Commentary

This stereo preamplifier was developed for both analog and digital disc playback in response to the advent of digital audio and digital discs.

The circuit configuration combines the Accuphase traditional A-class push-pull amplifier stage with the cascode method to improve the element characteristics before NFB is applied.
The cascode connection provides good performance, especially at high frequencies, with little feedback. In addition, linearity is maintained up to the operating limit of the device, distortion is reduced, and the signal-to-noise ratio is improved. By using A class push-pull, linearity and distortion characteristics are further improved, and element characteristics before NFB is applied are improved.

The basic circuit configuration of the unit amplifier is the same as that of the head amplifier, equalizer amplifier, and high-level amplifier. It is basically 3-stage amplification. It consists of 2 stages of input cascode push-pull differential amplification with 8 elements and 2 stages of cascode push-pull amplification with 4 elements.
In addition, it is equipped with an IC-based DC servo circuit that detects DC generated at the output and returns it to the input to prevent DC drift. This DC servo system and precise design make DC drift almost zero, and all signal systems from the MC input to the output are directly connected to eliminate coloring.

The power supply section uses a dedicated power transformer for the left and right channels, making it a completely electrical and structural construction.
In order to reduce impedance over a wide range, each unit amplifier is equipped with a dedicated constant-voltage power supply for a total of six multiple power supplies. In addition, the constant-voltage power supply section of the equalizer amplifier is a cascode differential amplifier to obtain high gain and increase constant-voltage effect.
The unit amplifier and the constant-voltage power supply are housed in a thick aluminum housing and connected at the shortest distance to prevent the source impedance from increasing in the high-frequency range.

To prevent high-frequency degradation and instability by routing the signal path for functions such as input switching and tape monitoring, the C-280 is equipped with relays in the signal path where a switch is required, and these relays are switched electronically controlled by logic circuits.
The relay used for this purpose is a sealed type relay filled with nitrogen gas, which has been developed especially for audio and communication equipment. The contact is a crossbar twin system of gold-plated and silver-palladium alloy. It has low contact resistance and high durability.
In addition, four low-level signal paths are used in parallel for greater reliability.

The equalizer amplifier section uses carefully selected elements to further reduce noise.
The Silbird Mica Capacitor is specially developed for audio applications. The capacitor has a special processing and structure in which a silver thin film is formed on a carefully selected high-purity piece of mica by the screen method and a glass pattern (thin glass layer) is printed. This increases the corona discharge starting voltage and allows the signal to pass stably even under high voltage pulses. The lead wire, which affects sound quality, is connected directly to the edge of the silver film to prevent degradation.

The head amplifier for the MC cartridge uses a special flat amplifier inserted in front of the equalizer amplifier.
A low-noise element is used for the input circuit and at the same time, the impedance of the NFB loop is reduced to greatly reduce resistance noise. In order to avoid the influence of noise generated by the DC servo, feedback is not directly to the - side of the differential amplifier, but is instead fed back to the base of the low-current source, thus reducing the influence of servo noise.

To support various MC cartridges, the MC input impedance can be switched to 3 stages.
It also includes a gain selector switch for low power MC cartridges.

Equipped with 600 Ω balanced output terminal.

The six unit amplifiers for left and right channels and the constant voltage power supply are housed in a thick aluminum housing.
The board is fixed with screws, and the output element of class A amplifier and the element of constant voltage power supply are fixed in close contact with the housing for heat dissipation, preventing resonance due to induction and vibration.

Two tape recorders can be connected for recording, playback and monitoring.

Equipped with a 3-step rotary attenuator, it can be switched between -20dB, -30dB and - ∞.

Equipped with left and right independent attenuator level control.
Accurate adjustment is possible in 0.5 dB steps from 0 to -6dB, and then in 1 dB steps up to -14dB.

Equipped with a subsonic filter that cuts only subsonic noise of 10 hz or less.

A compensator replaces the tone control.
The frequency response is automatically corrected according to the position of the volume, and two types of response can be selected with the switch.

Using a high-grade rhodium plated input / output jack, it has excellent durability with a hardness of 1 μ m thickness that can withstand 100 million sliding cycles.

It uses a front panel of gold-like scratch hairline and Persimmon wood case.

Model Rating

Type	Stereo Preamplifier
Frequency Response	Tuner, Line, Tape Play : 1.0 Hz to 500 kHz + 0 -3.0 dB 20 Hz ~ 20 kHz + 0 -0.2 dB Disc : 20 Hz-20 kHz ± 0.2 dB
Total Harmonic Distortion Factor (New IHF)	0.005% (20 Hz to 20 kHz at all input terminals)
Input Sensitivity / Impedance (Rated Output / New IHF Output 0.5 v)	Disc (Head amp off) MM : 2.0mV/0.5mV/47k Ω Disc (Head amp + 26 dB) : 0.1mV/0.025mV/10 Ω, 30 Ω, 100 Ω Disc (Head amp + 32 dB) : 0.05mV/0.0125mV/10 Ω, 30 Ω, 100 Ω Tuner, Line, Tape Play : 126mV/31.5mV/20k Ω
Rated Output / Impedance	Output (unbalanced) : 2.0V/1.0 ohm Output (balanced) : 2.0V/600 ohm Tape rec : 126mV/200 Ω (at disc)
S/N / Input conversion noise	Input Short IFF-A Correction Input Terminal Rated Input S/N Input Conversion Noise EIA S/N Disc(Head amp off)MM: 85dB -140dBV 82dB Disc(Head amp +26dB): 72dB -152dBV 76dB Disc(Head amp +32dB): 66dB -152dBV 76dB Tuner, Line, Tape Play : 110dB -128dBV 88dB
Maximum power level (0.005% distortion rate, 20 Hz to 20 kHz)	Output(balanced):10.0V Output(unbalanced):10.0V Tape Rec : 19.0 V in AD
Disc Maximum Input Voltage (1 kHz, 0.005% distortion)	Disc(Head amp off):300mV Disc(Head amp +26dB):15mV Disc(Head amp +32dB):7.5mV
Minimum Load Impedance	Output (unbalanced) : 1k Ω Tape Rec : 10k Ω
Gain	Tuner, Line, Tape Play → output : 24 db Tuner, Line, Tape Play → Tape rec output : 0 dB Disc (Head amp off) MM → output : 60 dB Disc (Head amp off) MM → Tape rec output : 36 dB Head amp is switchable between + 26 dB and + 32 dB
Loudness Compensator (volume control -30dB)	1:+3dB(100Hz) 2 : + 8 dB at 100 Hz, + 6 dB at 20 kHz
Subsonic Filter	10 Hz, -18dB/oct
Attenuator	-20dB, -30dB, ∞
Semiconductors used	Transistors : 225 FET : 16 IC : 33 Diodes : 167
Power Supply	100 VAC / 117 V / 220 V / 240 V, 50Hz/60Hz
Power Consumption	75W
External Dimensions	468 mm wide x 171 mm high x 396 mm deep
Weight	18.1kg