Audio Database

AU-7900
Commentary

This is a pre-main amplifier that realizes a power with a margin and a low distortion factor by using an original input / output terminal structure and a design policy focusing on the power supply.

The power amplifier section uses a pure complimentary service circuit directly connected to all stages.
The first stage dual transistors is adopted for the first stage, and Darlington 3-stage configuration with active broad is adopted for the drive stage. In addition, the power supply for the pre-drive stage is separated from the power supply for the output stage. This improves linearity and reduces the occurrence of non-linear distortion even when a transient signal enters.

The output stage uses a parallel push-pull the output stage. The AU 7900 combines four power transistors with the same characteristics per channel and improves the linearity of current characteristics to achieve a low distortion rate and high output.

The equalizer uses an NF-type equalizer circuit consisting of four low-noise transistors. The first stage input is a differential amplifier with a good DC balance. The + / - 2 power supply with a total high voltage of 51 v is supplied in a well-regulated manner to ensure a sufficient margin.
A carefully selected CR element is used for the equalizer element.

The Phono1 input allows 3-step switching of impedance.

The input terminal on the back has a unique structure in which terminals are mounted upward.
With this structure, it is not necessary to use a shield wire from the input terminal to the selector, and degradation of high-frequency characteristics due to stray capacitance can be prevented.

The preamp section and main amplifier section can be separated by a switch on the back panel.

Low-noise transistors and high-precision CR elements are used in the tone control circuit. These parts and switches are directly mounted on a single board to prevent lead wires and shield wires from being routed.
The circuit configuration is an NF type with differential amplification at the first stage and stable operation with ± 2 power supply. The variable resistor of the AU 7900 tone control uses a detent volume with the same performance as the attenuator and less error.
The tone control is Sansui's own triple tone control for adjusting the mid-range sound quality, and the tone selector for three levels of Bass and Treble turnover frequency.

Equipped with a tone defeat function.
In defeat mode, the tone control and filter circuits are bypassed, and the signal from the buffer amplifier after the volume is fed directly to the main amplifier.
The AU 7900 can be switched between two types : tone control and filter circuit, or only filter circuit.

The power supply section uses a large power transformer and combines a silicon diode for rectification and a 15,000 μ Fx2 large-capacity power supply capacitor to provide ample room.
In addition, this large power supply unit supplies a stabilized power supply to all block stages except for the power stage and drive stage, and supplies ± 2 power supply with good regulation and strong resistance to load fluctuation at the time of Dainyu force.

As a protective circuit, an output terminal DC detection circuit combining an electronic circuit and a relay, an abnormal temperature detection circuit, and an ASO (safe operating area) detection current limiter circuit are mounted.

By adopting a chassis-less structure and a one point grounding system in which a power transformer is installed independently, the deterioration of sound quality is reduced.

A spare AC outlet is installed.

Equipped with a two step switching loudness control, you can switch between the normal high and low boost and the correction boost for only the low range.

Equipped with low filter and high filter.

Equipped with a muting function.
The AU 7900 is a 2-step switching type of -15dB and -30dB.

Equipped with a mode selector, switching between reverse, normal and mono is possible.

Model Rating
Type Pre-main amplifier
Effective power
Both channel operation :

Single channel operation :		 75W + 75W (4 ω, 8 ω, 20 Hz to 20 kHz)
80W + 80W (4 ω, 8 ω, 1 kHz)
130W + 130W (4 Ω)
80W + 80W (8 Ω)
Total harmonic distortion factor 0.1% or Less (at Rated Output)
0.08% or Less (at 1/2 Output)
Cross modulation distortion factor 0.1% or less (70 Hz : 7 kHz = 4 : 1, SMPTE)
Power Band With (IHF) 5 Hz ~ 40 kHz (8 Ω)
Frequency Response (at 1W) 10 Hz ~ 50 kHz + 0 -1.5 dB
RIAA deviation 30 Hz to 15 kHz ± 0.3 dB
Damping factor 80 (8 Ω)
Channel Separation (1 kHz) Phono1, 2 : 50 dB or more
Aux1, 2, Tuner, Tape1, 2 : 55 dB or more
Power amp : 60 dB or higher
Hum and noise (IHF) Phono1, 2 : 75 dB or more
Aux1, 2, Tuner, Tape1, 2 : 90 dB or more
Power amp : 100 dB or higher
Phono maximum allowable input 250 mv (1 khz, THD0.1% or less)
Input Sensitivity / Impedance (1 kHz) Phono1 : 2.5mV/30k Ω, 50k Ω, 100k Ω
Phono2 : 2.5mV/50k Ω
Aux1, 2, Tuner, Tape1, 2 (PIN) : 130mV/50k Ω
Tape1(DIN):130mV
Power amp : 700mV/50k Ω
Recorded output Tape rec1, 2 (PIN) : 130 mV
Tape rec1(DIN):30mV
Tone control Bass : ± 13 dB (50 Hz)
Midrange : ± 5 dB (1.5 kHz)
Treble : ± 13 dB (15 kHz)
Tone selector Bass : 600 Hz, 300 Hz, 150 Hz
Treble : 2 kHz, 4 kHz, 8 kHz
Loudness Low, High Boost : + 8 dB at 10 kHz, + 10 dB at 50 Hz
Low Boost:+10dB(50Hz)
Low filter -3dB, 20 Hz (12dB/oct.)
-3dB, 60 Hz (12dB/oct.)
High filter -3dB, 7 kHz (6dB/oct.)
-3dB, 12 kHz (12dB/oct.
Muting 0 db, -15dB, -30dB
Power consumption 176W (rated)
490W maximum
External dimensions Width 430x Height 132x Depth 340 mm
Weight 14.2kg