Proprius Monoblock Power Amplifier

Monoblock Power Amplifier

"Truly great audio components never happen by accident..."

"...the ambience of the recording venue, the physical size of it, the placement of the instruments within it, the timbre of the individual instruments, and the depiction of what the instruments are playing is so believable and so natural-sounding that I find myself, without a hint of self-consciousness or irony, applauding after each LP I play. The Proprius is, simply, a superb amp." (Paul Szabady, Stereo Times)

Real class-AB discrete transistor amplifiers made small. 45 watts into 4 Ω; 25 watts into 8 Ω. With rugged 15 amp 200 watt output transistors the Proprius drives even the most difficult of speakers.

For more information continue reading after the purchasing options below.

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Single/Mono £649.50
Stereo Pair £1299

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(Power supply self-adjusts: 100-240Vac)
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Specification Reviews

"Clever-Simple-Reliable Value-For-Money Amps" If you "applaud value for money products and clever simple designs that work reliably year after year" (see HiFi Pig review by Ian Ringstead), these are the power amps for you!

Clever - Arrange to Taste

typical Proprius amplifier based power amp

With volume controls that go all the way, these power amps can be driven by a separate preamp (for example: the excellent Majestic pre-DAC in the picture above) by parking the volume at full — or their own volume controls can be used in a minimalst system driven by a fixed output source.

As a stereo pair the volume controls can be used to balance the stereo image if there's no balance control anywhere else.

Bi-amping, tri-amping, and surround sound will benefit by using the Proprius volume controls, allowing you to easily adjust the tonal balance and/or audio image placement.

Monoblock Power Amp with Power Supply

A single Proprius monoblock power amplifier with its outboard power supply

Simple (and Smart) Design

Power amps don't have to be brutish - they can be neat and compact as the Proprius shows - and at only 20.5 cm deep by 5.5 cm high by 10.5 cm wide, the Proprius is definitely neat and compact.

Proprius power amp side view

Cool running doesn't always require dark, drab and dusty heat-sink fins - clean lines are possible as the Proprius heat-exchanger case shows. It doesn't overheat even when used for long intervals at maximum output.

Easy to connect and get up and running, the Proprius can be used with "single-ended" and "balanced" sources using the same single input XLR socket (left). We've got a full range of interconnect cables making it simple to obtain the right one for your Proprius amps.

Proprius power amp rear panel

Connect power supply to the "48V" connector - both signal and power connectors are locking - and attach the speaker cable: the speaker terminals accept 4mm plugs; 8mm spades; or bare wire to 4mm diameter.

The Proprius power amplifiers can be used free-standing wherever you want, or mounted on the back of speakers, making them into "powered monitors" using the supplied mounting kit.

Proprius power amp speaker mounted

A Proprius monoblock amp attached to the rear of an Harbeth M20 monitor loudspeaker


The last thing we want is to be repairing the things we make - it's so counter-productive.

So that should give you a clue to why the Proprius power amp has a number of features which make it reliable.

Proprius power amp under the lid

Lid's off the Proprius showing its truly linear class-AB circuit

We don't want it to break when you forget yourself, and although we don't advise on you being wreckless, we did lots of naughty things ourselves to test the reliability of the Proprius.

Knowing we were going to do this, we made the power devices as rugged as we could - yet we also needed them to be gentle-giants to resolve all the subtleties contained in your music.

Take a look at the giant output transistors bolted to the heat-exchanger chassis in the above shot. They're rated at 200 Watts; 15 Amps and 260 volts! They can stand 40 times more power than Nyquist's power amplifier theorem states is neccessary.

It means we don't need to employ a protection circuit which is always meddling with your transient highs.

Instead, it's intelligent remote power supply can detect when things are getting out of hand when it immediately shuts down and restarts when the problem goes away. But that's not going to be during normal listening.

And then we added the final "failsafe" - a fuse (just in case).

Swipe down for Specifications and Reviews.


Get the best out of your Slee. Discover the ways ...

Frequently purchased with...

Majestic DAC / Preamp Libran Balanced Audio Interconnects

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Power output (rms) measured26 Watts into 8 Ohms; 45.5 Watts into 4 Ohms (ref 1% THD+N at 1kHz)
Power bandwidth at rated power into 8 Ohms10Hz - 47kHz (-3dB), 20Hz - 25kHz (-1dB)
Power bandwidth at rated power into 4 Ohms12Hz - 40kHz (-3dB)
Total harmonic distortion and noise at 8 Ohms100Hz: 0.043%; 1kHz: 0.047%; 5kHz: 0.061%; 10kHz: 0.085%; 20kHz: 0.125%
Total harmonic distortion and noise at 4 Ohms100Hz: 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 typeClass Ab; Bias current: 60mA (approx.)
Input sensitivity for rated output575mV rms
Short circuit protectionFuse and power supply shut-down (short circuits should be avoided to prevent component stresses)
FuseF2A 5mm x 20mm (glass "quick-blow" fuse rated at 2A)
Power supply48V 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

Power amp specifications subject to alteration without notice in keeping with our continuous improvement policy.


Read Paul Szabady's Proprius Amplifier Review has much in common with the best tube amps, pure Class A transistor designs, and other exotic highly natural-sounding amps. Its advantage is its smaller size, ease of use, much cooler running, and lower price.
Paul Szabady, Stereo Times, July 2019

Hi-Fi Pig Outstanding Product Award

For an output rating of only 25 watts per channel and being so small in stature I couldn’t believe how loud and good these little amps sounded. My resident Triangle Titus EZ’s are relatively efficient but there was loads of power in reserve and I never felt the amps would run out of steam in my average sized living room. . . The sound is very clean, smooth but detailed and three dimensional. I didn’t feel any part of the frequency range suffered and on a very wide range of music types they excelled in all cases.
Ian Ringstead, HiFi Pig, July 2015

TNT-Audio Magazine

Having 'downgraded' from some serious valve amplication costing several times the asking price, the GSP combo didn't sound out-of-their-depth at all. No transistor amp (and only a couple of valve amps) has ever matched the sheer musicality and transparency of the sublime Yamamoto but the Majestic/ Proprius were not a million miles behind, sounding unusually sweet and open for a transistor design and certainly well able to control the bass of the M15 which has a tendency to looseness with many amps regardless of output. The monoblocks, hard against the speakers gave lovely soundstanging with great left-right definition if a little less depth and 'air' (hateful word) than the Yamamoto. Their control, detail and overall lack of grain or other nasties made them both easy to listen to and well able to demonstrate the difference between mixes and different components.
Geoff Husband, TNT-Audio July 2015

Hi-Fi Pig Magazine

This is about music; this is real class Ab – not D or T; this is about ease of listening, and an amazingly good amount of audio level despite the low power numbers; all those sudden bursts that many an amp needs hundreds of Watts in reserve to deliver with any meaning, are catered for with comfort here, even when I connected them to my 83dB sensitivity 15ohm LS3/5a speakers. . . This had grunt, with an exceptionally dynamic and energetic soundstage from the deepest lows to the tizzling tops. . . That this 21st Century £1299 all-in package can sound as good as it does is indicative of Slee’s compulsion with creating the very best. . . This is real class in a pint pot, and well worth the price.
Janine Elliot, HiFi Pig, May 2015

Hi-Fi Plus Magazine Cover

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

Hi-Fi Stars Magazine Cover

So small may be their physical appearance, so great are their musical charisma and playfulness. Amplifiers must in future be measured in my system with the Graham Slee Proprius...
Hi-Fi Stars, März 2014 - Mai 2014 - Amplifiers Graham Slee Proprius Mono (translated from German)