Musical Fidelity AMS Primo line preamplifier

Musical Fidelity's Tri-Vista kWP, introduced in 2003, was an impressive, high-tech, "statement" audiophile preamplifier. Its outboard power supply weighed almost 56 lbs—more than most power amplifiers—and its hybrid circuitry included miniature military-grade vacuum tubes. As I said in my review of it in the January 2004 Stereophile, the kWP's chassis and innards were overbuilt, the measured performance impressive, and any sonic signature imposed on the signal was subtle and, essentially, inconsequential.

But while the kWP's fit and finish were jewel-like, its appearance was cartoonish. The front panel was dominated by two large knobs about as subtle in their susceptibility to alternate interpretation as the butter nipples topping the pancakes in IHOP's TV ads. The gargantuan remote control, apparently milled from a solid block of army tank, made every visitor to my listening room laugh. Most of us, when we buy pricey audio gear, aren't going for laughs. In fact, Musical Fidelity's Tri-Vista kWP preamp and kW monoblock amplifiers was the homeliest bunch of expensive audio components I've ever owned.

I loved how they sounded, especially together. That was good enough for me. But I never understood how someone with Musical Fidelity chief Antony Michaelson's finely honed aesthetic sensibilities could produce such ungraceful-looking kit, or why, with so many years in the business, his company had yet to develop a unified look for its premier products.

That all changed with the introduction of the Titan power amplifier, which I reviewed in June 2009. But while its attractive art-deco looks are unmistakable, they've so far had little effect on the design of other Musical Fidelity products. Rather, the company's sleek new AMS Primo preamplifier ($10,999), launched at about the same time as the Titan, introduced a new, understatedly elegant look that's now found throughout MF's AMS and M6 series of components.

Built in the UK
The AMS Primo is a fully balanced, class-A preamplifier with 14 dual-triode tubes (ECC81/12AT7). It employs zero global feedback and weighs 37 lbs. An internal enclosure of mu-metal shields the signal-carrying circuits from the two onboard power supplies, one each for the signal path and the control functions. The power supply rectification is dual-mono and solid-state. Each channel has its own fully regulated high-voltage and heater circuits.

Why so many tubes? According to Antony Michaelson, because the Primo is fully balanced in operation from input to output, it includes four mono amplifiers attenuated via a four-gang ALPS volume control. Low-current, small-signal tubes don't drive loads effectively, becoming variably nonlinear when presented with an ever-changing load such as a volume pot. When you change the volume, you change the tubes' operating conditions, hence their performance. Paralleling the tubes gives you twice the current capability in each stage, as well as lower the coupling impedance between them, both of which produce greater linearity, "average out" tube performance, and better deal with tube aging. Michaelson warns against tube rolling with the Primo: the linear performance of its zero-global-feedback design depends on carefully matched sets of tubes.

Peer through one of the two mesh-covered vents on the top plate and all you'll see will be the crowns of the tubes—seven per channel—isolated by a secondary enclosure that runs from just behind the large, central volume control all the way to the rear plate, on which are mounted the various high-quality input and output jacks and other connectors. All circuit boards and associated wiring are hidden beneath the internal subchassis, making for an extremely neat and orderly look that complements the Primo's exterior.

The tidy and understated front panel is dominated by that big, motorized volume knob at the center. There are also two rows of small pushbuttons, each surmounted by a small blue LED. The five to the right of the volume knob select among the five inputs: CD, Tuner, Aux 1, Aux 2, and Tape. The three to the left are a single Power/Standby/Mute button, as well as ones for Gain and Tape Monitor. Gain is used to program the individual inputs to compensate for the 6dB difference in gain between the balanced and single-ended inputs. The panel's simplicity and ease of use are welcome, but the lack of an LED on the volume knob makes it difficult to ascertain the setting from a distance or in the dark.

Musical Fidelity's inclusion of a tape loop is an interesting choice in 2010. Few audiophiles today use tape recorders, and fewer still have decks that include separate record and playback heads, but those who do will welcome this loop. The Tape input, of course, can be used for any source, as can the other four inputs.

The rear panel is equally orderly. Each of the five inputs has both high-quality single-ended RCA and balanced XLR inputs, selectable via a slider switch. (The Tape Out jacks are single-ended only.) Trigger in and out jacks allow the Primo to be turned on via remote trigger, and to turn on other remote-triggered products, including MF's Titan and AMS power amps. The Primo is built in the UK.

Included is an equally sleek-looking, ergonomically pleasing remote control machined from a solid billet of aluminum—large, but not tank-like. While not backlit, the remote's 10 buttons are logically placed on its spacious surface, and I found their functions easy to memorize. In addition to repeating the front panel's Input, Gain, and Power controls, there are also Volume Up and Down and Mute buttons.

