Commentary

A stereo preamplifier developed with the ultimate aim based on the development technology obtained with the C-280 and C-280L.

The line amplifier uses a balanced transmission method.
This circuit basically consists of three differential amplifiers, with amp1 receiving the input and amp2/amp3 receiving the output. When balanced, the + - and + - inputs of amp1 are amplified and input to amp2/amp3, respectively. These two amplifiers feed back their outputs to the other side, so that a + - symmetric signal is sent out with low impedance.
The advantage of this circuit is that even if one side of the output is grounded, both amplifiers can operate and double the output. Therefore, when an unbalanced connection is made by a normal phono jack, both input and output can operate normally by simply connecting one of them to the ground line.
This configuration ensures that the amplifier configuration remains the same regardless of whether it is balanced or unbalanced.

Each unit amplifier of the line amplifier is composed of AccuPhase original symmetrical push-pull circuit.
The input circuit uses a cascode source follower to achieve stable operation up to very high frequencies, and the output stage uses a complimentary service Darlington push-pull to achieve low output impedance and excellent linearity.
All of these unit amplifiers are directly connected. In order to suppress shock noise caused by the DC component generated in the output, DC drift is prevented by a DC servo circuit.

The power supply section uses a four transformer completely independent power supply with separate left and right equalizer, so that the total capacity of the filter condenser reaches 120,000 μ F.
In addition, all unit amplifiers are equipped with a wide-band constant voltage power supply to completely prevent mutual interference between amplifiers.

The volume adjuster uses a specially developed adjuster for the C-280V.
The CP (conductive plastic) element used in the studio is used for the resistor. The CP resistor realizes low contact resistance and low distortion factor by shaping the surface to mirror surface by high temperature pressure after printing the resistor element.
The brush that contacts the resistor is a gold-plated multi-contact type. It is integrated with the external terminal to eliminate the metal joint surface, reducing the strain of different metals. In addition, the rotation method is not a method in which the brush rotates, but a structure in which it is fixed and the resistor rotates. This greatly reduces the internal contact surface and eliminates the contact grease, improving durability.
In addition, an 8 mm diameter brass shaft is supported by an aluminum cutting bearing, and four elements are each housed in a high-precision aluminum cutting case for complete shielding to improve sound quality.
The variable method is a continuously variable type, and the tracking error is within 0.5 dB measured at a position of -60dB.

A logic relay control circuit is used for signal switching. Relays are installed where switching is necessary and are controlled electronically with a logic circuit. This circuit method reduces unnecessary signal routing and prevents signal degradation.
The relay used in this case employs a completely sealed structure filled with nitrogen gas, which has been developed especially for audio and communication equipment. The contact points that affect sound quality are gold and silver-palladium alloy cross-bar twin type with low contact resistance and high durability.

The equalizer amplifier uses an all-circuit symmetrical push-pull circuit.
It is equipped with a dedicated input circuit that matches each cartridge of MM/MC, and consists of a high S/N FET element at MM input.
In addition, since the MC input receives small signals with low impedance, a differential input circuit with low-noise elements is constructed. By reducing the impedance of the NFB loop, low-noise reproduction is possible.
In addition, in order to match the characteristics of the cartridge, the MC input impedance is switchable into 3 types and the gain can be selected from 2 types.

A total of 6 unit amplifiers on the left and right sides of the line input, line output and phono equalizers are each equipped with a dedicated constant-voltage power supply. They are housed in a thick aluminum housing and are of high construction.
The output and constant voltage power supply elements are firmly fixed to the housing for heat dissipation, preventing resonance due to vibration.
The overall structure consists of an 8 mm thick rigid aluminum framework to which a thick glass-epoxy motherboard is firmly attached and the aluminum housing is screwed in.

The input / output jacks are made with a rhodium plated characteristic jack, which has a proven track record as a connector for communication devices and computers and is highly durable.

It is equipped with a 5-step rotary attenuator so that the volume can be adjusted instantly without changing the volume control.

