Conrad Johnson MF2500 with Elna Golden Caps upgrade

Rumor had it that if the MF2500 amplifier had gotten any better in development, Conrad-Johnson would have had to include it in their “Premier” series. However, C-J’s intention was to hold to the lower price of their established MF series, and so they have. Rated at 240Wpc and retailing for $3495, the ‘2500 is the core model of Conrad-Johnson’s current range of “MF” power amplifiers. Its companion MF2250 offers 120Wpc, while the MF5600 delivers 120Wx5 for multichannel home-theater applications.

Technical story
Like many Conrad-Johnson designs, the MF2500’s higher performance goals have been attained through experience and the refinement of established circuits, not by some magic new topology that’s claimed to solve all previously known audio-fidelity problems. Thus the input stage is a standard differential circuit executed with FETs. The second stage comprises a buffered voltage-gain amplifier, and uses bipolar transistors. It drives a regular class-A/B push-pull bipolar output stage, which is direct-coupled to the speaker. The output stage is run at quite a low idle power, which partly explains why the amplifier takes a little time to get up to speed from a cold start (footnote 1). But it does mean that the amplifier does not run too hot to touch.

While some of the MF2500’s technology is derived from the older MF2300A (reviewed in October 1996), the big change has been from FET to bipolar output devices. In this circuit, at least, bipolars confer greater gain stability with output load and current variations, imparting a more consistent and more dynamic sound quality. The MF2500 has high-current, resin-encapsulated Sanken power transistors: two 2SC3281 and two SA1302 devices are paralleled for each channel’s output stage. These transistors have good thermal properties, high-quality electrical connections to their junctions, and are endowed with good safe-operating areas. This means the amplifier can be protected with simple quick-blow fuses.

C-J favors a predominantly low-order distortion spectrum in which some second harmonic is designed to dominate the distortion residual, this in the MF2500 courtesy of the input FETs. Minimal loop negative feedback is used, in this case just 12.5dB, or approximately 4x. (Some amplifiers may run with x1000 to x10,000 of feedback.) As Lew Johnson explained, “the more the performance of the basic circuits was improved from the new MF series, the more important was the upgrading of individual component quality. Then negative feedback could be reduced yet again.”

The MF2500’s regulation topology is of Premier quality. For example, the input stage has its own transformer tap stabilized at a voltage higher than the output stage requirement. Discrete open-loop regulators have audiophile-grade polystyrene and polypropylene capacitors. Only the main current reservoirs are electrolytic, but here there is no choice. Voltage-reference and feedback-control resistors are costly metal-foil Vishays. Six 10,000µF Nichicon electrolytics form the central reservoirs for both channels, giving a total of 30,000µF per DC supply phase and ensuring good current delivery at low frequencies.

The massive EI-lamination transformer has split secondary sections to feed the various regulators. I also counted no fewer than 22 of C-J’s exclusive lead-foil, large-value polystyrene capacitors, plus eight higher-value, audiophile-grade polypropylene caps.

Hardware
The MF2500’s 3/8″ alloy front panel is anodized in C-J’s traditional gold, while the rest is finished in satin-black textured enamel. The baseplate can be removed to access the underside of the printed circuit board. The only control is a front-panel power switch, and there’s no soft start. The heavy transformer is affixed to the right side of the chassis, while the single, external, full-size finned heatsink is to the left. The inputs are single-ended phono sockets, the outputs 5-way binding posts, all gold-plated. Thermal protection is via a self-resetting thermal trip circuit. The power cord is a captive three-core type.

Listening
I found I preferred to leave the MF2500 on most of the time, its low idle power proving a blessing. The amplifier also benefited from extended break-in.

In some respects, the MF2500 gave an understated view of musical events—with casual listening it’s possible to dismiss it on grounds of a lack of forthrightness for its rather measured pace, and an almost rich, distant presentation. Yet, over the long term, and along with its even-tempered if slightly sweet tonal balance, it was the very absence of a forced or excitable character, false hardness, or excessive attack that endeared the MF2500 to me. There was a sense of ease; aural fatigue was significantly lower than usual for this class of amplifier. Just when I thought I’d got about as much out of the MF2500 as I was going to get, it got better by stages.

Elements of this sound—almost a tube experience—had been dimly evident in earlier solid-state amplifiers from C-J. But some of the necessary tonal consistency, fine textures, vitality, and clarity were not expressed well enough to make those models truly viable alternatives. Their FET output stages suffered from some variations in both clarity and tonal color with reproduced sound level, this to some degree also dependent on the loudspeaker load and the program complexity.

