The Krell FPB600 Amplifier
Noel T. Keen
14 April 1999

FPB600.jpg (6099 bytes)Specifications

Krell FPB600 amplifier, $12,500.
Krell Industries, Inc., 45 Connair Road, Orange, CT 06477 USA.
Direct coupled, current mode, bipolar transistor stereo amplifier.
Fully balanced circuitry from input to output.
Power transformer capacity, 8 kVA.
Nominal output rating: 600 watts/ch into 8 ohms, 1200 watts/ch into 4 ohms, 2400 watts/ch into 2 ohms.
Fully regulated input and output supplies and 'sustained plateau biasing', permitting full class A operation and low or high output levels but allowing the amplifier to run cool at idle or low input.
Weight: 180 lbs.
Warranty, 5 years, transferable.


I have had the Krell FPB600 in my listening room for about 18 months, certainly enough time to become familiar with it and recognize its strengths and weaknesses. At the time of purchase, the Krell to my ears easily beat two competing solid state amps and two tube amplifiers that are highly reputed. I recently obtained the KR Enterprise VT8000MK amplifiers ($24,400 retail). These are an all tube, zero feedback, push/pull design and put out a maximum of 300 watts per monoblock into 2, 4 or 8 ohms. The KR amps match or better other tube amps in important areas such as imaging and holography and are killer in the mid/treble frequencies. As such, they afforded an excellent and rigorous comparison with the Krell FPB600 solid-state amplifier.

Krell is one of the most familiar and highly regarded names in high end audio—it is sort of the BMW of the high end—performance comes first and this has led to a cache whereby other products frequently are compared to Krell. This is of some importance, given that when you buy a high-end component you are also buying a high-end company. Krell’s reputation is to build extremely durable, reliable and well engineered products (as with BMW, some would say over-engineered!). Krell is accordingly able to offer 5-year warranties on their products and the warranties are transferable to other buyers. I have owned four Krell products and had a problem with only one of them, a KSA200 amplifier that had its ground shaken loose by UPS gorillas several years ago. Krell fixed this promptly; the amp was back within 10 days and worked ever after.

The Krell FPB 600 has performed flawlessly from the time it came out of the shipping box. Krell is also an innovative company. It started the production of high-end single box CD players with the KPS products, a trend that others have now adopted. Similarly, it led the move of top end companies into ‘affordable’ products such as the 300 series of amplifiers and CD players, a trend that has also been copied by several others. Krell is usually at the forefront of engineering, as I will discuss in more detail for the FPB600 amp. These are all factors contributing to the traditionally high re-sale value of Krell products.

The FPB 600 amplifier is a stereo device. The amp contains two huge toroidal transformers with a total of 8 kilowatts capacity, some sort of record I would say at the price point! This also means that the amp is heavy, about 180 pounds, and long, about 26 inches front to back, and 10 inches high. The amplifier is pure class A and direct coupled and also has fully regulated input and output sections, unusual for a high powered amplifier. All of these features, however, would be expected to contribute to good sound.

The arrays of specially selected Motorola bipolar transistors collectively put out a claimed 600 watts/channel at 8 ohms impedance, doubling as impedance is halved down to at least two ohms. The definitive review on the Krell 600 in Stereophile by Martin Colloms measured output as an actual 935 watts/ch into 8 ohms before clipping! Thus the amp is a powerhouse. Two sets of high quality speaker terminals with thumbscrews are provided as outputs and both balanced and unbalanced inputs are available. Krell, however, maintains that the amp performs much better with balanced inputs.

Krell also uses a clever strategy to permit full class A operation without burning your house down. Their patented variable biasing system allows the amp to run in 7 different bias levels depending upon input demand. Thus, at idle or during low volume material, the amp loafs along at something like 30 watts/ ch maximum. However, if all hell breaks loose on the musical program, the amp has an anticipator circuit that detects this and rapidly ramps up the biasing to the required level. If input again decreases, the bias level also decreases—clever! Neither I nor anyone else I know has audibly detected this technology at work, but it does have the major advantage of allowing a full blown class A amp to run relatively cool and save on your power bill.

Krell amps have stereotypically been able to handle any speakers or program material and to provide incredible extension and resolution of bass frequencies. Their Achilles’ heel has been in the mid and high frequencies, where historically they could elicit visions of mid-fi cat howling, largely I suppose due to odd order distortions from the bipolar output transistors. The company has been working on that and it is generally agreed that the KSA series of amps provided improvement and further improvement has also been claimed for the FPB series of amps. Let’s find out, using the KR VT8000MK tube amps as a very high bar for comparison!

The System

Von Schweikert VR-8 speakers with the silver internal wire option were used throughout. These extremely revealing speakers were set up in a large room and placed about 6 feet from one side wall and more than 15 feet from the other. Only 26 inches from the front wall proved the best bass compromise in an irregular California style split level living area of greater than 25,000 cubic feet. The back wall was not parallel, with the furthest point about 45 feet away. The floor has a moderately thick carpet and the ceiling is open wood-beamed cathedral, angling up and away from the speaker position. The speakers sound good in this location without room treatment, almost a necessity due to Wife Acceptance Factor (WAF) (not to say that the usual tricks wouldn’t improve things). The speakers were separated by about 12 feet center to center and the listening position was about 11 feet from the speaker centerline. Only slight toe-in proved the best compromise for imaging and sound staging since the speakers are not particularly directional.

