Interview with Bruce Rozenblit
Transcendent Sound, Inc.
by Richard Seah
Part I - Introduction
OTL. These three letters, which stand for
"Output-Transformer-Less", did not have any meaning for
me until mid-1996, when I read a review of two OTL tube
amplifiers. I didn't remember most of what the reviewer wrote,
mainly because I tend not to pay much attention to things that
are beyond my budget, in this case US$6,295 for one model and
US$11,950 for the other.
I did remember one thing though. The reviewer talked about how
heavenly the amps sounded and he mentioned something about
I was intrigued. And the term OTL stuck at the back of my
mind. Shortly after, I came across another OTL amplifier, by
Transcendent Sound Inc. Out of curiosity, I did an internet
search and discovered that it was a lot more affordable, at
US$2,995. (This model has since been replaced with an improved
version at $3595). I had to find out more.
Thus began my e-mail acquaintance with Bruce Rozenblit,
designer of the Transcendent Amplifier. Bruce tells me that OTL
amps produce excellent sound because they do away with the output
transformer, which is a major source of distortion.
The downside, however, is that conventional OTL amps are
problematic. They generate a tremendous amount of heat. This, in
turn, causes the tubes to burn out and sometimes blow up! Such
amps draw lots of current and you may have to specially install a
dedicated high current circuit before you can even plug it in.
This was confirmed when I re-read the reviewers article where
he states: "OTLs tend to be space heaters/power hogs. You
should certainly consider, as a minimum, a dedicated circuit,
such as the one I have installed in my reference room, where a
separate 20-amp circuit services the power amps."
These are by no means minor inconveniences. But that's not
all. While OTLs are regarded as ideal for electrostatic
loudspeakers, they didn't usually don't do a good job at driving
dynamic loudspeakers. Moreover, the bass response tended to be
In short, OTLs were excellent sounding, but problematic
amplifiers with severe limitations. Which is why they never
became popular - except among the most dedicated audiophiles -
although OTL technology has been around some 45 years, since the
No problems now, says Bruce, who claims to have built the
world's first practical OTL amp, one that you can simply plug and
play. I think the best word to describe it is
For starters, his Transcendent amp generates only 275 watts of
heat for the stereo version, which is less than 100 watts warmer
than a conventional tube amp. In comparison, other OTLs generate
between 1,200 and 1,800 watts of heat!
You don't need a special high-current circuit to use the
Transcendents. Best of all, it will drive most loudspeakers -
electrostatic, dynamic, ribbon, horn, whatever - provided the
impedance does not drop below 4 ohms. Bruce has even managed to
put considerable punch and power into the bass.
Bruce has made OTL technology accessible to those of us who
will not tolerate a whole lot of inconveniences - plus high
expenses - just to enjoy music.
His achievements have been recognized by the US patents
office, which awarded him a patent at the end of last year. This
was the first OTL patent awarded since 1964.
Unfortunately, the audiophile community is still largely in
the dark about his work. That's why we like to tell you more
about the Transcendent OTL amplifier, and about the man who
designed it, Bruce Rozenblit.
Part II - The Interview
(RS) Bruce, can you begin by telling us a bit about
yourself. What got you started in building audio equipment? How
old were you then and what was the first thing you built?
- (BR) I first became involved with electronics when I was
no more than 10 years old. An uncle of mine bought me a
crystal radio kit and I was hooked. Throughout my whole
childhood, I was fascinated with science of all kinds.
Mostly, I was very curious about how things worked. I
couldn't wait for appliances around the house to break so
I could take them apart. Sometimes, I helped them break
so I could have an excuse to get inside them.
- I became involved with audio when I was about 13. There
were some older boys across the street that had some
audio equipment and I thought that this was the neatest
stuff in the world. I use to dream of owning my own Hi-Fi
gear but it wasn't until I was 18 before I had enough
money to buy any. I bought a Heathkit amp and tuner, then
a turntable and a pair of headphones.