In the case of the MF2500, two key aspects have become firmly established. First, this amplifier offered the ability to play all the way to full output with consistent quality. Second, while the MF2500 won’t put C-J’s Premier Eight A monoblocks out of business, or even the smaller Premier Eleven A, its sound conveyed much of the general nature of a fine tube amplifier—a classic type of moderate negative feedback, wide bandwidth, and linear, push-pull design—coupled with greater dynamic range and sheer load-driving grunt.

Tonal colors were natural, never strident, thin, or hard. While the bass didn’t have the kind of deeply extended impact of the biggest, most costly reference amplifiers, it was satisfying. Not only was the bass powerful—more than sufficient to do justice to cathedral organ at full stretch, and able to carry the low-frequency presence of a large symphony orchestra—it was also articulate, in terms of both detail and tune-playing. The bass also had a kind of bouncy, springy recovery that lent drive and interest to rhythm bass lines on jazz and classic rock.

Through the midrange, the MF2500 had a thermionic tonal poise that appeared to have a payoff in terms of natural image depth and perspective. While the amplifier’s transparency was rated fairly well, the natural tonality helped create coherent, focused well-layered images. Stage width also rated above average, if a little more like that of a fine moving-coil cartridge than a CD source.

In the upper frequencies, the amplifier remained even tempered, well balanced, and even quite delicately resolved. String tone was nicely presented, and no false edge or breathiness was added to vocal sibilants. In fact, vocals in general were handled with near-tubelike quality and good clarity, and the inner harmonies of massed choir were well read, with well-maintained perspectives.

Within that slightly “relaxed” temperament, the MF2500 nevertheless managed to pack quite a punch—this is a bighearted amplifier, and sounds it. As it continued to break-in and I grew more familiar with its balance, musical values asserted themselves. Pace, rhythm, and timing, plus dynamic resolution, were all rated significantly above average.

On my personal scoring scale, this amplifier achieved a commendable 22 points, a significantly good result for type and price.

Conclusion
Conrad-Johnson’s MF2500 is a successful product on many grounds. It closely approaches the classic tonal character of a fine tubed amplifier—a C-J hallmark—and holds on to that quality over a great range of output power and possible speaker loading. Not easy to accomplish.

It also avoids the cheap, quick thrill of a bright, fast, forward sound. Instead, it builds on quality and depth of performance to achieve its commendable rating. Its sound is musically entertaining without being hyperactive, while its broad, bighearted approach is easy on the ears. All kinds of music were handled with equanimity.

In the lab, the MF2500’s high output power and good load tolerance were confirmed. The amplifier could dump up to 900Wpc into 2 ohm loads for short-term peaks, and cruise happily at around 300Wpc driving 8 ohm loads. This matches the attainment of significantly more costly amplifiers. Except for the modest channel separation at high frequencies, no test result gave rise to concern.

Well-built and well-finished, with effective if old-fashioned circuit protection, the MF2500 idled at quite low power and could be safely left on for extended listening periods. This musically well-balanced powerhouse represents fine dollar value, and wins a warm recommendation.

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Conrad-Johnson MF2500 Amplifier

by Todd Warnke

(Top to bottom) Conrad-Johnson MF2250, MF2500, MF5600 amplifiers.

Review at a Glance Sound “Superb bass extension with very good definition” and treble that avoids “typical solid-state nasties”; its “defining characteristic” is its ability to re-create “music being made by real people in real space.” Features Beefy 240Wpc output via bi-polar and FET transistors; Cardas RCAs and five-way speaker binding posts; captive power cord. Use “No current issues, no heat issues, no compatibility issues. Just hook ‘er up and let ‘er rip.” Value A powerful and fine-sounding solid-state amp by one of the royal makers of audio electronics is a good value indeed.

Back in dark days of the early ‘70s, when transistors, like so many Visigoths, were sacking the centers of high fidelity, Bill Conrad and Lew Johnson, then a couple of Federal Reserve economists, joined forces to help keep the music-loving community safe from barbarism. And from their first products, all of which were tubed, the combined Conrad-Johnson Design, Inc. has been about musical flow and not mega-buck looks or glossy ads. Through the ‘70s and into the ‘80s, Bill and Lew held the musical high ground with thermionic values. But then, like the Romans of ancient day, they learned to assimilate the good in their old enemies and began to design with transistors.

