or the system contains several mains-powered components.
The XLR input is selected using the FBA-800’s front-panel input control.
Unbalanced RCA Input
A conventional stereo RCA input allows the FBA-800 to be used with an unbalanced preamplifier.
This provides flexibility when upgrading an existing system or partnering the amplifier with equipment that does not offer balanced XLR outputs.
Only one input type should be selected and used at a time.
Front-Panel Input Selection
A front-panel control selects between the RCA and XLR input connections.
This allows both compatible preamplifier outputs to remain physically connected when required, although only the selected input will be used for playback.
Volume and source selection are controlled by the connected preamplifier rather than by the FBA-800.
Selectable 6dB Input Attenuation
A second front-panel control provides 6dB of input attenuation.
Engaging attenuation reduces the FBA-800’s effective input sensitivity, helping it match preamplifiers with a relatively high output level or systems that otherwise become loud within a very small movement of the volume control.
The standard setting should normally be tried first. The attenuated position can then be selected when it provides a more useful volume-control range or better system-level matching.
What Does 6dB Attenuation Do?
Input attenuation reduces the level of the signal entering the power amplifier. It does not change the FBA-800’s maximum available power.
Instead, the preamplifier will need to provide a higher output level before the FBA-800 reaches the same listening volume.
This can make volume adjustment less sensitive when using a high-output preamplifier or particularly efficient loudspeakers.
735mV Input Sensitivity
Sugden publishes an input sensitivity of 735mV for full output in the standard gain setting.
A compatible preamplifier should therefore be capable of supplying sufficient clean output voltage while providing a properly controlled volume adjustment.
The 6dB attenuation setting can be used when the connected preamplifier produces more output than the system requires.
A Separate Preamplifier Is Required
The FBA-800 is a power amplifier and does not provide source selection, a listening-volume control, phono amplification, digital conversion or network streaming.
It must normally be connected to a suitable preamplifier with a variable analogue output.
A streamer or DAC with a properly implemented variable output may also be suitable, although its volume control, output level and power-on behaviour should be checked before direct connection.
Do Not Connect a Fixed-Level Source Directly
A conventional fixed-level CD player, DAC or streamer should not be connected directly to the FBA-800.
Without a suitable volume control, the power amplifier could receive the source’s full output level immediately, producing dangerously high playback volume and risking damage to the loudspeakers.
Use a dedicated preamplifier or a source component with a reliable variable output and appropriate power-on protection.
DC-Coupled Power-Amplifier Circuit
The FBA-800 is DC coupled through its amplifier circuit.
This avoids placing conventional coupling capacitors at several points within the audio path and supports the amplifier’s extended low-frequency response.
A servo-lock circuit controls unwanted DC conditions while allowing the audio signal to remain directly coupled.
Servo Lock for Low-Frequency Stability
The servo-lock circuit monitors the amplifier’s DC operating conditions.
Its purpose is to maintain stability while preserving the low-frequency information carried by the audio signal.
Sugden credits the arrangement with supporting the FBA-800’s detailed and extended bass performance.
DC Output Protection
A dedicated protection circuit monitors the FBA-800 for unwanted DC at its loudspeaker outputs.
Excessive DC can place a connected loudspeaker at risk, so the protection system disconnects the output when an unsafe operating condition is detected.
The protection circuit forms part of the amplifier’s soft-switching system.
Low-Resistance MOSFET Output Switching
The protection circuit uses extremely low-resistance MOSFET switching devices rather than a conventional mechanical output relay.
These devices provide a static electrical connection between the amplifier and loudspeakers during normal operation.
The system is designed to protect the connected equipment while minimising the electrical resistance placed between the amplifier output and loudspeaker terminals.
Soft Power Switching
The MOSFET protection circuit also soft switches the power-amplifier output during start-up and shutdown.
This delays the loudspeaker connection until the amplifier’s operating conditions have stabilised and disconnects it cleanly during power-down.
The process helps prevent unwanted noises from reaching sensitive loudspeakers when the amplifier is switched on or off.
Custom Bifilar-Wound Transformer
The FBA-800 uses a single custom-made bifilar-wound mains transformer.
Its four separate windings supply the amplifier’s floating-bridge channels and associated power circuits.
The transformer forms the heart of the FBA-800’s low-impedance power supply and has been developed specifically for the amplifier.
Low-Impedance Power Supply
The power supply is designed to present low impedance to the amplifier circuits.
This allows current to be delivered rapidly when the musical signal demands it and helps maintain stable operation as the connected loudspeaker load changes.
