TAS Legacy: Harman Kardon Citation II Power Amplifier

Tubed power amplifiers
TAS Legacy: Harman Kardon Citation II Power Amplifier

The Citation II was Stu Hegeman’s penultimate masterpiece, inspired by his unwavering desire to build a power amplifier equal to his stunningly great Citation I preamplifier. I consider the Citation II power amplifier to be one of the most original designs in audio history, introducing technology that had never before been seen in an audio product. The Citation II’s high-gain, wide-bandwidth video-pentode design yielded performance that was difficult to believe.

Introduced in 1959, the Citation II sold for $159 in kit form or $229 assembled (about $2000 in 2017 dollars).

Stu Hegeman’s circuits produced a sound that had a life and a breadth and image depth that were stunning. They were the ultimate in soundstaging and sense of immersion. The Citation II’s circuit was most elaborate, the thinking behind it exceptionally advanced, the sound spectacular! It consisted of a pentode input stage, followed by a pair of 12BY7 video pentodes as a differential phase splitter (the video pentode supplied a bandwidth of well over a megahertz in the input stage). The video pentodes drove the grids of the output tubes to full power. This had never been done before, and it resulted in a wide power response that was second to none. The output stage utilized a pair of KT88s for each channel, each valve having its own bias control as well as an AC balance control. (A small meter at the chassis rear measured bias and AC balance.) Three feedback loops provided 32dB of overall feedback with unconditional stability. The output transformers were superb, built by the Freed company in New York. Huge, well-potted units, they boasted extremely wide response thanks to the ultra-low leakage inductance. Only the highest-grade core materials were used, which lowered the effect of core distortion to a region well below the limit of human hearing. With feedback, the Citation II transformers were capable of high-frequency response up to 270kHz, a response that most designers of the time thought could not be achieved.