2024.05.31 - Stereophile.com - Balanced Audio Technology REX 500 power amplifier

There was a period in the 1970s when many pop ballads that should have had understated arrangements instead turned grandiose and even maudlin. Take Gilbert O'Sullivan's sensational single "Nothing Rhymed" (a track that went deep for a pop hit, referencing famine, duty, and morality). Soon after the start, O'Sullivan's piano is overshadowed by a loud, saccharine string section.

Another example is "Lotte," German singer Stephan Sulke's portrayal of a dying love affair. The devastatingly wistful chanson is burdened by a mawkish orchestral track—the equivalent of glitterbombing an Edward Hopper painting.

Contrast this with Roberta Flack's definitive version of Ewan MacColl's "The First Time Ever I Saw Your Face." Apart from Flack's voice and her emotional delivery, the gently strummed guitar and quiet piano do all the heavy lifting. An unhurried double bass and a couple of minimally bowed string instruments leave swaths of negative space, helping to give her interpretation its hushed, reverent character.

I reflected on all this after spending several months with Balanced Audio Technology's REX 500 solid state power amplifier ($25,000), which has more in common with the Roberta Flack track than with the bombast of "Nothing Rhymed." I don't mean to say that the REX's sonics are understated—that might imply shyness, and it definitely isn't a shy-sounding product, but it's a far cry from the amplifier equivalent of O'Sullivan's bombast. But enough about this for now: more after you've met the amplifier in question.

Attack of the forklift

You'll need help moving and unboxing this beast, which is deeper (23.5") than it is wide (19") and weighs a grueling 140lb. I experienced its bulk and weight more than once because the first time I received the REX 500, it had to go back for repair. What seemed at first like an inconsequential tear in the shipping box was, upon closer scrutiny, the likely result of a forklift blade punching through the cardboard and ramming the REX's right side. The bend in the casework looked like it was part of the swooping design of the flanks, and the damage to the heatsink was invisible except when the top cover was removed. What was easily noticed was a loud hum from the speakers. Hours of troubleshooting got me nowhere. The buzz was impervious to different outlets, power conditioners, interconnects, power cables, and assorted sources. It did go away when I substituted either my Krell or Anthem amplifiers. At that point, I knew that the REX needed a bench check, so back it went to the Wilmington, Delaware, BAT cave.

The company determined that a hard impact had dislodged one of the SuperPak capacitors and broken some solder joints. (The build quality of the REX 500—all aluminum except for a thick steel bottom plate that supports the supersized transformers—is sterling (footnote 1), but in a deathmatch with a carelessly driven 9000lb forklift, it lost.)

About 10 days later, I took possession of a replacement Rex 500. Other than my poor back, after that, all was well.

Visually, the REX means business. Forty-eight heatsink fins adorn each side, hidden behind the fascia when you view it straight-on. The front of the amplifier is almost but not perfectly symmetrical: On the left edge is a 1"-wide vertical billeted-aluminum strip, mounted at about a 20° angle toward the rear, milled to read "Rex 500" with a swoopy, vaguely art deco–style R. Positioned in the middle of the front plate is a springy, narrow, 2"-tall power switch. The blue LED just to its right, slightly off-center, has a simple function: It merely indicates whether the power is off or on—it doesn't flash during the roughly eight-second turn-on sequence or turn red when the protection circuit kicks in. Protruding from the back panel is a curved metal handle that's intended to assist in lifting the REX—but it's not enough by itself. BAT advises using a rope or cloth sling tied around the front so that two people can lift the REX from the box and move it into place.

Also on the back, you'll find loudspeaker binding posts, a pair of XLR inputs, and two each—one per channel—IEC connectors and fuse holders.

BAT makes a point of avoiding fuses inside its amplifiers: "The best-sounding fuse is no fuse at all," the company says. BAT's auto-protection circuit does away with the need to replace internal fuses. The absence of internal fuses has the secondary benefit of allowing the REX to handle higher currents without crippling the circuit's operation.

Those fuse holders you see? They're in line with the AC, to protect the whole amplifier from external power surges. Why two? Because the REX 500 has separate left and right power cords and power transformers.

Speaking of protection circuits, if your speaker cables are terminated with spade lugs, as my AudioQuest Thunderbird Zeros are, I'd advise caution. The posts are close enough together that large spades are only separated by a millimeter or two. Once, with the amp turned on and everything connected, I decided to tidy up the speaker cables a bit. As I moved the first one, its positive and negative lugs torqued around the binding post stem ... and touched.

The only anomaly I experienced during my time with the REX involved the auto-protection circuit, which kicked in seemingly at random three or four times over a few months. The puzzled BAT team asked if it shared the AC line with another piece of equipment with a large power draw, such as a refrigerator. It didn't. In fact, my listening room has a dedicated line for audio, with a pair of 20A circuits. Perhaps, we speculated, intermittent voltage drops or surges caused the cutouts.

Then again, since I began using the new room more than a year ago, no other straight-to-the-wall amplifier seemed affected by voltage irregularities.

None of this is worth obsessing over considering that the BAT's better-safe-than-sorry protection circuit activated infrequently and that getting the amp back to normal was always a quick affair (footnote 2).