It is equipped with left and right independent attenuator type level control which can control the orientation in any listening position with perfect channel balance. It can be adjusted accurately in 0.5 db steps from 0 to -6dB and in 1 db intervals from -14dB.
The resistance element uses a mirror finished low-distortion type.

A phase switch is provided to adjust the absolute phase by inverting the phase of the whole device.
The switching method only exchanges the + and - of the balance amplifier. There is no load circuit like a phase inverter, so there is no sound quality degradation.

Equipped with equipped with a compensator switch to improve the sense of volume at low volume.
The sound quality is automatically adjusted according to the position of the volume regulator to maintain a natural balance.

Equipped with a subsonic filter.

The front panel is made of Accuphase traditional thick aluminum with gold-style scratch hairline, and the body is stored in a Persimmon wood case.

Model Rating

Type	Stereo Preamplifier
Frequency Response	balanced input (CD, Line) : 1.0 Hz to 350 kHz + 0 -3.0 dB 20 Hz ~ 20 kHz + 0 -0.2 dB CD, Tuner, Line, Tape Play : 1.0 Hz to 350 kHz + 0 -3.0 dB 20 Hz ~ 20 kHz + 0 -0.2 dB AD : 20 Hz to 20 kHz ± 0.2 dB
Total Harmonic Distortion Factor	0.005% (at all input terminals)
Input sensitivity / impedance (at rated output / 0.5 V output)	AD MM : 2.0mV/0.5mV/47k Ω AD MC (26 dB) : 0.1mV/0.025mV/10 Ω, 30 Ω, 100 Ω AD MC (32 dB) : 0.05mV/0.0125mV/10 Ω, 30 Ω, 100 Ω Balanced : 252mV/63mV/40k Ω (20k Ω / 20k Ω) Unbalanced : 252mV/63mV/20k Ω
Rated Output / Impedance	Balanced output : 2.0V/10 ω (5 ω / 5 ω) Unbalanced output : 2.0V/5 ohm Tape rec : 252mV/200 Ω
S/N / Input conversion noise	Input Short IFF-A Correction Input Terminal Rated Input S/N Input Conversion Noise EIA S/N AD MM: 90dB -140dBV 86dB AD MC(26dB): 80dB -154dBV 85dB AD MC(32dB): 75dB -154dBV 85dB Balanced: 111dB -123dBV 95dB Unbalanced: 111dB -123dBV 95dB
Maximum power level (0.005% distortion rate, 20 Hz to 20 kHz)	Balanced output:7.0V Unbalanced output:7.0V Tape rec:19.0V
AD Maximum Input Voltage (1 kHz, 0.005% distortion)	MM:300mV MC(26dB):15mV MC(32dB):7.5mV
Minimum Load Impedance	Balanced output : 600 ω (300 ω / 300 ω) Unbalanced output : 600 ohm Tape rec : 10k Ω
Gain	Balanced input → Balanced output : 18 dB Balanced input → Unbalanced output : 18 dB Unbalanced input → Balanced output : 18 dB Unbalanced input → Unbalanced output : 18 dB Unbalanced input → rec output : 0 dB AD MM input → Balanced output : 54 dB AD MM input → Unbalanced output : 54 dB AD MM input → rec output : 36 dB AD MC (26/32dB) input → Balanced output : 80/86dB AD MC (26/32dB) input → Unbalanced output : 80/86dB AD MC (26/32dB) input → rec output : 62/68dB
Loudness Compensator (Volume -30dB)	1:+3dB(100Hz) 2 : + 8 dB at 100 Hz, + 6 dB at 20 kHz
Subsonic Filter	10 Hz, -18dB/oct
Attenuator	-6dB, -20dB, -30dB, ∞
Semiconductors used	Transistors : 216 FET : 15 IC : 18 pcs Diodes : 158
Power Supply Voltage	100 VAC / 117 V / 220 V / 240 V, 50Hz/60Hz
Power Consumption	53W
External Dimensions	468 mm wide x 171 mm high x 415 mm deep
Weight	25.2kg