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Krell Full Power Balanced 350mc monoblock amplifier

By Jonathan Scull • Posted: Aug 10, 2000

With its latest series of FPB (Full Power Balanced) amplifiers, Krell is taking careful aim at the seam between classic high-power two-channel systems and quality multi-channel installations where sound is yet paramount. Nevertheless, Krell founder Dan D'Agostino was adamant: Krell's Class A components were designed for music playback. "I'm a purist, like you, Jonathan!" he told me.

The sleek, good-looking 350Mc monoblock sits in the middle of the FPB series. Specified power is 350Wpc into 8 ohms, 700 big ones into 4, and a whopping 1400W into punishing 2 ohm loads. 250W and 650W monoblocks are also available, while two-channel models with slightly different cosmetics offer 200WPc, 300Wpc and 600Wpc.

The monoblocks are available in symmetrical pairs, with left- or right-side heatsinks, and also as Theater Pack Three and Five combos. A serious two-zone preamplifier—the KCT—and a CD player—the KPS28c—join the upgraded top-of-the-line KPS25sc CD/preamp to round out Krell's Class A Series.

While both a KCT and KPS28c were on hand during the review period so that I could experiment with Krell's CAST (Current Audio Signal Transmission) system, I evaluated the 350Mc as a classic pair of two-channel audiophile amplifiers.

Core Technologies
There are a number of technological pillars on which Krell base their Class A designs. First, fully balanced circuit topology with positive and negative signal paths symmetrically referenced to ground, which results in greater noise rejection and less "cable interference."

"The primary advantage of input-to-output balanced operation is control." says D'Agostino. "Independent symmetrical circuit paths individually amplify the positive and negative signals, locking the speaker into a push-pull relationship. FPB series amplifiers exercise absolute control over the most unruly speakers, even those with exotic impedance curves and disruptive back-electromotive forces (EMF). These amplifiers overcome the speakers natural resistance to motion and forces them to perform the precise required movements." Very Krell.

Tech pillar two: Regulation is good, and the more of it, the better. According to their TechSynopsis: "With Krell's Active Regulation, any change of supply voltage is instantly corrected, whether due to variations in the AC line voltage or to changes in output stage current delivery." The paperwork goes on: "The power supply relies on large banks of capacitors to support it during demanding dynamic passages. Unfortunately, the capacitor's passive nature dictates that there will always be a time lag between the precise moment the power supply needs current and when it's supplied. The result is the output rail voltages cycle back and forth from linear to non-linear states, depending on the demands of the signal and the load."

The output stage in all FPB amps is fully and actively regulated through a "free-floating feedback arrangement" where "ultra-fast regulators" operate independently of the output stage while monitoring its condition and responding to even the smallest drop in current or voltage with "essentially instantaneous regulator response."

Hand-in-hand, Motorola microprocessors control optimization of Krell's Sustained Plateau Bias II system. The updated circuitry "evaluates" both the musical signal and the speaker impedance and passes that information along to the microprocessors charged with dynamically adjusting the bias plateaus to keep the amp in class-A "during any musical passage or speaker condition." Each bias plateau is sustained from 30 to 90 seconds, so the output stage is always ready to rumble. "Jonathan," Dan enthused, "it's simple. Class-A is the most linear, lowest distortion mode of operation—full current for full power is always available!"

The claimed result is class-A performance with less heat and power consumption than other more conventional designs. The output devices are anything but ordinary, of course. The 350Mc makes use of 28 Motorola high-speed, high-current bipolar transistors per chassis. No overall loop negative feedback is used, but there are "nested" local feedback loops.

The microprocessors also implement sophisticated protection schemes for a variety of conditions—short circuits, excessive heat, "irregular regulator voltage," or excessive DC at the outputs. (All FPB amps are direct-coupled from input to output.) When an "aberration" is detected, the FPB amps disengage their input and output stages, and no one gets hurt.

Low-impedance pathway Current Mode Gain stages are used throughout all components in Krell's Class A line, implemented on military-grade multi-layer circuit boards. (See the CAST section for further details.)

The whole package is contemporary and sharp-looking, big but not massive, and surprisingly well-proportioned. The slightly concave front panel looks rich and attractive, the polished and anodized finish contains an optical brightener giving it a special luster. The convex vertical center section carries an elegant, silver Power button with a triangle of blue LEDs mounted above showing operating status. The Standby LED illuminates when the back panel power breaker switch is engaged, powering up the input circuitry. The Bias LED shines when the Power button is pressed and Sustained Plateau Bias II comes up, and the Regulator LED indicates the regulator is providing power to the output stage. A teeny-tiny red LED in the center of the triangle lights up when the CAST connection is active. The LEDs flash to indicate fault conditions, but can be turned off completely if they bug you.

