Frequently asked questions about
What is OTL? What is an OTL amplifier?
OTL stands for Output Transformer-Less. It is a term used to
describe amplifiers, particularly tube amplifiers, that do not have an output
transformer. Such amplifiers are commonly called OTL tube amplifiers.
What's the point of not having an output transformer?
The output transformer is a major source of distortion. Amplifiers
without the output transformer therefore produce cleaner, less distorted
Moreover, it is extremely difficult - and therefore very expensive - to
make a good output transformer. Because of this, the cost of the output
transformer often accounts for a very big part of the total cost of a tube
Some excellent sounding tube amplifiers have output transformers that are
painstakingly hand wound using very pure silver wires. These amplifiers
may cost tens of thousands of dollars, or even over a hundred thousand dollars.
They are way beyond the reach of average audiophiles.
Adding to the high cost is the fact that good output transformers are bulky
and very heavy. Audiophiles therefore end up paying a lot of money for shipping
Isn't the output transformer necessary?
The main function of the output transformer is to change or transform the
high impedence / high voltage / low current signal of the tube to the low
impedence / low voltage / high current signal needed to drive loudspeakers.
One way to do without a transformer is to build loudspeakers of very high
impedence. In the 1950s, for example, there were some 500-ohm and 600-ohm
loudspeakers. But these did not prove popular and most loudspeakers today
have an impedence load of 8-ohm or less.
Another way is to use a capacitor for the job of transforming the signal.
Though the capacitor is not totally distortion free, it is much lower in
distortion than an output transformer. A capacitor is also less bulky and
less expensive. Most OTL amplifiers have a capacitor between the tubes and
Yet another way is to design special circuits such that there is no longer
any need for either output transformers or capacitors. This allows the tubes
to be connected directly to the loudspeakers. Nothing comes in between (except
the speaker cable). Such designs ensure that the purest, distortion free
signal reaches the loudspeakers. They ensure the best possible sound amplification,
at relatively low costs.
Transcendent Sound is presently the only OTL amplifier incorporating this
design, where the tubes are connected directly to the loudspeakers.
Why aren't all tube amplifiers made this way?
Because it is not easy to design a good OTL circuit. Audio engineers
and designers have been trying to build OTL amplifiers since the 1950s,
but they met with only limited success. While they managed to produce some
excellent sounding amplifiers, these amplifiers came with a lot of problems
What were these problems and limitations?
The biggest problem was poor reliability. The early OTL amplifiers
produced extreme high heat, which often caused the tubes to fail or even
This was because they used plenty of output tubes to take over the job of
the output transformer. Early OTL amplifiers - and even modern OTL amplifiers
based on these early designs - typically have between 8, or 16 or more output
tubes per channel. Such an amplifier would generate between 1200 watts and
1800 watts of heat, which is about as much heat as that generated by home
heaters used in winter. No wonder the tubes exploded.
Other problems included:
- High maintenance cost because of the need to constantly replace lots
- Impractical to use.
Because of the extreme heat generated, powerful air-conditioners are needed
to keep the environment cool. And air-conditioners introduce hum, which
is not what audiophiles want to hear!
Moreover, many OTL amps required a very high current circuit (at least
20 amps). Most homes do not have such circuits and you will need to call
in the electrician to specially install one. Another disadvantage here
is that such amps cannot be plugged into a power line conditioner.
- Weak bass, lacking depth, punch and power;
- Inability to drive dynamic or cone speakers. The early OTL amps worked
best with electrostatic speakers, but could not drive most other types
Can these problems be solved?
Yes. And they have been solved.
Bruce Rozenblit, designer of the Transcendent Sound OTL amplifier, has solved
ALL of these problems and has built an OTL amp that:
Can drive all types of loudspeakers, including electrostatics, cone,
horn and ribbon, provided the impedence does not fall below 4 ohm.
- Runs relatively cool, generating only about 275 watts of heat, compared
with the 1,200 watts to 1,800 watts produced by other OTL amps
- Is highly reliable. The tubes will last for years because of the much
lower heat level.
- Is not expensive to maintain because when the time finally comes, there
will only be a few output tubes to change - 4 tubes per channel in the
stereo amp, 8 per channel in the monoblocks. Other OTL designs typically
have 8, 16 or more tubes per channel.
Can be plugged into any wall socket or power line conditioner.
Has deep and powerful, punchy bass.
Are there any other problems left?
Only one small limitation. OTL amps will not be able to drive
speakers whose impedence drop much below 4 ohm. It still works fine at,
say, 3.5 ohm. But 2 ohm or 1 ohm speakers are out.
What is the sound of an OTL tube amp like?
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.
What output tubes are used in the Transcendent
The new generation of Transcendent amplifiers uses Svletana EL
509 tubes. This is an extremely stable and reliable tube. It is widely available
and not costly to replace. Matched pairs are not necessary, thus halving
Will my speakers be damaged if the tubes fail?
No. Absolutely not.
Even in the earlier OTL designs where tubes blew constantly, there have
never been cases of loudspeakers being damaged.
The Transcendent Amp is very reliable and the likelihood of a tube blowing
up is extremely small. The ciruit is fuse protected and will shut down instantly
should any trouble arise.
Moreover, it is designed such that, in the unlikely event of an electrical
short circuit, the amount of current reaching your speakers will be much
less than what is needded to cause damage. It is a very safe design.
How much will it cost?
The Transcendent OTL amp comes in two versions. The 25W stereo
sells for US$3,595, while a pair of 80W monoblocks sells for US$6,995. This
is very much cheaper than other OTL amps or good quality conventional tube
If you have further questions, please e-mail:
Bruce Rozenblit of Transcendent
Richard Seah of The Soul fo Music