oh yeah, here's another review from Don Sachs, the tech who does most (all?) of the Harmon-Kardon Citation rebuilds and restorations for Jim McShane.
He bought one of my SP14 kits in January, and when he finished it, he decided he needed to build several for his existing customers,
because in his own words . . . .
This is the finest preamp that I have ever heard!
Here's what Don has to say about the preamp . . . and how he is adding his own knowledge of premium parts to improve it even further . . .
This is the first preamp I have ever heard that I thought was every bit as good a piece of gear as a fully restored Citation II amplifier.
Really, this preamp is so good that it is scary. It is based on the 6SN7 tube, which has always been one of my favorites.
The bigger octal tubes like the 6SN7 have a much more relaxed, fuller, and richer sound than the smaller tubes
like the 12AX7, 12AU7 or 12AT7 tubes used in many other preamps. You get all of the detail with no brightness.
The preamp is based on the SP14 board from Roy Mottram. Here is a link to Roy’s SP14 page: http://www.tubes4hifi.com/SP14.htm
I looked at Roy’s page and decided to build my version because his basic design is absolutely fantastic.
This is essentially a $3000-$4000+ commercial preamp built for $1600-$1900. It is a cost no object design without
all the ridiculous markup and hype of the high end industry. I have spent numerous hours experimenting with parts choices
based on my many years of experience restoring vintage tube gear. I know what parts sound very good in various positions in the circuit.
This preamp is incredibly good for a number of reasons. First, the audio circuit itself is a wonderful, but simple design that has
a very short signal path and really only one coupling capacitor per channel at the output. The tubes are direct coupled.
The 6SN7 is a fantastic audio tube as noted above. If you are used to the sound of small tubes you are in for a treat,
and you most likely will never go back. Small tubes just aren’t going to cut it for you!
Second, the power supply is a fully dual-mono design. Each channel has its own B+ (high voltage) and filament supplies.
This gives amazing stereo separation because each channel is independent. The B+ power supplies are tube rectified
(the BEST way to go in a tube preamp, none of that solid-state hash) which gives a soft-start and simplifies the design
compared to a solid-state design, and it also sounds better! Then the B+ is fed to a separate voltage regulator for each channel
that eliminates all variation in the supply, and reduces the noise by at least 20db, making for a deep, totally dark background.
The two filament supplies (one for each channel) use ultra-fast schottky diodes, and then the filament voltage is regulated
by a low-noise, low-impedance device that is completely isolated from the audio signals by floating the 6 volt supply about 70 volts
above ground so there is no hum whatsoever. The 6 volt DC regulated filament supplies are driven by completely separate transformer windings
for each channel. In fact there are separate filament and high voltage transformers. The power supply and filament supply design gives
stunning channel separation and the blackest background that I have ever heard in ANY preamp, tube or SS.