Assembled PC board shown. The Dual Class-A II Output is offered as a bare or fully-assembled PC Board.

The "Dual Class-A II Output" is a high current audio line amplifier capable of driving low impedance headphones or long, highly-loaded, audio cables.

A unique circuit topology which emulates the original Dual Class-A pre-drives a discrete pair of BD139 and BD140 power transistors to provide a highly-linear, low distortion, Class-A, open loop output.

The DCAO2 improves on the original Dual Class-A to provide voltage gain and the option of overall, global feedback for a very low impedance "0 Ohm" output and power extension from 100 mW to >1W.

A Vbe "voltage mirror" bias technique and "common mode" pre-drive circuit topology provide very low distortion, low DC offset and tight bias current control. DC bias current control is closed loop with thermal feedback taken from the device heatsink. With the exception of a single interstage film capacitor the design is fully DC-coupled and does not require a servo to provide DC stability.

The Class-A power output stage open loop has an output  impedance that is <2 Ohms. When running closed loop, the output impedance is approximately 40 mΩ for a true "0 Ohm" output.  In either configuration the DCAO2 is stable with highly-capacitive loads.

The inputs are fully-balanced with  THAT1240 (INA134) or THAT1246 (INA137) line receivers.

Input jumpers permit both channels to (1) operate fully independently with single-ended outputs, (2) in parallel for distribution amplifiers or paralleled output, or (3) cross-coupled for fully-balanced outputs and line "re-balancing" applications.

A volume control insert point with an optional film (or electrolytic) inter-stage coupling capacitor are provided for use as a stereo headphone amplifier.

The Dual Class-A II Output has many applications:

• A stereo class-A headphone amp.
• A stereo (unbalanced) class-A line output.
• Mono fully-balanced line output with cross-coupled differential input.
• A high-current line "re-balancer."
• Stereo (single-ended) or Mono (differential) transformer driver.
• Dual output distribution amp.
• Parallel operation for double the output current without external ballast resistors.
• Passive EQ network driver.
• Control Room Monitor Switcher/Amplifier driver.
• A Simple Direct-Coupled 10W Class-A Power Amp.
• A few more things we haven't thought of yet.

Schematics, performance, construction and other information for the Dual Class-A Output can be found here on the Pro Audio Design Forum.

The Dual Class-A II is sold either as a bare PC board which does not include any components or an assembled and fully-tested module.

For DIY builds a Mouser Electronics Project Manager bill-of-materials are available here.