To preamp or not to preamp

Because my equipment console is only 15" deep—about 8" too short for the REX—I used an impressively beefy Finite Elemente Pagode HD-10 amplifier stand, centered on the floor between the speakers. Atop the amplifier, separated from it by a 4"-tall Townshend Seismic Podium that allowed for good separation, I placed the REX's preamplifier brethren, the tubed VK-90. BAT had sent the VK-90 along with the REX 500 in the expectation that it would prove an ideal pairing.

I decided to evaluate the amp in three ways. First, I went without a preamp, using the volume control of my main source, an Aurender A20 digital transport and server. Then I listened to the REX with the Aurender connected via the line stage of my Benchmark HPA4. Finally, I pressed the VK-90 into service, again using the A20 as a source.

While I did indeed prefer the sound with the BAT preamp in the chain by a small margin, I spent the least amount of time with that combination because I wanted to home in on the sonics of the REX without the wild card the VK-90 presented.

BAT co-founder Steve Bednarski told me he's keen on listening with a preamp in the system. I asked him why: What's the downside to feeding a power amp the output stage of a DAC or other digital source? "A well-designed active line stage will provide greater current delivery from its output stage than almost any source omponent," Bednarski emailed back. "Current delivery gives a sense of liveliness and joie de vivre to the reproduction of music. For instance, a CD player that uses an op-amp–based output stage will provide weak current delivery. When such an output stage is connected directly to an amplifier, the music can sound flat and uninvolving." (footnote 3)

Bednarski wanted me to experience the REX with a tubed line stage like the VK-90 because he feels that combining a valve preamp with a solid state amplifier can better flesh out colors and textures. This regard for tubes goes back to the company's very beginning, in 1995. For almost three decades, BAT has built high-end valve and solid state components, the latter culminating (for now) with the REX 500, BAT's flagship power amp.

Is it me, or is it hot in here?

The REX 500 is a class-AB amplifier that uses 24 high-current output devices per channel, giving it stupendous power: 500Wpc into 8 ohms, 1000Wpc into 4 ohms. (There's no official rating into 2 ohms.) Considering that amplifiers have to produce only a few continuous watts to make most speakers sing (very low-sensitivity and low-impedance loads excluded), you might think that such specs are overkill. To justify such a beast, Bednarski cited JA's measurements of Wilson Audio's Alexx V speakers, whose equivalent peak dissipation resistance (EPDR) drops below 2 ohms over most of the midrange and mid-treble, with minimum values elsewhere in the 1 ohm neighborhood.

"That's the type of load for which an amplifier like the REX 500 will be a requirement," Bednarski said. He allowed that for most speakers, "you'll never need the power" that his amp delivers—but that doesn't mean that speakers with 4 to 8 ohm specs and 90+dB sensitivity won't benefit from a little of that REX elixir. In that application, "the speakers can reproduce the dynamics of a drum kit or the crescendo of a symphony orchestra with greater realism." Think of it this way, Bednarski proposed: "I've driven to the top of Pikes Peak a number of times. It's humbling how little power you have in reserve. You can think of the dynamic peaks in music as scaling mountains. The current delivery of the REX 500 allows peaks to be scaled with ease."

As you might expect, the REX is quite the space heater, using 400W at idle and 3000W full tilt. After a day's worth of music in the 80–85dB range at the listening position, I brought out my touchless infrared thermometer and measured 111°F on the top cover and 124°F around the fins. That's about 10°–12° hotter than my Krell but well within BAT's operating standards.

Despite the temperatures emanating from the aluminum chassis, the price of running the REX isn't prohibitive. At my request, team BAT calculated that based on an average electricity rate of 15 cents per kilowatt, when using the amp four hours a day, the owner would spend around $100 a year. That not likely to deter someone who's in a position to drop 25 grand on an amplifier.

A cut (or two) above

To learn more about BAT's design approach and about the REX 500 in particular, I read the very informative white paper that Bednarski emailed me (footnote 4). These are the principles on which the REX 500 was designed.

Don't restrict the signal. Restricting signal flow, BAT says, results in "loss of detail, anemic bass, and lackluster dynamics." Other amplifiers restrict signal flow by having too many voltage-gain stages, each with too little quiescent current—as little as 2–4mA. That keeps those products affordable, because boosting quiescent current means an increase in the required power; that in turn necessitates bigger and more expensive power transformers and filters. The REX 500 contains only two gain stages, one of which runs at a whopping 200mA per channel. Who needs negative feedback? Some years ago, Bednarski and his business partner, audio designer Victor Khomenko, built a prototype amplifier with feedback controls. They and a cadre of testers found that as little as 3dB of negative feedback audibly shrank the soundstage and restricted air around reproduced voices. "Notwithstanding the improvements that negative feedback brings to an amplifier's measured performance, every listener preferred the zero feedback position," BAT says. Consequently, the REX 500 uses no global feedback.

