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a dual-mono purist-designed push-pull masterpiece from Decware



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M o d e l   Z M A






 





See the complete review in TONE AUDIO Magazine No.71






  • 40 watt class-A mono-blocks.
  • Transformer Balanced XLR Inputs & RCA.f
  • Input level control makes it possible to use with no preamp.
  • Ultra cool bias adjustment - no tools and can be done while listening at any time.
  • Large Meters - let you maintain PERFECT tube matching without effort.
  • Exotic parts - cost no object design.
  • Compatible with a wide range of speakers
  • KT66 Ultra-linear output stage is also compatible with other tubes.
  • Dual Mono design
  • Lifetime warranty





One of the things that makes the Zen Mystery Amp unique is it's insane power supplies.
 
A typical tube amplifier this size uses power supply capacitors totaling about 80 micro-farad. 

The Mystery Amp has 4500 micro-farads.  Yes, that's about 50 times more than normal but we did mention that this was a cost no object design and the massive capacitors come with some huge sonic benefits.  Each amplifier's power supply has enough stored energy to start your car and melt all four tires to the garage floor!

In fact, these capacitors are so large that the amplifier sounds as if it's being run on batteries.




Steve Deckert - owner DECWARE High Fidelity Engineering
 





The ZMA comes with your choice of input configuration, knobs and different hardwood base designs and colors.





 



You can select from a Black Figured Base OR a Contemporary Style Base like the walnut one shown above. Different woods can be selected from within the shopping cart. (The Black Figured base is only available in Black)


There are also some knob choices.  From left to right are Ebony, Gold, Black, Cream, Walnut.  You can configure your amp with any of these knobs from within the shopping cart.




 

The Zen Mystery Amp comes standard with a single pair of single-ended RCA inputs.  You may optionally have your amplifier configured with an additional pair of fully transformer balanced XLR inputs from within the shopping cart.



We carefully hand select and match the tubes for these amplifiers. The amplifiers are tested and burned-in with the tubes you get.

These include four Russian Military Grade 10,000 hour 6N23P-EV's with gold grids, four KT66's and two OA3's per amplifier.
  (Note the amplifier is also compatible with 6N1P / 6922 / 6DJ8 / 7DJ8 as well as KT77, EL34, 7027, 5881, 6L6)



The Zen Mystery Amp's bias system makes sure you know if your matched tubes really match, let you force them to match when they don't and keep your tubes from failing prematurely not to mention sounding their best throughout their entire life cycle.





THE ZMA ADJUSTABLE BIAS SYSTEM
  • The Zen Mystery Amp features 4 independent bias supplies and 4 precision current meters, one for each output tube, so that adjusting one has zero effect on the others!
  • The meters give you a real time view of how each tube is doing and how well they match each other.
  • A simple balance control between each pair of meters lets you match the tubes at any time while the amplifier is on.
  • The meters also show you if a tube is going bad and which one - at any power level.
  • A visual power indicator, the meters begin to dance when the amp reaches maximum output.
  • True Analog movements in each meter make it possible to predict tube performance by simply watching the meters behavior during warm up.



The Mystery Amp came directly from our signature Zen TORII mono-blocks.  We wanted something with similar performance, but in a single chassis and at a reduced cost.  By building two mono amps in a single chassis, eliminating the fancy transformer covers, reducing the power a bit, using smaller meters and finally by making balanced XLR inputs optional ~ it was possible to maintain the same cost no object design and internal parts used in our $12000 mono-blocks.

The Mono-blocks are unique in that they have a massive power supply with caps so large it is possible to eliminate the choke and any dropping resistors feeding the output stage.  Unless you see an amp with similar size caps, you can be certain that it has at least one if not both of these current limiting necessities.  This is why the Mono's sound better than almost everything.  Well, on that note, the Mystery Amp explores this zen approach a little further by doubling the capacitor size and eliminating the dropping resistor that feeds the input stage.  Instead a vacuum regulator tube was used to drop the voltage with no heat and 20 times the ripple rejection of the resistor it replaces.  This was a personal goal of mine because it makes this the first tube amp power supply in the world that doesn't use a choke or any current robbing resistors to supply both the input and output stages of the amp.  Consequently there is no heat or parts to fail.  The real impressive part however is the liquidity and speed that results from eliminating these parts.

This is about as close to pure DC power as you'll ever get in a 500 volt amplifier.  As a result, the Mystery Amp will have about 50 times the immunity to dirty power from your wall outlet.





The other nice thing about this amplifier is how it handles the output tube bias adjustments.  We've all seen tube amps that have blown up due to "bias runaway" when the plate inside the tube gets red hot and causes the tube and sometimes the amp to fail.  This happens because many amplifiers share a bias supply between more than one tube.  Setting the bias wrong, or using mismatched tubes can trigger this event.  It's a big reason people who like cathode biased amps, like cathode biased amps better than fixed bias amps.  Anyway, an adjustable fixed bias amp can actually be as reliable and sound as good or better by simply doing it right. 

