日本Phasemation(协同电子) MA-2000 旗舰级真空管单声道后级放大器

300B parallel single-ended stage for fixed-bias single pure class-A operation

In order to compose the single pure class-A circuit so that the innate goodness of 300B is expressed with the simplest operation, two fixed bias power supplies are allocated individually for the ended-tube of PSVANE WE300B and therefore the setup of the optimum operating point and stable operation are secured. Consequently, this strong power greater than 25W outputted by a parallel operation can be supplied for driving a speaker. By adding the power supply system having a timer control (for 1 minute) at the time of turning the power supply on, the operation with further high reliability is achieved.

SOVTEK 2A3 for the transformer-coupled circuit driving PSVANE WE300B ended-tube optimally

We utilize 2A3 which is well-established even as an ended tube for a transformer–coupled circuit to drive the ended tube 300B, set 2A3 to the optimum operating point, and secure the DC stability by supplying the individual fixed bias to 300B parallel circuits. Therefore, the speaker is driven with unshakable stability.

Composing the non-feedback amplifier by adopting triode tubes in all stages

We compose the non-feedback amplifier by adopting triode tubes, 12AX7SRPP+2A3+300B, in all stages. Therefore, the ability for reproducing dynamic and very expressive music is provided only possible from the non-feedback circuit.

Adopting the output transformer and inter-stage transformer made by LUNDAHL in Sweden

Because the inter-stage and output transformers made by LUNDAHL in Sweden are proper to compose this equipment and suited for the sound quality to be aimed by us, we have adopted them.　The fixed bias circuits are allocated individually for the ended-tube of 300B with utilizing the independent winding structure at the secondary side of the inter-stage transformer and therefore the setup of the optimum operating point and stable operation are secured by reading the meter directly.　The independent terminals able to drive an 8 or 4 ohm speaker are provided by utilizing the independent winding structure at the secondary side of the output transformer.

Composed of two independent power choke transformers

Because the power supplies from the independent power choke transformer are individually supplied for the driver stage (2A3) and power stage (300B parallel single-ended), the mutual interference between the input signal amplifier circuit and the speaker drive circuit is purged. Therefore, signals of various levels from infinitesimal signals to high power signals are amplified clearly.

Thoroughgoing design for decreasing the noise

We have adopted the thoroughgoing design for decreasing the noise as follows:

-Rectifier tube 5R4G which does not generate a switching noise in principle against a high voltage
power supply
-Rectifier circuits composing of choke transformers
-DC ignition for heaters driven by complete DC power supplies for all tubes.

Timer indicator and appearance

The color of the timer indicator is red at the time of standby and changes to blue after 1 minute as a starting time. In addition, the impressive front glass with placing the two-color LED indicator on the front panel is designed as a stately appearance.

MA-2000 Block diagram