Setup and Use
With the exception of the Gain button, setting up the AMS Primo was straightforward. Just plug in your sources, select balanced or single-ended input for each, run single-ended or balanced cables out, plug in the power cord, turn on, and play. To set a single-ended input for additional gain, just select that input and push Gain. The Primo will then remember to raise the gain each time you select that particular single-ended input, even if you lose power. I drove the Titan in balanced mode and ran a combination of balanced and single-ended sources to feed the Primo, which performed without a glitch for the many months it was in and out of the system.

The eternal question
Anyone who's vacillated between tubes and transistors in his or her long and sometimes confusing audiophile journey can't blame a manufacturer for doing likewise. Antony Michaelson began with tubes—1977's highly acclaimed Michaelson & Austin TVA-1 power amp is now a classic. Later he switched to solid-state exclusively for Musical Fidelity's premium products, and then to hybrid designs using military-grade, low-noise, high-tech, long-lived mini-tubes, hardwiring into his circuit such tubes as the metal-cased nuvistor triode and the glass-cased 5703WB. Tubes were also used in MF's X series. Musical Fidelity claims to have made and sold more electronic components employing small-signal tubes than anyone else in the past 30 years.

The Primo's published specs boast a very low THD+noise of 0.005% from 20Hz to 20kHz, a signal/noise ratio greater than 105dB, and a relatively wideband frequency response of 5Hz–50kHz, –0.5dB. The claimed maximum output voltage of 32V (!) and the 30dB overload margin are impressive. In fact, many of the Primo's specs read like those of a good solid-state design.

Between the Primo's high output and the volume control's taper, I never had to turn the knob above 11 o'clock—even when using a single-ended input without additional gain. Beyond that, it was too loud. But with no signal, turning the pot almost all the way up produced through my speakers nothing but the faintest suggestion of hiss. "Tube rush" was nonexistent.

After breaking in and warming up the Primo with the Sooloos music server set to Swim (ie, Shuffle) mode, the first LP up was a test pressing of a vinyl reissue of Donald Johanos and the Dallas Symphony's justifiably famous recording of Rachmaninoff's Symphonic Dances, engineered by David B. Hancock in 1967. The entire series of Johanos/DSO recordings are audiophile spectaculars recorded with four custom ribbon microphones at 30ips in Southern Methodist University's McFarlin Auditorium. The original LP edition, on Vox Turnabout, was made from the master tapes, and David B. Hancock's initials are inscribed in the dead wax. Unfortunately, Vox, a budget label, pressed on poor-quality vinyl; quiet copies were hard to find. Still, if you see any in the $1 bins or at your local Goodwill, don't pass them up!

The new reissue of Symphonic Dances (2 45rpm LPs, Vox/Turnabout/Analogue Productions) beats the original in every way. It's as warm and harmonically complex, but sounds more open and less boxy, and dynamically it's in a different league entirely. Through the AMS Primo the imaging and soundstaging were ultra-expansive, the harmonics richly and vividly drawn, the dynamics thunderous. Most significant, the bottom-end extension and control, particularly in the big timpani wallops, gave up nothing obvious to solid-state sound, nor did the Primo's top-end extension sound truncated. Mostly, the midrange plumped up, but by a subtle amount that never caused bloat, or produced ballooning images like those in a Macy's Thanksgiving Day Parade. At no time did I think, Yes, this is really pleasing, but I wish I had more weight and/or definition on bottom, or better top-end extension or transient attack. The Primo was fast and extended on top, and well defined and pleasingly punchy on bottom, with enough weight to anchor the low-frequency end of every kind of music.

Going from the twice-as-expensive darTZeel NHB-18NS solid-state preamplifier's precise, some might say clinical, control and finely drawn images to the Primo's harmonically vivid, exuberantly drawn sound pictures was jarring and invigorating. The Primo elicited from me an immediate and enormous "Wow!" that I still felt even after weeks of listening. No solid-state device in my experience can produce the sense of effortless musical flow that all-tube designs—even poorly designed, tonally colored ones—deliver with ease.

Yet, clearly, the Primo's overall sound was altogether different from that of the darTZeel NHB-18NS: wetter, stickier, slappier (not sloppier), and altogether more generous. But unlike with some other tube preamps, with the Primo I never had a sense of darkness creating a desire for more light, or a lessening of bottom-end resolve creating a desire for more intense wallop.

Musical Fidelity Ltd.
US distributor: Tempo Sales & Marketing
PO Box 541443
Waltham, MA 02454
(617) 314-9296