Output transformerless
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Output transformerless (OTL) is a type of
vacuum tube A vacuum tube, electron tube, valve (British usage), or tube (North America), is a device that controls electric current flow in a high vacuum between electrodes to which an electric voltage, potential difference has been applied. The type kn ...
audio
power amplifier An audio power amplifier (or power amp) is an electronic amplifier that amplifies low-power electronic audio signals, such as the signal from a radio receiver or an electric guitar pickup, to a level that is high enough for driving loudspea ...
, which omits an
output transformer A variety of types of electrical transformer are made for different purposes. Despite their design differences, the various types employ the same basic principle as discovered in 1831 by Michael Faraday, and share several key functional part ...
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 A loudspeaker (commonly referred to as a speaker or speaker driver) is an electroacoustic transducer that converts an electrical audio signal into a corresponding sound. A ''speaker system'', also often simply referred to as a "speaker" or " ...
. Instead, OTLs use one of two primary methods for output stage coupling: direct coupling (DC) or
capacitive coupling Capacitive coupling is the transfer of energy within an electrical network or between distant networks by means of displacement current between circuit(s) nodes, induced by the electric field. This coupling can have an intentional or accidental ...
(AC).


Additional definitions

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) — distinguishes OTL designs lacking an output coupling capacitor (directly coupled) * DC-OTL — denotes a directly coupled design; synonymous with OCL * AC-OTL — denotes an AC (capacitively) coupled design * Z-OTL — denotes a small class of variants, based on US Patent 5,612,646. While strictly speaking not transformerless, it avoids a bulky audio frequency transformer and its frequency restrictions, and essentially modulates the high impedance output of a vacuum stage with a high frequency carrier so it can pass through a small multi-winding high frequency transformer to be applied to a low impedance speaker load.


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 electric 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 An electrolytic capacitor is a polarized capacitor whose anode or positive plate is made of a metal that forms an insulating oxide layer through anodization. This oxide layer acts as the dielectric of the capacitor. A solid, liquid, or gel el ...
- which is interposed between the amplifier's output section and the loudspeaker.


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 Circlotron valve amplifier is a type of power amplifier utilizing symmetrical cathode-coupled bridge layout of the output stage. Original circlotrons of 1950s used output transformers to couple relatively high output impedance of vacuum tubes to lo ...
'' type (Examples: Atma-Sphere Music Systems, Atma-Sphere's patented technolog

/ref> Joule Electra, Tenor et al.) * ''Totem-pole'' type (Many examples, including SEPP and the White Cathode Follower) * ''H-bridge'' type (Few examples) * ''Single-ended'' type (Examples: Transcendent Sound SE-OTL, various low-power headphone amplifiers)


OTL Applications

OTL power amplifiers for driving loudspeakers require multiple tubes in parallel to obtain the required drive current. An alternative is to use high impedance loudspeakers (now rare, but the Philips produced 400 and 800 ohm speakers, such as type number: AD4690/M800). OTL headphone amplifiers are more common, as typical headphones require the current that a single pair of tubes can provide. OTL designs are sometimes also used when Line driver, driving long communication or interconnect cables, when a predictable and low output impedance is required.


References

{{DEFAULTSORT:Output Transformerless Vacuum tubes