The acronym 'OTL' refers to a tube amplifier that is Output TransformerLess.
Regular tube amplifiers have very high voltages in them and by contrast most loudspeakers are low voltage. In order to convert from the high voltage of tubes to the low voltage of a speaker an electronic device known as a transformer is used. Transformers are large arrangements of metal that also have large amounts of wire in them. The wire is arranged in two sets of WINDINGs on the metal of the transformer (called the CORE), so there is an input and an output. The input winding is called the PRIMARY winding and the output is called the SECONDARY winding. OTL amplifiers do not use a transformer, and sidestep many of the barriers to high fidelity reproduction that transformers cause (increased distortion, reduced bandwidth, plus the transformer absorbs power).
Single-Ended vs. Push-Pull
There is a debate about the relative merits of Single-Ended amplifiers vs. Push-Pull. Single-Ended amplifiers owe their 'magical' properties to the way the output transformer is used and to the lack of loop feedback (we won't cover the zero feedback issue here). There is only a single power tube in a Single-Ended amp, connected to the output transformer. As it draws DC power through the transformer, it sets up a magnetic field in the transformer. This current (and field) is at one half of the total current possible when the amp is at rest. At no time does the magnetic field in the output transformer have to reverse polarity- it merely changes in strength.
It can take a small amount of energy to reverse the polarity of a magnetic field in an output transformer; this energy loss is known as 'hysteresis loss'. The energy to reverse the field comes from the signal. If you never reverse the field, this problem goes away. Thus in Single-Ended amplifiers it is relatively easy to make small changes in the current through the transformer. This accounts for the fine inner detail that Single Ended amplifiers are known for.
Push-pull amps by contrast have more bandwidth and power, as the dual power tubes produce opposing magnetic fields in the transformer (while the amp is idling), resulting in no magnetic field. This increases the amount of power and bandwidth the transformer is capable of, but at a price: low level detail. The major issues for small signals occur at the zero crossings: when the signal goes from negative to positive and back again. It takes energy to reverse the field (however small) in the transformer, and this energy requirement results in increased distortion. Thus push pull amplifiers lack the low level detail that Single-Ended amps have in spades.
Eliminating the transformer eliminates this issue and any arguments for single ended operation. Removal of the transformer from the signal path also reduces other degradations of the signal. There is distributed capacitance in the windings (loading the tubes), series inductance (which can contribute to distortion), hysteresis loss (meaning that anywhere up to 20-25% of the amplifier power is used to create heat) and resistive loss in the windings as well.
These issues cause the transformer to inhibit bass, dynamics, and bandwidth. Detail is lost and tone colors are obscured. In larger output transformers it is almost impossible to get both the bass and the treble right at the same time due to these issues.
OTL technology allows this to be corrected. The lack of a transformer means that the amplifier can deliver the signal with the same speed as a transistor amplifier, but with the sonic benefit typical of tube amplifiers.
The History of OTL Amplifiers
In the 1960s and 1970s OTLs developed a reputation for unreliability, primarily due to a circuit developed by Julius Futterman and the later marketed by New York Audio Labs (Harvey Rosenburg). The Futterman circuit was for many years the most publicly visible OTL, and was known for stability issues (caused by positive feedback nested within a global negative feedback loop). When in oscillation, (which could be caused by overload, component failure or even layout problems), the amp had a tendency to destroy itself. For many years the public has associated the weaknesses of the Futterman circuit with OTLs in general. Fortunately modern OTLs have solved the earlier problems of the Futterman by (for the most part) using entirely different circuitry.
In fact, every manufacturer who has ever attempted to produce a Futterman amplifier has ultimately gone out of business. The public is very demanding of reliability. For 30 years, the 'Futterman legacy' has been the largest marketing issue any OTL manufacturer has had to face.
At this point no accurate history of OTLs can ignore Atma-Sphere Music Systems. Founded in August of 1977, Atma-Sphere was created around its radical new approach to OTL technology and a dedication to State of the Art in audio amplification.
The First Practical OTL
Atma-Sphere's biggest claim to fame is the introduction of the world's first reliable and practical OTL. This was accomplished by using a fully symmetrical output circuit (known as the Circlotron; first devised in 1954 by Cecil Hall), which resulted in low distortion and 1/2 the output impedance. The low distortion means that little or no feedback is required, resulting in a very stable amplifier. Atma-Sphere is also the first to offer an OTL amplifier in a fully balanced (differential) configuration, allowing for balanced and single-ended inputs (and in fact offered the first balanced-line products for use in the home).
A further innovation was the first use of a fully symmetrical driver circuit. The design was quite successful; Atma-Sphere is now the largest and oldest manufacturer of OTLs worldwide.Furthermore, there have been only three patents issued to OTL manufacturers since the 1950s; two of them belong to Atma-Sphere.
Atma-Sphere OTLs can drive a wider range of speakers than had been previously possible due to the reduced output impedance. Use with 8-ohm speakers is commonplace (and in the case of our larger amplifiers, 4 ohm speakers too), with far greater performance than other technologies. OTLs are a very practical choice for discerning audiophiles.