Aurex SB-Λ 77

Commentary

Pre-main amplifier with clean drive and Λ loop.

It is equipped with Aurex's unique clean drive.
In the Clean Drive method, a distortion component detection code is added to the negative terminal side of the speaker. This detects the distortion component generated in the speaker and feeds it back into the amplifier. Then, the feedback amount is adjusted to cancel the distortion component in the amplifier. This eliminates the distortion caused by the transmission characteristics of the speaker cable and the back electromotive force generated in the speaker, and improves the dynamic characteristics.
Since there is almost no current flow in this detection cord, there is no voltage drop, and the distortion component can be detected at the speaker terminal. Since the cord also detects the impedance with the speaker cable, the voltage at the speaker input terminal becomes equal to the output voltage of the amplifier, and it is a constant voltage drive.
By adopting this clean drive method, the harmonic distortion factor when connected to a speaker is reduced to less than 1 / 10, and intermodulation distortion is also greatly improved. In addition, since the impedance of the amplifier and the speaker cable can be completely cancelled, the impedance seen from the speaker input terminal toward the amplifier is 0, and the damping factor at the speaker input terminal is infinite.

In order to improve the sound quality of the power = output loop, a Λ loop using copper is used for all elements and wires used in this path.
In order to realize the Λ loop, the aluminum electrolytic capacitor uses a copper film Λ capacitor, the power transistor collector uses a copper substrate, the emitter-base collector terminal uses soft copper wire, and copper wire is used for wiring, while the emitter resistor, which has been a problem in the past, uses a newly developed Λ resistor.
It was impossible to use copper as a resistor because of its low specific resistance. However, in the Λ resistor, copper foil is used as a resistor to realize a non-inductive winding similar to the Λ capacitor. Even when a current flows, the magnetic flux is cancelled out and behaves like a pure resistor. Therefore, the emitter resistance value is constant up to high frequencies, the reactance is low, and the transient characteristics are improved.
In addition, the generation of electrical noise due to vibration mode, which is common in film capacitors, has been solved by the vibration-free technology realized in Λ capacitors.

The equalizer amplifier section consists of a first stage circuit in which a dual transistor is connected to the dual FET 2SK146 with a cascode bootstrap, and a second stage differential amplifier in which a cascode bootstrap is connected.
The MM/MC switching is a gain switching method that reduces the number of contacts and suppresses sound quality degradation. The MC is also an ICL configuration that uses FET as an input to improve sound quality.

There is a simple 2-amp configuration between the phono inputs and the speaker jacks, and a DC configuration without a coupling capacitor between the Tuner/Aux/Tape inputs and the speaker jacks. This allows the equalizer output to be directly connected to the main input when in the direct position, thus providing a function that combines the direct equalizer and tone defeat.

The power amplifier is a low-distortion DC power amplifier with a first stage differential circuit connected to the dual FET 2SK150 with a dual transistor cascode bootstrap and a second stage differential amplifier connected to the cascode bootstrap.

A copper-evaporated Λ capacitor with simplified vibration mode and low impedance, and a VX capacitor, an improved version of the V capacitor, are used at important points in the circuit.

A newly developed electrolytic capacitor (15,000 μ F, 63 WV) is used for the power supply.
This condenser is designed to achieve low impedance and high sound quality by carefully selecting the materials used, using paste with fast response speed, low-magnification electrode foil by special etching, chemical conversion film by special method, and structure to improve sound quality.

Power transistors have a super-f with a transition frequency of 60 MHz or higher_TTransistors 2SC2565A and 2SA1095A are used. In order to prevent electrode distortion, soft copper wire is used for collector substrate, base and emitter resistance without using magnetic material. Thus, stable amplification with little distortion is possible up to very high frequencies.

A large toroidal transformer is used for the power supply section. The transformer has a three power supply system in which the secondary side is separated into a voltage amplifier stage, a power amplifier stage, a protection circuit and a lamp.

Equipped with a Rec selector switch, the recording source can be selected separately from the source being played.

Unlike general NF-type circuits, the tone circuit uses a CR-type tone circuit with separate amplifiers and tone elements.

Equipped with a high-speed protector exclusively for clean drive circuits, it protects the circuit even when the distortion detection code is miswired and notifies by flashing the power indicator.

Model Rating

Type	Pre-main amplifier
Power Amplifier Unit
Continuous output	100W + 100W (8 Ω, 20 Hz to 20 kHz) 105W + 105W (8 ω, 1 kHz)
Total harmonic distortion factor	0.007% (at rated output)
Cross modulation distortion factor	0.007% (at rated output)
Frequency characteristic	DC ~ 300 khz + 0 -3dB
Output Bandwidth (IHF, 0.05% distortion factor)	5 Hz ~ 80 kHz (8 Ω, both channel drive)
Speaker impedance	8 Ω ~ 16 Ω
Preamplifier Section
Input Sensitivity / Impedance	Phono MM : 2.5mV/47k Ω Phono MC : 0.25mV/100 Ω Tuner, Aux, Tape play : 200mV/50k Ω
Recorded output	200mV
Frequency characteristic	DC ~ 300 khz + 0 -3dB
Total harmonic distortion factor	0.007%
Tone control	Bass : ± 8 dB (100 Hz) Treble : ± 8 dB (10 kHz)
Subsonic filter	16 Hz, 6dB/oct.
Muting	-20dB
Equalizer deviation	20 Hz to 20 kHz ± 0.2 dB
Phono maximum allowable input (1 kHz, 0.01% distortion rate)	MM:300mV MC:20mV
Signal-to-noise ratio (IHF-A network, short circuit)	Phono MM:90dB Phono MC:71dB Aux:108dB
<General>
Semiconductor used	Transistor : 80 IC : 2 Diode : 40 pcs FET : 18
Power supply voltage	100 VAC, 50Hz/60Hz
Power consumption	170W
External dimensions	Width 420x Height 151x Depth 399 mm
Weight	11.5kg
Attachment	Four clean drive distortion detection cables (3.5m)