The Motif line, begun about 1983, was the first of the counterstrike designs, and was well received. Well built and well designed, Motif equipment set the standard for musical solid-state gear. The acquisition of Sonographe, around the same time, led to an entry-level solid-state line. And around 1987, a solid-state CD player became the first non-tube product to carry the C-J name. That player was soon joined by an amp and preamp, which completed the subordination of transistors to the C-J aesthetic. And all of this brings us to the latest of the silicon-state C-J products, the MF2500 amp.

Under the hood

Every reviewer, regardless of technical skill — and keep in mind the old adage, “those who can, do, and those who can’t, review” — is required to open the case, just so he can say “under the hood” or some such thing in his review. The packed-but-clean and well-dressed circuit board of the MF2500 bears witness to the skill of those who hand assemble each amp. The circuitry inside the case uses both bi-polar and FET transistors. In the voltage gain section the MF2500 uses FETs because, according to C-J, FETs have a distortion characteristic that, like tubes and unlike bi-polars, minimizes odd-order harmonics. On the other hand, the output stage, where low output impedance is of primary importance, bi-polars are used since they have about one-quarter the output impedance of FETs. Conrad-Johnson feels that this combines the better distortion characteristics of FETs and the drive capability (and hence better bass response) of bi-polars. As for numbers, the amp puts out a burly 240 watts into an 8-ohm load.

At 7 1/2″ high and 19″ wide, the 3/8″ thick, champagne faceplate gives the amp a substantial look, and at 16″ deep and 56 pounds, the amp is substantial. Besides the C-J logo, a power switch mounted in an oval cutout which is bisected by a vertical groove in the faceplate, and a red LED power indicator, the MF2500’s fascia is clean. Around back, the straightforward theme is carried on. A captive power cord and fuse holders are on one side while the other has a pair of Cardas RCA input jacks and a single pair of Cardas five-way binding posts per channel. Since the circuit board is laid out east/west rather than the typical north/south, heat-sink fins are located on only one side of the amp.

Sound

Now that we’ve covered history and specs, we can get to the heart of the issue — namely, how the MF2500 sounds, and for that I’m going to steal a line from Michael Fremer. In the July ‘99 issue of Stereophile (coincidentally, in a review of a C-J product), Fremer commented that great products have “soul,” and that the soul they have comes from the soul of their designer. Well, the MF2500 has a large measure of the soul that Bill and Lew have given all their products. In short, it sounds like a modern C-J design.

Beginning tonally and at the bottom, the MF2500 has superb bass extension with very good definition. A disc I play a lot (for the music thereon, but also to test the bass) is Roy Hargrove’s Habana [Verve 314 537 563-2]. The opening track, “O My Seh Yeh,” uses a bass drum and bass guitar to set the heartbeat that drives the tune. With the C-J MF2500 taking the place of my reference Blue Circle BC6 amp, the bass had better depth and a bit more slam, which gave this track greater presence as well as better definition of the recording location. But it was also just a slight amount less developed and less textured. Considering that my in-house reference for bass tonal development is the BC6, and that the MF2500 had better depth and slam, the C-J acquitted it self quite well.

Associated EquipmentLoudspeakers – Dunlavy SC-III, Merlin VSM-SE, Totem Arro. Amplifiers – Assemblage ST-40, Blue Circle BC6, Warner Imaging VTE-201S. Preamplifier – BAT VK-3i. Digital – JVC 1050 CD player used as transport, Dodson DA-217 Mk II DAC. Analog – Rega Planar 25 turntable, Rega RB600 tonearm, Dynavector Karat 17D2 Mk II cartridge. Interconnects and speaker cables – Audio Magic Sorcerer, Cardas Golden Cross, Cardas Neutral Reference. Accessories – Golden Sound DH Cones; VansEvers Reference 85 power conditioner; Audio Magic and VansEvers power cords; SoundRack Reference stand.

The mids of MF2500, as you would suppose, were well served. On the Hargrove disc, the guitar parts (courtesy of the under-appreciated Russell Malone), add tonal depth and richness to the session. As with any recorded guitar, too much emphasis on the snap of the strings, and the tonal richness is lost, while too much emphasis on the middle of the note, and the recording loses pace and clarity, a factor of special import in this New York-cum-Havana session. The story here is that the C-J gets this balance just right, which makes all music, not just this disc, a very natural-sounding and moving event.

