Review: Leben RS28CX and RS-100 Preamplifiers

pic from Audio Asylum: RS-100 on top of RS-28CX

Review: Leben RS-28CX and RS-100 Preamplifiers
Vinyl Talk: Leben vs AN Kit vs Softone/ICL

As you know, because of my preference for SET, I have always been interested in boutique Japanese tube manufacturers. Even after the pinnacle of Kondo, now my reference, my interest in other manufacturers remain.

I have to admit that my interest in Leben previously had not been particularly great. Contrary to others, I am not exactly a fan of its golden looks. Leben makes only push-pull amps and I am not interested in those. Hence, the only area left for me to explore are their preamplifiers. As a vinyl man, the one I was interested have always been the RS-28CX, but my journey started with a borrowed sample of the line preamp RS-100, a product that evolved from the RS-28CX. I was grateful for that opportunity, which allowed me to gain the all-important initial insight. Ancillary equipment used in evaluation:

Digital: Audio Research CD2 as transport/Genesis Digital Lens/Musical Fidelity M1 DAC
Analogue 1: Kuzma Stabi S/Stogi S + Midas Denon DL-103
SUT: Denon AU-1000.
Phonoamps: Audio Note Kit MM Phonostage (older version); Softone/ICL Model 4
Connection to preamp: I used my best cable, the incomparable Kondo KSL-LPz.
Preamp: Leben RS-100 line preamp (info) on loan to me
Amp: Elekit 8230 amp (2 wpc 2A3 amp)
Loudspeakers: Tannoy Canterbury.

RS-100 Line Preamplifier (Official literature)
Like its bigger brother RS-28CX, from which it is derived, the RS-100 uses 2x 6CG7 in SRPP mode for amplification, but its tube rectification is by 6X5 in lieu of 5Y3. There is also a "U" version that employs 12AU7 instead of 6CG7. You can find out all the details from Jeff Day's review in Positive Feedback. It is a handsome unit and I prefer its size and hammertone finish of the top plate to that in its brethren.

The loan period coincided with my evaluation of the Audio Note MM Phonostage Kit, and you can read about that in my previous article. Here I'd add some brief descriptions. The sound of the Leben is utterly lucid in the entire frequency spectrum. Noise level is extremely low. The sound is neutral, without much tube coloration, which is how I like it. Dynamics have an unforced quality, with full, clear bass, good rhythm and pace and microdynamics. In my 200 ft LR and with my flea powered 2A3 amp I need to turn the volume knob to 11-12 o'clock to get the volume I need. This is in marked contrast with my other preamps from Japanese SET manufacturers; with the Kondo and Wavac line preamps, the volume is plenty loud already at 9 o'clock. Considering the SET amps all have very high input sensitivity, this is understandable as Leben does not make SET amps, rather higher-powered PP amps.

Since this is purely a line stage, there is a direct in feature, which was what I used. There is a set of variable output (with a knob at the back) but as you'd expect it did not sound quite as good as the fixed output. Unusually, there is also a Stereo/Reverse knob!

I did do a brief comparison with Kondo. No, there was no miracle. The Kondo just had more air and that je ne sais quoi quality, and was even more dynamic. But, keep it in perspective, the RS-100 remains a wonderful piece.

RS-28CX Full-function Preamplifier (official literature)
This flagship preamplifer has been in existence for much longer. For details, read the same Jeff day's review in 6moons.

By chance a second-hand unit became available here and I purchased it. You might remember that I have reported a little on this preamp's phonostage performance in my recent article on LFD (here).

Where the RS-28CX really diifer from the RS-100: Not surprisingly, while the sonic signature is close to the RS-100 (that is not much signature), there are important differences not explored Jeff Day's reviews. The integral part of the difference is due to the difference in gain structure of the line stage: the RS-100 has 23 db of gain, with maximum output at 53V; the RS-28CX line has only a little more gain at 25.2 db but the maximum output is much increased at 80V.

So while the line sections of the two superficially seem similar, they behave quite differently. With the same equipment, I need only to turn the RS-28CX's volume pot to about 9 o'clock to get the volume I want, bringing the preamp in line with my Kondo and Wavac. The reason for this is undoubtedly due to the presence of the phono section (keep in mind Kondo and Wavac all have separate phonostages).

This major difference in gain structure also makes the sound different from the RS-100. The higher gain undoubtedly is more suitable for my SET amps: the sound is more dynamic, more nuanced at low volume, more vital. Not a huge difference, but instantly audible. It is still not quite Kondo, but the difference narrows a little.

I think if you are a SET amp user, or your amp is low-powered, or have inefficient loudspeakers, even if you do not play vinyl, the RS28CX will likely be a better choice than RS-100. But for Leben amplifiers the RS-100 may be as suitable for digital-only users. Given Leben's integrated amplifiers are basically amps with passive volume, I'd also advise adding a Leben preamp and use the integrated as an amp (volume at maximum) if you want to upgrade the sound.

The phonostage: On Leben's website and in the reviews, there is a lot of hooplah on the superiority of the NFB-less CR type phonostage. IMHO this is over-hyped. CR types are actually not uncommon in modern tube phonostages; my Softone/ICL Model 4 is just such a device.

I had a hard time removing the tube shields of the 2x 12AT7 in my unit. Someone had applied some gluey substance to the tubes and after drying the shields were hard to remove. I had to chip away carefully. I wonder whether this was done at the factory or by a previous owner. I replaced the stock green-latter GE tubes with much older GE black-plate 6201's to great effect.

The gain of the phonostage is spec'ed at a VERY low 20.2 db. Together with the 21 db gain of Denon AU-1000 SUT, the gain of the phono section is only 41.2 db. But let numbers fool you, the truly amazing thing about it is that it sounds like twice that! With either my 0.3 Denon DL-103 or 0.47 Ortofon Kontrapunkt C, I only need to advance the volume knob to barely past 9 o'clock, about the same as using digital sources (here thanks to the high gain line section). It is also superbly quiet. Wonderful!

Which leads me to ruminate yet again. I have seen so many vinyl people calculating gain based on manufacturers' data and debate theoretically on whether a certain phonoamp is suitable, but let me tell you many of the (mostly transistor) devices just sound like less. Or shall we say there are quite a bit of inflated figures out there. Theory and data is one thing; listening is another.

Comparison with AN Kit MM Phonostage and Softone/ICL Model 4: Not to mince words, used with the Denon AU-1000 SUT, the sound of the Leben RS-28CX phonostage is decidedly superior to either high-value phonoamps. In almost all parameters, so no more elaboration.

Previous owner's setup
RS-28CX is seen here with re-issue McIntosh 275, driving Tannoy Stirling. Sound was not bad!