A U D I O
P A P E R
The TABOO of Amplifier Design
2005 by Steve Deckert
Some how I
seem to frequently fine myself exploring the dark side of
analogue circuit design. (Not usually on purpose either) I find it fascinating and challenging
to twist the "rules" to the edge of operational stability
just to see what happens. The most recent distraction of
this nature began in December
when Paul (the instigator) bugged Dave to make him another
amp out of spare parts laying around his shop. Dave is heavy
into his own guitar amp business now and together they rummaged
through his shop to find an old original Zen amp chassis, a couple
output transformers and even an old original power transformer.
This was perfect for Paul because his ongoing experiment in
his basement was about one zen amp short of being one zen amp richer.
I don't know how many he has down there, but frequently he
will hook up as many as 20 speakers at the same time.
They had all
the parts in front of them to toss together another SE84C but decided
to wire it in Pentode rather than in Triode. As with all of
Paul's new toys, it made it's way over to our studio for
a listening test. In the rush to get it done they decided
to try leaving the negative feedback out or just forgot to add
it I'm not sure... but the end result sounded like a giant tweeter.
a terribly good debut at my place, he then took it over to Dennis's
house for the official "Dennis Test" and
hoped for better luck over there. Desperately rolling tubes
trying to get it listenable it continued to show up both here and
at Dennis's house repeatedly over the next month. While it
had improved somewhat and did some things very interestingly good,
it still remained unlistenable.
escape on night from the bazillion things I should be doing,
and curious why Paul's new little amp just wasn't happening I snuck
it over to the shop to see what was going on. It's always
fascinating to hear a different amplifier and puzzle over why it
sounds the way it does. This one was no exception, because
we rarely hear something this far off for any length of time. Basically
the frequency balance was the only thing I could fault with it as
everything else it was doing was good.
liked the sound of Pentodes but Triodes are far easier to get a
good transparent sound out of. The difference between
them is linearity. The Triode output device is usually very
linear making it possible to run with no feedback. The Pentodes
amplification efficiency increases with frequency so the midrange
is louder than the bass and the treble is louder than the midrange.
To get it flat some manipulation must take place.
The most common
way to manipulate the frequency balance of the Pentode is with global
negative (degenerative) feedback. By applying a sample of
the output from the amplifier back into the input stage out-of-phase you can achieve a flat response. Since the natural tenancy
of the Pentode is to amplify high frequencies more than low frequencies,
the out-of-phase signal that comes back from the output stage will
be stronger at high frequencies thus creating more cancellation
at the input stage at those high frequencies.
fixes the problem, when compared to a Triode with no feedback it
usually lacks depth and transparency. However, as you might
imagine, since the Pentode is more efficient, has more power with
a natural tenancy to become still more efficient as the frequencies
increase it is also a subjectively "faster" device. It
is capable of adding a presence to the midrange and treble (because
of this speed) that is less apparent in Triodes. This natural
of Pentodes would normally make the result more transparent than
triodes but that transparency is often undone by the added parts in the circuit
needed to manipulate the response so very few Pentode amps actually
sound great when compared to Triodes. Given both types expertly
implemented, the Triodes are generally smoother, have more warmth,
and tend to be more forgiving and musical. However, the flip
side is that the Pentode can be largely more honest, faster, and
have more power. Bass character on a great Pentode amp, especially
push-pull is still a good reference or benchmark. A 100 watt
PP Pentode amp can make the bass of even the best solid state amps
So, back to
the story - I tried to add feedback to Paul's little pentode
amp but it screamed loudly every time I tried. I noticed that
the output transformers were wired backwards at the primaries making
the grounded secondary out-of-phase so adding feedback simply created
a perpetual oscillator. Rather than fixing it right, I elected
to simply unground the secondary of the output transformers and
let them float. This is where the Taboo started.
In the Zen
Triodes I always float the transformers so they have no reference
to ground. Because of the extreme stability and simplicity
of the circuit design in a Zen Triode floating the transformers
adds a large degree of magic to the signature of these amps. In
particular the soundstage and imaging become more surreal. Trying
this in a Pentode amp or any amp with feedback is asking for trouble.
For one thing, whatever feedback scheme you design for the
amp, ie., how many dB of feedback and what type of RC network you
might arrange will completely change without the ground.
After a few
hours, I got a pretty decent signature out of the amp with what
ended up being positive (regenerative) global feedback. The
following evening Paul showed up and I explained that Dennis and
I couldn't take listening to his amp anymore so we gave it an enema
and here it is! As we listened a couple things became
apparent. It was fun to see the little Zen amp (visually identical)
get so amazingly loud compared to it's triode counterpart. It
was also apparent that this new little amp had the spank and speed
that when combined with the added power came together to add a level
of excitement to most
of the music we listened to.
Paul's comment was that it has some of the signature of all
the amps we've made, the Torii being the closest. But even
the Torii didn't have quite the spank of this little amp?