- I attended the University and got a Bachelor of Science
degree in Electrical Engineering. I got B's mostly. I was
more interested in building projects than studying. When
I got out of school, I couldn't find work locally in
electronics, so I worked in the power engineering
industry. This caused me a lot of frustration. I didn't
work with electronics for 7 or 8 years. Then at around
age 29 or 30, I started getting involved again. I
discovered Audio Amateur Publications who put out a
series of magazines devoted solely to home construction
of high quality audio equipment. I started reading and
dreaming again. I ordered some kits and got back into the
- Up until this time, I had no interest in vacuum tubes. In
fact, I use to laugh at them. Who needs such an
antiquated technology. Then it happened. I was shopping
in a thrift store looking for bargains and I came across
an old, rusty, Eico 35 watt per channel tube amp. I had
to have it.
- At this time (1987) I knew absolutely nothing about
vacuum tubes and I mean NOTHING. In order to analyze the
circuit, I made a pictorial diagram of how everything was
physically connected. From this pictorial, I developed a
schematic. I still didn't know what it meant. I went to
the library to get some basic information just so I could
even interpret the symbols properly.
- After some study and I had a minimal idea what was going
on, I cleaned up the chassis, repainted it and the
transformers, got new parts and put the thing together. I
worked feverishly into the night for days. I had to know
how this thing was going to sound. I hooked it up to my
speakers, and I was in hi-fi heaven. I was awe struck at
the sonic quality of this little amp. That day was a
major a turning point in my life. Incidentally, the amp
is still in operation at a friends house. It has
performed flawlessly for over 10 years.
- I had to learn everything that I could about tubes.
Fortunately, less than one mile from my home stands the
Linda Hall Library. This is one of the greatest and most
comprehensive scientific/technical libraries in the world
- no exaggeration. I spent many Saturday afternoons
pouring over the engineering records looking for articles
and books that would teach me about tubes. The library
was my teacher, my mentor. Slowly, after much study,
things started falling into place and I started to learn.
- When I read those old articles from the 30s, 40s and 50s,
I felt like I was young man back in that era learning
about tubes as if it was new technology at the time. I
was so enchanted with tubes, that I felt like I was
experiencing the breakthroughs that were published as if
I were reading them as a contemporary of the period, as
if it was my time then. Maybe it was.
- My job(s) continued to give me much frustration. I have a
lot of creative energy and I have to do something with
it. Audio Amateur started a magazine called Glass Audio
then. That was the answer. I would design tube projects
that would be published in the magazine. This would give
me a creative outlet. Something I could focus my energy
on. I didn't make any money doing it, I did it for fun.
Some of us are born to create. I have no talents in the
fine arts, but I can work with technology. I am a born
design engineer. I must create new things. That's what
(RS) What attracted you to OTL amplifiers? Can you recall your
first experience of an OTL amp?
- (BR) During my studies at the library, I came across
designs for OTL amplifiers. The concept fascinated me and
I thought that it would make an interesting project for
Glass Audio magazine. So I designed one in 1989 and it
was published in late 1990. The sound was unbelievably
good. I knew I was on to something. I developed another
design in 1990 which was even better. My technology was
in its infancy but I knew I was heading on a path to
- In Jan 1991, I decided to go into business for myself and
formed my company. I kept developing better and better
OTLs but it wasn't until the summer of 1994 that I
developed the breakthrough that really catapulted the
design and formed the basis of my patent. Up to that
point, I had achieved good reliability and low heat but
the amps all had weak bass. The new circuitry solved that
problem and I new I had an amp that could do battle with
the best of them.
(RS) You mean you built your first OTL amp without first
having heard one?
- (BR) Yes, I was attracted by the technology. I figured it
would sound good. Then, when I built it and heard it, I
was hooked on the sound.
(RS) How would you describe the sound of an OTL amp?
- (BR) OTLs have a distinctive sound. They do not sound
like a conventional tube amp. They do not sound like a
transistor amp. They capture the best of both types along
with the ability to develop incredible three dimensional
imaging that no other amp can produce. People that are
expecting a traditional tube sound from an OTL will be
disappointed. People that are the most happy with an OTL
are looking for extreme accuracy and realism. They do not
want any coloration of any kind.
