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The Moscode 402Au


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Moscode 402Au
Tube Stereophonic Amplifier

The Moscode 402Au, now with gold plated circuit board, exhibits all the best qualities you’d expect from an audiophile amplifier: deep, tightly controlled bass; open, layered, textured midrange; extended, open, sweet sounding highs, all in a stable, realistic soundstage. As with most tube gear, female vocalists are especially enticing. Thanks to the hybrid design acoustic bass is delivered with an amazing sense of realism, giving dimension to the notes, avoiding one-note-bass evident in many all-tube/transformer coupled designs. The 402Au does well with acoustic or electronic instruments including the wide variety of music that comes with the territory. The 402Au serves up music with a realism that makes you forget you're listening to an audio system.

Try the 402Au in your own system and I know you'll have a hard time falling back to lesser amplifiers.

NOTE: All pictures indentified below & left click open to full-size shots of original 401HR.  To return, simply click the full-size photo.



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Tubes, Transistors and Melding
I’ve always understood the advantages of using the best characteristics of tubes and transistors in one package for the best bang for your audiophile buck. Back in 1984, after manufacturing the Julius Futterman OTL amplifier, I developed the first hybrid tube/MOSFET power amplifier on the market with a tube driver stage and MOSFET power output devices. That was the New York Audio Labs Moscode 300. It led to a series of memorable Moscode products made in conjunction with a man of expanded horizons, the late and missed-by all Harvey Rosenberg.

The 402Au delivers an articulate, life-like, dynamic, musical 200Wpc using a class A tube driver stage coupled to MOSFET power outputs. It’s a low-feedback dual-mono, dual-regulated hybrid design on hard-wired elastomer-mount circuit boards. Moscode quality goes deep; there’s no off-board speaker wiring, and just four wires per channel carry the audio signal. Moscode Maxi Caps are used, and MIT coupling caps, Edison Price Music Posts and Cardas connectors and internal wiring are all standard.



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Tube Driver Stage
One of the reason tubes are best for voltage amplification is they’re natural high-voltage devices that act in a linear fashion. The circuit itself is a simple, 2-stage, class-A triode driver stage. The first stage is open-loop with no negative feedback, just like in many preamplifiers. The output of the first stage, summed with negative feedback from the output, feeds the second driver stage capacitively-coupled to the MOSFET power module. The two-stage design provides the voltage swing needed for the powerful 200W/ channel 402Au.

In the 402Au, each stage of amplification takes its power from a “virtual” independent source created with local MOSFET voltage regulation. A separate high voltage transformer further isolates the driver stage from power line disturbances. We use a passively-regulated ultra-quiet tube filament supply. Passive regulation is quieter than standard 3 terminal filament regulators which allows for a big, airy soundstage and exceptional low level detail.



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MOSFET Output Stage
MOSFETs behave like tubes in the sense that they’re high impedance devices that tubes can drive directly without a second ‘helper’ transistor, so the circuit is simpler. In audio design, less is still and forever more.

The 402’s output is a "follower" circuit, i.e. the output closely follows the input signal. Followers add or amplify current rather than voltage, although there is a slight voltage loss in all follower stages. We compensate with a feedback loop around the driver and power output stages. There’s only one “pole point” in the feedback loop, and that ensures stable operation and eliminates an output coil/resistor network. An optically-coupled floating-bias circuit keeps the MOSFETs properly biased, and two large Solen/Châteauroux film capacitors (Moscode Maxi Caps) bypass the main electrolytics--a whopping 176,000 mF--for a sweeter top end. A servo eliminates DC-offset that’s commonly found in solid-state amps.

Moscode quality goes deep; there’s no off-board speaker wiring, and just four wires per channel carry the audio signal. Moscode Maxi Caps are used, and MIT coupling caps, Edison Price Music Posts and Cardas RCA connectors and internal wiring are all standard.



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Perils of Output Transformers & the Rule of Ten
I’ve always loved tube sound, but I have problems with output transformer tube amplifiers. Transformers need a lot of iron for good, tight bass at higher power levels, and that means more wire, and that means more stray inductance and capacitance that interferes with the transfer function of the output transformer at high frequencies. Transformers exhibit phase shift very close to the outer limits of the audio band. I think most transformers obscure musical detail because of the unavoidable compromises in high power tube output transformers.

Did I mention that you have to send the audio signal through an enormous amount of wire? Hundreds, even thousands of feet of it? Did I mention that magnetic cores also exhibit non-linear behavior like hysteresis and saturation at power extremes that color the sound?