Around back in a vertical array, just next to a carrying handle, are two pair of speaker binding posts. These are great connectors and make child's play of lashing up all manner of recalcitrant mind-of-their-own cables. Between the upper and lower set you'll find a balanced XLR input next to the CAST connector. Over to the right are a pair of Krell Link connections (Krell Link allows remote operation of the amps), with a heavy-gauge captive power cord below.

When powering up from Standby, the amps default to the CAST input if this is connected, even if you've got an XLR cable plugged in. There is no single-ended input, although adapters are available from Krell. They just don't believe in 'em.

CASTing about for better sound
CAST (Current Audio Signal Transmission) is the keystone upon which all Krell's Class A series components are built. CAST inverts the voltage-mode wisdom that signal transfer from source to load requires a low-impedance output and a high-impedance input. Instead, CAST operates in current mode, with the signal communicated from a high-impedance source to a low-impedance load.

Why is this important? According to Krell, interconnects can "significantly" affect overall signal quality, their impedance—resistance, inductance, and capacitance—reacting with the source and load impedances to change the signal voltage at the amplifier's input. According to the Krell white paper on CAST: "This alteration can amount to as much as a few percent, depending on the exact nature of the source, cable, and load impedances. This may not sound like much, but in an industry where 'goodness' is often measured in hundredths and thousandths of one percent, it's really quite a lot. As cables get longer, these effects increase proportionately. The sonic results of these interactions range from mild to severe, and they form a large part of the mysterious art of 'matching' components and cables for a desired sonic result.

"The idea is basically simple," the documentation goes on, "even if the implementation is a bit complicated: The preamp has a high output impedance measuring hundreds of millions of ohms and delivers current rather than voltage to the load. The amplifier's input senses and amplifies this current, which is not altered by the cable's impedance as these impedances and their reactive effects will be at least a million times less than the total transmission system impedance. In short, cable impedances and nonlinearities are totally eliminated from the picture.

"CAST has another important aspect. It really doesn't matter how long the connecting cable is because the cable's reactive effects are still never going to be more than a negligible part of the total circuit impedance. This means that preamplifiers and power amplifiers can be placed where needed, unconstrained by cabling problems."

Well hey now, that sounds like the ideal solution! So I spent a fair amount of time playing the KCT two-zone preamplifier and KPS28c CD player into the 350Mcs using both CAST and "standard" interconnects. Here's where I come down on the matter: CAST technology works, no doubt about it. The black, discreet cables are relatively small in diameter and easy to run. They're perfect for custom installs where a snakepit of wires is a no-no. And they are neutral-sounding, as they should be, given the technical specs.

But—and I have a big one—I preferred the sound of the 350Mc on Synergistic Research Designers Reference with Active Shielding. Why? Well, I found I could "tune" the sound of the amps right to the top of the sonic bubble by swapping interconnects and speaker cable.

The KPS28c CD player was another matter. Look, I'm just totally spoiled by 24/192 upsamplers, and 16/44.1 just doesn't do it for me anymore. Even so, when I connected the KPS28c to the KCT with CAST connections, the Krell player gave the Accuphase DP-75V and the dCS Elgar/972 combination a good run for their money. It fell behind when it was connected with standard line-level interconnects of whatever brand or type (RCA or XLR) either into the KCT or into the other preamplifiers on hand.

So if you're going for Ultimate Sound, work those cables, baby. If you're going for Ultimate Convenience, go CAST and don't look back.

Kvelling!
Sometimes I find it interesting to imagine a single word to describe a component under test. The 350Mc was a snap that way; it was totally organic.

Yes, I agree, that needs further elucidation. I caught a glimpse of it listening to Golden Earrings by the Ray Bryant Trio (VICJ-60212), one of the latest and most intoxicating XRCD2 releases yet by Alan Yoshida and Akira Taguchi at JVC Hollywood. This is a Rudy Van Gelder recording, bless his soul, in "big mono" as Taguchi calls it. The sound was big, meaty and round, lovely, airy, and transparent. You want to know what organic is? Play this recording. Love this recording as it wraps itself DNA-like into your mind and body. Try and stop hummin' the tune a hundred, a thousand times—it's the very essence of jazz.