(RS) From my experience, it seems that OTL amps are better
known for their problems than for their excellent sound. For
example, they generate a tremendous amount of heat which causes
the tubes to blow or burn out. Some even require that you install
a special high-current circuit in your house before you can plug
it in. Can you comment on this.
- (BR) I have been fighting this image since I started. I
tell people I have an OTL that doesn't blow up and they
laugh at me. I'm sorry to say this and it is probably
going to make some people mad but the caliber of modern
day tube designers isn't any where near where it was in
the 50s. Instead of degreed engineers, most are
experimenters with no formal training who don't
understand the theory behind the circuitry. When you
don't have the proper training, all you can do is take an
established design and modify it. It is almost impossible
to come up with something new. It's like changing your
appearance with clothes or make up. You are still the
same person underneath.
- The reason of all the other OTLs suffer from these same
problems is because they are all basically the same
design. There is more to being a circuit designer than
selecting a 3-inch piece of wire or using a 1/4-inch
thick chassis. All of my designs (there have been 8 over
a 5 year period) have been reliable.
- I had some problems finding the best output tube but the
circuitry always held up. Now with the introduction of
the new Svetlana EL509 and some further improvements that
I have made, the reliability has been pushed to levels as
great as tube gear of the past generation.
- I have heard many stories from dealers and audiophiles
about the low reliability of modern day tube equipment. I
have repaired many pieces of old tube gear that was 30
years old or more that finally broke. Why is it that the
old fashioned stuff could last for decades and the new
stuff breaks all the time? It is all in the design and
implementation. That is called engineering and that is
what I do.
(RS) You have managed to overcome those problems. I believe
your Transcendent amp is the first, or one of the first, OTL amps
that is practical and user-friendly. What made you think those
problems could be solved in the first place? And what made you
believe that you are the person who could solve them?
- (BR) I embarked on a journey and I didn't know where the
journey would take me. When I began the project, I
approached it from a completely practical viewpoint. I
knew that just about any tube amp sounds good. The
challenge with the OTL was to get the heat down and the
reliability up. I knew good sound would follow. Many
contemporary designers of audio equipment don't even
consider practical matters and they produce gear that you
can't even fit into a room.
- I didn't know what I would ultimately come up with. I had
already designed many excellent projects so I had learned
to trust my intuition and instincts. I let whatever ideas
that inspired me direct my work. The biggest battle in
harnessing creativity is to trust your intuition. Most
engineers can't do that, let alone admit that such a
thing as intuition exists. Tubes are my element, my
medium. I just let myself go, being convinced that
wherever I ended up would be a good place.
(RS) Were there times when you felt like giving up?
- (BR) Giving up was not an option. I didn't treat the work
as trying to reach some absolute goal. When you are on a
journey, the fun of it is the travelling. I treated the
work as an evolutionary process that takes me wherever it
takes me. By maintaining this attitude, failed ideas
caused little frustration and just increased my knowledge
of what worked and what didn't.
(RS) I understand you were recently awarded all 20 claims in
your patent. Does this mean you have solved ALL the problems of
- (BR) The only trade off that I can't solve is that the
OTL doesn't do well with low impedance speakers. This is
because tubes have a maximum current that they can pass.
Transistors can pass tremendous amounts of current to the
point that they can easily burn themselves up. With the
OTL, power output start dropping very quickly when
speaker impedance falls below 4 ohms. It is best to use 8
ohm speakers. Unfortunately, many speakers are 4 ohm
because solid state amps can double their power when
impedance drops from 8 to 4 ohms.
(RS) What is your design philosophy? Many people believe, for
example, that a good amplifier needs pure class A circuitry, or
expensive components, or point-to-point wiring without circuit
boards. Some believe in single-ended design, some believe in
push-pull. And so on. What do you believe in? Do you believe that
OTL design is absolutely necessary? Or can you build an equally
good - or better - conventional amplifier with an output
- (BR) This is where the rest of the world is way off
track. The answer is: "There is no best way, there
are just ways." There is no ultimate circuitry, or
resistor, or wire or technique or anything. There are
just successful applications of technology and
unsuccessful ones. It is all in the implementation. Many
people involved with audio for some reason are on a quest
to find a "holy grail." They are searching for
THE answer. It doesn't exist. I use whatever provides
outstanding performance at minimal cost.
