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 sound.

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 amplifier.

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 as well.

Isn't the output transformer necessary?

Not absolutely.

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 the speakers.

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 and limitations.

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 explode!

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:

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.

  • 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 OTL amp?

    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 replacment costs.

    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 amps.

    If you have further questions, please e-mail:

    Bruce Rozenblit of Transcendent Sound

    Richard Seah of The Soul fo Music

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