This blog post, focusing on the Leben CS-600 and CS-600X tube amplifiers, began as a thorough introduction to vintage tubes after I bought a Leben CS-600 in 2024.
It expanded to cover other Leben Hi-Fi Stereo Company amplifiers, from early models to their latest tube-based products, with a focus on integrated amplifier designs. In addition, it turned out to be the beginning of the Objects for an Ideal Home series.
Key features of the Leben CS-600
- Excellent build quality – I expected a lot, but the amp exceeded those expectations -with substantial weight, robust build, refined finish, and exceptionally smooth, precise controls
- Aesthetically superior
- A completely silent background, regardless of the input source
- Numerous inputs supporting a wide range of configurations
- The option to connect a dedicated preamplifier via the ‘pre-amp in’ inputs
- Tape out, which I use to connect a USB sound card to do sampling from vinyl records
- High resale value.
- Balance and bass boost function (which I don’t use)
- Headphones output
- Compatible with a wide range of output tubes
- 6CS7 input tubes very reasonably priced when I got it
- Sufficient power output for a wide range of speakers (28 watts with EL34 family and 32 watts with 6L6 family) of output tubes
- Heavy-duty power transformers
- Wide range of impedance settings to match different speakers
- Possibility of bypassing several circuits using the tape-in connections
- Rock-solid stability and flawless operation
- Differences between the Leben CS-600 and CS-600X
Input and output tubes in the Leben CS-600 vs CS-600X
Both the Leben CS-600 and CS-600X have four driver tubes facing the listener – behind these are four output tubes. In the driver stage of the CS-600X there are two 12AU7A and two 12BH7A tubes; in the Leben CS-600 all driver tubes are of the 6CS7 variety.
In the CS-600X the distribution of driver/preamplifier tubes is: 12AU7A; 12HB7A; 12AU7A and 12BH7A.
In addition, there’s a 6CJ3 damper tube in the CS-600 and a 6CM3/6DN3 damper tube in the CS-600X.
A word of warning on changing tubes
From the Leben CS-600 manual: “Before removing a top cover for replacing tubes, please confirm the power cable is disconnected and the tubes are cooled down”.
Yoshi Hontani’s on tube placements
I’ve corresponded with Yoshi Hontani, Leben’s Japanese distributor and general contact for the English-speaking world, who explained tube positions this way: The two left tubes are for the left channel; and the two right tubes are for the right channel.
This gives us: Tube 1 (from the left) is the first stage for the left channel; Tube 2 (from the left) is the second stage for the left channel; Tube 3 (from the left) is the first stage for the right channel; Tube 4 (from the left) is the second stage for the right channel.
The selection of driver tubes for the CS-600X is much broader, offering both the option of NOS (New Old Stock) tubes and modern production tubes. Many seem to prefer the sonic signature of the older brother because it’s more transparent, precise, airy with a wide sound stage, whereas the CS-600X is more lush, warm and what could be termed tube-like in character.
Another difference between the two is the choice of damper tube used in the amplifier design. One uses a 6CJ3 damper tube, while the other uses a 6DN3 damper tube.
Tube upgrade set for Leben CS-600
Bypassing the input selector in the 600 and 600X
In a recent review of the CS-600X it’s suggested to use the tape-in input to bypass the input selector for improved sound by a shorter signal chain:
“Incidentally, you’ll get better sound from the CS600X using the tape monitor input rather than the regular line inputs. Why? Because the tape monitor bypasses the main selector, resulting in a shorter more direct signal path. Try it!”
With a setup of this caliber, and considering the price of the amplifier, I thought it wise to ask Leben directly if this was a safe and recommended way of connecting things.
“In a recent review in (Stereonet) it’s suggested to use the tape-in input to bypass the input selector for better sound, see below. Is this considered safe / beneficial use of the function if no recording device is connected to the tape-out?
Yoshi Hontani replied quickly:
“Yes, this is true. You can use the TAPE MONITOR directly to receive a Phono input (for example).”
“I’m planning some comparative listening sessions to hear whether this setup makes a meaningful difference.
