Graham Slee Proprius Power Amplifier Monoblock
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Graham Slee Proprius Power Amplifier Monoblock

  (1 Review)
£541.25 Exc. VAT |  


Graham Slee Proprius Power Amplifier Monoblock

  • Sold as a single monoblock or a stereo pair with included power supplies


"HiFidelity played an Acoustic Solid turntable with a Hadcock arm into Electrocompaniet amps and to two pairs of Usher Dancer floorstanders. Prototypes of Graham Slee's tiny mono power amps were also on show, which made a very interesting comparison. Despite the EC amps being several times more powerful, the Slees lost no sense of energy and dynamism.

Hi-Fi+, Dublin High-End 2012 Show Report

The Proprius "mighty-midget" Mono-Block power amplifier delivers 25 watts into 8 Ohms (RMS) and 45 watts into 4 Ohms.

It's a mono-block amp so two are required for stereo. We made it this way for a variety of reasons — one of which allows them to be used remotely at, or fixed to, the loudspeakers for powered monitor performance from your favourite passive speakers — see speaker mount tab. The great thing about mono-blocks is they give you true dual-mono amplification for improved stereo separation.

They're perfect for use with high sensitivity speakers where quality comes before quantity. However, they're not short on quantity and sound louder than their 25 watt rating would suggest, even using speakers as low as 83dB sensitivity — including the Stirling Broadcast/BBC 11 Ohm LS3/5A. 

The balanced XLR input works just as well single ended with shorter interconnects so using them side by side like a normal amp isn't a problem — you can find sutable interconnects in our Cables & DIY section.

For remote or speaker mounted placement requiring long cable runs, you'll need a source with balanced outs and our balanced audio cables.

Unusually for a power amp, the Proprius is equiped with its own volume control meaning it can be used direct from a fixed output source. When using it with a control preamp the volume control is put "out of circuit" by parking it at full rotation.

The input is based on the balanced topology by David Birt of the BBC, modified to "all-inverting" making it free of common mode distortion. Absolute phase is accomplished by "phase swapping". 

Speaker Mount 

Speaker mounting brackets included with every Proprius mono-block amplifier. 

With the availability of balanced output sources, professional users often prefer to use powered monitors enabling them to run a balanced "mic" cable feed, instead of bi-wiring with bulky speaker cable. However, with powered monitors you're limited to the quality of amp built-in by the speaker manufacturer. If you could find a better compact mono-block power amp to fix to the back of a better conventional "passive" speaker, you step up the performance a number of notches.

CAUTION: Care should be taken to ensure speaker crossover components are not damaged by drilling into or screw points piercing through speaker cabinets. 


  • 25 Watts into 8 Ohms / 45 Watts into 4 Ohms
  • Class AB biased discrete transistor power amp - not class D or T!
  • Rugged 15 Amp 30 MHz output transistors
  • No DC offset on output - safe to leave speakers connected
  • Balanced or single-ended line level input
  • High-end professional gold plated XLRs on input and power
  • Can be used free standing - or remotely on loudspeakers (brackets supplied)
  • Individual volume control can be "parked" at full when controlled from a preamp
  • Effective head radiator case for cool class AB running
  • Individual 120 Watt remote switchmode power supply with each Proprius 


  • Power output (rms) measured: 26 Watts into 8 Ohms; 45.5 Watts into 4 Ohms (ref 1% THD+N at 1kHz)
  • Power bandwidth at rated power into 8 Ohms: 10Hz - 47kHz (-3dB), 20Hz - 25kHz (-1dB)
  • Power bandwidth at rated power into 4 Ohms: 12Hz - 40kHz (-3dB)
  • Total harmonic distortion and noise at 8 Ohms: 100Hz: 0.043%; 1kHz: 0.047%; 5kHz: 0.061%; 10kHz: 0.085%; 20kHz: 0.125%
  • Total harmonic distortion and noise at 4 Ohms: 100Hz: 0.066%; 1kHz: 0.071%; 5kHz: 0.094%; 10kHz: 0.129%; 20kHz: 0.181%
  • Signal to noise ratio in dB: "A-weighted": 87dB; 22Hz-22kHz "Q-pk": 80dB
  • Bias type: Class Ab; Bias current: 60mA (approx.)
  • Input sensitivity for rated output: 575mV rms
  • Short circuit protection: Fuse and power supply shut-down (short circuits should be avoided to prevent component stresses)
  • Fuse: F2A 5mm x 20mm (glass "quick-blow" fuse rated at 2A)
  • Power supply: 48V DC, 120 Watt - self adjusting to AC mains 100V - 240V, 47 - 63 Hz; IEC cord supplied
  • Size (approx.): Amp W105 x H55 x D205 mm; Power supply W65 H40 D170 mm

Average Rating (1 Review):  
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Proprius Monoblocks
20 February 2020  | 

Do not be fooled by the physical size and simplicity of the external design; listening to this amplifier shows off the honesty behind the Graham Slee products.
Why listen to an amplifier, when it should simply be part of a chain of components that allows you to immerse yourself in your favourite recordings? This amplifier is so neutral (and unassuming), that you can do precisely that. After decades of searching, I have found as near perfection as I believe I ever can.

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