Puresound M845 Mono Power Amplifiers
The M845 is a single-ended triode (SET) mono power amplifier built around the classic 845 valve. Operating in pure Class A, it delivers a highly expressive and dynamic sound, designed to partner medium to high efficiency loudspeakers.
Key features:
• single-ended triode (SET) mono power amplifier
• 845 output valve
• Class A operation
• 27 watts output per channel
• custom in-house output transformer
• 6SN7 driver stage with interstage transformer
• zero negative feedback design
• direct-coupled circuit (no coupling capacitors)
Design and performance:
The M845 is built around the 845 triode, originally developed by RCA in 1931. Known for its linearity and power handling, it uses a graphite anode capable of dissipating high energy while maintaining stability across the frequency range.
A key challenge with 845 designs is managing the high voltage and filament requirements. The M845 addresses this with a carefully engineered power supply, using a hybrid Graetz bridge for rectification and a CLC filter with a substantial smoothing choke to ensure stable current delivery.
The driver stage is based on a 6SN7 triode, coupled via a custom wideband interstage transformer. The circuit is fully direct-coupled, eliminating coupling capacitors and preserving signal integrity.
Materials and construction:
• 845 output triode with graphite anode
• 6SN7 driver valve
• custom-designed output transformer
• hybrid Graetz bridge rectification
• CLC filtered high-voltage power supply
• wideband interstage transformer
The output transformer has been extensively developed in-house, with careful selection of core materials and winding techniques to maintain bandwidth and efficiency.
Sound characteristics:
• highly dynamic and expressive presentation
• rich tonal colour and harmonic detail
• strong sense of energy and immediacy
• excellent instrument texture and realism
• wide bandwidth with natural flow
With no negative feedback and a fully direct-coupled design, the M845 prioritises purity and immediacy of signal, resulting in a highly engaging listening experience.
Build and usability:
Each amplifier is a dedicated monoblock, designed to work as a pair in a stereo system. Due to the nature of SET amplification, it is best matched with efficient loudspeakers to achieve optimal performance.
Specifications:
• output power: 27W (Class A)
• topology: single-ended triode (SET)
• output valve: 845
• driver valve: 6SN7
• circuit: direct-coupled, no negative feedback
Brand and design:
Puresound is a UK-based company focused on delivering high-performance valve amplification. The M845 is designed by Guy Seargent, whose background includes work with Audio Innovations, Voyd Turntables, Heybrook, and JPW, as well as training in Japan with Audio Note / Kondo.
Note:
The M845 operates at very high voltages and is designed for use with appropriate loudspeakers. Careful system matching is essential to achieve the best performance from this amplifier.
