LUXMAN L-68A

Commentary

Duo Beta plus-X pre-main amplifier that combines a duo beta circuit with a newly developed plus-X power supply.

The duo-Beta circuit is used to realize a superior amplifier by taking advantage of the advantages of NFB and preventing TIM distortion due to excess NFB.
By adding an appropriate amount of NFB and DC servo to the amplifier circuit that realizes a practical level even with bare characteristics, excellent medium and high frequency and tight low frequency reproduction are realized, and harmful 1/f noise and flicker noise are removed.

The power supply section uses a common feedback circuit (which feeds back changes in the output voltage and keeps it constant) and a plus-X power supply that has the advantages of an unstabilized power supply and eliminates its disadvantages.
The conventional stabilized power supply consists of a feedback circuit and uses a large amount of feedback, whereas the plus-X regulator uses an appropriate amount of feedback and has frequency characteristics.
As a result, it is possible to realize a low-frequency band with good reproducibility and damping by avoiding excessive feedback in the audible band and making use of the bare characteristics of the power supply itself (optimum error-amplifier), and by controlling frequencies to improve stability and controlling force by providing strong feedback only in the low-frequency band.

In order to realize a plus-X regulator, we are improving the bare characteristics of the power supply section.
The transformer employs a large power transformer that makes full use of Lux's transformer technology. The winding diameter is large to prevent power loss. In addition, an audio block capacitor (15,000 μ Fx2) is used in the smoothing circuit to create a power circuit with excellent bare characteristics.
In addition, the polarity is unified by aligning the winding direction of the power transformer on both the primary and secondary sides, and the polarity of all non-polar capacitors used in the power supply circuit is also unified.

All stages use push-pull circuit with excellent linearity for the power stage.
In addition to the pure complimentary service differential push-pull input stage using FET with the same characteristics of P channel and N channel, all stages use high-ft (high-cutoff frequencies) and low-Cob (collector-output capacitance) transistors and FET for push-pull. The distortion factor in bare characteristics has been reduced to 1/5 to 1/10 of conventional ones, and the frequency characteristics have been improved by approximately 10 times.

The output stage employs a power MOS FET that does not generate switching distortion in principle.
Power MOS FETs have characteristics such as no carrier accumulation effect (which contributes to switching distortion) that cannot be avoided in bipolar transistors and no phase shift. They also boast excellent high-frequency characteristics and linearity. They also have excellent characteristics as power amplifying devices, such as high gain, simplified drive stages, and excellent stability.
In the L-68A, a large amount of idle current is supplied to the power MOS FET to set the optimum operating condition, and ultra-high-speed transistors are also used in the driver stage to be combined, completing the amplifier circuit that does not generate switching distortion.

Duo-beta circuit is also used for the equalizer circuit.
The circuit configuration employs a cascode-connected bootstrap differential current mirror input stage with a carefully selected low-noise FET. It is driven with a constant current to improve SVRR (ripple rejection ratio) and has a structure excellent in CMRR (in-phase suppression ratio). In addition, a constant current load cascode amplifier is connected and a shunt circuit is provided to suppress gain.
In the output stage, a single-ended push-pull circuit with A-class operation is adopted to obtain high stability as well as low distortion and low output impedance.
As a result, the distortion factor in bare characteristics has been reduced to 1/5 to 1/10 of the conventional one, and the frequency characteristics have also been realized with about 10 times the bandwidth. As a finishing touch, the duo-beta circuit realizes excellent sound quality.

The MC head amplifier section uses a DC amplifier configuration with N-channel FET parallel connection.
In addition, by installing an impedance switch, the optimum load condition of the MC cartridge can be selected and the gain is automatically adjusted.

For tone control, LUX system NF type with turn-over frequency switching circuit is adopted with smooth changing characteristics and no waviness.
In addition, an independent configuration is adopted in which the front stage of this tone control is used as an FET cascode input buffer. When it is not necessary, it can be completely separated from the signal path by the tone bypass switch.

By controlling the polarity of film capacitors one by one and applying bias, even foil vibration is eliminated completely, and clear sound quality without coloring is achieved.
In order to minimize the influence of the metal chassis on the circuit, the chassis corresponding to the circuit board is hollowed out, and overcurrent distortion is completely eliminated by using a natural plated rosewood cabinet with minimal aluminum shielding.

Equipped with a phono straight function, functions not required for disk playback can be disconnected from the wiring.

Equipped with a line-phase sensor, you can check the polarity of the AC power supply.

Model Rating

Type	Integrated amplifier
Continuous effective output	110W + 110W (8 Ω, both channel operation, 20 Hz to 20 kHz)
Total harmonic distortion factor	0.008% or Less (8 Ω, -3dB, 20 hz to 20 khz)
Cross modulation distortion factor	0.008% or Less (8 Ω, -3dB, 60 hz : 7 khz = 4 : 1)
Frequency characteristic	Phono MM, MC-1 : 20 Hz to 20 kHz ± 0.2 dB Tuner, aux1, 2, monitor1, 2 : 10 Hz to 100 kHz -1dB
Input Sensitivity / Impedance	Phono MM : 2.0mV/50k Ω Phono MC-1 : 2.0mV/100 Ω (direct to equalizer) Phono MC-2 : 0.07 mV ~ 0.2 mV (Impedance Switching) Tuner, aux1, 2, monitor1, 2 : 280mV/40k Ω
SN ratio (IHF-A, input short)	Phono MM : 80 dB or more Phono MC-1 : 80 dB or more Tuner, aux1, 2, monitor1, 2 : 100 dB or more
Tone control	LUX System NF Type with Bending Point Frequency Switching Low-pass curve point : 150 Hz, 300 Hz, 600 Hz High-frequency bending point : 1.5 kHz, 3 kHz, 6 kHz
Attachment	Phono straight switch Subsonic Filter (15 hz, off, 30 hz) High-Cut Filter (9 khz, 0 ff, 15 khz) Low Boost Switch (70 Hz, off, 150 Hz) Tape monitor switch Tape dubbing & rec off switch Speaker selector Headphone jack Line phase sensor
AC outlet	Switched : Route 3, max300W Unswitched-1 line, max200W
Power supply voltage	100 VAC, 50Hz/60Hz
Power consumption	300W (Electrical Appliance and Material Control Law)
External dimensions	Width 478x Height 194x Depth 378 mm
Weight	15.5kg