October 2008

Atma-Sphere MP-3 Mk III Preamplifier

by Vade Forrester

If this is the autumn of two-channel audio, it’s especially ironic that more stereo preamps than ever compete for the hard-earned bucks of listeners. They span known technologies, from gainless passive units to models using solid-state devices, tube technology, or a mixture of both to provide gain. Features range from very basic -- just a manual volume control and perhaps an input selector -- to quite elaborate remote controls. Circuit topology can be either balanced or unbalanced. Preamp chassis may be constructed of steel, aluminum, acrylic, wood, or anything else a designer favors. Volume controls can be simple potentiometers or stepped discrete resistor switches, either manual or remote controlled.

So what are poor listeners to do? How can we possibly choose from the wide variety of preamps available today? There’s no easy answer. Once upon a time, I would have advised you to select a few preamps that matched your desired feature set and budget and then visit your local audio specialist for advice and to borrow some loaner units to try in your system. However, with brick-and-mortar audio stores becoming scarce, if (hopefully) not extinct, that advice may not be feasible unless you live in a major metropolitan area.

Atma-Sphere’s proprietor Ralph Karsten is no stranger to the cyber pages of SoundStage!, having received an almost monotonously long string of Reviewers’ Choice accolades for both amps and preamps, including one from me for the S-30 Mk III amplifier, which still graces my system. Over the past year or so, Ralph has been updating his product line to Mk III versions, and the last of the updates is the MP-3 Mk III preamp, the budget model in Atma-Sphere’s line. It sells for $4550 USD in line-stage form or $5150 with an internal phono stage. Those aren’t exorbitant amounts; Atma-Sphere’s top-of-the-line MP-1 Mk III, with its external power supply, sells for $10,950 without a phono stage and $12,100 with.

The MP-3 Mk III is like a Model T Ford: black, sturdy, and solid. It's a bit of a hair-shirt design, with a balanced differential circuit and, except for two tape loops, only balanced XLR input and output jacks. And there’s no remote control, even as an option. The MP-3 Mk III is the proverbial purist design, which eschews any features that don’t contribute to its basic job of amplifying line and phono sources.

Measuring 17" wide (the front panel is 19" wide) by 13" deep, including knobs and connectors, by 6" high and weighing in at a solid 20 pounds, the MP-3 Mk III is finished in a black crackle paint, which covers everything except the front handles, even the unusually large knob on the 23-position hand-wired volume control. There are two other knobs in addition to the volume control: left- and right-channel gain-trim controls, which serve as balance controls and allow adjustment of overall gain. They are not in the signal path. All other functions are handled by toggle switches. On the left side of the front panel is a switch labeled Main/Aux. If the MP-3 Mk III has a built-in phono stage (as the test unit did), that’s what this input will be; otherwise it’s another line-level input. Moving to the right we find another toggle labeled Aux 1/Aux 2. This lets you switch between two line-level inputs. When the left-most switch is in the Aux position, the input is determined by the Aux 1/Aux 2 switch. Two tape loops use unbalanced inputs and outputs, so if you have an unbalanced source like a tuner, you could connect it to one of the tape-loop inputs.

Continuing to traverse the front panel from left to right, we next come to two toggle switches for the two tape loops, switching between source and tape output. Next is a phase switch, which toggles the absolute phase of the entire preamp. Except for the power switch at the right end of the front panel, located almost under one of the front-panel handles, that’s the switch complement. In the center of the front panel are three red lights mounted in a vertical line. Two are DC offset indicators, one for each channel, which indicate if the preamp is producing DC output, which is not good. In normal use, these may flash on and off during warm-up, then go off afterwards. If one of the lights stays on, there’s a problem with the preamp. The bottom-most light is a pilot light, which stays on while the unit is powered up.

On the rear panel, again starting on the left side, we find an IEC connector for the power cord. Atma-Sphere shipped a hefty-looking cord with the MP-3 Mk III, which I used for my testing, but the manual encourages the use of any aftermarket cord you favor. Moving to the right, we see two XLR jacks that comprise the preamp’s single pair of output jacks. A second set of output jacks is optional at $75 -- a good investment if you think you might want to drive a subwoofer as well as the main amplifier.

