home entertainment 2007

Today is Monday, February 5, and
it’s so buttercupping cold outside
that the custodian couldn’t get
our school’s oil burner started.
Consequently, my daughter is home
for the day, playing on the rug in front
of the fireplace. (Santa brought a
wooden castle and a fine selection of
medieval figurines, some of which are
headed for the dungeon as we speak.)
I’m at my desk in the music room, on
the upwind side of the house—and the
wind is murder. The west wall is cold.
The north wall is cold. The floorboards
are cold. But the air
inside is warm as toast:
I’m driving my Quad
ESL speakers with a
Joule Electra VZN-80
amplifier ($12,000) that
isn’t at all bashful about
squandering a goodly
amount of energy as
heat. I can’t think of a
more delightful quality
for an amp to have, at
least on a day like this.
The Joule Electra’s
warmth comes from
more than just its use of
vacuum tubes, and
more than just its class-
A design. It runs warm
because it’s an outputtransformerless
(OTL)
tube amplifier.
That’s a really horrid oversimplification.
Let me back up a bit…
As a power source, an audio tube has
a naturally high impedance: It isn’t
good at supplying high current for a
given voltage, so it has a hard time
developing power across a load, at least
on its own. A loudspeaker, on the other
hand, is a low-impedance load, and it
requires a fair amount of current in
order to do any real work.
The classic means for bridging those
two otherwise unbridgeable qualities is
an output transformer, the primary coil
of which is also used to conduct DC to
one or more of the tube’s electrodes.
The drawbacks of such a thing are
obvious, and while there exists a wide
range of quality from the good ones to
the bad ones and back, it simply can’t
IMAGES: ART DUDLEY
LISTENING
Art Dudley
Joule Electra VZN-80 tubed power amplifier
be denied that output transformers
tend to compress amplitude, limit
bandwidth, shift phase, and ring like
cowbells (mu!) to one extent or another.
Good ones also cost a lot of money.
For as long as output transformers
have existed, there have been designers
who’ve tried to dispense with them
altogether. None is better known than
New Yorker Julius Futterman, whose
pioneering work in the 1950s gained
sufficient fame that, among some hobbyists,
the terms OTL amp and Futterman
amp are used interchangeably—to
The Joule Electra VZN-80 amplifier, seen here without its acrylic top plate.
the great discomfiture of still others.
Futterman’s predecessors in the
OTL genre strove to create an output
section whose impedance is low
enough to drive a loudspeaker directly—and,
of course, the most natural
way to reduce a tube’s output impedance
is to configure it as a cathode follower.
That’s precisely the trick they
used, in all of the very first OTL amps.
But because a cathode follower
requires a much larger input signal than
other configurations—thus opening the
door to the same level of distortion that
one hoped to avoid by ditching the
output trannie in the first place—none
of those early amps can honestly be
considered successful. Besides, when a
designer goes from a single-ended
cathode-follower output to a push-pull
cathode-follower output, in a reason-
able effort toward generating reasonable
power, he or she doubles the output
impedance by comparison. Back to
the starting line.
Futterman’s idea was to use two separate
output tubes—or groups thereof—in
such a way that one was a cathode follower
and the other was a regular “common
cathode” output device. That
arrangement, known as a single-ended
push-pull (SEPP) output section, had
been used to create OTL amplifiers in
the past, but Futterman added a clever
twist: He tied the cathode resistor of his
full-wave, auto-bias input
tube to the top of the
loudspeaker load, to create
a signal-imbalanced
drive for the inherently
signal-imbalanced SEPP
output section, thus producing
a signal-balanced
waveform of reasonably
high current. Notwithstanding
certain drawbacks,
such as a lack of
immunity from DC offset
and the need for at
least some negative feedback,
Futterman’s solution
was a good one, and
his amplifiers are generally
considered the first
truly successful commercial
OTLs.
A Different Architecture
Throughout this time, there was yet
another, very different output architecture
floating 1 around in Tubeland: the
bridge amplifier—or, as the Electro-
Voice company called it in 1954, the
Circlotron. In this remarkably elegant
design, two output tubes are arranged in
a push-pull cathode-follower scheme
(ie, the loudspeaker load appears in the
cathode circuits of both tubes), with
their anodes tied to two separate power
supplies that float above ground.
The Circlotron wasn’t conceived as
an OTL, although its inherently low
output impedance (think: cathode
drive) was touted with the suggestion
1 Believe it or not, that’s a joke—albeit not a very funny
one.
www.Stereophile.com, May 2007 33