Commentary

Based on the balanced transmission technology developed with C-280V, this is a preamplifier that aims for the highest quality by reviewing all circuits, thoroughly examining all materials and parts, and polishing them thoroughly.

A balanced transmission circuit is used for the line amplifier. Balanced transmission is a method of simultaneously transmitting positive and negative symmetrical signals whose phases are inverted with each other. It has an excellent ability to remove common mode noise components.
The C-290 consists of three differential amplifiers. When balanced, the + - input of amp1 is amplified and fed to amp2 and amp3. These two amplifiers feed back their outputs to the other side, providing a symmetrical + - signal with low impedance.
The feature of this circuit is that the + - symmetric signal is floating from the ground. Even if one side of the output is grounded, both amplifiers operate and the output voltage does not change. Therefore, in a normal unbalanced connection, either input or output is simply connected to the ground line.

All unit amplifiers are composed of Acuphase original symmetric push-pull circuits, and the input circuit is cascode source follower, which realizes stable operation up to ultra-high frequencies.
In addition, the next-stage differential complimentary service push-pull circuit is made into a module. The module is made of a alumina ceramics substrate with good heat conduction and high-frequency characteristics. As a result, low noise, high thermal and electrical stability and reliability are improved.
The complimentary service Darlington push-pull stage maintains low output impedance and excellent linearity up to large amplitude.

The printed circuit board is made of PRO (Polyphenylene Oxide) resin, a glass cloth substrate.
This material has almost the same level of electrical properties as the polytetrafluoroethylene (fluorine) resin base material. It has a low dielectric constant, excellent high-frequency characteristics, and performance is compensated up to 12 GHz.
In addition, a gold plate is applied to the copper foil surface to further improve sound quality.

The power transformer and filter condenser are independent on the left and right and are completely monophonic. In addition, all unit amplifiers are equipped with wide-band constant voltage power supply to prevent mutual interference between amplifiers.
The power transformer uses an amorphous core with excellent frequency characteristics. Amorphous alloy ribbons are obtained by ultra-quenching molten metal. Amorphous alloys are amorphous metals with no crystal structure. Therefore, they have superior magnetic and mechanical properties compared to metals with crystal structure. The main feature of amorphous alloy ribbons is that they have superior overcurrent characteristics because they have a high maximum magnetic flux density. Temperature changes have little influence on the core because they have a high Curie point (temperature at which magnetic characteristics change abruptly) unique to magnetic circuits. Heat generation is small because iron losses such as hysteresis loss and overcurrent loss are small.
The amorphous material used for C-290 is an Fe-based core with a thickness of 25 μ m.

The volume uses a four link volume controller that uses a CP (conductive plastic) element as a resistor, which is also used for the C-280V.
After the resistance element is printed, the surface of the CP resistor is finished into a mirror surface by high temperature pressure shaping to realize low contact resistance and low distortion factor. Also, the brush that contacts the resistor is a gold-plated multi-contact type, and it is integrated with the external terminal to eliminate the metal joint surface, reducing distortion caused by contact between different metals.
The rotation method is not a general method in which a brush rotates, but adopts a structure in which a resistor rotates. This greatly reduces the internal contact points and eliminates the need for contact grease, improving durability.
An 8 mm diameter extra-thick 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 tracking error is less than 0.5 dB measured at -60dB.

A logic relay control circuit is used to switch the input signal.
Relays are installed where switching is necessary and electronically controlling them with a logic circuit, we prevent the deterioration of sound quality caused by the routing of signal paths and the deterioration of contact points caused by direct switching with a selector.
The relay used in this circuit has a completely sealed structure filled with nitrogen gas, which has been developed especially for audio and communication equipment. Gold and silver-palladium alloy cross-bar twin type contact points affect sound quality, providing low contact resistance and high durability.

Each of the 4-unit left and right amplifiers, such as line and balance amplifier circuits, is equipped with a dedicated constant-voltage power supply unit. To prevent these unit amplifiers from interfering with each other, they are housed in a thick aluminum housing. The output and constant-voltage power supply elements are firmly fixed to the housing for heat dissipation, and resonance due to vibration is also prevented.
The overall structure consists of an 8 mm thick rigid aluminum framework to which a thick glass-epoxy motherboard is firmly secured and on which an aluminum housing is screwed.

It is equipped with a 1-dB-step attenuator that can control the orientation in any listening position with perfect channel balance.
The operation can be changed from 0 to -6dB and - ∞ with the left and right push buttons respectively, and it is displayed on the text display on the panel.

It has a phase switch which inverts the phase of the whole device.
The switching method is simply to replace the + - of the balance amplifier input, so there is no additional circuit like a phase inverter, so there is no sound quality degradation.

It is equipped with a compensator switch to improve the sense of volume when the volume is low.
It automatically adjusts the response according to the position of the volume control to maintain a natural balance.

Equipped with a subsonic filter to cut ultra-low frequency noise.

6N high-purity copper wire is used for the power cord that makes up the power circuit and has a great effect on sound quality.
The connector type power supply also supports commercially available power cords.

The C-280 is designed exclusively for the CD line, but it can be used for recording with a special phono equalizer unit (sold separately).

The line and balance unit amplifier can be mounted on the C-280V.
By replacing four C-280V line balance amplifier units, you can enjoy the change of sound quality.

Model Rating

Refer to AD-290 of other categories for AD input characteristics.
Type	Stereo Preamplifier
Frequency Response	balanced input (CD, Line) : 3 Hz to 350 kHz + 0 -3.0 dB 20 Hz ~ 20 kHz + 0 -0.2 dB CD, Tuner, Line, Tape Play : 3 Hz to 350 kHz + 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 (at rated output / 0.5 V output)	Balanced : 252mV/63mV/40k Ω (20k Ω / 20k Ω) Unbalanced : 252mV/63mV/20k Ω
Rated Output / Impedance	Balanced output : 2.0V/50 ohm Unbalanced output : 2.0V/50 ohm Tape rec : 252mV/200 Ω
S/N / Input conversion noise	Input Terminal Input Short IFF-A Correction Rated Output S/N Input Conversion Noise EIA S/N Balanced: 111dB -123dBV 95dB Unbalanced: 111dB -123dBV 95dB
Maximum power level (0.005% distortion rate, 20 Hz to 20 kHz)	Balanced output:8.0V Unbalanced output:8.0V Tape rec:9.5V
Minimum Load Impedance	Balanced output : 600 ohms 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
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	-20dB
Power Supply Voltage	100 VAC / 117 V / 220 V / 240 V, 50Hz/60Hz
Power Consumption	30W
Maximum External Dimensions	475 mm wide x 149 mm high x 405 mm deep When adding AD-290 : Depth 414 mm
Weight	23.8kg When adding AD-290 : 24.8 kg
Sold Separately	Dedicated Phono Equalizer Unit AD-290 (¥ 200,000)