Continuing our journey across the rear panel, we next come to the four RCA jacks used for the tape-monitor connections. You may wonder, as I did, why there are two tape loops. The reason is that the Tape 2 output is inverted while Tape 1 is not. Then we find three pairs of XLR jacks, which are the preamp’s inputs. If there’s a phono section, the right-most jacks will be used for it. That means your tonearm cable must be terminated with XLR plugs. Operationally, that’s not a problem; moving-coil cartridges are inherently balanced. There’s no ground-wire connection like you find on most preamps, either -- the ground connection is provided via the shield in the balanced cables, while the positive and negative conductors are inside the shield, protected from electronic nasties.

Above the phono input jacks is a screw-terminal strip that lets you connect loading resistors for the phono cartridge. The default input impedance is 47k ohms, but if you screw resistors onto the terminal strip, they will replace the standard load. I like that arrangement; it lets you try different brand resistors, which have different sonic characteristics.

Underneath the MP-3 Mk III, three rubber feet support the sturdy chassis -- two in front and one in the rear. The use of three feet means the chassis won’t wobble.

Inside the chassis, there are a lot of tubes: six 12AT7s for the phono section and another used as a regulator, along with two 12AU7s and two 6SN7s for the line section. All of these tubes are in current production and readily available. However, if you like to try different brands of tubes to tailor the sound of a component, the instruction sheet sternly warns you not to try new-old-stock (NOS) 12AT7 tubes, claiming they are usually too noisy.

The review sample had optional V-Cap Teflon capacitors installed, a $1500 option that Karsten tells me is ordered on virtually every unit built. I can personally attest to the effectiveness of V-Caps; they provide an amazing amount of detail without any sonic drawback. You can get the V-Caps in the line stage only, or in the line stage and phono stage, which was the configuration I received. So the cost of the review unit was $6650 -- not cheap, but it does include a phono stage.

Speaking of the phono stage, it’s really nice to have this included in the same chassis as the line stage. For those whose equipment racks are chock-full of gear, not having to devote a shelf to a separate phono preamp is a definite benefit. The phono stage’s gain is 50dB, and if that seems a little low for use with a moving-coil cartridge, keep in mind that the MP-3 Mk III is specified to work with cartridges that have as low as a 0.24mV output. For cartridges with outputs lower than 0.3mV, an optional $750 step-up transformer is available.

The line section’s gain is 15dB, and its input impedance is 500k ohms unbalanced or 1000k ohms balanced, both much higher than standard loads. I can’t think of any source equipment that won’t comfortably work with the MP-3 Mk III. This preamp will drive power amplifier loads as low as 600 ohms.

Setup

Like any tubed component, the MP-3 Mk III needs ventilation, so I placed it on a shelf on my equipment rack with several inches clearance between the top of the preamp and the shelf above it. I tried several cables and found the latest Clarity Cables interconnects sounded particularly open and extended. Atma-Sphere was kind enough to make a balanced tonearm cable for me to use during the review.

Although the manual is very helpful and informative, it has no drawings or pictures, a strange omission for a unit with so many tubes. While experienced users should have no trouble replacing tubes, inexperienced users could conceivably mix up tube locations. To get inside the chassis to replace tubes, you must remove 18 (!) small machine screws. I twice had to replace noisy 12 AT7s and wished for easier access to the inside of the chassis. The preamp is shipped with the tubes installed in their sockets. Sometimes that can be a problem, but the tube sockets used for the MP-3 Mk III grip the tube pins so tightly that I doubt there’s any chance a tube could accidentally come out.

I burned in the MP-3 Mk III's line stage for around 200 hours and then burned in the phono stage a like amount of time using a KAB burn-in box to reduce the signal from a CD to MC levels. I used alligator clips connected to the cartridge clips on my tonearm to route the burn-in signal through the tonearm cable and then into the MP-3 Mk III’s phono section, assuring that everything in the circuit was burned in. For the review, I used 562-ohm Dale resistors, which I had on hand. I’d try other values and other brands if the preamp were around for a longer period. My cartridge’s 0.65mV output was well above the MP-3 Mk III’s recommended minimum level.