The speakers were coupled to the concrete slab floor with the high quality brass spikes supplied by VSR. These produced the greatest increase in focus and sound staging of any speaker I have ever heard relative to performance unspiked and floating on the carpet! CDs only were used as source material through a Krell KPS-20iL CD player run directly into the amplifiers through Harmonic Technology Truth Link unbalanced interconnects. For some auditioning, Audio-Magic Sorcerer balanced interconnects were used with the Krell amplifier. Performance of the excellent 20-iL CD player was elevated another few notches by an ElectraGlide ReferenceGlide power cord. My reference speaker cables have recently become the top line Harmonic Technology pro-9 copper cables bi-wired to the mid/tweeter and bass modules of the VR8es. These cables afford new levels of detail and resolution and fully deserve the accolades many reviewers have given them.

I also recently obtained the highly rated Arcici Suspense equipment rack (see review in this issue), which was used for all amplifiers (a total of about 360 pounds!). The CD player was similarly placed on a BDR Shelf for the Source with #3 cones and set onto the top shelf of the Arcici. I found that a mixture of #1 and #2 Vibrapods between the shelf and the top of the Arcici rack also provided an improvement in tonality. Components were plugged directly into a Magnan Signature power cord with strip, in turn plugged into a dedicated 30 amp circuit. CDs were liberally tweaked with Compact Dynamics Optrix spray on the playing surface and CD Upgrade discs, as well as Eco-3 antistatic formulation sprayed on the label side. CDs were also routinely demagnetized with the Bedini Ultraclarifier before play and the Audio Prism CD Blacklight was used. Yes, green paint on the edges of CDs made an improvement, and I have evolved to the product from LAT, that does not smear when discs are treated with Optrix. All of these tweaks gave audible improvements that varied somewhat in degree, depending on source material. The system was occasionally treated with the latest Purist Audio break-in disc. Finally, monthly cleaning of all terminals on cables/interconnects/power cords with ‘Pro Gold’ was beneficial.


Suffice it to say, the Krell 600 is magnificent in the bass and mid-bass frequencies, probably better than any amplifier at or below its price point for providing extended but fast and tight bass. In addition, the amp has incredible presence--the ability to make you think you are there—no quarter mile away music or struggling to handle the next transient here! Some might call this effortless—the amp just spits out whatever comes in, regardless of how raucous it might be.

Soundstaging, both vertical and horizontal is immense, and imaging is excellent, regardless of complexity of material or pyrotechnics. With 1200 watts per channel available at the nominal 4 ohms impedance and 96 dB/w/m of the VR-8 speakers, headroom is not an issue with the Krell. Indeed I have never managed to make it clip, and to do so would I suppose require being outside the house or having a set of very good earplugs. The Krell will also beat most solid state amplifiers and many tube amps for mid and treble purity and transparency, but I will say more about that momentarily.

The Bad News

I have run interesting comparisons of the Krell 600 with the KR Enterprise VT8000MK amplifiers in my system and, suffice it to say, the two are very different. In some respects, the KR amps clearly beat the Krell, which they perhaps should, retailing for twice as much. The secret of the KR amps is outstanding house-built input and output tubes, the latter actually producing significant current - unheard of with conventional tubes. The KR amps are accordingly very linear and excel in the mid and treble frequencies and to my ears sound as good in this regard as any amps I have ever heard. The 8000es can be characterized by the three ‘Ts, tone, timber and transparency, all of which the amps have in spades, easily beating the Krell on most material.

The KR amps detail well and also provide wonderful holographic realism—those indefinable cues that we all perceive in real music, and which create the illusion for a sound system that you are in the concert hall and the music is live. These characters made the KR amps excel and easily better the Krell on ‘small scale’ music such as jazz, blues, small ensembles, soloists, small singing groups, solo piano etc. Reproduction of cymbals, bells and such was marvelous.

For instance, the Golden String All Star Percussion Ensemble CD (GSCD 005) has lots of bells, drums and other percussion sounds that generally are not as well reproduced by the Krell 600. Listening to tracks from this CD through the 8000es was a revelation for clarity and rendition (and made me think that 16/44.1 is not as bad in the treble as we’ve been led to believe—amps have clearly been part of the problem!). Similarly, vocals such as on Muddy Waters’ famous ‘Folk Singer’ album (Mo Fi MFSL UDCD 593), Patricia Barber on ‘Café Blue’ (Premonition 737-2) and Margo Timmins’ smokey voice on the Cowboy Junkies ‘Trinity Sessions’ (BMG 8568-2-R) were all rendered with incredible clarity, timber, emotion and presence.