MOSFETs aren’t the perfect solution either, but their errors tend to be power-related issues (harmonic distortion) rather than frequency response or phase shift issues because of stray capacitance and inductance. THD is no big deal, it can be easily controlled with a little feedback.

The Moscode Rule of Thumb is the Rule of Ten. Make the circuit work well to 10 times its highest or lowest frequencies. MOSFETs go down to DC so the low frequency limit of the amp is defined by the capacitive coupling to the output stage, optimized for 1/10 the lowest frequency passed. The high frequency limits of MOSFETs are defined by their inherent capacitance within and around the devices. However the limits are way over 200kHz, or 10 times 20kHz, the top of the audio band.

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Power Supply

You need a robust power supply because large demands in the bass frequencies can create ripples in the power supply of poorly regulated amplifiers. The ripples modulate the audio signal in unfortunate ways, usually resulting in a thick, hazy, unstable soundstage and poorly controlled bass.

The power supply should ideally behave as if it has infinite power reserves. And you do that with voltage regulation. Regulation keeps a constant supply voltage regardless of power line fluctuations (within normal limits) and circuit demands, thus the driver tubes operate in a consistent electrical environment. When your fridge switches on, your speakers will never know about it.

Voltage regulation eliminates interaction between the driver and power output stage. Each stage takes its power from a “virtual” independent source created with local voltage regulation. A separate high voltage transformer isolates the driver stage from any output power demands. When dynamic material makes heavy demands the voltage at each individual stage of regulation is the same as at idle. With unregulated supplies the voltage drops as power demands rise, leading to subsonic pulsing as the supply tracks the varying power demands and line fluctuations. There’s no need for any kind of power conditioning with the 402Au. A well designed power supply doesn’t need any help.

The 402Au output power supplies store a huge amount of energy, 88,000mF of capacitance per channel so it can drive any speaker with ease. There are dual-mono supplies for each of the two output stages, so power demands on one channel won’t affect the other. Dual-mono double-regulated high-voltage DC power supplies yield over 120dB of channel-to-channel power supply isolation.

Aside from star grounding for best ground noise suppression, the 402Au has quiet, heavy-duty power transformers specified for 50Hz rather than 60HZ to get more iron in the power supply. And that’s the build quality we apply to every operating part of the 402Au.




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Other “Hybrid” Topologies
Some amplifiers are solid-state designs with a single stage of tube amplification tacked on before the output stage. To me, that’s the same as using a tube preamp with a solid-state power amp. You’re still listening to a solid-state power amp! While it may warm up the sound, the approach doesn’t take full advantage of tubes’ strongest suit. They’re high-voltage devices that generate the wide voltage swings needed for high power output.




  Build Quality
• Well designed ventilation assures long life
• Oversized internal power module heatsinks
• Chassis covers heatsink -- no rough edges
• 1/2" thick faceplate
o Fine-grain anodized finish in Black or Clear
• 14-gauge steel chassis
• 14-gauge heavy-duty power cord supplied
• Cardas input connectors
• Cardas internal wiring
• Edison Price Music Posts
o O2-free copper heavy-duty 5-way binding posts directly soldered to output – No output wiring
o Minimizes number of solder joints.
• Minimalist audio wiring
o Only 4 wires per channel carry audio signal
o Two ground, two hot
• All audio wiring to circuit boards hard-wired by hand for maximum signal transfer
• Fully shock-mounted circuit board using elastomer mounts
o Absorbs acoustical and resonant energy
o Reduces microphonic effects
• Audio path wiring soldered directly to pc board for better low-level information
• No off-board speaker wiring for better low level detail
• Compact output modules for better low level detail, more stability
• 6 output devices/channel– runs lower impedance speakers, longer life.





Support Circuitry
• Soft start controlled warm-up
o Eliminates dimming house lights
o Eliminates stress on tubes and other components
• Warm up muting
o Tubes warm up and stabilize before output stage is engaged
• Servo eliminates DC offset common in solid-state amps
• Extensive voltage regulation independent of power line fluctuations and noise
• No power conditioners required
• Remote 12 Volt power-on circuit allows remote on/off control



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Tubes and Tuning
The 402Au runs using a wide range of front-end driver tubes. Just flip down the power-interlocked front panel for access. There’s no need to turn off the power as the amplifier will instantly shut off as if you pushed the power switch button. Power will be restored when the panel is flipped up and snapped into place. For those who want to experiment, tune your system’s sound by changing the outer pair of tubes first; they provide the voltage gain and make the most difference in sound. An Auto Filament Switcher detects center-tapped filament tubes like 12AU7s and switches instantly from a 2- to a 3-wire filament circuit. The outside pair of tubes must always have the same filament voltage, as must the inside pair. Having so many tubes choices means you’ll never have worries about finding great tubes for great sound for years to come.

