OTL Amplifiers


If you’re a true audiophile, you’re sure to already know that finding the right amplifier is key. When faced with a choice, most audiophiles will automatically opt for OTL amplifiers due to the many benefits that they offer. In fact, when achieving optimal sound quality couldn’t be more important, there’s arguably no better option than an Output TransformerLess amp.

What Should I Know About OTL Amplifiers?

OTL is an acronym which stands for “output transformerless”. OTL amps stand apart from regular tube amplifiers which use high voltages and therefore require a transformer to convert to a speaker’s low voltage. With no transformer in its design, an OTL amplifier sidesteps a lot of the obstacles caused by transformers that impact on high-fidelity reproduction such as reduced bandwidth, increased distortion and increased power absorption.

OTL Vs SET Tube Amplifiers

A SET (or Single-Ended) tube amplifier is the next-best thing to an OTL amp, but it still cannot match up to the quality of sound that the OTL amplifier can offer.

A Single Ended amp features just one power tube which connects to an output transformer. As DC power is drawn through the transformer, a magnetic field is established which is at half of the amp’s resting total possible current. There is no need for the magnetic field at any time to reverse its polarity – only its strength changes. Therefore, making small current changes via the transformer is relatively simple.

As a result, SET amps offer better inner lower-level detail than push-pull amplifiers and this makes them a popular choice. However, merely having a transformer in the signal path at all can result in signal degradations and this is why the OTL amp offers a far better quality of sound since it eliminates the transformer from the equation.

Due to distributed capacitance within the windings, series inductance, resistive loss, and hysteresis loss, the transformer can inhibit bandwidth, dynamics and bass, thus causing tone colours to be obscured and detail to be lost. In transformers with a larger output, it’s virtually impossible to get the treble and bass right simultaneously because of these issues.

Meanwhile, OTL technology can correct this. As there is no transformer, amplifiers can deliver signals at an identical speed to a transistor amplifier while also boasting the sonic benefit that tube amplifiers offer.

A History Of OTL Amplifiers

During the 1960s-1970s, OTL amplifiers were said to be unreliable, mostly because of the Futterman circuit which was, for a considerable period of time, the most visible OTL on the market and well-known for its stability problems. When the amp was in oscillation (something that could be caused by anything from layout problems or component failure to overload) it was prone to destroying itself. So, it wasn’t too surprising that the public linked the problems associated with this circuit will all OTLs. In recent years, though, the most advanced OTLs have overcome the Futterman circuit’s early problems by utilising completely different circuitry.

When looking at OTLs’ history, though, it would be impossible to overlook Atma-Sphere OTL Amplifiers. First established in 1977, Atma-Sphere was founded around a brand new and radical approach to output transformerless technology, with a commitment to ensuring cutting edge audio amplification. It was Atma-Sphere that introduced the first practical and reliable OTL to the world using the Circlotron symmetrical output circuit to create low distortion and reduce output impedance by half, thus allowing no feedback to be required for a more stable amp.

This revolutionary music producing circuit was designed and patented by Ralph Karsten, the founder and owner of Atma-Sphere, who is a true titan in the audio world. His interest in the concept of OTL amplifiers was fostered early in his childhood, and his goal to give tubes a matching ability to solid-state through elimination of the transformer was an ongoing one. One night, he was contemplating the idea as he went to sleep, and in his dream he saw the full schematic solution that told him how an OTL should be made. On waking, he set to work on building it and amazingly, it worked!

Atma-Sphere’s OTL amplifier was also the very first to offer a fully balanced configuration, while being the first to adopt the use of fully symmetrical driver circuits. Standing testament to the quality and performance of Atma-Sphere’s amps is the fact that only 3 patents have ever been issued to manufacturers of OTL since the 50s and two of those have been granted to Atma-Sphere. The proof of this brand’s quality is right there!

The Atma-Sphere OTL Difference

OTLs that are made by Atma-Sphere are capable of driving a much broader spectrum of speakers than previously had been possible because of the reduction in output impedance they guarantee. For this reason, Atma-Sphere OTLs are commonly used with 8-ohm speakers, with 4-ohm speakers being frequently used with the larger Atma-Sphere amplifiers. However, it is with the 16-ohm speaker that Atma-Sphere’s OTLs really excel, producing a quality of sound that is virtually impossible to beat with any other type of amp.

Why Choose An Atma-Sphere OTL Amplifier?

If you’re a true audiophile, there is absolutely no question that an Atma-Sphere OTL amplifier is the one for you. After all, the Atma-Sphere brand has more than four decades of experience in leading the way in the audio industry, and was the creator of the first ever practical and reliable, fully balanced OTL in the world.

As if that wasn’t enough to persuade you, it’s important to note that all of Atma-Sphere’s amps are crafted by hand, being made-to-order to suit the precise needs and requirements of each customer. Each component used in their construction is selected manually and will often be custom-manufactured specifically for the brand, resulting in products that offer complete uniqueness together with an unparalleled ability to reliably reproduce music to the highest possible level. In short, if you’re determined to achieve truly high-end sound, an Atma-Sphere OTL amplifier should be your top priority.

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