In the Mystery Amp, I created independent bias supplies for each output tube so that when you adjust one tube, it does not effect the other tubes.  That way you only ever have to adjust it once, instead of ten times until you happen to hit it just right.  Also a lot of time was spent defining windows of adjustment that worked with all the recommended output tubes without allowing the user to get into too much trouble should he/she miss adjust it.  In fact, most of the time it will impossible to get into trouble ensuring long happy tube life. 

The biggest thing is output tube matching in push pull amplifiers for linear output and max power, so to that effect rather than have a bias adjustment for each tube, we have one for the pair and one as a balance control located between the meters so that the user can perfectly match the meters at any time in less than a second. Each channel is set up this way. You'll find this handy since tubes drift with temperature and this allows you to counter it on the fly as you listen without tools.  It takes all the pain in the ass out of owning a fixed bias amplifier.  Of course the meters make great tube testers and clipping indicators allowing you to see if any one tube is prematurely clipping relative to the others despite being perfectly matched at idle.


The voicing on this amplifier is more particular than anything I've done, short of the TORII Mono's.  I say that because I have a very strong preference for the 6N23P input stage as that is the only tube I like in this amp.  Cryo-treated with silver leads, and flat stacked beeswax film caps are used for all internal coupling, and 2 watt carbon composition resistors throughout the signal path.  Factory Tube compliment is the Tungsol KT66, and OA3 and for the driver stage a pair of Russian 6N23P-EV.

Another delightful thing about this amplifier is that is has zero negative feedback so the imaging and sound stage depth are unrestricted and capable of rending whatever is on the recording.

In the end, it is every bit the fidelity and performance of the Zen TORII Mono's with a bit sweeter top end.  On most speakers greater than 86dB there was no noticeable difference in power between the Mystery Amp and the Mono's.

The OA3 feeding the input stage combined with double the capacitance of the Mono's gives the Mystery Amp unparalleled accuracy and liquidity.

NOTE:  There is also a forum thread on the Mystery Amp that I started early in the design process.



-Steve Deckert







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  • 120/240 volt 50/60Hz
  • INPUT TUBE CHOICES: 6N23P, 6922, 6N1P, 6DJ8, 7DJ8
  • OUTPUT TUBE CHOICES: EL34, 6CA7, KT77, KT66, 7027, 5881, 6L6G
  • INPUTS: OPTIONAL TRANSFORMER BALANCED XLR INPUTS
  • INPUTS: SINGLE-ENDED RCA INPUT
  • OUTPUTS: 4, 8, or 16 OHM SPEAKERS
  • IDLE CURRENT: 40~60 MILS PER OUTPUT TUBE - ADJUSTABLE
  • HIGH B+ VOLTAGE: 410 VDC WITH LOAD
  • INPUT IMPEDANCE: 100 K OHMS
  • INPUT SENSITIVITY: FULL POWER @ 2.0 VOLTS ON RCA INPUT
  • POWER INTO 4 , 8 or 16 OHMS: 38 WATTS PER CHANNEL
  • NOISE: -90dB
  • PRECISION CURRENT METERS 0 to 100 MA
  • WINDOWED BIAS CONTROL to ADJ OUTPUT TUBES
  • DYNAMIC BIAS BALANCE CONTROL FOR PERFECT MATCHING of EVEN UNMATCHED TUBES
  • 600V CRYO TREATED COPPER FOIL BEES WAX - SIGNAL CAPACITORS
  • 20 POS STEPPED ATTENUATOR w/ GOLD CONTACTS for INPUT LEVEL (GAIN) CONTROL
  • 4400 UF 500V LABORATORY GRADE POWER SUPPLY CAPS
  • TOP GRADE GOLD TEFLON INPUT JACKS
  • RECTIFICATION: ULTRA FAST RECOVERY 3 AMP 1000V
  • MACHINED STEEL CHASSIS with POWDER COAT LIFETIME FINISHES
  • TOP GRADE FUSED IEC CONNECTOR for REMOVABLE POWER CORD
  • SOFT START- NO STANDBY NEEDED
  • SHIPS WITH:
  • PREMIUM GRADE KT66 OUTPUT TUBES
  • PREMIUM GRADE 6N23P/6N1P INPUT TUBES
  • OA3/VR75 REGULATOR TUBES
  • SIZE: 19-1/8 " DEEP x 13-3/4 " WIDE x 8-3/4 " HIGH
  • NET WEIGHT: 43 lbs.
  • WARRANTY: LIFETIME TO ORIGINAL OWNER / 90 DAYS ON TUBES








  • ZEN MYSTERY AMP
    OWNER'S MANUAL



















     MODEL ZMA
      40 watts x 2

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