Output transformerless (OTL) is a term which describes a subset of vacuum tube audio power amplifier topologies, all of which omit an output transformer for the purpose of greater linearity and fidelity. Conventional vacuum tube amplifier designs rely upon an output transformer to couple the amplifier's output stage to the loudspeaker. Instead, OTLs use one of two primary methods for output stage coupling: direct coupling (DC) or capacitive coupling (AC).
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There is some contention with respect to applying the broader term "OTL" to capacitively coupled designs and variants. The need to delineate these designs from their directly coupled counterparts has led to the informal adoption of several additional terms, including:
- OCL (output capacitorless) - which distinguishes OTL designs lacking an output coupling capacitor (directly coupled)
- DC-OTL - which denotes a directly coupled design; synonymous with OCL
- AC-OTL - which denotes an AC (capacitively) coupled design
- Z-OTL - which denotes a small class of variants
Differentiation of designs
By coupling methods: direct versus capacitive coupling and variants
Background: The output coupling method of a vacuum tube amplifier generally serves two basic purposes:
- Negation (blocking) of high DC voltages in the output section to prevent a damaging flow of direct electrical current through the loudspeaker.
- Matching the relatively high output impedance of the conventional vacuum tube to the relatively low impedance of conventional loudspeakers.
Direct coupled designs
In direct coupled OTL designs, both the necessary blocking of DC and matching of impedances are accomplished, respectively, through the topology of the amplifier's output section and the selection of vacuum tube types with sufficiently low impedance to allow effective power transfer to the loudspeaker. Typically, direct coupled OTL amplifiers will have a user-adjustable DC offset control, which allows the user to trim off any residual DC voltage residing at the amplifier's output terminals prior to operation. Servo-controlled variants also exist.
Capacitively coupled designs
Like the direct coupled designs, capacitively coupled designs rely on the selection of tube types with a sufficiently low impedance to effect the transfer of power to the loudspeaker. However, unlike direct coupled designs, capacitively coupled designs do not have inherent DC blocking by virtue of their topology. Instead, DC voltage in the output section is blocked by an output coupling capacitor - typically a large-value (3000-6000μF) electrolytic capacitor - which is interposed between the amplifier's output section and the loudspeaker.
While the vast majority of designs in service can be clearly designated as directly or capacitively coupled, there are some rare, novel exceptions which may defy definition in standard terms. These designs employ what would appear to be a type of transformer at the output, but due to the atypical design and operation of this output coupling device (transformer), these designs are currently included in the larger group of OTL amplifiers, although the inclusion is often contentious.
Among engineers, designers and enthusiasts there is continuing and often lively debate on this subject and, as yet, no firm technical consensus exists as to how these designs should be classified.
By output section topology
There are several practical approaches to the design of an OTL amplifier's output section, each with their own respective strengths and weaknesses. While certain topologies lend themselves well to direct coupling, others are more suitable for capacitive coupling. The various designs in service may thus be grouped based upon their common output section topologies. Common topologies include:
- Futterman type and variants (Examples: Counterpoint, Fourier, Julius Futterman, New York Audio Labs (NYAL), Prodigy Audio Labs., Silvaweld et al.)
- Circlotron type (Examples: Atma-Sphere Music Systems, Joule Electra, Tenor et al.)
- Totem-pole type (Many examples)
- H-bridge type (Few examples)
- Single-ended type (Examples: Transcendent Sound SE-OTL, various low-power headphone amplifiers)