- It is highly unfortunate that a culture of terribly
expensive parts has evolved over the last 20 years in
audio. People think that if you don't have $10 resistors
in your circuit then it can't possibly be high end
- I feel the main reason this "parts culture" has
evolved is that there has been so little innovation in
circuit design that the only way one manufacturer can
distinguish his product his competitor is to say that he
uses brand XYZ parts while the other guy uses those awful
brand ABC parts. The circuitry used in most contemporary
tube equipment is about the same. This wasn't the case in
the 50's and 60's. There was wonderful diversity in
design during that period.
- I have a customer who has owned many very expensive
amplifiers and was dissatisfied by all. He bought my amp
and loves it. He says its the best amp he has ever had.
He took the bottom off, and didn't see a chassis full of
$10 resistors, $30 capacitors and $20-a-foot hook up wire
and therefore is convinced that the amp is not performing
anywhere near its capability. As good as the amp is, he
can't accept it for what it is. Even though he freely
admits that it is the best he has ever owned, he has to
modify it. It must have those exotic parts!
- This is an example of the psychology that 20 years of
successful marketing can produce. Many times these highly
expensive parts do alter the character of the sound, but
is the alteration an improvement or just a change? If you
just paid $200 for a new toy and it changed the
performance of your equipment, it is a very small step to
believe that the change was for the better to justify the
$200 you just spent. If I were to use these items on a
blanket level, my products would cost 2 to 3 times as
much. Perhaps in the future I will offer such a product
to satisfy customers who want to spend that much but
right now, I am trying to emphasize value.
- If I have a philosophy, it is to make the circuit as
simple as possible. I try to strip out all unnecessary
circuits and components. Long ago a friend used to always
tell me, "simplicity is the keynote of design."
I have never forgotten and he was very, very, right.
- It is possible to create high performance audio equipment
using a variety of techniques. The end result is
determined by what parameters and characteristics you
want to maximize at the expense of others and at what
cost? There are always compromises. This is the challenge
- There are certain advantages that eliminating the output
transformer can produce. It creates a sonic signature
that is unique to the genre. I prefer it. Some people
prefer single ended sound. That is their choice.
(RS) Your earlier amp used the Russian 6C33C-B tubes, which,
as you saw at the CES in Las Vegas, are starting to become quite
popular. But now you have switched to the EL509. Why?
- (BR) The Svetlana EL509 is a vastly superior tube. I used
the 6C33CB because it was the only tube at the time that
could take the pounding. The 6AS7 (which I used in my
first OTL design) sounded a lot better but it would not
hold up. The 6C33CB is a very imprecise tube. The plate
current in a random sample of eight tubes can vary 10 to
one. That's a 1000 % variation. This is intolerable for a
manufacturer. Fully 20% of the tubes that I buy are
unusable because they are so far out of spec. There are
two separate triodes in each tube with common electrical
connections. Each triode's specs. are usually
dramatically different. The problem, I think, is in the
grid circuit. After all of the trouble I go through to
weed them out, about 5 to 7% will fail in three or four
months. When you get a set of good ones, however, they
will last for years, and are very stable. The problem is
the consistency. The 6C33CB is also a rather hard
sounding tube. In order to make the amp sound smooth, I
had to restrict the bandwidth to 20 kHz.
- The EL509 does not suffer from these problems. I don't
have to pretest and sort them out. They don't have to be
purchased in matched pairs. They have a much more stable
cathode. The tube is more musical sounding so I can let
the bandwidth extend to beyond 50 kHz. The power is
reduced by 20% which is minimal but the waste heat is
reduced by almost 100 watts per cabinet. The tube is in
ample supply. In short, it sounds better, is easier to
work with, much more stable and easier for the consumer
to live with.
(RS) How well have your amps been selling so far?
- (BR) Sales have been ramping up. Most of my sales come
from customer referrals. Someone buys it, tells all of
his friends, they buy it, etc. I would like to see sales
go higher for obvious reasons.
