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Moscode 402Au Amplifier Review

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Moscode 402Au Amplifier

I recall being in a discussion years ago with a long time audio manufacturer who mentioned a person nicknamed “Gizmo”. I blanked, “Uh…,” while trying to cover my profound ignorance. Harvey “Gizmo” Rosenberg, the colorful owner of New York Audio Labs, made his mark as a manufacturer of Output Transformerless (OTL) audiophile amps. It was only after researching his name that I heard of his antics including delivering amps from NYAL to customers along with diapers! I do recall from long term memory having perused an audio magazine and seeing a picture of an enigmatic character wearing a kilt and tube headdress. At the time I had dismissed Harvey’s colorful antics as the actions of an audio show weirdo. That wasn’t the most astute conclusion to draw in regards to the man who helped Julius Futterman preserve the OTL design. I’m working hard hereto make up for it.

It is a good thing that personalities in audio are tied to recordings and equipment, so that those who discover new gear may also absorb some of the culture and history of audiophilia. Such it was that I was led to a greater appreciation of the electrical and “theatrical” aspects of amplifier design when I arranged for a review of the Moscode 402Au amplifier. In truth, it was not the journalistically opportunistic antics of “Dr. Gizmo”, the history of OTLs or the tenure of Moscode’s amp production that I held foremost in my mind as I emerged from a particular demonstration at CES 2009 – it was the sound of the amplifier. The Moscode 402Au struck me as one of the better sounding amps at the show, and thus became the impetus for this article.

The commemorative amplifier dedicated to Harvey Rosenberg was the 401HR. It’s rare that an amp memorializes an individual, but George Kaye has made sure via the 401HR that Harvey Rosenberg will not be forgotten. It seems Harvey pulled Futterman’s disheveled notes and ever changing models of amps together to preserve his work on the OTL. The amps had been made literally one at a time by Julius, never realizing a final form for production. Harvey, acquiring the rights to the OTL circuit, brought that to the equation after Julius’ death.

Harvey was keen on taking the OTL design one step further, by using MOSFETS in the output stage. A MOSFET, short for “metal oxide semiconductor field effect transistor”, is used to amplify or switch signals. George Kaye originated the Moscode’s tube+MOSFET concept, and he took it to Harvey of NYAL and took on the title as the NYAL chief engineer while further developing the concept. In other words, George and Gizmo took what were the strengths of the OTL and mated them to the strengths of a solid-state design. Thus born Moscode, a design utilizing triode tubes in coscode arrangement to control the signal being utilized by the MOSFET, resulting in a hybrid (“MOS” + “code”).

Ok, so it’s a hybrid amp – so what? One must remember that this was pretty radical thinking nearly 25 years ago in 1984! We don’t even look twice at hybrid cars, as they have become so common. A hybrid amp was anything but ordinary back then; it’s had incredible longevity and sonic merit to be implemented a quarter century later!

George is adamant about the fact that the Moscode 402Au is not just a tube gain stage slapped onto a solid-state power amplifier; rather, it is a tube amplifier with a MOSFET output stage. It’s obvious from the depth of the discussion in the Owner’s Manual that he wants people to appreciate the thoroughness of the design of the amp, not just its sound. For the three pages which comprise the discussion of the circuit the manual uses technical nomenclature, extolling the amp’s topology. Discussed is the fact that the tube driver stage and the MOSFET output stage are complimentary as both are high impedance devices. Further, the use of MOSFETs eliminate the need for Output Transformers, which George insists add unnecessary inductance and capacitance, “…and that,” states George, “interferes with the transfer function of the output transformer at high frequencies. They exhibit phase shift within the limits of the audio band.” This hinders them from revealing the maximum musical detail. MOSFETs, while not perfect, have power-related (harmonic distortion) as opposed to frequency response errors. George uses feedback to control for THD. His design rule, his “Rule of Ten”, is to, “Make the circuit work well to 10 times the highest or lowest frequencies it will handle…” A little overkill is a good thing!


Case in point, the “Au” in the name stands the use of gold plated, three-ounce circuit boards. These improve the musicality of the amplifier supposedly. The redesigned circuit layout offers advantages over the HR version (for Harvey Rosenberg) in the areas of lower noise pickup, clearer highs, tighter bass and better low-level detail.

All this extra attention to build quality reminds me of the garage on my first house, the ridge beam of which was bowed to the point of cracking. I was disgusted at the flimsy 1”x8” which had been used. Over forty years it couldn’t take the weight of winter snowfalls. My solution was to overkill on the repair, so that it would never budge again. It was recommended that a 2”x8” be used. I said, “Put a 2”x12” in there,” and they did. When the contractor was finished he smiled, “You could put a VW on top of that roof!” Good! Ten years later, whenever I drive by the old house I look at that garage roof line – it’s still straight as an arrow! I think George feels the same way about building amps which will last.

