A U D I O... P A P E R
Zen Taboo designer's notes
2005 by Steve Deckert
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.
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.
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
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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