A strong power supply is particularly important in a pure Class A design because the amplifier maintains substantial standing current even when no music is playing.
Professional-Audio Smoothing Capacitors
The main smoothing capacitors used by the output stage were developed for professional audio applications.
Sugden selected them for their ripple-handling characteristics and long operating life.
The capacitors store and smooth the power supplied to the output stage, helping reduce unwanted electrical noise and maintaining stable current delivery.
Four Output Transistors Per Channel
Each stereo channel uses four multi-emitter power transistors within its bridged output stage.
Sugden publishes an output-current capability of 26 amps for each device.
This figure describes the capability of the individual transistor rather than the amplifier’s continuous output into a loudspeaker, but it indicates the substantial component specification used within the output stage.
Multi-Way Locking Speaker Terminals
One stereo pair of multi-way locking binding posts is provided for connecting passive loudspeakers.
The terminals accommodate compatible terminated or unterminated loudspeaker cable.
Correct positive and negative polarity should be maintained between the amplifier and both speakers.
One Pair of Passive Loudspeakers
The FBA-800 is designed to power one conventional stereo pair of passive loudspeakers.
It does not provide independently selectable Speaker A and Speaker B connections.
Using external speaker-switching equipment or connecting multiple speakers can alter the load presented to the amplifier and should be discussed with Audio T before installation.
Wide Published Frequency Response
Sugden publishes a flat frequency response from 20Hz to 80kHz at 0dB.
The wider amplifier bandwidth extends from 6Hz to 113kHz.
This wide-bandwidth design allows the amplifier to maintain consistent amplitude and phase behaviour throughout and beyond the audible frequency range.
Greater Than 96dB Signal-to-Noise Ratio
The FBA-800 has a published signal-to-noise ratio greater than 96dB.
A low noise floor helps preserve quiet musical information, spatial detail and subtle changes in level.
System noise will also depend on the connected preamplifier, source components, cables, mains environment and earthing arrangements.
Handmade in West Yorkshire
Sugden designs and manufactures its products at its factory in Heckmondwike, West Yorkshire.
Each amplifier is handmade by a skilled member of the company’s small production team, with Sugden retaining control of processes including design, engineering, assembly and final testing.
This traditional manufacturing approach supports careful quality control and long-term product serviceability.
Substantial Sapphire Construction
The FBA-800 measures 430mm wide, 206mm high and 460mm deep.
Sugden publishes a gross packed weight of 25kg, so a strong and stable equipment support is required.
Allow additional depth behind the amplifier for the mains lead, interconnects and speaker cables without forcing them into tight bends.
Ventilation and Placement
Pure Class A power amplifiers generate substantial heat during normal operation, including when music is playing quietly.
The FBA-800 should be placed on a strong, level and well-ventilated equipment rack with generous clearance around its casework.
Avoid installing it inside a tightly enclosed cabinet, directly above another heat-producing component or beneath equipment that could obstruct airflow.
Do not cover the amplifier or block its ventilation openings.
Key Features
- Sugden’s most advanced stereo power amplifier
- Part of the flagship Sapphire Series
- Handmade in Heckmondwike, West Yorkshire
- Pure Class A output operation
- Floating-bridge power-amplifier architecture
- Four 20W amplifier sections within one stereo chassis
- Two amplifier sections working together per channel
- Bridged power-transistor output stages
- Fully balanced operation from input to output
- 40W per channel into 8 ohms
- Both channels operating for the published power rating
- Stereo unbalanced RCA input
- Stereo balanced XLR input
- Front-panel RCA or XLR input selection
- Front-panel 6dB input attenuation
- 735mV sensitivity for full output
- DC-coupled amplifier circuit
- Servo-lock DC control
- Dedicated DC output protection
- Low-resistance MOSFET protection switching
- Soft loudspeaker connection during start-up and shutdown
- Custom bifilar-wound mains transformer
- Four separate transformer windings
- Low-impedance power-supply design
- Professional-audio smoothing capacitors
- Four multi-emitter power transistors per channel
- 26-amp published capability per output device
- One stereo pair of multi-way locking binding posts
- 20Hz to 80kHz published flat frequency response
- 6Hz to 113kHz published bandwidth
- Greater than 96dB signal-to-noise ratio
Frequently Asked Questions
What is the Sugden FBA-800?
It is Sugden’s flagship pure Class A stereo power amplifier and forms part of the company’s Sapphire Series.
Is the FBA-800 an integrated amplifier?
