Audio Database

M-300
Commentary

A stereo power amplifier using duo-beta plusX circuit.

In order to control the NFB of the amplifier circuit appropriately, we developed a duo-beta circuit and adopted it in the M-300.
This is a practical amplifier with bare characteristics, and an appropriate amount of NFB and DC servo are added as a finishing touch of sound quality. Thus, suppression of reproduced sound due to excessive NFB and transient distortion such as TIM are suppressed.
In addition, the M-300 also introduces the concept of appropriate control into the power supply circuit as a power supply duo-beta plusX.
On the assumption that there is a large power supply in the plusX, it has a loose controlling force to achieve both sound quality and stable operation.

M-300 can be used by switching between A class and AB class.
Since class A operation amplifies the signal waveform as it is, it is free from switching distortion and crossover distortion switching distortion and crossover distortion and has no cause of distortion other than the linearity of the device itself. Ideal class A operation is realized by adopting LAPT with excellent high-frequency characteristics for the device, duo-beta for the amplifier circuit, and plusX for the power supply circuit.
In the Class AB operation, in pursuit of bare characteristics, transient distortion is below the measurement limit, and high output that cannot be achieved in Class A operation is achieved.
When switching between Class A and Class B, the sound is not interrupted and no noise is generated even during performance.

The circuit configuration consists of a push-pull circuit for all amplifier stages from the input stage to the output stage, and realizes an amplifier with excellent characteristics as a premise of duo-beta circuit.
The input stage consists of Pch-type and Nch-type complimentary service FETs with cascode, bootstrap and differential push-pull configuration. The pre-driver stage consists of transistors with excellent high-frequency characteristics with cascode connection, and the driver stage and power stage consist of push-pull configuration with 3-stage Darlington connection.
As an output device, a high-speed power transistor LAP-T with excellent linearity and high-frequency characteristics is used as a triple push-pull configuration for current flow.
By refining these circuits, both static and dynamic characteristics have been improved, and an appropriate amount of NFB and DC servo have been added.

The power supply section has a twin monaural configuration in which the left and right sides are completely independent, providing excellent linearity over the entire band and excellent crosstalk characteristics.
Two power transformers for Rch and Lch are used with thorough winding control. First, the power stage and drive stage are separately wound, and the direction of winding is unified for both primary and secondary sides. This is because it is confirmed that the reproduced sound is affected by the potential (polarity) between the ground and the beginning and end of winding of the transformer.
In addition, four filter capacitors (left and right) of 12,000 μ F capacity dedicated for audio are used to reduce the impedance of the power supply.
A rush current prevention circuit is used to prevent the rush current that increases as the capacity of the power supply increases.

A speaker protection circuit and an amplifier protection circuit are mounted.
A built-in speaker protection circuit detects the DC output at the output terminal and disconnects the output. In addition, even if there is a large input when the speaker cord is short-circuited, the amplifier circuit is protected or a current protection circuit is installed.

The elements used are carefully examined and used by strictly controlling sound quality independently.
Film capacitors, which are generally considered to be non-polar, are also subjected to strict sound quality control by adjusting their polarity. The input attenuator uses a precision continuously variable detent volume with excellent sound quality and operational feeling.
In addition, we have adopted a gold-plated pin jack with low contact resistance for the input terminal of terminals and a large screw type for the speaker terminal.
The wiring is arranged in the shortest distance, and materials with high sound quality such as oxygen-free copper and 500 core shielded cable are selected. In addition, the power bus made of copper plate is adopted in order to suppress the rise of the power supply impedance on the patterned surface of the substrate, taking advantage of the power supply performance.

Equipped with a line phase sensor, the polarity of the equipment is unified by this polarity detection function to improve sound quality.

Equipped with an indicator that lights up at the clip point of the output, peak can be monitored.
It is normally green. When the output power exceeds approximately 40W in class A operation and approximately 170W in class AB operation, it is replaced with red.

A thick wooden box is used for the cabinet and it is plated with natural rosewood.

Model Rating
Type Duo Beta plusX power amplifier
Continuous effective output
(20 Hz to 20 kHz, simultaneous operation for both channels)
Class A : 40W + 40W (8 Ω)
Class AB : 270W + 270W (4 Ω)
170W + 170W (8 Ω)
Total harmonic distortion factor (8 Ω
20 Hz to 20 kHz, -3dB)
Class A : 0.005% or less
Class AB : 0.005% or less
Intermodulation distortion factor (8 Ω
60 Hz : 7 kHz = 4 : 1, -3dB)
Class A : 0.005% or less
Class AB : 0.005% or less
Output bandwidth Class A : 10 Hz to 100kH-3dB (Distortion Factor 0.015%)
Class AB : 10 Hz to 100 kHz -3dB (Distortion Factor 0.04%)
Frequency characteristic 10 hz to 100 kHz -1dB (both class A and class AB)
Input sensitivity Class A : 0.45 V
Class AB : 1 V
Input impedance 50k Ω (both class A and class AB)
Signal-to-noise ratio (IHF-A corrected) Class A : 115 dB
Class AB : 120 dB
Residual noise (IHF-A corrected) 30 μ V (both Class A and Class AB)
Damping factor 120 (1 kHz, 8 Ω, both Class A and Class AB)
Protection circuit DC drift detection speaker protection circuit
Overcurrent detection amplifier protection circuit
Attachment Circuit operation switch (indicator)
Peak indicator
Input Attenuator
Headphone jack
Speaker OFF switch (indicator)
Line phase sensor
Power supply voltage 100 VAC, 50Hz/60Hz
Power consumption (Electrical Appliance and Material Control Law) Class A : 420W
Class AB : 430W
External dimensions Width 478x Height 214x Depth 484 mm
Weight 30kg