On tube-rolling / tweaking: Blessing and curse
Of course these characteristics change with different tube combinations, and that’s one of the charms that attracts some people to tube amplifiers. Over time our hearing evolves, our preferences shift, our speakers can change, our RIAAs and DACs, the synergy of which can be tuned by different choices of preamplifier and driver tubes.
Admittedly, this can also be a curse, an ongoing tweaking and split testing potentially leading to an overemphasis on voicing at the expense of the actual listening experience.
In a sense, I think Leben’s motto ‘A Motion Sound’ is an attempt to transcend the more analytical layers of the Hi-Fi experience – I find people usually reflect on a movie before and after watching it, not during.
Supplies of 6CS7 tubes have dried up lately
The supply of NOS (New Old Stock) 6CS7 tubes has quickly been depleted during the last couple of years. “The Tube Store,” the American reseller of vacuum tubes has addressed this issue on their website, providing a notice stating that their supply is extremely limited in comparison to earlier, and as a result they are no longer able to offer matched pairs or quads of 6CS7.
“Note to Leben [CS-600] amplifier owners, we do not have matched sets at this time. These tubes are very difficult to find. Only odds and ends of various brands available,” The Tube Store, August, 2023.
In general this phenomenon is called “Vacuum Tube Extinction” and is something one should be aware of when investing in a tube amplifier.
The CS-600X manual has a section on why 12AU7A and 12BH7A tubes were chosen for the new first stage of amplification:
A huge quantity of these tubes (12AU7A and 12BH7A) were produced by many manufacturers in the past and many high quality NOS tubes are available on market, therefore, you have a chance to try several different NOS tubes by replacement. Apart from it, these tubes are still being produced for musical instruments and for audio amplifiers as well, and there is no fear of replacement tubes in future as well.
To sum up, only 6CS7 tubes can be used in the CS-600 , and only 12AU7A (ECC82) and 12BH7A tubes can be used in the CS-600X.
Circuitry in the CS-600X from the CS-600X manual
“After the first stage amplification by SRPP through the first triode unit of 12AU7A and 12BH7A, make a phase reverse by Mullard type cathode connection through the second triode units, then transmit to the last stage.
Self-biasing circuit (Auto-biasing circuit) will protect the output tubes EL34 (or 6L6) from damage. A characteristic of the damper tube(6CM3/6DN3) is to raise a high voltage gradually, which will affect very much to extend a life of the output tubes.(6DW4B also can take place of 6CJ3).
The Bass Booster of CS600X is controlled by capacitors arranged on the “NFB loop”, that is a very rare method. Bass(or treble) control is
normally controlled by a tone control circuit, however, the above method makes the circuit much simpler.
Furthermore, in order to cut a negative influence of the Bass booster to the middle frequency, 15Kohm resistor(filter) is arranged in the circuit.”
Text quoted from Leben CS-600X manual.
Leben CS-600 factory specifications
Tubes employed:
4 X 6L6GC (5881) (output tubes)
4 X 6CS7 (input / driver tubes)
1 X 6CJ3 (6CM3)
Output power: 32W X 2 (6L6GC)
Frequency response: 10 Hz – 100 kHz (-0.1 dB).
THD: 0.7% -(1KHz/10W)
Input sensitivity: 900 mV at 20 W.
Output impedance: 4/6/8/16 ohms (selectable)
Headphone output: 1000 mW.
Dimensions/size: 450 (W) x 360 (D) x 142 (H) mm.
Weight: 22.5 kg.
As far as I know, there haven’t been any changes to these specifications.
Tube settings table / overview of output tubes
6L6, KT77, 6L6GB, 350B, KT66, KT88, 6550A
There is a wide selection of tubes currently in production from different manufacturers. In addition to the 6L6/5881 and EL34, both amplifiers are compatible with KT77, 6L6GB, 350B, KT66, KT88 and 6550A tubes.
Some of the tubes are considerably taller than the classic 6L6/5881 and EL34, so you’ll need to remove the lid to use these. In this way, the design accommodates both tube collectors who were lucky (or clever) to buy NOS tubes when the prices were at a completely different level from what they are now for the best tubes from the best brands.
