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.
SPECIFICATIONS:
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 |