Output transformerless (OTL) is a type of vacuum tube audio power amplifier, which omits 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). 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