There are some other recordings that slice deeply into my musical soul, that I become, you might say. The cover of "Bags Groove" from the XRCD of the same name, for instance, or another late-breaking XRCD2, Bill Evans' Green Dolphin Street (VICJ-60372). These beautifully mastered monos make evident certain properties that with stereo recordings become less critically important, tonal balance and overall harmonic structure, for example.

Take the bass solo just after four minutes into "Green Dolphin." Notes: "There's a roundness, an acoustic rightness that's light, lively, big, powerful and deep. An impressive in-control and comprehensive bottom end and midbass transition into a detailed, textured, super organic nubbly-with-detail midrange. Transitioning up smoothly, the upper registers are sweet and velvety, attractive with texture, no bite; it's all very intimate and musical." The mono image set up between the JMLab Utopias was huge yet very precisely defined, the seductive tonal balance drawing me into the music instantly yet profoundly.

Or try "Rent Party" (in stereo) from The Timekeepers (JVCXR-0206-2). The inner beauty and "believability" of this recording lies in the sleight-of-hand of reproducing the "real" sound of two pianos in your listening space. In this regard, the Krells were superb. The fullness of tone produced not by the 350s but by two pianos filled our listening space. Let's not even mention the jazzectasy produced when the bass and drum come in at that perfectly formed moment at 5:26, and on to the end of this golden slice of musical pie. Notes: "The 'mesh' between Basie and Peterson is perfectly set up. When the drums and bass come in, it's already a well-oiled machine dragging you deeper into the music, like putting your nose into a heady 50-year-old Armagnac." The Krell amps excelled at retrieving the pace and timing, the very nature of the music.

Just so our world-music readers don't think I've given up the Trip Hop ghost, I'll turn now to "Shining" on Peter Kruder's Peace Orchestra (G-CD 004) Notes: "That voice, she's so focused, luminous, round and real, it's just fantastic! The Krells lay her out stunningly. Contact! Plaintive, poignant, sweet and ambient, just plain killer. Aural candy, but not at all euphonically flawed." The low bass was deeply resonant, the powerful fundamentals filleting out the bottom end. The very top was, it has to be said, just a tad hashy, but not too bad. Considering that those highs are in the recording, the Krells in fact ought to leave their hashiness intact, as they do. Organic doesn't mean rolled-off, although the 350Mcs could sound mighty sweet. (Exactly the opposite of my expectations, if truth be known.)

At 1:50 into "Universal Highness" from Thievery Corporation's Sounds from the Thievery Hi-Fi (Eighteenth Street Lounge Music ESL5) you'll find a great drum solo. If your body don't shuck'n'jive to it, if the physicality and palpability doesn't move you, if the slap of palm on taut drumhead doesn't leave you slack-jawed in amazement, then seek help immediately. Again, the amps proved utterly organic in this way. I was spastically boppin' around in the Ribbon Chair until I caught K-10 giving me the look!

Notes: " 'Incident at Gate 7' lays out a female vocal that floats buoyantly well to the rear of a huge soundstage. It's a thing of great beauty, supported by lush midrange and a jumpin' drum beat with bass deeper and tighter than Schwartzenegger's butt!" (Yes! How do I really feel!)

And you think I'm nuts? Listen to Moby on his new album Play (V2 63881-27049-2). Track four, "Why Does My Heart Feel So Bad," proved an emotive experience indeed. Moby's got an interesting way of mixing music, lyrics and recorded sounds bordering on performance art. (Catch his act on Sessions at West 54th Street on A&E.) At times he plays with repetition in the same way Andy Warhol explored multiple images. Moby manages it all through a state of induced satori, anything but boring. And somehow, that joy in the music came through smashingly well on the Krell monoblocks. I listened to the entire CD, mouth agape, drool cup handy, digging it deep.

Coming down from spiritual enlightenment, I marveled how the Krells knocked out such big, capable bass. They dug down and got the basics right, creating an organic, earthy foundation for the music. (When enjoying bass quality like this, most audiophiles smile and nod at each other in some group acknowledgement of prowess. Yah mon!)

The Krells set up a large, airy soundstage, not particularly forward or rearward in aspect. The presentation was—all together now—organic, thank you (organic, in the sense of fading naturally into and out of the acoustics of our well-treated room). The soundstage edge boundaries weren't so finely defined on their own, unless delineated by the recording, in which case, of course, the Krells delivered a virtuoso performance. I've heard this effect with a few other amps in the past, and I like it.