A few weeks
pass during which time we continue evaluating Paul's little amp.
It still wasn't perfect. The Zen Triodes did several things
better in the midrange. But I have to admit that what Paul's amp did right
was really starting to get me worked up. He finally took it back
home and in the days that followed I found I really missed listening
to it. So back to the shop to make another one. Of course
building a second from scratch allowed me to make some overall changes
that I hoped would be improvements. I got it done, and while
almost exactly the same amp, it failed to sound like what I remembered
Paul's amp sounding like. It was however stone quiet now,
and had adjustable feedback so you could tweak the tone.
I worked on
voicing the feedback network for hours at a time
whenever I had a spare moment until I had probably 30 hours of time
into it and had tried a couple dozen different feedback designs.
By this point I was becoming very happy with it and felt that
it did many things better than Paul's. We listened to it for
the next week or so and it's potential became intoxicating. I
let it become an obsession for awhile and began to completely explore
the range of possibilities it may have hidden inside.
To keep the signal path clean,
all the work was done in the feedback loop. Around a hundred
man hours later I found myself starting to spin my wheels. The
stability of the amp was perhaps the main factor since it had floating
outputs with feedback. Along the way I tried negative feedback
schemes, positive feedback schemes, and even developed one that
had both positive and negative feedback at the same time. I
was blown away that it actually worked ... but the
of this rambling, if there is one, is that you don't just build
what you think is going to be a great amp and expect it to blow
away a Zen Triode. I've been trying for 10 years. This
is just one story of that attempt, of which there have been hundreds.
of this long distraction came when I reached the point where
nothing else could be done or get any better with the amp. It
was getting great comments both here at the shop and down at the
studio and I was starting to salivate over the possibility of marketing
it. The final test came one night when I put it in the reference
listening room at the shop. I drove my corner horns with it,
and used my vinyl rig as the source. Thinking I would be able
to just spin a few tracks here and there and decide if it's a winner
or not I found myself there hours later still frozen to my listening
chair in total amazement. Without question it had pushed it's
way up to a level that easily surpassed all of our other amplifiers.
It had imaging and presence that was outside my ability to
describe with all the common adjectives.
in the reference room for a week and we all listened to it and were
in agreement it was something very special. Then one day I
decided to take it out and play it out on the test bench so I could
listen to the radio while I worked on other things. I discovered
that somewhere along the way it lost it's huge power advantage over
the Zen that it originally had. 6 clean unclippable watts
was now dirty after about 2 making it not any louder than a Zen
Triode would have been. I didn't notice this in the reference
room because the corner horns were 94+ dB efficient. With
a saddened heart I put it aside and pondered what to do. This
lead to a few more sessions with the soldering iron trying to get
back what was lost and at the same time hopefully add some stability
to it. For example, the amp had to have an earth ground to
work. The speaker wires could not be removed while the amp
was playing during that few seconds after you unplug the power cord.
If they were, you would get shocked from a wild oscillation
that occurred in the split second you removed the wires... stuff
The end result
- or should say where it ended - was wiring it back "by the
book" with grounded outputs and conventional negative feedback.
Now stable, it sounds real nice, but nothing magic is happening
that would make it any different from the norm. I learned
again that yes, it is possible to make amplifiers that blow away
a Zen triode in all categories, but only in the otherworld where
the rules are flipped upsidedown. In the otherworld, you may
not be able to replicate the result from system to system without unpleasant
surprises. There was no way, at least at that point, that I
ccould see something like this even make it's way out of the starting
gate as a finished product ready for sale. It's almost like
the Devil of Ohms Law had manifested itself disguised as an angle of
like this happen, I usually find out that a higher power is trying
to give me answers and I'm not listening.
a tube amp builder and want to experience the tease of a taboo circuit when
it was in development - here it is. May the force be with you.
that the output tube in the above schematic is shown wired incorrectly.
Use the pin out shown in the schematic below.
This is the
circuit exactly as I listened to it for that week in the reference
room. The usable clean power of this variation is far below
the 6 watts it can have, but also qualifies as one of the best sounding
amplifiers I've ever heard. Notice that all the voicing was
done in the feedback circuit on cathode of the input stage. The
output stage cathodes are also carefully manipulated. The
network on the input stage evolved from over 30 different variations,
some simple, some even more complex. Even though the performance
of this amp is unpredictable, I will probably remake it for myself
just for those evenings when I want something really outrageously
good in my reference room that I can't sell.
Start with this
circuit, you may have to remove the .0047 cap to ground, but start
with it in. I found the SV83 output tube to be far more musical in the
midrange than the EL84 in this circuit. Not necessary
true in the final version. Use the best 12AX7 you can
find for the driver stage. You'll also notice the low voltages
used on the driver stage placing the operational points of the tube
on the fringe of it's curves. The choke is simply another
output transformer with the secondary floating.
or not a production amplifier actually came out of this experimenting
and I found ways to make it stable and sound good too. We
named it after this design experience... the Taboo.
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