Unison Research Simply 845

The Unison Simply 845.

23 Watts. Nothing More Required.

There is a particular kind of confidence that goes into designing a single-ended amplifier around the Unison Research Simply 845. You are committing to a tube type that runs at high plate voltage, requires transformers of exceptional quality to make properly, and produces a relatively modest output power by conventional standards. You are betting that what those 23 watts sound like will matter more to the right listener than what 100 watts of something lesser sounds like. Unison Research has been making that bet since the original Absolute 845 in 1994 — widely regarded as one of the finest valve amplifiers ever built — and the Simply 845 is the current expression of that lineage in an integrated amplifier that doesn't require a second mortgage.

It is, by any honest measure, the amplifier that most of the people who buy it stop looking after. That's a high bar. The Simply 845 clears it.

The 845 Tube — Why It Matters

The 845 is a directly-heated triode — meaning the cathode is the filament itself, rather than a separate heater element warming an oxide-coated cathode. This is the same operating principle used in the earliest audio tubes, and it remains among the most sonically transparent architectures in tube amplifier design. The directly-heated triode's simplicity of construction translates directly into the simplicity and purity of the signal it amplifies.

The 845 specifically is a transmitting tube — designed originally for continuous duty at high power levels in radio transmitters — which means it is built to tolerances and mechanical standards that typical audio tubes are not. The result is a tube that is exceptionally linear, highly durable, and capable of delivering its output with an effortlessness that smaller, lighter-duty tubes simply cannot match at equivalent power levels. The Unison Research-branded 845 tubes in the Simply 845 are self-biasing, which means setup is genuinely straightforward: install the tubes, connect the cables, and play music.

The Transformers

In a single-ended amplifier, the output transformer is not just important — it is the amplifier. Its quality sets the ceiling on everything the circuit can achieve. Unison Research winds their output transformers by hand, in-house, one at a time. The core material is a specially selected ferromagnetic lamination stack — thinner than standard, chosen for its linearity under the intense magnetic fields of a high-power SE output stage. The insulation between layers is very thin Mylar and polypropylene, chosen to minimize parasitic capacitance and magnetic flux dispersion, both of which would compromise frequency extension and raise harmonic distortion. The result, on paper, is a transformer with extended bandwidth and very low distortion. The result in listening is the kind of bass control and treble air that separates a properly designed SET from one that merely uses the right tube type.

The Circuit

All stages of the Simply 845 — both preamplifier and power output — operate in pure Class A. There are no transitions, no crossover points, no moments where the circuit changes its operating mode based on signal level. What you hear at low volume and what you hear at moderate volume are produced by the same circuit operating in the same way.

The preamplifier stage uses two ECC82/12AU7 tubes fed by a stabilized linear power supply designed specifically to isolate the sensitive input stage from mains noise. All signal-path capacitors use polypropylene dielectrics — the correct choice for minimizing dielectric absorption and its associated smearing of transient information. The printed circuit boards use pure electrolytic copper traces at three times standard thickness, reducing resistance and the mechanical stress that standard-weight traces can develop over time.

The ALPS RK27 motorized potentiometer handles volume. It is among the most respected volume controls available at any price — Unison Research chose it deliberately, and it shows in the channel balance and tracking accuracy at all volume settings.

The Design

The Simply 845 looks like nothing else in a listening room. The chassis is matt black steel with ribbed heatsinks, and the front panel and side trim are sculpted from solid wood — Walnut or Cherry, chosen and shaped by hand. The 845 tubes themselves are enormous — larger than most amplifiers' output transformers — and they glow with a filament intensity that is genuinely dramatic. The stainless steel heat shield between the tubes and the circuit reflects heat away from the sensitive components below while adding another layer of visual architecture to the amplifier's already distinctive appearance.

It is heavy. 30.5 kilograms, or 67 pounds. Two people to move it. Worth it.

A Word on Speaker Matching

23 watts of single-ended Class A power is not the same thing as 23 watts of push-pull Class AB power. The linearity of the output stage means the Simply 845 drives speakers with an authority and control that the power figure alone doesn't predict. HiFi Pig's reviewer drove 87dB and 88dB sensitivity speakers — neither of them SET-friendly by reputation — without ever feeling short of power. Twittering Machines drove the DeVore O/96 and the Perlisten S5t. The practical lower limit is somewhere around 86-87dB with a benign impedance curve. Higher sensitivity speakers — 90dB and above — will give the Simply 845 more room to breathe and reward the pairing further.

This is not an amplifier for 4-ohm, low-sensitivity loudspeakers. For everything else, the question is how far you want to take it.

Press Recognition

Twittering Machines reviewed the Simply 845 in depth over more than a month of listening with multiple loudspeakers including the DeVore O/96 and Perlisten S5t. The verdict: capable of drama, delicacy, and danger that transcends reproduction and turns the act of listening into a full-fledged experience. The reviewer noted a particular gift for delicacy — surprising, given the amplifier's physical imposingness — and concluded that with sensible speakers, the Simply 845 delivers something that goes beyond what the technical description prepares you for.
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HiFi Pig gave it a thorough workout through 87dB and 88dB speakers — neither typically considered SET-friendly — and found it never wanting for power or control. The reviewer described it as alive in a way that heavy amplifiers rarely are, with a naturalness that made extended listening sessions feel effortless.
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HomeTheaterReview's Ken Kessler called the Simply 845 an Ongaku for a fraction of the price at its original UK launch — a comparison that has held up in the years since. He wrote that design choices both inside and out speak to certain people, and that those people will be drawn to it like a moth to a flame.
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Specifications

• Type — Pure Class A single-ended integrated tube amplifier
• Output tubes — 2× 845 directly-heated triode (Unison Research branded, self-biasing)
• Preamp tubes — 4× ECC82 / 12AU7
• Maximum output power — 23W per channel
• Bandwidth — 20Hz – 50kHz (+0dB / -1dB at 12W)
• Feedback — 14dB
• Output impedance — 6 ohm
• Input impedance — 47kΩ / 100pF
• Inputs — 4× line level RCA
• Outputs — 1× tape RCA; 1× subwoofer RCA stereo
• Volume control — ALPS RK27 motorized potentiometer
• Output transformers — Handmade in-house; Mylar and polypropylene insulation; special ferromagnetic lamination core
• Power consumption — 300 VA
• Dimensions — 37 × 57 × 26 cm (W × D × H)
• Weight — 30.5 kg / 67 lbs
• Finish — Walnut or Cherry woodwork; matt black chassis
• Origin — Handcrafted in Italy

Questions about speaker matching, or how the Simply 845 compares to other SET amplifiers we carry? We're happy to help you think it through.