No. It is a power amplifier and normally requires a separate preamplifier to control source selection and listening volume.
Does the FBA-800 have a volume control?
No. Volume must be controlled by a suitable connected preamplifier or compatible variable-output source.
Is the Sugden FBA-800 pure Class A?
Yes. Its floating-bridge output stages operate in pure Class A.
How much power does the FBA-800 produce?
Sugden publishes an output of 40W per channel into 8 ohms with both channels working.
Why does Sugden describe four 20W amplifiers?
The floating-bridge design uses two 20W amplifier sections together for each stereo channel, providing four amplifier sections in total.
What is a floating-bridge amplifier?
It is a circuit in which two amplifier sections work together for each channel to drive a bridged output stage. Sugden uses this arrangement to increase available output, reduce noise and improve load tolerance.
Is the FBA-800 fully balanced?
Yes. Sugden states that the amplifier operates in a fully balanced mode from its input to its output.
Does the amplifier have XLR inputs?
Yes. One stereo balanced XLR input is provided.
Does it also have RCA inputs?
Yes. One stereo unbalanced RCA input is provided.
Can the RCA and XLR inputs be connected together?
Both connections can remain attached when required, but the active input must be selected using the front-panel control.
What does the front-panel input selector do?
It selects whether the FBA-800 receives its signal through the RCA or balanced XLR input.
What does the 6dB gain control do?
It attenuates the incoming signal by 6dB, allowing the amplifier to be matched more easily with high-output preamplifiers or efficient loudspeakers.
Does 6dB attenuation reduce the amplifier’s maximum power?
No. It reduces input sensitivity, so a higher signal level is required from the preamplifier before the FBA-800 reaches full output.
What is the input sensitivity?
Sugden publishes a sensitivity of 735mV for full output in the standard gain setting.
Can a streamer be connected directly?
Only when the streamer has a reliable variable analogue output that can safely control listening volume. A fixed-output streamer must be connected through a preamplifier.
Can a CD player be connected directly?
A conventional fixed-output CD player should not be connected directly because the FBA-800 has no volume control. Use a suitable preamplifier.
Does it have a phono stage?
No. A turntable requires a phono stage and preamplifier before its signal reaches the FBA-800.
Does it include a DAC or network streamer?
No. The FBA-800 is a dedicated analogue power amplifier.
What is DC coupling?
DC coupling provides a direct electrical signal path through the amplifier without conventional interstage coupling capacitors.
What does the servo-lock circuit do?
It controls unwanted DC conditions within the directly coupled amplifier while preserving low-frequency signal information.
Does the FBA-800 protect connected loudspeakers?
Yes. It includes a DC protection circuit with low-resistance MOSFET switching and soft connection during start-up and shutdown.
How many pairs of speakers can be connected?
The amplifier provides one stereo pair of loudspeaker terminals and does not have independently selectable Speaker A and Speaker B outputs.
Will 40W be enough for my loudspeakers?
That depends on loudspeaker sensitivity, impedance, room size and preferred listening level. A demonstration with your chosen speakers is strongly recommended.
Does the FBA-800 run hot?
Pure Class A operation generates substantial heat, so the amplifier requires generous ventilation and should not be installed in a tightly enclosed cabinet.
What is its frequency response?
Sugden publishes a flat frequency response from 20Hz to 80kHz.
What is the published bandwidth?
The published bandwidth extends from 6Hz to 113kHz.
What is the signal-to-noise ratio?
Sugden publishes a signal-to-noise ratio greater than 96dB.
Where is the FBA-800 manufactured?
It is handmade at Sugden’s factory in Heckmondwike, West Yorkshire.
How large is the Sugden FBA-800?
It measures 430mm wide, 206mm high and 460mm deep.
How much does the FBA-800 weigh?
Sugden publishes a gross packed weight of 25kg.
Related Products and Audio T Guides
Why Buy From Audio T?
- Independent advice from experienced Hi-Fi specialists
- Careful power amplifier and loudspeaker matching
- Help selecting a compatible preamplifier
- Balanced XLR and unbalanced RCA connection advice
- Input-sensitivity and system-gain guidance
- In-store demonstrations at participating branches
- System positioning and Class A ventilation advice
- Cable and equipment-support recommendations
- Home demonstrations and product loans where available
- Click and Reserve from participating stores
- Nationwide UK delivery on eligible products
- Ongoing after-sales support
Contact your local Audio T store to arrange a demonstration or discuss matching the Sugden Sapphire FBA-800 to your preamplifier, loudspeakers and listening room.