George continues his tutorial in the manual on the topic of developing a robustpower supply. The goal is to simulate infinite power reserves. As a junkie of high-output amplifier, that sounds very good to me! The amps are to act as if normal power line fluctuations do not exist, accomplished by way of voltage regulation. According to the manual, “Voltage regulation eliminates interaction between the driver and power output stages.” Neither stage can influence the other unduly in terms of demand. Consequently, there is no need for power conditioning when using the 402Au.George intones, “A well designed power supply doesn’t need any help.” The amp stores 88,000mF of capacitance per channel, as well as sporting dual-mono supplies for each of the output stages. I like the fact that the heavy-duty power transformers are specified for 50Hz rather than 60Hz, “…to get more iron into the power supply.” Sounds like a discussion of a Harley Davidson; bigger, beefier is often a good thing – in cycles and amps.

The International

I recall a movie trailer by the title, “The International”. It appears to be yet another of those mind-numbingly monotonous chase-them-across-the-globe action films starring yet another cloned, dark haired man of nebulous ethnicity operating as a look-think-shoot-to-kill type protagonist. I found the latest Bond movie, “Quantum of Solace”, to be a tremendous Yawn; insipid plot with action sequences shot so hyper-fastthat it wasn’t realistic at all, but in fact was laughable. I found myself laughing at the lunacy of men swinging on ropes, crashing several floors to the ground, shooting all the while. After seeing that, I would recommend an alternative title for the movie, such as “The Intentional?”

The aesthetics of the Moscode 402Au are international, and quite intentional in appearance. Most prominent is a front brushed aluminum face plate with a smooth cornered rectangular “window” to see the pair of driver stage tubes inside. The stock unit came with a pair of Sovtek 6H30Pi toward the inside and a pair of EH 6GU7 follower tubes outermost. A narrow “floating” glass shield spans the gap, in the center of which is etched an illuminated blue LED “Moscode” logo. This flashes intermittently in standby mode as the unit powers up gently to conserve tubes, and remains lit as the unit clicks into operational mode. The logo’s intensity can be varied by a control at the back of the amp. Note that the face of the amp is never to be rested against the body while moving it, as the thin glass plate can break.

The logo turns red if the amp’s internal sensors detect overheating. Should this happen, the amp remains in stasis until the unit is cool enough, then it restarts. If this happens regularly it’s a good bet that you are not giving the amp enough breathing space; move it so that it can have better ventilation.

Rugged black structural metal casing with large gaps rests over the side mounted heat sinks providing nifty hand holds for hauling the amps around. The entire front face of the amp is hinged at the bottom and drops down (a gentle, evenly applied tug frees it) to allow direct access to the tubes. Now that’s a sweet feature!However, caution must be exercised. While the majority of the time the amps will properly sit on a shelf, in some instances I can imagine them resting atop a preamp or similar component. When I first set them up for burn-in, I had them directly atop the venerable VAC Renaissance Signature Preamplifier MkII, a two-chassis design. The chasses on the VAC pieces feature oversized faceplates. I am so happy that I usually dothings cautiously, deliberately!As I lowered the faceplate of the 402Au I noticed that if lowered to its resting position it its lower edge would have hit the fantastically ornate facade of the VAC pre! I shuddered to think of what the monolithic aluminum would have done to the high gloss face of the VAC had I lowered it quickly! The solution was simple; merely slide the 402Au amps forward about two inches so that the faceplates would not make contact. Once the tubes were replaced, I slid the amps back into place. When setting up gear with unusual features it always pays to think through the process prior to acting.

Compared to the Cambridge Audio Azur 840W solid-state amps on hand the Moscode was Spartan in terms of features. One could hardly have two amps more different in terms of design. The Azur is all about configurability taken to the extreme. The Moscode is the antithesis, with a wide open, sparsely populated backside. Giant lettering marks the Left and Right Edison Price binding posts, the same ones used by Bruce Thigpen for his Eminent Technology speakers. Thank you, George! You understand the audiophile’s need for clarity when assembling a rig!

Another word of caution is in order, one which is clearly stated in the manual. These are solid copper posts, and can be stripped fairly easily if not treated with diligence. Aggressive tightening may harm them; respect the material they are made of, or else you’ll be looking at a repair. These accept bare wire, banana plugs, spades or a combination of the two. I have used a set each of spades and WBT bananas on the review amps. I have no issue with these posts; they work well and the equipment I have used with them has sounded good.

There are two operating modes, Stereo and Vertical Biamp. A toggle switch above the L/R INPUTS determines the mode. The “Stereo” position is center, “Mute” is left, and “Bi-Amp” is right. A twelve-volt remote trigger is supplied. Aside from the 15-Amp IEC and the fuse, that’s it. The 402Au certainly seems from a design standpoint to embody the OTL heritage of simplification for a dedicated task. That is, until you open it up! Then it looks as busy and advanced as any solid-state amp.

Moscode 402Au Amplifier

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