The MP-3 Mk III requires at least 30 seconds to stabilize after it’s turned on. During that period, the DC offset lights blink on and off, indicating the presence of DC at the outputs. The user is sternly warned not to turn on the power amp until the MP-3 Mk III has stabilized and the light show has died down. That’s good advice for any preamp, but critically important with the MP-3 Mk III.

I began listening with the power cord that came with the MP-3 Mk III, but when I substituted a Purist Audio Design Venustas power cord for the one that came with the Atma-Sphere preamp, I noted improvements across the board. There was more powerful bass, greater openness and spaciousness, and even truer tonality. I hesitate to say the Venustas power cord transformed the MP-3 Mk III’s sound, but it made it perceptibly better. For me, it would be well worth its $1000 cost.

The sound

The MP-3 Mk III had a scintillating, sparkling sound that delightfully complemented the MaxxHorn Lumination speakers I reviewed last month and have become my new reference speakers. Instrumental tonality was depicted with great complexity and detail, and it sounded ravishing. Chris Jones’s acoustic guitar on the CD Roadhouses and Automobiles (Stockfisch SFR 357.6027.2) sounded as realistic (and achingly beautiful) as I’ve ever heard a guitar recording sound. When I’ve played Jones’s solo-guitar piece "The Last Fallen Leaf" for visitors, there was often stunned silence after it was over, so gorgeous and touching was the performance.

On the low end of the spectrum, bass was agile and dynamic, but it lacked a smidgen of weight. On "No Sanctuary Here" from Roadhouses and Automobiles I missed some of the depth and growl of the electric bass that underpin this piece so effectively. Don’t get me wrong -- those qualities were present and depicted with unusual detail. It’s just that they lacked the last iota of weight. The bass drum on Jordi Savall and colleagues' performance of "Folia: Rodrigo Martinez" on the La Folia CD (AliaVox AV 9805) was deep and detailed, but it lacked a bit of its accustomed impact.

Moving to the other end of the audio spectrum, the high frequencies were quite extended. Argento’s "For the Angel, Israfel" on Reference Recordings’ CD Casa Guidi (Reference Recordings RR–100) begins with solo orchestral chimes that rang out with great purity and definition. The measured decay as the vibrations in the metal chimes slowly damped out to inaudibility was particularly well portrayed.

Extended high-frequency response often goes hand in hand with excellent soundstaging, and the MP-3 Mk III was no exception to this rule. I played A Sei Voci’s performance of Allegri’s "Miserere" on the eponymously named CD [Naïve E8909], and the spatial image was breathtaking. Each voice rang out with amazing purity from its own well-defined space, and the realism of the soundstage was quite vivid. Soprano Ruth Holton sang much of the baroque ornamentation unique to this version of the piece, and it was a simple matter to hear how her voice projected those difficult sections from her position in the body of the group. She remained firmly located in the group, but when she called on her vocal resources, her power almost equaled that of the rest of the group.

Anytime I describe a component as having "scintillating, sparkling sound," I expect it will have excellent dynamics. To assess that, I turned to Eric Mongrain’s Fates CD [Prophase Music MVDA4585], a master class for guitar players. Mongrain uses his acoustic guitar as much as a percussion instrument as for classic melody/harmony production. On the title cut, the MP-3 Mk III defined both microdynamic and macrodynamic variations in volume with delicate precision, making them trivially easy to follow. The former were illustrated by the continuous small changes in volume level as Mongrain developed the theme of the piece, while the latter were demonstrated as he whacked the body of the guitar for percussive effect. Both were exceptionally realistic. I wonder how long one of his guitars lasts under such abuse!

Very dynamic and detailed, the phono section exhibited noteworthy tonal purity and accuracy. On a recent foray to Half Price Books, I scored a 93¢ near-mint copy of Sheffield Labs’ direct-to-disk LP of Harry James’s The King James Version [Sheffield Lab-3]. I don’t think the state of the recording art has advanced much since Sheffield Labs direct-to-disk recordings were made, so renewing my acquaintance with James’s band was lots of fun, and sure enough the music still sounded eerily realistic. Having played in stage bands, I can attest that the MP-3 Mk III rendered this recording’s instrumental timbres in spot-on fashion.