But the 350Mcs aren't just bassmaster acoustic "place holders". Nope, try track 11, "Run On," from Moby's Play and dig that midrange-rich gospel vocal backup. Moby mixes a lot of information into his music, and the 350s pulled all that information quite beautifully. There was nothing mushy or indecisive about the presentation. Everything sounded coherent yet textured, part of a musical whole, complementing the sonics rather than fighting it.

To illustrate, let's be-bop to one of my favorite pianists, Alfred Brendel, playing Haydn's Piano Sonata in b, one of my all-time favorite piano works. Notes: "I find myself listening to the action of the piano, noticing it like never before, as part of the process of making the sound. The Krells nail it. The detail's there even as the sound remains transparent and sweet in the upper mids and highs. The upper registers are wonderfully laden with harmonics, there's an airiness, a graciousness in the sweep of the music that's very engaging and involving."

CAST your fate to the wind
Incredibly, I found, the Krell 350Mc monoblocks were more self-effacing than the slightly-more-expensive Linn Klimax, my current solid-state reference. The Krells actually drew less attention to themselves from the midrange on up through those sweet, engaging yet detailed highs; attractive enough to pull me right into the music, but fairly subtle about it all. Imagine.

The bottom end was powerful, just what you'd expect from a Krell. It was impressive, no doubt about it. In fact, I couldn't help being aware of how great the bass was. Is that a good thing or not? I dunno, you might feel inclined to argue about it, but not me. I just enjoyed it.

The Linns had slightly better microdynamics, the Krells slightly better macrodynamics. The Klimax sound a touch "white" and more transparent on top in comparison to the Krells, the 350Mc were slightly darker and seductively sweeter.

So...is it all just a matter of taste? Yep. And if your tastes runs to their sound, don't even hesitate. The FPB 350mc is a classic class-A design if there ever was one. Highly recommended!

Specifications

Description: Solid-state monoblock power amplifier with "Sustained Bias Plateau II," class-A output stage and optional remote control. Inputs: one balanced (XLR), one Krell CAST (4-pin bayonet). Loudspeaker outputs: two pairs. Output power: 350W into 8 ohms (25.4dBW), 700W into 4 ohms (25.4dBW), 1400W into 2 ohms (25.4dBW). Output voltage: 60V RMS (170V peak-peak). Voltage gain: 26.4dB. Frequency response: 20Hz-20kHz, +0dB, -0.05dB; 0.1Hz-240kHz, +0db, -3dB. S/N Ratio: 118dB (A-weighted, no reference level given)). THD (balanced drive, unweighted): less than 0.03% at 1kHz and 20kHz (no reference level given). Input impedance (XLR): 100k ohms. Input sensitivity: 2.6V RMS for maximum power. Power consumption: 37.5W (standby), 175W (Idle), 3kW (maximum).
Dimensions: 12.5" W by 10.5" H by 19.1" D. Weight: 98 lbs.
Serial numbers of units reviewed: 12500030087 and 12600030088 (auditioning), 12500050100 (measuring).
Price: $17,500/pair. Approximate number of dealers: 125.
Manufacturer: Krell Industries, 45 Connair Road, Orange, CT 06477. Tel: (203) 799-9954. Fax: (203) 799-9796. Website: www.krellonline.com .

Associated components

Digital Playback: Accuphase DP-75V 192/24 upsampling CD player, dCS 972 digital-digital converter and dCS Elgar D/A processor running at 24/192, Marantz SA-1 SACD player.
Preamplifiers: Mark Levinson No.32 Reference, Krell KCT.
Power Amplifiers: Linn Klimax Solo 500s, Forsell Statement.
Loudspeakers: JMLab Utopias.
Interconnect: Krell CAST, Synergistic Designers Reference Active Shielding, Cardas Golden Reference, Linn Silver.
Loudspeaker Cables: Synergistic Designers Reference, Tara The One, Audioquest Everest, Cardas Golden Cross, XLO The Limited.
Power cords: Synergistic Designers Reference Master Couplers Squared.
Digital datalinks: XLO The Limited RCA and AES/EBU.
Accessories: ASC Studio Traps, Argent RoomLenses, API Power Wedge Ultra 116 and Ultra Enhancer, PS Audio P300 Power Plant, PolyCrystal amp stand, equipment racks, cones and cable towers, Signal Guard platforms, Black Diamond Racing shelves and cones, Nordost Pulsar Points, Bright Star Air Mass and Big Rock combo.

—Jonathan Scull

Measurements - (John Atkinson)

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