- Audio is an extremely tough market. I liken it to being a
baby turtle that has just hatched on the beach along with
1000 others. As soon as you start to move towards the
water, every manner and type of predator will do
everything possible to eat you. If you somehow make it to
the water and begin to feed, no one will pay any
attention to you until you are large enough to make a big
pot of turtle soup. I'm at the stage where I have made it
to the water and begun to feed but I am too small to
throw into the pot.
(RS) I understand you are now working on a pre-amp. In what
way will it be different from other existing pre-amps? When do
you expect it to be ready?
- (BR) Now that my book is done, I will have time to
develop new products. I have no intention of producing
another me too anything. My pre-amps will utilize new
circuitry. That raises the question of another possible
patent application which is a lot of work. The question
will be if it is different enough from the prior art to
justify the application and there are a lot more pre-amp
designs to go through on a patent search than OTLs. I
have several ideas to explore. I will soon start on that
(RS) What about other products, like DACs? Or a budget
amplifier or an absolutely state-of-the-art cost-no-object
- (BR) I am an analog circuit designer. I don't have much
experience with digital circuits. It would take me at
least 6 months and probably a year of study before I
could come up with something. There are some very fine
digital converters on the market. I like to work with
tubes, not micro chips. If my heart isn't in it, I won't
do the kind of work that I expect of myself.
- As far as a super amp, that is still possible. I am
motivated by efficiency and cost / performance. I don't
see the sense in spending 500% more for 1% improvement in
(RS) You mentioned having completed your book. I understand
it's on amplifier design. What can we expect from it? Are you
going to teach people how to build amplifiers? Will you be giving
away any trade secrets? What's the title of the book? And when
will it be out?
- (BR) The book should be out by mid year or sooner, I
hope. It is being published by Audio Amateur Corp,
(publishers of Glass Audio). The title will be The
Beginners Guide To Tube Audio Design.
- What I have attempted to do is write a book for lay
people that teaches them how to design tube circuits. It
is not a cookbook of circuits. It is a teaching book. I
have tried to relate the fundamentals of a college degree
in a practical, intuitive way so people can understand
the material. The material is generic but there is plenty
there to get involved with. I give the reader the benefit
of my experience in showing ways to solve design
- Even if you don't want to do your own designs, the book
will yield a great deal of knowledge about circuitry in
general. Hopefully, it will help audiophiles gain a solid
understanding of electronics and tubes so they can better
enjoy the hobby, make informed and intelligent purchases
of audio equipment, and not be taken by the many snake
oil salesmen and con artists that unfortunately
proliferate this business. I really want people to get
involved and build their own gear. It is a lot of fun,
very rewarding, gets people away from the television set.
The world needs more doing and less watching.
(RS) Where do you see yourself 5 or 10 years from now? Do you
think you will radically influence the way future amplifiers are
built? Do you see yourself becoming famous, perhaps like another
- (BR) I don't know where I'll be in 10 years. In the US,
success is measured by your financial accomplishments,
not your technical or artistic achievements. I am already
assured a place in audio history with the issuance of my
patent. This validates the claim that I have
significantly advanced the state of the art. Whether or
not I can get the rest of the world to care is another
- The cruel reality of the marketplace is that I don't have
the financial resources to buy myself an image as large
as any of the established audio companies. Even if I had
the money, I may not have the marketing savvy to pull it
off. Luck and being in the right place at the right time
has a lot to do with becoming a financial success. All I
can do is try, give it my best and see where the journey
Part III - Loudspeaker/Amplifier Safety
(RS) Most of the claims Bruce makes about having
solved the problems of OTL design can be easily verified. We can
see that he uses few tubes, we can feel that the tubes aren't too
hot, we can hear whether or not the amplifier can drive dynamic
speakers, or whether it delivers bass punch. Another common
worry, however, is that an tube fault might cause the
loudspeakers to burn. How can we be sure this won't happen? Bruce
(BR) There is absolutely no possibility of damage to the
consumers loudspeakers. This will require a technical answer.
