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Amphion One25A

Active Monitors By Phil Ward
Published May 2024

Amphion One25A

We put Amphion’s first ever three‑way monitor design to the test.

Finnish speaker and amplifier company Amphion have carved out a niche in the professional monitoring market with their much admired range of two‑way nearfield speakers, such as the Two15 I reviewed back in 2017. Their reputation for thoughtful and innovative electro‑acoustic engineering is well deserved, and their approach translates into highly effective monitoring. But it has always felt as though there were a couple of elements missing from the Amphion range: active drive, and a three‑way speaker. Of course, Amphion have had their own range of amplifiers for a while, and subwoofers too, and these arguably fill the gaps. Now, though, they’ve brought everything together into the subject of this review: the three‑way, active One25A.

Before you get too excited, a couple of health warnings. Firstly the One25A is not an inexpensive monitor (I’d deploy the term ‘aspirational’) and secondly, it is very much at the large end of the nearfield monitoring spectrum — it’s a midfield, really. At 41kg, it is also outrageously heavy. So heavy, in fact, that instead of testing them in my normal garden studio and large acoustic measuring space, I had to take a different approach. My local recording and rehearsal complex, Brighton Electric, very generously offered their Studio 2 control room for listening.

First Look

The One25A’s visual appearance is unmistakably Amphion, and none the worse for that. I’ve always admired the simplicity of the Amphion aesthetic, with its combination of dark, matte cabinet surfaces and aluminium driver diaphragms set off by the whiter‑than‑white tweeter waveguide. It has the look of a high‑precision, professional tool, and I rather like that.

Its dimensions are 316 x 510 x 487mm (HWD). Compact, it’s not. And if you want to know why it’s so heavy, as well as its cabinet being constructed from 25mm thick, heavily braced MDF, the bass driver alone weighs 10kg. The chassis of the bass driver is even incorporated into the cabinet bracing. Furthermore, the One25A also incorporates numerous constructional measures designed to ensure that the midrange driver and tweeter are mechanically and acoustically isolated from the bass driver. The narrow perforated grille on the front of the enclosure, for example, terminates a foam‑filled air‑gap slot that runs diagonally through the cabinet from the front through to the rear side. The slot and its internal damping ensure separation between the driver elements of the One25A. Furthermore, the diagonal geometry of the slot results in the separate bass and midrange enclosures being asymmetric, which helps discourage internal standing waves. And when I asked Amphion about the weight of the One25A, the response was that it wasn’t really something they considered. When you set out to make a no‑compromise active monitor, it weighs what it weighs.

Bolted to the rear panel of the cabinet is a filter, EQ and three‑way amplification module housed in a large folded‑steel enclosure. The amplification is rated at 205W each for the mid driver and tweeter, and a generous 700W for the bass driver. So, even though Class‑D technology is known for its light weight, amps supplying a total 1.11kW were never going to be featherweight. The crossover filters are all fourth‑order (24dB/octave) types, and rather than employing active op‑amp chips, are implemented using passive networks buffered on their inputs and outputs. The whole electronics module is removable to enable the monitors to be soffit‑mounted, and Amphion are additionally planning a rackmount version of the module. On its underside are a mains power input and switch, a balanced XLR input, and a stepped knob that offers a ±8dB range of LF equalisation profiles to provide some compensation of low‑frequency level depending on the monitor’s installation with regard to room boundaries. The electronics module offers no other connection or configuration facilities.

The Low Down

Like the bass/mid drivers in Amphion’s passive two‑way monitors, the One25A bass drivers come from Norwegian specialists SEAS. The bass driver is a nominally 25cm (10‑inch) unit, designed specifically for low‑frequency duties alone. That’s clear from the extremely generous roll‑surround fitted to the driver and the fact that its motor system (magnet, pole‑piece, top plate and voice coil) provide ±14mm of linear diaphragm excursion — around twice that of smaller bass/mid drivers. But the driver’s motor system is not only impressive in terms of diaphragm excursion: its voice‑coil is also unusually large at 56mm in diameter (getting on for twice the more usual 30mm). The driver is clearly designed to generate low bass at high volume levels with minimal compression.

And if that wasn’t enough, the motor system also incorporates a copper cap on its pole‑piece, which functions to reduce the voice‑coil inductance and the degree to which inductance changes with voice‑coil movement. I’ll unpack that a little more. Voice‑coil inductance results in a resistance to the flow of electrical current that increases with frequency, and in many speaker drivers, the inductance changes depending on the position of the voice coil. And as voice‑coil inductance influences a speaker’s frequency response, having it change in response to the input signal (because it’s the input...

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