Commentary

This control amplifier is based on the balanced transmission technology of C-290, and is composed of a current feedback amplifier circuit to further enhance various characteristics.
Although it is a pure line amplifier configuration, it is also possible to play an analog disk with the special phono equalizer unit AD 275 (sold separately).

The line amplifier uses a balanced transmission method.
Balanced transmission is a method of simultaneously transmitting positive and negative symmetric signals whose phases are inverted with each other. It has an excellent ability to remove common mode noise components.
In this circuit, two pairs of amplifiers feed back their outputs to the other side. The + - symmetric signal is sent out with low impedance. Since the + - symmetric signal is a floating circuit from the ground, even if one side of the output is grounded, both amplifiers operate and the output voltage does not change.

The balance output uses a current feedback amplifier circuit and consists of discrete parts with an emphasis on sound quality.
This circuit detects a current by reducing the impedance of the input terminal on the feedback side, and converts the current to I-V using a transimpedance amplifier to produce an output signal.
Since the impedance of the feedback input part is extremely low, phase rotation hardly occurs in this circuit, and there is almost no need for phase compensation. Therefore, this circuit has excellent response characteristics to the input signal, and is useful for music playback containing many pulse components.

The amplifier circuit consists of 4-unit left and right amplifiers for line input and balance output. It is housed in an aluminum housing equipped with a wide-band constant-voltage power supply, and is fixed to a glass-epoxy motherboard to prevent resonance and mutual interference caused by vibration.
In addition, the power transformer, filter and condenser are also independent on the left and right, and the frame structure of 8 mm thick hard aluminum is made into a perfect monophonic structure, which reduces electrical interference and mechanical vibration.

The power cord uses 6N high-purity copper wire.
In addition, it is possible to connect a commercially available cord by connector method.

In order to configure the shortest and straight signal path without routing the signal path, a logic relay control method is adopted which controls the relay electronically.
The relay used here is a closed-type relay for the communication industry, with gold-plated cross-bar twin contacts for low contact resistance and high durability.

It is equipped with a phase switch which inverts the phase of the whole device.
There is no degradation of sound quality because the switching method is only to exchange + and - of the balance amplifier input.

The tone control uses an additive active filter tone control that is used in full-scale graphic equalizers.
This tone control allows the flat signal to pass straight through, and adjusts the tone from the flat signal by making different characteristics as needed.

There is an amplifier for headphones only.
You can turn off the output to the speakers and adjust the headphone output with the main volume.

The volume uses a resistor with especially low distortion.
Volume control by remote control is done by the combination of this volume controller and small motor with clutch.

Model Rating

Type	Stereo Preamplifier
Frequency Response	Balanced/Unbalanced Input: 3 Hz to 350,000 Hz + 0 -3.0 dB 20 Hz ~ 20 kHz + 0 -0.2 dB
Total Harmonic Distortion Factor	0.005% (at all input terminals)
Input Sensitivity / Impedance (Rated Output / Output 0.5 v)	Balanced : 252mV/63mV/40k Ω Unbalanced : 252mV/63mV/20k Ω
Rated Output / Impedance	Balanced/Unbalanced Output : 2.0V/50 ohm Tape Rec (at AD input) : 252mV/200 Ω
S/N / Input Conversion Noise	Input short, IEC-A correction Balanced:114dB/-126dBV Unbalanced:116dB/-128dBV EIA S/N Balanced:96dB Unbalanced:96dB
Maximum power level (0.005% strain rate)	Balanced/Unbalanced Output:6.0V Tape Rec (AD input) : 6.0 V
Minimum Load Impedance	Balanced/Unbalanced Output : 600 Ω Tape Rec : 10k Ω
Gain	Balanced/Unbalanced Input → Balanced/Unbalanced Output : 18 dB
Compensator	1:+3dB(100Hz) 2:+6dB(100Hz) 3 : + 6 dB at 100 Hz, + 6 dB at 20 kHz
Tone Controls	Turnover frequency and variable range Bass : 300 Hz, ± 10 dB (50 Hz) Treble : 3 kHz, ± 10 dB (20 kHz)
Subsonic Filter	10 Hz, -18dB/oct
Attenuator	-6dB, -20dB, -30dB
Headphone jack	Applicable Impedance, 4 Ω to 100 Ω
Power Supply Voltage	AC100V
Power Consumption	20W
External Dimensions	Normal : width 475x height 150x depth 404 mm When adding AD-275 : Depth 413 mm
Weight	Normal : 19.7 kg When adding AD-275 : 20.4 kg
Sold Separately	Dedicated Phono Equalizer Unit AD-275 (¥ 100,000)
Included : Remote Commander RC-10
Remote Control Method	Infrared Pulse Method
Power Supply	DC3V dry battery : UM-4 (IEC designation R03) 2 pieces
Maximum External Dimensions	66 mm wide x 175 mm high x 20 mm deep
Weight	190g (including dry batteries)