I may be a latecomer to this thread, but I'm pretty darned pleased with my modded HK Citation Twelve. I bought it from a fellow AK'er and it's been my main office/workshop testing amp. It is fantastic with a Carver C-1 BillD modded front end.
It is a 60 WPC MOSFET power amp based on Nelson Pass’ modification of Harman Kardon Citation 12. This was the first power amp Mr. Pass designed.
The sonic signature of this amp is best characterized by this paragraph from Nelson Pass original article: “The amplifier is sonically a significant improvement over the original, particularly in the high end where the Citation 12's veiled characteristic is replaced by a detailed, somewhat sweet sound. The imaging and midrange definition are also much improved; but the bass response (one of the Citation 12's strongest points) remains much the same…”
What’s left from the original Citation 12 is chassis, two power transformers, fuse holders and some power wires. Original PCBs were replaced with MF-12C ones purchased from diyaudio member Tazz. Schematic is an improved “complementary version” of Nelson Pass original design and can be found here:
https://www.passdiy.com/pdf/citation.pdf
The last owner, who did the mods, used 1% metal film resistors, mica and polypropylene capacitors and Nichicon Muse and ELNA Silmic II electrolytes. Transistors were matched by HFE as close as possible. Input jacks were upgraded to gold –plated ones and output terminals to heavy-duty 5-was binding posts. Input signal wire is now shielded and output wire is 16 AWG UP-OCC copper.
Rectifiers in the power supply were replaced with IXYS HEXFREDs and filtering capacity bumped up to 16400 mkF per channel. The original power cord was replaced with IEC socket featuring built-in power switch.
The original Citation Twelve generated quite a bit of hum which originated from power transformers and from loosely attached top cover. To minimize that hum, he installed an aluminum plate between transformers to provide some electromagnetic isolation and also serve as support for the cover. In addition, several new screws were added to firmly attach cover to the main chassis body and new rubber feet were installed. All these mods greatly diminished hum but not eliminated it completely. However, this hum is not heard in speakers: in my system the amp is dead quiet.
Original AA article by Nelson Pass
https://www.passdiy.com/project/amplifiers/build-a-mosfet-citation-12
The replacement MOSFET boards used are
Citation 12 complimentary Mosfet version PCB's
DIY Audio thread on this amp
Pass Citation 12
Another person doing the same mods but without the tazz MF-12C board
https://www.passdiy.com/gallery/projects/132/fbj1