6CJ3 damper tubes in Leben amplifiers
The Leben CS-600 and the Leben CS-600X both employ so-called damper tubes in their designs. The CS-600 utilizes a 6CJ3 tube, and the CS-600X uses a 6CM3.
EL34 RFT tubes including measurements on Roetest V10 tester
My first experiment with another type of output tubes is four New Old Stock EL34 tubes by the German manufacturer RFT marked ‘Philips’. The tubes are now on their way from Croatia.
Testing them out is a project for the near future. RFT stands for Rundfunk- und Fernmelde-Technik. Below are the measurements for each EL34 tube performed by a professional on a Roetest V10 tube tester. I’m looking forward to listening to these and compare them to the stock 6L6 Sovtek tubes the amplifier came with from the factory.
Current setup
Leben CS-600: Sylvania, General Electric, Raytheon, Dumont, Tung Sol 6CS7 tubes, black and gray plates+ Russian 6P3S-E output tubes (Reflector, New Old stock), Philips EL34, Sovtek 6L6GC / 5881. 6CJ3-family damper tube (General Electric, Sylvania, Raytheon).
Speakers: Harbeth M30.1 speakers. I use Snell K II speakers for reference.
Tonearm cable: Yannis Tomé Audio Cables, Coppercord, Silverord Duo24.
Interconnect: Yannis Tomé Audio Cables, Clio silver, Coppercord, Silvercord Duo 24. For reference: Belden 8402.
RIAA: Leben RS30-EQ RIIA (Hytron 6X5GT rectifier tube + two Telefunken ECC81s)
Turntable: Project X2, with upgraded RCA connectors
Cartridge: Goldring 2400
Speaker cables: Mogami 3103. Pearl Street Station speaker cables (Duelund-inspired), solid-core copper in cotton and linseed oil.
Other amps: For reference: Kenwood M1 power amplifier.
A Short History of Japanese Leben Hi-Fi Stereo Company
Leben Hi-Fi Stereo Company, established in 1995, is a Japanese manufacturer of high-end tube-based preamplifiers, power amplifiers and integrated amplifiers, and, so far, a single tube-based RIAA, the Leben RS30-EQ.
Leben is a German loan-word that translates into ‘to live’, ‘to be alive’, ‘lively’ or simply ‘to exist’. Newer models contain the tag-line ‘A motion sound‘ adding a feeling of experiencing music in a Haruki Murakami-like atmosphere.
Taku Hyodo, Leben’s founder
Mr. Taku Hyodo, an experienced audio engineer with a past working for Luxman, is a collector of new old stock vacuum tubes, for example 6X5GT, 3CSS, 12AT7, E288CC and 6L6GCs, in addition to being a music lover with a professional background playing the guitar.
At 60, Mr. Taku Hyodo left Luxman in 1979 to establish his own company, “Kouri Denki Corporation”. He kept designing and constructing amplifiers under the ‘KFH’ brand name – primarily custom-made products for café, bars and restaurants, also a somewhat Haruki Murakami-like project.
The first product to hit the market, the ‘Triode 33’
Mr. Taku Hyodo’s first commercial product to see a broader market was the ‘Triode 33’ power amplifier released in 1991 followed by the RS-35A 6L6GC tube-based power amplifier under the Leben brand in 1995.
Leben released the RS-28C preamplifier (equipped with E288CC tubes) in 1998. To cater to a wider range of audiophiles, Leben subsequently introduced a series of more affordable amplifiers: the CS-200, CS-250, CS-300, and CS-300X, released in 2001, 2002, 2003, and 2005, respectively.
Broader access to the products from Leben
These amplifiers allowed a broader audience to experience the unique sound quality associated with Leben products.
In 2005 Leben released the Leben CS-600 amplifier featuring four 6CS7 tubes for the driver stage and four 5881 or EL34 output tubes.
The CS-600 was succeeded by the CS-600X in 2019, with the design remaining mostly the same, except for the type of driver tubes.
Point-to-point wired tube amplifiers are rarely seen nowadays, as they require a skill set few modern workers have in the world of general mass production.
Leben is one of few amplifier manufacturers that feature amplifier designs of this type thanks to a a small number of highly skilled employees working in the company’s quarters in Nishi-Koya in Amagasi City, Japan.