A U D I O... P A P E R

Zen Taboo designer's notes
April 2005 by Steve Deckert

The Taboo is a 6 watt per channel stereo amplifier that uses a single EL84 or SV83 output tube per channel.  The Class A outputs are wired in single ended pentode driven by a single 12AX7 dual triode.  Only a single coupling cap lives in it's signal path and everything is hard wired point-to-point with parts leads and no hook-up wire.  Built from the same size chassis and with many of the same parts, the Taboo has all the hallmarks of it's successful Triode sisters, the SE84C and SE84CS.

To understand the Taboo you really have to be familiar with the incredible success of it's sister amps.  With an unusually ballsy two watts per channel and benchmark transparency they are difficult to top.  There is a real magic to the simplicity of using a single tube per channel.  It has a certain purity about it that never gets boring.  Until now getting over the two watt platue and keeping the sound as close to unchanged as possible ment using larger or more output tubes per channel.  The small envelope of the EL84 and SV83 output tubes has for whatever reason allways demonstrated itself as having better subjective speed and delicacy when compared to anything larger.

The other problem is that Triodes have a certain sound about them that you really can't argue with.  Pentodes on the other hand are seldom as good simply because they can be non-linear and usually require some type of negative feedback which collapses soundstage depth proportinately with the amount of feedback used.  Ironically the very thing that works to destroy an amplifiers transparency improves it's specifications on paper by lowering total harmonic distortion.  

There is already a paper (The TABOO of Amplifier Design ) written covering the more casul design phases of this amplifier including the spontaneous circimstances that brought it to be.  They touch on the things that Pentodes do better than Triodes.  You will find out about my attempts to capture the spank and detail of the Pentodes signature without losing any of the tone or body of the Triodes I've become so spoiled by.  And do all this without losing any of the benchmark imaging of our Triode amps... A tall order that required some new approaches to feedback design.  The paper covers the first several months of that design process.

Since the Taboo is using many of the same parts as the SE84CS (Zen Triode Select) and the SE84CS has become so many audiophiles favorite amp - the amp to beat, this was another attempt to do just that.  In the final judgement it's always going to be a subjective call largely based on system synegy, but the Taboo and Select are neck in neck.  What the Taboo brings to the table is a fresh signature that is equally as good as the Triode, perhaps better in some ways with almost 3 times the power and imaging that is as good or better.  It gives music a lively sound and has a certain honesty about it that you grow to love.

When the Taboo was completed the final prototype was sent around the country to our forum members who owned SE84CS amps for beta testing and evaluation.  It was sent out in it's "Standard" configuration and was favorably received by all who got to hear it.  The production amp has the one additional but rather outragous feature of what I call "Lucid" mode.  The amp can be switched on the fly between "Standard" and "Lucid" modes giving you two different amps at the flip of a switch.  Unlike the Zen Triodes where a signature adjusting switch was no more than a change in the bias of the input stage, the Taboo has a very unique output stage design that incorporates two configuraions.  The result is a predictable signature and image very similar to the SE84CS when the amp is in "Standard" mode.  In "Lucid" mode spacial information is enhanced and the sound becomes almost animated as all apparent boundries are removed.  

This was accomplished by drawing a compromise between floating output transformers and a more classic grounded design.  The feedback on floating outputs is very tricky and always unstable during gear swaps.  By partially floating the outputs, a stable design was possible but the advantages in sound of the floating outputs remain.  The Zen Triodes all used a floating output design which gives them more air a detail and enhances imaging.  This is a feature we couldn't afford to loose.

The feedback circuit in parallel with the cathode of the input stage is a 15.6K reference to ground for the output transformer.  At the same time a parallel reference to ground  can be seen through the impedance of the secondary plus 4.7 ohms.  This is a fairly esoteric reference that works in harmony with the feedback circuit.  The feedback circuit is carefully voiced to keep the output stage linear at any point in it's adjustment window.  There is one for each channel that allow you to fine tune the feedback on the fly for any given recording, although the most common use is to find that magic spot where the amp becomes synergistic with a given set of speakers and then leave it there.

Completing the output design, switch A ties both semi-floating secondaries together with a series resistance between each coil of 9.4 ohms (4.7 x 2).  This modifies the ground reference further and allows both channels to become actively aware of each other.  The resulting channel to channel synergy is something impressively special that only the Taboo can accomplish when in "Lucid" mode.

Because of the semi-floating output design the input sensitivity is around 2.5 to 3 volts if a 12AX7 is used and that is the stock factory tube shipped with the amps.  This means unlike the Triode amps that can all be run to clipping with no preamp, the Taboo can not be clipped with the standard 2 volts of most CD players.  If you have speakers that are 94 dB or higher it is likely that you would still have plenty of volume even without using all of the available power from the amp.  Of course many modified CD players and DACs have higher than 2 volt output levels so if you have one you would not need a preamp regardless of speaker efficiency.

Having said that, the Taboo was designed in harmony with out latest preamp called the SE84CSP and the two together are very synergistic and flexible.  They also match in appearance so they look like they belong together.  Adding the CSP to your Taboo will bring out the best in the amplifier.  Increased weight, dynamics, tone and image are just some of the benefits of using a great tube preamp with the Taboo.  

The power supply in the Taboo features a buffered choke fed from a 5Y3GT recitifer that is actually matches the output transformers because it is one.  The secondary is open.  This improved power supply creates the quietest amp we've ever produced with the average AC hum being less than 0.4mv with virtually no noise.  This is particularaly impressive on high efficiency speakers of 96dB on up because with this amp, you'll have to check to see if the amp is even on when you place your ear in front of the speaker.  The amp itself runs noticably cooler than it's Triode sisters and is no harder on tubes.

In the Taboo, the SV83's used in our Triode amps sound very good.  Glass smooth with great micro detail.  A good pair of EL84's have more spank and presence.  The difference between the two is fairly large while on the Triode amps the difference is only nominal.  A 12AX7 input stage is used in the Taboo instead of the 6922/6DJ8's used in our Triode amps.  The 12AX7 is better suited for this application both in voltage swing and in signature.  It is also perhaps the most popular preamp tube ever made with excellent variety and availability for tube rollers.

I can't imagine anyone not being impressed with this amp, especially on high resolution high efficiency speakers.  Even the hardcore among you with high dollar treasures are sure to be subtilly itimidated by this amplifier's incredible sound.  I know it does it for me.

Happy listening!


Steve Deckert




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