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
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
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
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
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
• 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.