Leben RS-28CX Pre-Amplifier

Available by special order, the RS-28CX pre-amplifier with phono stage is equipped with the CR type equaliser based on GE (General Electric / USA) 12AT7 authorised by "JAN" (Joint Army & Navy) standard from US Army and can reproduce a very high quality and clear sound without adding any NFB (Negative Feedback).

To avoid compromising on sound quality, one of the solutions we adapted to totally protect the sensitive CR type equaliser from negative effects was to develop a separate power supply unit. Likewise, some of the finest available high-end audio components were selected for the RS28C to keep the deflection of the RIAA curve below ±0.3%. Of course, the phono equalizer is only one of the strengths of the RS28C. Another notable feature is a very strong output voltage created by the dual-triode tube 6CG7 from General Electric (USA) used for the line stage.

Leben further selected the best quality tubes out of these and constructed a powerful SRPP type amplification which can supply power amplifiers with a maximum 80V of clean output signal. In considering the fact that output voltage of a standard preamplifier is more or less 40V, the RS-28C can output about two times as much voltage which will drive power amplifiers with very low input sensitivity. Likewise, when using the RS-28C with a high sensitivity power amplifier, a 5-step attenuator is present in order to eliminate residual noise from speakers. Leben also included two outputs (one fixed and one variable) in order to cope with requirements for adding a subwoofer.

Background:

There are two major types of RIAA phono equalisation systems, one is NFB (Negative Feedback) type and the other is the CR type. A representative pre-amplifier equipped with NFB type equalizer is the MARANTZ Model 7. This is a 3-step amplification NF equaliser based on 12AX7, which applies NFB from the 3rd stage cathode to the 1st stage cathode, (so-called "K-K type"). The advantage of the NFB type is that it does not require a high gain level and is not very sensitive to tube quality. However, the impedance of NFB at high frequency range becomes very low and results in a negative effect - the sound becomes hard and dry. The NFB type is usually used as it is an easier way to attain a good signal-to-noise ratio.

As for the CR type, a masterpiece preamplifier equipped with the CR type equaliser is the vintage Harman-Kardon Model "Citation IV". CR type equalisation requires a high gain level and is much more sensitive to tube quality. Since tube quality will be directly amplified, the finest possible quality tubes are needed for this type of equaliser. Moreover, proper implementation is more difficult. However, the CR type is much more superior sonically to the NFB type, being very stable in the high frequency range and is able to reproduce a very high quality sound especially when high quality valves are used.

Leben RS-28CX pre-amplifier - phono stage - rear panel.

Features:

• General Electric (USA) J.A.N. Standard Tube 12AT7: The Phono equaliser circuit is of the CR type RIAA equalisation design with zero feedback in order to bring out the superior characteristic of the 12AT7 which is authorised by J.A.N. (Joint Army & Navy) standard.
• General Electric (USA) High Output Tube 6CG7: The line stage circuit uses the twin-triode tube 6CG7 of General Electric (USA) SRPP system and can supply a signal of up to 80V to the power amplifier. Therefore the RS-28CX can drive all kinds of power amplifiers of low-sensitivity or low-driving capabilities.
• High-Grade Parts: The highest quality available components (capacitors, resistors, etc) are used for all essential parts which affect the sound quality.
• Independent Power Supply Unit: In order to shut-out any influences from the power transformer to the main circuit, the dedicated power supply unit is housed in a separate chassis.
• Muting 30 minutes after power-on.
• Natural wooden side panels.

Awards:

• France - Revue du Son 5-STARS - July / August, 2006

Leben RS-28CX pre and phono amplifier.

Specifications:

• Valves: 2x 6CG7 (GE); 2x 12AT7 (GE); 1x 5Y3WGT
• Gain: 20.2dB
• Phono Stage: RIAA curve less than ±0.3dB
• Phono Stage: Max. input 400mV (2kHz)
• Phono Stage: Gain 25.2dB
• Frequency Response: 10Hz - 50kHz (-3dB)
• Max. Output 80V (Variable) & 80V(fixed)
• Output Impedance: 580 Ohms
• Remaining Noise: 0.03mV
• Power Supply: 240V / 50Hz
• Weight: Main Unit: 7 kg (approx); Power Unit: 6.5 kg (approx)
• Dimensions: Main Unit: 35.2cm x 24cm x 17.5cm; Power Unit: 23.3cm x 24cm x 17.5cm (W x D x H - approx.)

Leben RS-28CX with the CS-660P power amplifier.