What is OTL and why use it?

The acronym 'OTL' refers to a tube amplifier that is Output TransformerLess. Regular tube amplifiers run at very high voltage and most loudspeakers need low voltage. In order to convert from one to the other an electronic device known as an Output 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. The use of the transformer limits bandwidth (both low and high frequencies are affected), absorbs (some) power, and it is a source of distortion. OTLs sidestep many of the barriers to high fidelity reproduction that transformers cause. Increased dynamic range, wider bandwith, improved low level detail and greater transparency (due to lowered distortion) are some of the more obvious benefits.

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 their output transformer is used and to the use of zero feedback (negative feedback has its own set of issues, but we will cover that elsewhere). There is only a single power tube in a Single-Ended amp, which is 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. As a consequence, to pass a singnal through it, the magnetic field of the transformer never has to be reversed- only changed in intensity. Thus 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 cancellation at rest- no magnetic field. This increases the amount of power and bandwidth the transformer is capable of, but at a price: low level detail, since the waveform has to reverse the polarity of the magnetic field in the transformer, and such action requires a little bit of extra energy, which comes from the signal itself. This is a major issues for small signals as they can be lost or distorted as a result. Thus push-pull amplifiers lack the low level detail that Single-Ended amps have in spades.

Eliminating the transformer eliminates this issue, and with it any arguments for single ended operation.
Removal of the transformer from the signal path also reduces other degradations. There is distributed capacitance in the windings (loading the tubes), series inductance (which can contribute to distortion), hysterysis 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 ampliifers.
OTLs have suffered their own issues over the years, primarily due to the earlier efforts of Julius Futterman and the later failings of New York Audio Labs (Harvey Rosenburg). The Futterman circuit was for many years the most publically visible OTL, and it was prone to reliability problems, in particular: extreme oscillation. When in oscillation, (which could be caused by overload, tube or component failure or even layout problems), the amp had a tendancy to destroy itself. In fact, every manufacturer who has attempted to produce a Futterman amplifier has met with demise or has been forced to stop production. The fact is that the circuit is so unrefined that it should never have seen the light of day. For many years the public has associated the weaknesses of the Futterman circuit with OTLs in general.

At this point no accurate history of OTLs can ignore Ralph Karsten. In August of 1977, he created a radical new approach to OTL technology- and with it the world's first truely reliable and practical OTL. This was accomplished by using a fully symmetrical output circuit (known as the Circlotron), which resulted in low distortion. The low distortion meant that little or no feedback was required, resulting in a very stable amplifier. He was also the first to offer an OTL amplifier in a fully balanced (differential) configuration, at the same time introducing the first use of a fully symettrical drive circuit for the output section.

The design has been quite successful; Ralph Karsten has produced OTLs for over 27 years.
There have been only three patents issued to OTL manufacturers since the 1950s; two of them are written by Ralph Karsten.

Rendition's approach to OTL design allows for far greater performance then other technologies. OTLs are now a very practical choice for discerning audiophiles.

The Twenty Year Rule
Ralph Karsten developed The Twenty Year Rule 27 years ago as a design guide for quality. The idea is that audio equipment should run for twenty years without major service (other than tube replacement). It has several aspects:

• Use only in-production tubes, to help insure availability in 20 years
• Rate parts according to military derating curves for long life
• Use obtainable parts so long range servicability is assured

The original idea was simple enough: good audio gear should sound good *and* last a long time! Twenty years is a practical measure of reliability; after which likely service issues are filter capacitors and connectors, which cannot be controlled by the manufacturer. Thus the voltages chosen in the equipment are such that it is likely that replacement filter capacitors will be available. During the 1970s when Ralph Karsten developed this rule, higher voltage capacitors were getting harder to find. Further, tube types were becoming less diverse. It was apparent that tube types for his designs had to be limited to extemely common (and, naturally, available) tube types.

A further issue was that of reliability of the company itself. What happens if the company fails? Would parts be available? Naturally these concerns surface, as this has always problematic when servicing older equipment. For example, in vintage audio equipment, issues with transformers in audio are common; so we developed transformer designs to improve aftermarket possibilities should we not be around to provide replacements in the future.

This practice reduces the customer's risk. It also helps insure the viability of the company, since it makes replacement parts easier to provide. This aspect of the 20-Year Rule has been a key player in the success of Ralph Karsten's designs.

Aside from parts availability, the other concern is simple survival of the parts installed. This has been well addressed by the military, which has developed derating curves to insure maximum parts life. Rendition thus uses these guidelines for parts selection and design criteria. Careful selection of solder, connectors and sockets are based on similar concerns.

At this point the Twenty Year Rule has a proven track record, having served over 27 years in Ralph Karsten's design career. In the past, audio manufacturers that have broken tenants of this rule have suffered or vanished as a result. The Twenty Year rule helps us insure reliability and servicability (of course, a solid design helps with that also...), making our equipment a more sound (pun intended) investment.