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The Leben CS-660P Vacuum Tube Power Amplifier
Mr. Hyodo has a particular fondness for the KT66 as used in the vintage Quad II amplifier. As a result Hyodo San decided to build his ultimate interpretation of a KT66 amp (the '66' in CS-660P) using all he's learned about music as a musician and in his many years of vacuum tube design. Thus the CS-660P was born.

I'm certainly no expert on vacuum tube circuit design but I'll attempt to describe the basic circuit of the CS-660P to the best of my understanding from Leben exporter Mr. Hontani's description: The first stage of the CS-660P is an SRPP circuit using the dual-triode 6CS7 which has two different triode stages contained within one tube. The second stage of the 6CS7 is more powerful than the first and used as a driver while the first is used for the first stage of voltage amplification. The driver stage is a 'Mullard type' and the output stage a self-biasing push-pull pentode circuit. As you can see in the accompanying photo, the layout and point-to-point wiring used for the circuit is a work of art. The quality of materials and construction is superb as it has been with all Leben products.

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Hyodo San used a silicon diode instead of rectifier tube for its "high speed, large capacity, small consumption and small size". However, because the diode supplies a large voltage immediately after turning on power, a 6CJ3 "dumper tube" provides delayed ramp-up of the output tube voltages to warm the pentodes up gently and extend their operational life. Little green LEDs on the chassis near the base of each output tube are fed from the cathode voltage of each output tube and light up to indicate when the amp is ready to play music. As you can see, the Leben CS-660P is as beautiful on the outside as it is on the inside. Leben styling cues in general evoke vintage Harmon Kardon, McIntosh or Marantz equipment from the Golden Age of audio design. The champagne color of the CS-660P casework, the gold-plated chassis screws and the contrasting hardwood side panels are very tasteful and when all the vacuum tubes are aglow and the power indicators lit, the Leben CS-660P is quite a visual feast! For those with kids or furry creatures, Leben includes a protective tube cage tastefully finished in a nicely contrasting mocha shade to prevent singeing little appendages.

Life in 31 Flavors
The Leben CS660P ships with NOS Sylvania 6CS7s, 6CJ3s and Shuguang KT66s. While Hyodo San particularly enjoys the KT66, he thought that CS660P owners might enjoy experimenting with different tube types so the CS660P can use a variety of power tubes. Two switches on the rear allow to select between a plate voltage of 450V/DC (up) and 410V/DC (down) with one switch, between a cathode resistor at 680 ohms (up) and 460 ohms (down) with the other. By using different combinations of plate voltage and cathode resistor settings, the CS660P can take KT66s, EL34s, KT77s, KT88s, KT90s, KT100s, 5881s, 6L6GCs, 6550As or 350Bs.

This wide tube type compatibility provides the owner great flexibility in voicing the amplifier to their tastes, hence the header to this paragraph and also entire review alluding to the '31 Flavors' of the Baskin-Robbins ice cream franchise. Which flavor is best? Whichever one you like best! I've been having a blast trying the different possible vacuum tubes in the Leben CS-660P. So far I have tried the Shuguang KT66, the JJ KT77, the New Sensor Russian reissue of the 1957 Genalex Gold Lion KT88, the Svetlana Winged C EL34, the National Electric 5881 and the Valve Art 350B - only a small sampling of the many possible options. This isn't a tube review so I'm not going into the details but suffice it to say that from the 45 SET-like Valve Art 350B, the suave and richly musical Gold Lion KT88 to the articulate and musical Shuguang KT6 and warm and caramel Winged C EL34, they all sounded superb. There are lots of flavors to choose from and I found that I pretty much liked best whatever I happened to be listening to at the time. They're all really pretty amazing once plugged into the Leben's magic circuit.

For input/driver tubes I've used either NOS Sylvania or NOS Tungsol 6CS7s, both wonderful in subtly different ways I won't go into. The Leben CS-660P is very revealing of the sonic and musical attributes of the various tubes, which makes tube tasting a lot of fun. I got best results when I let new tubes break in for about 20 hours at which point their full 'bloom' and 'bouquet' developed. Because of the enormous flexibility to the voicing of the Leben CS-660P's musical and sonic characteristics, pinning down one 'sound' for review is a tricky proposition. I chose the stock Shuguang KT66s in combination with the Tungsol 6CS7 black plates as the reference point. I haven't really got to where I have a clear favorite combination– they each have their merits and are all bloody good. The Shuguang KT66s and Tungsol 6CS7s just happened to be in the amp when I started writing up my final impressions so that's what I used as my reference point.
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To make the CS-660P even more flexible, Hyodo San has included a stereo-mono selector to turn two CS-660Ps into 80- watt monaural amplifiers each. The CS-660P includes a 4/6/8/16 Ohm impedance selector switch that allows the owner to pick the best sounding setting for their particular loudspeakers without disconnecting cable leads. On the front panel is a volume control to adjust the output to match speaker sensitivity so you can use the full range of the volume control on your preamp. This is particularly handy for those with more sensitive speakers often limited to the first few clicks of the preamp volume control. Once having lived with this feature, you won't want to be without it.

As if that wasn't enough flexibility, you can also order the CS-660P slightly modified to boost bass by 2dB at 100Hz. The high frequencies of the CS-660P extend to 100KHz and the boosted bass version provides a more full-bodied balance with loudspeakers of a leaner overall balance than my reference Harbeths. The CS-300X and CS-600 integrated amps include a bass boost selector on their front panels which I have found very handy depending on room interactions and speaker voicing. I wish the CS-660P included this feature as well rather than requiring you to make the choice upon ordering. Perhaps you can convince Mr. Hyodo to include a switch for you when ordering.

The versatility of the Leben CS-660P means that you will have an easier time getting good results when dropping it into your system than with most other amplifiers and tube-rollers and tweakers will be endlessly entertained exploring all the possible options.