Output transformerless

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: Background: The output coupling method of a vacuum tube amplifier generally serves two basic purposes: 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.

There are several practical approaches to the design of an OTL amplifier's output section, each with their own respective strengths and weaknesses.

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 designs are sometimes also used when driving long communication or interconnect cables, when a predictable and low output impedance is required.