These ain't your sister's transistors. BAT uses only N-channel MOSFETs in the REX 500, deeming the P-channel variety "simply much slower." (footnote 5) The common combination of N-channel and P-channel transistors is a crutch, the BATmen say. "In the REX 500, both sides of the waveform are handled by identical devices in identical circuit configuration, assuring ultimate symmetry of the resulting signal."

This is perhaps even more radical than it sounds. As Mark Craven pointed out in his REX 500 review in Hi-Fi News, Stereophile's sibling publication, "While the amplifier does not use complementary PNP/NPN transistor pairs, neither is its design quasi-complementary in the fashion of so many early transistor amps from the 1960s. Instead, the REX 500 takes its design cues from the Circlotron triode tube circuit patented by Wiggins in the US in 1958, though others had published similar topologies earlier. In fact, all BAT's amplifiers, whether tube or solid state, have employed a modified form of the bridged Circlotron configuration." It is, in other words, an unusual and clever design.

Getting an earful

Specsmanship aside, how does the REX 500 perform as a music-making machine? Did it please the ear and gladden the heart?

There's often a point during break-in when, all of a sudden, a piece of music demands your attention. That's your clue that serious listening can begin. With the BAT amp, that piece of music was Maurice Ravel's Pavane Pour Une Infante Défunte, in a woodwinds-heavy version by Vince Mendoza and Germany's WDR Big Band (footnote 6) (Sketches, 16/44.1 FLAC, ACT Music/Tidal).

At the 1:11 point in that track, Charlie Mariano comes in with his alto sax. With lesser gear, this passage can sound as if a soprano vocalist is producing the first six notes. That's an interesting effect, but I prefer not to have to guess what instrument I'm listening to. The REX 500 left no confusion about the timbre.

Elevated levels of transparency and resolution were also evident on Paddy McAloon's masterful I Trawl the Megahertz (24/44.1 FLAC, Sony Music UK/Qobuz, footnote 7). I could hear every word of American guest Yvonne Connor's spoken part on the 22-minute title track—not necessarily a given. The BAT seemed to improve her enunciation. Soundstaging, too, was stellar. You could circle each instrument with a fine-tip Sharpie.

The BAT rendered the overtones of Alex de Grassi's acoustic-guitar strings on "Eulogy in a Low Voice" (The Bridge, FLAC 16/44.1, Tropo/Tidal) with velvety ease, and reproduced the shimmer and decay of all manner of cymbals beautifully, whether played by Simon Phillips, Buddy Rich, or John Bonham. Drum recordings revealed top-notch, grippy bass, the agility of a mountain goat, and phenomenal dynamics. The REX took every pianissimo–fortissimo transition and ran with it, making music seem effortless.

On David Bowie's "Bring Me the Disco King," off Reality (16/44.1 FLAC, Columbia/Qobuz), I sensed an increase in continuousness in the reproduction of lower piano notes—the same mosaic made with smaller bits of colored tile.

The upshot

The BAT REX 500 walks that happy line between reticence and bombast, giving preference to neither and varnishing nothing. It doesn't insert its own drama, but it renders sound that's both dramatic and refined—if that's what the recording calls for. Not to get too cute about it, but the REX is balanced in more than just topology.

Listening to this amplifier reminded me of the moment after I turned on and calibrated my first 4K television. I wasn't exactly slumming when I watched movies on the previous 1080p set, but with the higher-resolution screen, suddenly there was more there there.

Unfortunately for audiophiles, while a good 4K TV can now be had for less than $500, the REX costs 50 times that. I can get a little salty over skyrocketing equipment prices that seem to cater to oil sheiks and hedge-fund managers—Herb Reichert famously dubbed this Oligarch Audio—leaving us poor schlubs in the cold. On the other hand, quality costs money, and the REX 500, expensive as it is, comes in multiples below the most expensive competing products.

The REX 500 has superb articulation and slam, zero glare, grip that could embarrass a bench vice, and as much truthfulness as the best power amplifiers I've heard. If a $25,000 purchase makes you neither wince nor whimper, this powerhouse deserves to be on your shortlist. It's good to be the king.

Specifications

Description:

Fully balanced, dual-mono, class-AB stereo amplifier heavily biased into class-A with high-current differential driver stage, dual power transformers, no global negative feedback, and fuseless protection circuit. Power output: 500Wpc into 8 ohms, 1000Wpc into 4 ohms (both 27dBW). Input impedance: 215k ohms. Bandwidth: 3Hz–250kHz. Slew rate: 200V/µs. Gain not specified. Power consumption: 400W at idle, 3000W at full power.

Dimensions: 19" (482mm) W × 10" (254mm) H × 23.5" (597mm) D. Weight: 140lb (64 kg).

Finish: Silver or black anodized aluminum.

Serial number of unit reviewed: 0R5000062. Made in USA.

Price: $25,000. Number of US dealers: 13; sold online at MusicDirect.com. Warranty: 5 years.

Manufacturer: Balanced Audio Technology, 1300 First State Blvd., Suite A, Wilmington, DE 19804. Tel: (302) 999-8855. Email: info@balancedaudio.net. Web: balancedaudio.net.

Source : https://www.stereophile.com/content/balanced-audio-technology-rex-500-power-amplifier