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:
DC-OTL — denotes a directly coupled design; synonymous with OCL
AC-OTL — denotes an AC 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 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 electrolytic capacitor - 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
Circlotron type
Totem-pole type
H-bridge type
Single-ended type
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. 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 driving long communication or interconnect cables, when a predictable and low output impedance is required.
