Linear Tube Amplifier (ZOTL)

A linear audio amplifier includes a push-pull pair of vacuum tubes operating in a linear amplification mode coupled through a pair of dc-dc switching power converters to an external load impedance. Each power converter includes a transformer with one or more secondary windings that drive rectifier circuits, and the resultant dc voltage sources are loaded by their respective tubes.

The power input ports of two bridge power converters are connected in a series totem-pole fashion with the minus power input port of the top bridge connected to the plus power input port of the bottom bridge. A center-tapped fixed voltage source is connected across the two-bridge pair such that the positive voltage is connected to the plus power input port of the top bridge, and the negative voltage is connected to the minus power input port of the bottom bridge. One side of an external load impedance is connected through a noise filter to the junction point of the two bridges, and the other side of the external load is connected to the center tap of the fixed voltage source.

The output power ports of each converter drive the primaries of their respective converter transformers. The effective turns ratio between primary and secondary windings of these converter transformers determine the voltage/current step-up/step-down relationships between the tubes and the external load impedance. - Patent US 5612646 A.

Urban HIFI Audio manufactures and sells the linear tube amplifier in a number of models: microZOTL, headphone amplifier, the ZOTL 10, 10 watts amplifier, and the ZOTL 40, 40 watts.