We give our tubes a break with a soft start circuit for a controlled warm-up lasting about 45 seconds. They warm-up and stabilize before the outputs are engaged. This lowers stress on the tubes and other components. A servo eliminates DC-offset commonly found in solid-state amps, and a 12V power-on trigger allows for remote on/off control.




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Break-in & Maintenance
The 402Au takes about 25 to 35 hours to open up, and a good 100 hours on the clock before it can be considered broken-in. Keeping the 402Au tuned up just requires periodic replacement of low-cost small-signal tube pairs of which you can choose from many types. [see tube type list on actual 402Au] All the tubes and their support circuits are shock-mounted with elastomer to minimize microphonic interference so the circuit board will give a little when changing tubes.





402Au List of Features
• Low negative feedback
o Feedback loop in only one stage of amplification - improved high frequency clarity - bass response
o Allows direct coupling to the speaker
o No output inductor needed for stability
o First stage open-loop topology uses no global negative feedback
• Output transformerless
• Optically-coupled floating bias circuit maximizes output stage behavior
• Servo controlled output stage for 0 DC offset
• Dual-mono power supplies for each output channel
o Power demands from one channel leaves the other unaffected
o Yields a stable soundstage and consistent bass
• Dual-mono double-regulated high-voltage DC power supplies
o Over 120 dB of channel-to-channel power supply isolation
• Passively regulated ultra-quiet DC filament supply for amplification stage
o Quiet backgrounds
• Independent filament supply for driver tubes
• Filament supplies auto-sense tubes with 12V center-tap
o Auto switch circuitry
• Mono switch parallels both inputs for easy vertical bi-amping
• MIT coupling and Solen/Châteauroux bypass capacitors are used throughout



Moscode 402Au Specifications:

Power: 200Wpc at 8 Ohms: 200; 4 ohms: 300+, both channels operating
THD at full output: (tube dependent) with 6H30pi for V101 & 201, 0.15% or less at full power.
Noise (inputs shorted): < 1mv
Voltage Gain: (tube dependent) approx. 30-33 db.
Tube Complement:
• Choice of 6H30Pi, 6GU7, 6DJ8, 6922, 6FQ7, 5814, 7730, or 12AU7
o (2 each) 6H30Pi, 6GU7 supplied
• Must be same tube types in “mirrored” positions
o Inner tube pairs must be same type
o Outer pairs must be same type
Power Requirements:
Wattage idle: approx 120W (1 amp); Watts at full output: approx 960 (8 amps)
Available for 120V (USA), 230-240V (European), and 100V (Japan)
Dimensions: Chassis W: 17 1/2", D: 14 1/2", H: 6 1/4"  Faceplate width: 19"
Shipping Weight: Approx 52 lbs
Input: Cardas Gold RCA, unbalanced, 100K input impedance
Output: Edison Price Music Posts Standard
Power: IEC standard connector
Edge-Lit Logo Colors:
• Dimmed: Always on/power is being supplied to the amplifier
• Flashing: Warm-up cycle in progress (~ 45 seconds)
• Red: Overheat condition/amplifier shuts down
• During normal operation logo brightness is user controlled by a dimmer on the rear panel




Homage to Harvey Gizmo Rosenberg
I developed the original Moscode line of amplifiers while working with Harvey Rosenberg, the man behind New York Audio Labs. The great thing about Harvey was his true love of great sound through Gizmology, and he gave everything he had to champion Julius Futterman’s OTL amplifiers, and later the Moscodes. Those who knew him understood the lad was a bit special. He was a prolific writer, promoter and always spoke to the inner soul of those who, at the end of a long day, find joy, comfort and solace in great music -- and great gizmos! These are, after all, sophisticated toys for grown up boys as Harvey used to say. He died too young in July 2001 of a heart attack at 60. I dedicate the Moscode402Au to him.



And dont take our word for it - Try the 402Au in your OWN system
for 331/3 days, totally risk free.  If you dont like it for ANY
reason, we'll pick it up, the return shipping on us!
Yep, we're that confident that you'll love this amplifier.

It's risk free at Moscode!  Call to arrange your audition today! Send email to Moscode!  We look forward to hearing from you!