Instrumentals were also treated well by the KR8000 amps. Dean Peer’s ‘Ucross’ recording (Redstone RR91012) has never really sounded quite right through the Krell 600, with lots of boominess and poor string harmonics. I had erroneously chalked this up to the recording, but the 8000es did a credible job of reproducing Peer’s bass, including excellent articulation of harmonics in the lower registers. Similarly, Gino D’Auri on Flamenco Mystico (Golden Strings GSCD 016) was reproduced by the 8000 amps with stunning timber, realism and detail on the strings of his classical guitars, but the Krell 600 missed much of the harmonics and "you are there" holographic nature of the tube amps. "Difficult to do right" instruments such as the saxophone (try cuts from Jeremy Cohen on A Taste of Violin, Clarity CCD-1012), trombone (Shades of Brass, Mapleshade 03932) and piano (Schumann, track 7, DG Originals 447 451-2) were reproduced as well through the 8000es as I’ve ever heard. Accolades are also in order to the VR-8es for correctly reproducing the wonderful sounds of these bell weather instruments.

The Good News

Now for the counterpoint, or "don’t conclude that I have thrown out ss amplication"! When one starts to encounter music with anything resembling a ‘big’ character such as most pop groups or large orchestral compositions, especially involving deep bass with organs or synthesizers, the 8000es largely abandoned the scene and the big Krell came into its own.

It’s not that the 8000es don’t make bass. Its just that for "big" music with lots of instruments and dynamic passages, the 8000es fails to match the Krell’s strengths of producing huge dynamic jolts while providing great weight, presence, and the ability to keep instruments where they’re supposed to be in the soundstage. A lot of this probably revolves around the power and the great speed of the Krell amp, the latter an area where it excels and one in which no conventional tube amp with output transformers can hope to compete well.

Even on some vocals such as the new Gram Parsons GP/Grievous Angel CD (Reprise 9 26108-2), the KR amps certainly sounded detailed, but showed a rather sterile, analytical character in the mid-bass that was lacking in warmth relative to the solid states amp. The Krell amp was more musical to my ears on this CD, better portraying sound staging and the presence and magnitude of the music. When one gets to bigger music, such as the excellent Lost World CD from Michael Stearns (Hearts of Space, 11054-2), it is no contest. The KR amps did simply not match the major league drum whacks, synthesizer jolts, and room wide soundstaging afforded by the Krell amp. The same held for the excellent Calvary Grand Organ Dedication CD (no number listed), with the Bach piece from track 4 produced with pace, power, and immense sound staging by the Krell. This track has a large dynamic range, in excess of 40 dB I would say, easily leading the VT8000MK amps into clipping (and pretty harsh clipping at that, surprising for a tube amp!). On the other hand, the Krell, with its greater than 6 dB of extra headroom, fully regulated power supplies and 8 kw power transformers, zoomed through any and all of the pyrotechnics and in-room peaks of 100 dB or more with no complaint.

Finally, it should be emphasized that even on smaller scale music, the Krell’s superior vertical and horizontal sound staging, weight, immediacy and presence have great appeal. For example, on the Love Over Gold album of Dire Straits (Warner 9 23728-2), "Telegraph Road" was reproduced with much greater power and palpability by the Krell 600 than the KR amps. On big orchestral music such as tracks 15/16 of the outstanding Reference Recordings Tutti CD (RR906CD), the 8000 amps did a wonderful job on timber and inner detailing. However, when things get hot and the big drums come out along with massed instruments, the KR amps fail to convey the power and emotion of the music as well as the Krell amp.

Yes, maybe some of those massive bass drum and tympani whacks are overdone by the Krell and not entirely life-like, but boy they surely convey the emotion that up front seating at live music has, but the KRs don’t! On heavy rock music, it was no contest, perhaps not surprisingly. For example, Journey’s "Faithfully" (Greatest Hits, Columbia, CK44493) was reproduced via Krell with the power and emotion that I remember from hearing this piece performed live years ago. The KR amp was simply out of its league--nice but not convincing.

Bottom line

The Krell 600 earns the accolades it has received from all reviewers lucky enough to receive review samples. On music with complexity pace, rhythm and/or heavy bass/midbass, the Krell is hard to beat. On the other hand, with ‘small music’, the KR amps clearly prevail, so we have somewhat of a conundrum here--the Krell FPB 600 and KR Enterprise VT8000MK tube monoblocks are both great amplifiers when used with the right music.

In some ways the KRs are the best tube amps I have ever heard, retaining the conventional strengths of tubes with at least a modicum of bass power and definition, dynamics and pace. But if, in addition to jazz, blues, and vocals, you also like "big" music with lots of dynamics, massed instruments, and low bass frequencies, the 8000 amps will not perform as well as a top quality solid state amp such as the Krell 600.

For the overall eclectic mix of music I like, the Krell is the better single choice, but boy the KRs are killer on the stuff they do well. What then to do? I am keeping both amps and switching speaker cables as the mood hits me. While it is too early to say yet whether it will work, I have also been experimenting with what could be nirvana--passive bi-amping! Yes, with the KR amps (fortunately they have variable input sensitivity) feeding the mid/tweeter modules of the VR-8 speakers and the Krell 600 pushing the bass modules. There is promise here for extracting the best properties from both of these outstanding amplifiers.