Treble, out of the C-J mold, is well rendered, if stopping just the slightest bit short of the absolute top. The good news here is that this both avoids the typical solid-state nasties while having a near insignificant effect on detail retrieval. Jimmy Cobb’s brush work on Kind of Blue, in either vinyl or polycarb form [Classic Records CS-8153 and Columbia CK 64935], is absolutely essential to the feel of the session. With the MF2500 in the system, Cobb’s sense of floating beyond both time and emotion while offering comments on each is of the first rank. Still, on other recordings, such as Takemitsu’s From Me Flows What You Call Time [BBC Music Vol. II, No. 11], specifically where the bells and percussion cut through the orchestra, the tone of the highest frequencies was accentuated and just the slightest bit of the high overtones were down in level.

Dynamic contrasts, on both the micro and macro level, were very well rendered. On the new DCC reissue of Nat “King” Cole’s The Greatest Hits [DCC GZS-1127], “Orange Colored Sky” opens with soft piano, bass and voice orchestration. About 15 seconds in, the background singers and brass open up, and Ella, the Rat Dog, nearly jumps out of her skin. Using the C-J, this sudden jump was, as far as I can tell, 100% accurate — no compression, no shrinking of the stage, no smearing or squashing, just a big bang worthy of universe creation. As for the micro level, the sense of breath you hear, whether on a Margo Timmins vocal, a Hank Mobley sax solo, or a quiet Debussy piano passage, was not quite up to the same level of the Blue Circle BC6, but certainly in the same general class.

For me, the defining characteristic of the MF2500 was its ability to create a stage, another traditional C-J strength. While I’m not an imaging freak — I can and do enjoy music in mono, on headphones, in the car and from a boombox — I also enjoy the illusion of music being made by real people in real space, and this amp has the magic skill of creating images like nothing I’ve heard before. In fact, the MF2500 does spatial tricks that I thought were only the province of speakers. It’s not just a right-to-left thing alone, although this attribute is remarkable as images project at least three feet beyond the speakers. Nor is it the depth, which is almost as incredible. Nor is it even the way width and depth combine to give the smoothest, deepest and best-defined soundstage I’ve heard in my house, a soundstage that remains stable in the back corners and even under heavy loads. But it is also the way the speakers completely disappear as identifiable sound sources and become nothing more than tall furniture. Does this increase the musical experience? Well, it sure helps cement the illusion of a real space, and that does increase the musical reality, at least for me.

Clarity, a small stumbling block of C-J solid state amps in the past, was improved as compared to my recollections of past models. However, the MF2500, while very good, is still is not quite in the absolute first rank in this area. The stage it threw was world class, as I said, the best I’ve heard, with corners that remain stable regardless of load. But images in those deep corners lacked a bit of texture as compared to the best amps I had through here. To clarify at bit, I’m not calling the MF2500 muddy; it isn’t. Nor is it grainy, gray, golden-toned or cloudy. But the sense of openness, of having an unobstructed view of the back row that class-A solid-state or OTL amps offer is missing. Of course, to get this means putting up with class-A or OTL issues, and this C-J is about as issue-free an amp as I know. Seemingly, the MF2500 has no current issues, no heat issues, no compatibility issues. Just hook ‘er up and let ‘er rip.

Compare and conclude

After several months of listening to the MF2500, I brought out my Blue Circle BC6, a 25Wpc, single-ended, class-A, solid-stater for a more complete comparison. Putting the Blue Circle amp in helped to highlight the advantages of the MF2500’s 240 watts. While not a dynamic weakling, placed up against the MF2500, the BC6 was just not able to create the WHOMP that the C-J could. It also re-created a smaller soundstage than that of the MF2500. On the other hand, the BC6 accentuated the emotional subtext of music in a more involving and, in some ways, a more accurate manner than the MF2500. Also, while the stage was smaller, each player had a greater density with a fuller harmonic structure to their presence that more closely mimics reality. And, in spite of the denser harmonic presentation of the BC6, it also offers that unobstructed view I was just referring to.

But comparisons only go so far. Taken on its own, the Conrad-Johnson MF2500 is wonderful amp. It has tremendous power reserves, which it uses to superb musical effect. Dynamic contracts are fully and properly scaled. Staging is not only otherworldly, it maintains that dramatic spread and depth even when pushed to neighbor-annoyance levels. And, out of the classic C-J mold, it offers a grainless, smooth but not overly rounded view. It does slightly lean out harmonics, more so in the lower registers than in the all-important mids, but the shift is small enough that system matching can easily compensate. For example, with the Totem Arro, a slightly rich-sounding speaker, the harmonics took on a more even-handed balance.

But even taking these minor characteristics into account, the new Connie-J amp is a formidable product. The MF2500 gets a recommendation, especially for systems where loads of power are needed.