First of all, the protection scheme that I am using is an
improvement over the original used in 1989. The original
amplifier used 6AS7G output tubes. It had an extremely smooth
sound. But the tubes failed all over the place. They couldn't
take the pounding. There was never any damage to any speaker
when this occurred and it occurred dozens of times.
Secondly, the first design was published in Glass Audio
and many people, possibly hundreds, from all over the world
built the amp. I never received one letter from anyone about
speaker damage. There are dozens of amplifiers in the field
now, some for over a year, and there has never been any
damage anyone's speakers.
In order to understand how the protection works, it is
necessary to understand the mechanism of failure for a
loudspeaker. There are two modes of failure, heat and
mechanical. Excessive heat in the voice coil will melt it and
cause it to short out or open up. A mechanical failure can
occur if a sufficiently large pulse of current pushes the
diaphragm so hard that there is a rupture. Voice coil burn
out is more common.
Heat is the result of energy delivered into a substance,
in this case the voice coil. There is a fundamental
difference between power and energy.
Power is the RATE that energy is delivered. Example: If
you were to try and boil one litre of water in a pan with a
candle, it probably never would boil. One candle can't get
that much water hot enough. Let's measure the temperature of
the flame and see that it is 500 deg C.
Now, take 100 candles (assuming they can fit under the
pan) and we find that water quickly boils. Interestingly
enough, the temperature of the flames for 100 candles is
still 500 deg C. The reason the water boils is that there is
100 times the rate of energy delivered to the pan to cause it
to boil. The rate of delivery of energy with one candle is so
slow that the pans cools off as soon as the heat is applied
so it never boils. It will warm up a little, but it wont
Obviously, a certain amount of time is required before the
water will boil. If two hundred candles were used, the water
would boil in half the time. With 200 candles, the same
amount of heat is applied in half of the time because the
rate of delivery of energy or power is double.
An electrical analogy of this would correspond with
voltage replacing candle flame temperature and current
replacing the number of candles. Time is still time. So two
factors must be considered when examining overheating of a
voice coil. One is how much power is applied and two is for
how long. Both quantities, the rate of energy delivery and
the time length of energy delivery are vital. The net total
energy applied is always the product of rate of energy
delivery multiplied by time.
To the novice, it sounds like a disaster if 150 volts is
applied to a speaker. That depends for how long. You can
safely pass your finger through a candle flame and not be
injured. However, hold your finger in the flame long enough
and you will receive a severe burn. The increase in time
allowed more heat to be applied to your finger causing the
damage.The same amount of power is present in both instances.
This is the principle behind the speaker protection in my
A fuse is the simplest, most reliable type of overcorrect
protection available. Electronic circuits used for fault
protection take time to recognize the fault and then to act
it. The fuse will open faster than any protection circuit can
operate. The power supplies are fused. In the extremely rare
event of an output tube short, the surge of current causes
the fuse to blow almost instantaneously. There isn't enough
time for any damage to occur just as you can pass your finger
through a flame and not be burned.
Furthermore, the original design had nothing to limit the
current allowing a tremendous surge (probably in the range of
more than 1000 amps) and still there has never been any
speaker damage. The rise time of the fault is so fast that
the reactive properties of the loudspeaker keep any current
from flowing into it. The energy of the fault is dissipated
across the tube not the speaker. By the time the reactive
properties have had a chance to charge up and allow for
steady current flow, the fuse has long since blown. The event
is over with before it started.
The new design uses passive devices to limit the fault
current to 75 amps at most. This a reduction in power of one
hundred times. There is no way a sustained fault can be
maintained. The fuse will always blow in an instant. Not only
is the speaker fully protected, but the amplifier and output
tubes are also protected from damage. The protection scheme
is foolproof and has been thoroughly field tested.
SoundStage! wishes to thank Bruce
Rozenblit and Richard Seah for this interview.
If you would like to contact Bruce he can be reached via e-mail
Richard Seah lives in Singapore and can be reached at firstname.lastname@example.org
The interview also appears in the April/May
edition of High End Journal (in Singapore). This
interview is printed in SoundStage! with the permission
of Richard and Inkwell Publications, the publishers of High