August
2022

Country of Origin

UK

Gradient Box 2

Reviewer: Srajan Ebaen
Financial interests: click here
Main system: Sources: Retina 5K 27" iMac (4GHz quad-core with Turbo, 32GB RAM, 3TB FusionDrive, OSX Yosemite. iTunes 14.4), PureMusic 3.02, Audirvana 3, Qobuz, Tidal, Singxer SU-6 USB bridge, Sonnet Pasithea DAC; Active filter: icOn 80Hz/4th-order hi/lo-pass; Power amplifiers: Kinki Studio EX-B7 mono, Enleum AMP-23R; Headamp: Kinki Studio; Phones: HifiMan Susvara; Loudspeakers: Aurai Audio Lieutenant w. sound|kaos DSUB 15 on Carbide Audio footers, Audio Physic Codex, Cube Audio Nenuphar Cables: Complete loom of Allnic Audio ZL; Power delivery: Vibex Granada/Alhambra on all source components, Vibex One 11R on amps, Furutech DPS-4.1 between wall and conditioners; Equipment rack: Artesanía Audio Exoteryc double-wide 3-tier with optional glass shelves, Exoteryc Krion and glass amp stands; Sundry accessories: Acoustic System resonators, LessLoss Firewall for loudspeakers, Furutech NCF Signal Boosters; Room: 6 x 8m with open door behind listening seat
2nd system: Source: Soundaware D300Ref SD transport clock-slaved to Denafrips Terminator +; Preamp/filter: icOn 4Pro + 4th-order/40Hz hi-low pass; Amplifier: Goldmund Job 225; Loudspeakers: sound|kaos Vox 3awf, Dynaudio S18 sub; Power delivery: Furutech GTO 2D NCF; Equipment rack: Hifistay Mythology Transform X-Frame [on extended loan]; Sundry accessories: Audioquest Fog Lifters; Furutech NFC Clear Lines; Room: ~3.5 x 8m
Desktop system: Source: HP Z230 work station Win10/64; USB bridge: Singxer SU-2; Headamp/DAC: iFi iDSD Pro Signature;  Headphones: Final D-8000; Active speakers: DMAX SC5
Upstairs headfi/speaker system: Source: smsl Dp5 transport; DAC: Auralic Vega; Integrated amplifier: Schiit Jotunheim R; Phones: Raal-Requisite SR1a; Active DSP speakers: Fram Midi 120
2-channel video system: Source: Oppo BDP-105; All-in-One: Gold Note IS-1000 Deluxe; Loudspeakers: Zu Soul VI; Subwoofer: Zu Submission; Power delivery: Furutech eTP-8, Room: ~6x4m

Review component retail: TBA

Sub sandwich perfected? "A submarine sandwich—also known as sub, hoagie, torpedo, hero, Italian or grinder—is a cold or hot sandwich made from a cylindrical bread roll split lengthwise then filled with meats, cheeses, vegetables and condiments." We're no foodie channel. Our sub sandwich involves a subwoofer. To perfect integration with our main speakers, it too splits, just not bread but freqs. Those to the sub filter by low pass (that which lets the lows pass). Those which hit the speakers filter out bass via inverse high pass.

In the multi-channel space, pre/pros include all necessary bass management. They may even include finely calibrated digital delay to compensate for a sub's time alignment if it doesn't sit equidistant with the speakers; or suffers DSP latency. Now we delay the mains signal to insure that their sound arrives at the ear time-coincident with the sub's. Late bass be slow bass. Nobody should want that. Meanwhile in the refined 2-channel space, our preamps don't do any of it. At best they offer twinned pre-outs. Those condemn us to augmentation or 'tack-on' mode. It's when both speakers and sub see the same full-range signal. That of the sub gets cut by the sub's built-in low pass often preceded by an A/D converter. It's usually adjustable between 40-120Hz but in certain cases could span 25Hz to 200Hz.

The intrepid user attempts to set this built-in low pass so the sub rolls in where the speakers roll out naturally. In dim man caves there's much crossing of fingers, hoping for miracles and sweating proper phase to avoid a suck-out at the handover and achieve a seamless patch. In the worst case it's like vintage hippie jeans lengthened at the hem with a flowery band. Colorful such upcycled pantalones were but never of a piece. You hear that bass tacks on. It's not part of the original fabric. It's not fully integrated.

A better tailor filters the speakers in a mirror-image seam. He applies identical attenuation slopes at the same frequency hinge so neither sub nor speakers see a full-range signal. If done in the analog domain, this avoids digital latency from heavy DSP processing. Dual-filter mode is a compound win. We gain not just lower louder bass from a sub. We reduce bass stress on the main amp. Now that can be smaller/cheaper or better because we're not chasing max power. We reduce stress on the speakers whose excursions diminish for lower distortion. That creates less heat on their lower voice coils. Their impedance doesn't rise to become more resistant thus choke dynamic range. Of this, the REL concept of a high-level connection knows nothing. It still runs unfiltered mains. Those don't improve. It's basic stereo 2.1. It doesn't convert a 2-way into a proper 3-way with active bass¹. Remember that in 2-ways, bass and midrange share the same cone. That typically meets a 1" dome at ~2kHz. Relieving the mid/woofer of low bass runs it at lower distortion with better dynamics. These gains extend across its entire bandwidth all the way up to the tweeter. It's no marginal win. It goes well beyond more bass. Working smarter not harder, the monitor's mid/woofers perform better. Who doesn't want that?
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¹ Obviously if we start with a 3-way, we now convert it into a 4-way and so forth. Once speakers hit -3dB at 45Hz or lower, it becomes harder and harder to successfully tack on a subwoofer without unwanted overlap. The most common low-pass filter built into a subwoofer is a 4th-order Linkwitz-Riley. That defines by its -6dB point. If the lowest settable value is 40Hz or 50Hz, for such speakers that's already too high. 

Prototype motherboard without display.

If you now have an inverted When Harry met Sally moment—"I have what he's having"—you'll feel buggered. How to manage an active high pass when your purist high-end preamp plays snobbist to forget about subs? We return to the generic sandwich popularized in 1762's England by one John Montagu, 4th Earl of Sandwich. Legend has it that our fellow suffered a substantial gambling habit to spend hours on end at the card table. When he began to order two slices of bread with a central slab of beef to munch on while gambling, his companions began asking for what Sandwich was having. The rest is history. More than three centuries later, subwoofer fans not wishing to gamble on augmentation mode can once again count on Great Britain to lead the way. Here Hungarian expat Pál Nagy of icOn 4Pro fame designed more than mere condiment for our perfect sub sandwich. Somewhere in Manchester, he put together the whole stuffed shebang.