Audio Database

AU-707
Commentary

The first pre-main amplifier of the 707 series was developed in pursuit of physical characteristics under the development theme that new sound is produced by a novel circuit that understands the essence of audio.

When a program source with a large dynamic range and a dense music signal is reproduced by an amplifier with a large bare gain and a large amount of NF applied to improve the distortion, transient intermodulation distortion (Transient Intermodulation) occurs due to an unstable element such as delay in the amplifier circuit, and the resolution deteriorates. Transient Intermodulation Distortion (TIM) occurs at the summing point, which is the synthesis point of the input signal and NF signal, because the NF operation is momentarily lost due to the delay in the circuit, and an excessive input enters the amplifier. Therefore, it was difficult to achieve a wide range with conventional amplifiers.
In the AU 607, we examined the bare characteristics before applying NF, and expanded the frequency characteristics, improved the phase characteristics, and reduced the distortion factor. We also improved the phase characteristics up to several tens of MHz by combining a unique phase correction circuit with an appropriate stagger ratio for phase advance and phase delay. By applying the minimum necessary NF, the transient intermodulation distortion is improved. As well as excellent response in the ultra-low frequency range, response in the ultra-high frequency range is improved, and wide-range playback is enabled.

The power amplifier section uses a DC amplifier configuration.
A newly developed dual FET with the same characteristics is used in the first stage. This device has a feature that allows a large current to flow approximately four times that of conventional FETs. Taking advantage of this feature, it improves the slew rate and prevents the generation of transient intermodulation distortion at the summing point where the input signal and NF signal are synthesized.
The second stage employs a differential two stage amplification configuration using a low-noise transistor.
In this circuit, gain is suppressed as much as possible to improve bare characteristics. In addition, a unique current mirror circuit and cascode connection are used. In addition, current differential operation is performed from the single-end output of the second stage. The pre-drive stage is configured. Because this circuit is a push-pull circuit, the second harmonic wave is canceled out, and the circuit configuration is excellent in bare characteristics and distortion rate characteristics. Moreover, it is not affected by temperature characteristics and power supply fluctuation. The transistor of this pre-drive stage is carefully selected with a particularly small hoe (emitter grounding, output conductance). In addition, a series resistor is added to the emitter side to eliminate linearity deterioration due to hoe.
The output stage is a 3-stage Darlington configuration. This reduces output impedance and reduces distortion. In addition, the output transistor has good high-frequency and pulse characteristics, and stable operation with margin is obtained.
As for the drift of mid-point potential, which is a problem in DC amplifiers, the DC amplifier configuration is made after realizing stability of drift by suppressing power supply fluctuation thoroughly so that the first stage of the amplifier is not affected by the output, and improving the symmetry (pair characteristic) of the summing point.

The equalizer amplifier section consists of 8 transistors consisting of 1 stage differential with first stage current source, 1 stage class A amplifier with buffer and activator, and final stage pure complimentary service SEPP output-circuit. The signal system consists of 6 transistors. As with the DC power amplifier section, the circuit design is simple and substantial. Bare characteristics are emphasized, and low distortion is realized in a wide range outside the audible band (10 hz to 100 khz).
In particular, a low-noise transistor is carefully selected for the differential input, and a current source circuit is added to improve the CMRR (in-phase component rejection ratio) at the summing point where the input signal and the NF signal are combined to improve the dynamic characteristics.
In addition, a pure complimentary service SEPP circuit is used in the EQ output stage to obtain a high signal-to-noise ratio and sufficient dynamic merging together with sufficient gain.
The EQ element uses metal film resistors and capacitors with low errors to achieve excellent RIAA deviation.

The tone control section uses a 7-transistor configuration of differential amplification buffer amplifier consisting of differential amplification with first stage current source + 1-stage amplification + 3-transistor configuration.
This circuit is equipped with a tone selector that allows switch between high and low crossover frequencies for a wide range of tone adjustment, and with a tone defeat switch that provides flat frequency response with a flat amplifier that eliminates tone circuits.

Two power transformers are used in the power supply section to ensure that the left and right channels are completely independent, thereby preventing crosstalk to other channels and improving regulation.
In addition, an electrolytic capacitor of 15,000 μ Fx4 is installed as a power supply capacitor to supply ± 2 power supply with good regulation to each circuit. In addition, by adopting a metallized film capacitor and constant voltage power supply circuit, the internal impedance of the power supply, which affects the reproducibility of sound quality, is reduced. As a result, good articulation in the low sound range and good sound thickness and resolution in the middle and high sound range are obtained.

