Twochannel Reference Build — Planar Detail Room

For the listener who values transient speed, transparency, and stage depth over bass slam and easy placement, and who has a room of 200 square feet or more where the panels can stand well clear of the walls and a high-current amplifier to drive them. It rewards well-recorded acoustic, jazz, and orchestral material and is unforgiving of nearly everything else.

The Magnepan 3.7i is not a box; it is a 70-inch dipole panel carrying a true-ribbon tweeter and a quasi-ribbon midrange, radiating equally front and back into the room. Two facts about it set the rest of the build. It is a nominal 4-ohm load, and it converts only 86 dB from a watt at a metre, a low figure that falls further because a dipole cancels part of its own bass where the front and rear waves meet. A panel like this does not want a clever amplifier; it wants current, and a lot of it. That is the job the Parasound A21+ was chosen for. Into the 3.7i's 4-ohm load it supplies 500 watts per channel, twice what it gives into 8 ohms, and it holds together down to a 1.5-ohm minimum. Its Class A/AB output stage pairs that reserve with a grip, quantified as a damping factor around 1100, that keeps the ribbon and planar diaphragms moving only when the signal tells them to. On a speaker this insensitive, that pairing is what buys realistic level: roughly 108 to 110 dB at a three-metre seat before the supply is anywhere near its limit. The digital front end is stripped to two boxes on purpose. A Lampizator network transport pulls files off the wired network and passes only a reclocked S/PDIF stream to the Benchmark DAC3 HGC; nothing analogue happens until the converter. From there the DAC3's ES9028PRO stage and its own analogue volume drive the A21+ directly, through a 10-ohm output into the amplifier's 33-kilohm input. There is no separate line stage, and that omission is the resolution argument: with a measured 115 dB signal-to-noise figure at the amplifier and one fewer set of gain devices in the path, the quietest information on a recording is not masked before it reaches the panel. Everything here serves recovery of detail rather than weight or warmth. The ribbon's tiny moving mass starts and stops faster than a dome, holding pitch on massed strings and letting cymbal decay trail off to nothing, with usable output to 40 kHz; the dipole's null to the sides drops early sidewall reflections, so the stage sits well behind the panels and images with unusual depth once they stand three to five feet clear of the front wall. The pairing is coherent only because the amplifier can feed the panel. An underpowered amp does not make the 3.7i quieter, it makes it flat and lifeless, and that is the one substitution this system cannot survive.

The honesty cuts both ways. A dipole has no enclosure to load its bass, so below the 35 Hz figure the output tapers off gradually rather than stopping at a ported tuning point, and the felt bottom octave under synthesiser and pipe organ is simply not there; recovering it means adding a subwoofer and then solving the harder problem of blending a boxed, omnidirectional sub with a dipole's figure-eight pattern. The 86 dB sensitivity rules out low-power tube and flea-watt amplifiers outright, and under about 150 watts the panel sounds constrained. Placement is demanding: the 3.7i needs three to five feet behind it and a room of 200 square feet or more, so near-wall or small-room installation is off the table, and the vertical listening window is shallow enough that standing up dims the top end. Finally, a ribbon this resolving does not disguise a thin or aggressive master; it plays back exactly what the mastering engineer committed.

• 01

  Swap 3.7i → PerformaBe F228Be (−$3,995)

  For listeners who need real bass and wall-friendly placement, the Revel F228Be trades the ribbon's speed and the dipole stage for a dynamic floorstander: a 90 dB, 8-ohm load any amplifier drives easily, in-room extension to 27 Hz, and freedom to sit closer to the front wall. The system loses depth and transient delicacy but gains weight and flexibility.

• 02

  Swap A21+ → 3B³ (+$995)

  The Bryston 3B-cubed is the traditional Magnepan partner: 200 watts into 8 ohms and a 500 damping factor, with a drier, more clinical balance than the Parasound. It sharpens the leading edge slightly at the cost of the A21+'s deeper current reserve, so peak dynamics on large-scale material give way a little sooner in a big room.