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. 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, 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 driving long communication or interconnect cables, when a predictable and low output impedance is required. References Vacuum tubes