A DC detection circuit for the output terminal and an ASO (safe operation area) detection current limiter circuit are built in as protection circuits.

High-precision 4-row detent volume is used for the volume.

Equipped with a subsonic filter, it can cut unwanted noise in the ultra low frequency range.
In addition, it is equipped with a high filter and can cut high-frequency noise such as scratch noise.

The preamp section and main amplifier section can be separated.
DC input and AC input are mounted on the input terminal of the main amplifier section.

You can use 2 circuits of the tape deck and enjoy the tape play copy function.
In addition, air checks can be performed while listening to a record.

Equipped with an audio muting switch, the volume can be reduced by -20dB with one touch.

Equipped with a loudness function.
Other features include a center-click balance control, a headphone jack, a spare AC outlet, and a large ground jack.

Model Rating
Type DC pre-main amplifier
Power Amplifier Unit
Effective output (both channel operation) 85W + 85W (8 Ω, 20 hz to 20 khz, 0.03% THD)
85W + 85W (8 ohm, 1 khz, 0.003% THD)
Total harmonic distortion factor (20 Hz to 20 kHz at effective output) 0.03% or Less (8 Ω)
Intermodulation distortion factor (70 hz : 7 khz = 4 : 1) 0.03% or Less (8 Ω)
Output Bandwidth (IHF, both channel operation, THD 0.03%) 5 Hz ~ 50 kHz (8 Ω)
Damping factor (IHF, both channel operation, 1 kHz) 60 (8 Ω)
Frequency Response (1W) DC ~ 200 khz + 0 -3dB
Input Sensitivity / Impedance (1 kHz) 1V/47k Ω
Signal-to-noise ratio (IHF, A-network, short circuit) 115 dB or more
Channel Separation (1 kHz, Input Short) 75 dB or more
Preamplifier Section
Input Sensitivity / Impedance (1 kHz) Phono1, 2 : 2.5mV/47k Ω
Aux, Tuner, Tape play1, 2 : 150mV/47k Ω
Phono maximum allowable input (1 kHz, THD 0.01%) 320mV
Output Level (1 kHz) Tape rec1, 2 (Pin) : 150mV/47k Ω
Pre out : 1V/47k Ω
Max Pre-out : 10V/47k Ω (THD 0.05%)
Output Impedance (1 kHz) Tape rec1, 2 (Pin) : 600 Ω or less
Pre-out : 600 Ω or less
Total harmonic distortion rate (20 Hz to 20 kHz) 0.01% or Less (at 1 V Output)
0.1% or Less (at 10 v Output)
Intermodulation distortion factor (70 hz : 7 khz = 4 : 1) 0.01% or Less (at 1 V Output)
0.1% or Less (at 10 v Output)
Frequency characteristic 5 Hz to 50 kHz + 0 -1dB
RIAA deviation ± 0.2 dB (20 Hz to 20 kHz)
Signal-to-noise ratio (IHF, A-network, short circuit) Phono1 and 2 : 77 dB or more
Aux, Tuner, Tape play1, 2 : 100 dB or more
Channel Separation (1 kHz, Input Short) Phono1, 2 : 60 dB or more
Aux, Tuner, Tape play1, 2 : 65 dB or higher
Input Separation (1 kHz, Input Short) Phono1, 2-Tuner : 90 dB or more
Phono1, 2-Tape play1, 2 : 90 dB or more
Tape play1, 2-Tuner : 90 dB or more
Tape play1 - Taple play2 : 100 dB or more
Tone control Bass : ± 10 dB (50 Hz)
Tone Selector : 400 Hz, 200 Hz
Treble : ± 10 dB (15 kHz)
Tone selector : 6 kHz, 3 kHz
High filter 10 kHz (-3dB, 6dB/oct.)
Subsonic filter 16 Hz (-3dB, 6dB/oct.)
Loudness (Volume -30dB) + 10 dB (50 Hz), + 7 dB (10 kHz)
Audio Muting -20dB
<General>
AC outlet Power switch interlock : 1 system (100W)
Power switch not linked : 2 systems (250W)
Rated power consumption (Electrical Appliance and Material Control Law) 225W
External dimensions Width 430x Height 168x Depth 389 mm
Width 482x Height 168x Depth 418 mm (with rack-mount adapter)
Weight 16.8kg
Sold Separately Rack Mount Adapter BX-7 (¥ 3,000)