A-2:

2500mW pure Class A headphone amplifier/pre-amplifier

Fully negative feedback-free design using CAST technology

All discrete transistor design.

Overview:

All-transistor pure Class A feedback-free CAST current transfer circuit and output circuit

2 pairs of precisely paired 20W high power tubes (80W Pcm total) act as pure Class A outputs

Pure Class A output power up to 2500MW at 25 ohms, driving most headphones in pure Class A operation

12DB and 20DB gain for driving high and low sensitivity headphones with ease

Precisely matched tubes, OCC cables and other Hi-end grade components

3 sets of preamp inputs: RCA X2 /CAST X1

Preamp output : RCA

Headphone output : 6.3MM single ended X1

Two built-in tone settings, allowing the user to adjust the tone to suit the system or personal preference

Amplifiers of fully discrete design.

Impedance matching:

There are many different brands of DACs in the world, with different designs and potentially very different output impedances. To avoid the sound quality being affected by the different output impedances of different DACs, in the A-2 we have designed a pair of pure Class A diamond buffers on the input side, through which all the RCA input signals pass, so that the output impedance is constant and not affected by the high or low output impedance of external sources.

The diamond buffers operate in a pure Class A state and they have an extremely short signal flow, ensuring that the sound is not coloured and that there is no dryness or harshness.

Core:

The A-2 uses an all-transistor feedback-free CAST current transfer circuit, with an all-transistor design throughout (the OPA op-amp is a DC servo circuit).

The A-2 may look complex, but it is in fact the simplest design in the world, with many devices used for power processing. The simple and short signal processing channels result in a dramatic improvement in sound quality with very little colouration.

The A-2 uses a single stage CAST amplifier circuit with extremely low distortion and is mounted directly on the motherboard for better sound quality.

The CAST circuit is a transistorised amplifier with no feedback at all. Many audiophiles know that a negative feedback design provides better test specifications, but a feedback-free design provides better listening. This is a contradiction between traditional circuits. But the CAST circuit breaks new ground by offering extremely low tone coloration, a very neutral and natural sound with very low distortion and excellent linearity. As a result, it reproduces dynamics and detail without sounding rough and dry.

All signal gain stages operate in current transfer mode and all transistors operate in Class A condition.

A powerful pure Class A output stage:

The A-2 is designed with a pair of diamond-shaped output stages, each precisely matched to the other, eliminating the need for further human adjustment.

Using two pairs of 20W transistors, this pair of output stages has a Class A output power of 2500MW at 25 ohms and can drive most headphones in pure Class A operation.

Input mode:

Press the "Input" button to select the input signal source. A2 has 2 RCA inputs and 1 CAST input.

Gain mode:

Press the "Gain" button to select the local gain.

"L" means that the machine is operating in low gain mode (14DB).

"H" means that the machine is operating in high gain mode (20DB).

Output mode:

Press the "OUTPUT" button to select "HP , PRE".

"PRE" refers to the preamp signal from the RCA output at the back of the machine, for connection to an external amplifier.

"HP" means that the signal is output from the headphones on the front of the machine.

Volume knob :

Selects the A2 volume.

Set A-2 for a thicker, warmer sound:

Plug 2 to 4 shorting plugs into the shorting sockets shown below to give a slightly warmer sound.

The two sets of setting ports on the top of the diagram below are for the RCA inputs.

The two lower settings are for all inputs.

Any combination of the two settings can be used to make the sound more to the user's liking. (The power must be turned off and the power cord disconnected before opening the top cover)