There are two major types of RIAA phono equalization systems, one is NFB(Negative Feedback) type and the other is CR type.
A representative preamplifier equipped with NFB type equalizer is the MARANTZ Model: 7.
This is a 3-step amplification NF equalizer 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 equalizer is the vintage Harman-Kardon Model: “Citation IV”.
CR type equalization 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 equalizer. 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 tubes are used.
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RS-28CX is equipped with the CR type equalizer based on GE(General Electric/USA) 12AT7 authorized by J.A.N.(Joint Army & Navy) standard of US Army, and can reproduce a very high quality and clear sound without adding any NFB.
To avoid compromising on sound quality ,one of the solutions we adapted to totally protect the sensitive CR type equalizer 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 of 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 RS28C can output about two times as much voltage, which will drive power amplifiers with very low input sensitivity.
Likewise, for using the RS28C 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 Sub-Woofer.