To check how the MP-3 Mk III’s phono stage did vocals, I pulled out Eva Cassidy’s Songbird [S&P Records S&P-501]. "Fields of Gold" emerged with a smooth, detailed sound that was as engaging as I’ve heard this piece sound. Each nuance of Cassidy’s phrasing was unusually clear, and the phrasing was just spectacular. If you like girl-and-guitar recordings, you need to hear the MP-3 Mk III.

Unfortunately, I found the phono section’s noise level to be a bit high. Although noise didn’t intrude on the music, tube rush was audible -- barely -- at my listening position between selections or when the stylus was lifted off the record. I tried different tubes, but I still didn’t achieve the level of silence I’ve come to take for granted. Maybe I’m overly sensitive to such noise, but I think any noise I hear from the phono section is too much. LPs are noisy enough without any extra noise from the phono section. Even if the noise isn’t itself audible during even the quietest musical passages, it’s bound to obscure some of the finer details of recordings.

Comparison

I compared the MP-3 Mk III to my $5995 Audio Research LS26 and $2495 PH5 phono stage. The differences between the Audio Research gear and the MP-3 Mk III can be separated into two categories: sonics and convenience. The hybrid-circuit LS26 had an equally rich midrange, perhaps a little less high-frequency sparkle, and a bit more bass weight. That’s ironic, because when I reviewed the LS26, the only fault I found was a lack of bass weight. Then I changed the tubes, and suddenly that fault went away -- big time. The LS26’s openness and spaciousness equaled those of the MP-3 Mk III, which is to say that they were notable.

The PH5 phono stage is very quiet, which makes it sound more transparent. No noise is audible between selections or when the tonearm is sitting on its rest. Once you experience that sort of performance, it’s hard to accept less. Both the LS26 and PH5 chassis were just as difficult to open to replace tubes as the MP-3 Mk III's, but the former uses only two 6H30s, while the latter uses four 6922s. Retubing the Audio Research gear will be less stressful to your bank account.

On the convenience front, there’s no contest -- both ARC units have state-of-the-art remote controls that operate every feature. I can even switch phono loading remotely, albeit only to preselected resistances. But if convenience is a factor, I doubt that you’ve made it this far in the review. On the other hand, the MP-3 Mk III’s phono loading gives you lots more flexibility in choosing resistors.

The PH5 uses only unbalanced RCA connections, while the LS26 provides both balanced XLR and unbalanced RCA connections. And while LS26’s fully balanced circuit sounds better used that way, its unbalanced performance is quite close in quality. My favorite combination with the MaxxHorn speakers is the LS26 driving the unbalanced Art Audio PX-25 SET amp, a glorious match as long as you don’t need more than 6Wpc of power. This amp's expansive soundstage lights up like it’s illuminated from within, to borrow a well-worn audio cliché. And its bass is quite impressive, at least with the 103dB-sensitive MaxxHorn speakers.

Bottom line

There’s a lot to like about the Atma-Sphere MP-3 Mk III: flexible controls, rugged all-in-one chassis construction, easy-to-change phono loading, and, most of all, excellent sound. The slight lack of bass weight I noted could have been system dependent, and adding an accessory power cord as recommended in the manual made a noticeable improvement in the bass and elsewhere. I recognize that throwing a different power cord into the mix adds another product to the value equation, but it made a noticeable, if not earth-shattering, improvement in the sound.

The MP-3 Mk III’s phono stage is quite a bargain at its basic $600 price, and still a good value with $1000 of optional V-Caps added. It sounded quite lovely; and while I’d prefer a bit lower noise level, it might be well within your tolerance level.

Exciting, luminous sound with an expansive soundstage and spot-on instrumental timbres -- if those characteristics appeal to you as much as they did to me, you should put the Atma-Sphere MP-3 Mk III preamplifier on your must-hear list.

...Vade Forrester
vade@soundstage.com

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