The Unison Research P40 is an amplifier with push-pull circuit that consists of a stage with two power tubes each of which amplifies one half-wave, either positive or negative, of the audio signal. The special design of the output transformer then enables the correct reconstruction of the amplified signal at the output terminals.
The advantage of the push-pull system lies in its ability to produce greater output power than is obtainable from a single-ended design using the same type of valve. This advantage is determined by the greater efficiency of the push-pull system, and means lower heat dissipation for any given output power.
The disadvantage of push-pull stages is that, relying on the amplification of two separate half-wave signals by two separate amplifying devices, they must be particularly well engineered in order not to encounter significant distortion.
The P-40 boasts an impressive specification; 40 watts into a 6 ohm load with full-bandwidth distortion less than 0.2%. This performance is similar to that of a good amplifier using single-ended topology.