YAMAHA BX-1
¥ 330,000 (1 unit, around 1980)
Commentary
A monaural power amplifier with a pure current servo amplifier.
We have newly developed a pure current servo amplifier that does not affect sound quality by the power supply and installed it in the BX-1.
This circuit was developed with attention to the degree of influence (element sensitivity) of the amplification method and power supply of the amplifier on sound quality. It adopts a method in which signal current does not flow into the power supply or ground line at all. As a result, a constant current flows into the power supply line at all times, and it realizes 0 element sensitivity that is not affected by the power supply system such as L, C, and R components of the power supply and wiring or non-linear due to current change. Also, since the signal current does not flow into the ground, it is less affected by ground impedance, and crosstalk in the low-frequency range is improved. In addition, since the current is taken in and out from the load inside the amplifier, the loop of output current is small, and the influence of flux generated from this loop is reduced, and the influence from other sources is also reduced.
The basic circuit configuration of the BX-1 consists of a high-speed positive-phase 0 dB gain buffer amplifier, a high-speed inverted 0 dB gain buffer amplifier, and two class-A power amplifiers with a new linear transfer circuit. Four of these three types of amplifiers are combined to form a monaural power amplifier.
In the circuit operation, a high-speed positive-phase buffer amplifier and a high-speed inverting buffer amplifier are used to create a high-precision balanced signal, which is then input to two power amplifiers of exactly the same configuration and output as a floating output.
In this circuit, since the output is floating, the output current from one amplifier is always absorbed by the other amplifier. Therefore, in order for the current flowing through the common feed line to be a constant DC current independent of the output signal, the power supply current waveforms of the two amplifiers must be completely similar with no distortion to the output current. Therefore, the BX-1 uses a new linear transfer circuit to further reduce the distortion factor of the power supply current of the class A amplifier, and further reduce the current distortion to realize a pure current servo amplifier.
In the new linear transfer circuit, the voltage drop across the emitter resistor is kept constant, so that the change in the emitter current of each Nch and Pch transistor is 1/2 opposite in phase to the output current, improving the distortion of the current waveform.
Each unit amplifier is a special circuit according to the needs of each unit amplifier, and has five features in common : "high speed by feed-forward compensation," "emphasis on NFB band by 2-pole compensation," "expansion of dynamic range of voltage and current from input stage," "extensive use of cascode connection," and "full use of quality parts."
The positive-phase 0 dB amplifier is an ultrafast amplifier with a delay time of 10 ms by assembling a cascode bootstrap circuit with FET differential input and adopting a feed-forward compensation circuit. The inverted 0 dB amplifier is a high-speed inverting buffer with a delay time of 40 ms by using a current mirror complimentary service push-pull input, feed-forward compensation and 2-pole phase-compensation.
In an inverting amplifier, during the delay time between input and output, the phase difference between the input and output causes an overshoot in the opposite direction at the output edge (because punch-through occurs during the delay time). In order to reduce this effect, the amplifier section has a gain of 5 times and is attenuated to 1/5 at the output terminal to obtain a 0 dB amplifier.
The first stage of the pre-drive and drive amplifier section is a cross-coupled push-pull input circuit and achieves a maximum drive capacity of 40 mA. The pre-drive stage is differential-type for DC and emitter-grounded for AC, and the maximum output current is approximately 40 mA. This stage is class A for normal audio signals, and the current is canceled except for the current mirror section.
The drive circuit is cascode-connected with reference to the emitter potential of the drive transistor, and V of the power transistor.BEDrive transistor V byBEWill not change.
All stages of this pre-drive / drive amplifier are connected with a cascode.
The complimentary service stage consists of two sets of pure output triple push-pull stages with transistors of Pc150W, and the total collector loss of the sy passage stage is 1800W.
The power supply voltage and idling current are switched so that the rated output is 100W at both 8 Ω and 4 Ω loads.
As for the power supply to the output stage, AC from a large toroidal transformer is rectified by a high-current high-speed diode of trr200nS and smoothed by high-capacity electrolytic capacitors for audio equipment of + - 47,000 μ F each.
The power supply to the drive amplifier uses a low-noise constant-voltage power supply with output impedance of 1m Ω at 200 kHz and input / output insulation ratio of 100 dB or more at 200 kHz.
It is equipped with a protection circuit that operates when a DC voltage appears at the speaker terminals or when the temperature of the heat sink becomes too high. This circuit controls the transistor mute, speaker short relay and ± B cutoff relay with protection control.
The relay contacts do not enter the signal path at all, and the ± B cutoff relay also reduces the adverse effect on sound quality in the part where constant DC current flows.
In consideration of the influence of parts on sound quality, we have adopted a polypropylene film capacitor for each time constant in the circuit, a mica capacitor for phase compensation, a metal film resistor for all signal path resistors, a polypropylene polycarbonate composite capacitor for the output of constant voltage power supply, and a tantalum nitride resistor for the input NF resistor.
.JPG)
.JPG)
.JPG)
.JPG)
.JPG)
Model Rating
| Type | Monaural power amplifier | ||||||
| Rated Output (10 Hz ~ 20 kHz, Distortion 0.002%) | 100W (4 Ω / 8 Ω) | ||||||
| Total Harmonic Distortion Factor (4 Ω ・ 8 Ω) | 0.001% (10 Hz-20 kHz, 50W) 0.005%(100kHz) |
||||||
| Power bandwidth | 10 Hz ~ 100 kHz (Distortion 0.005%, 50W) | ||||||
| Input Sensitivity / Impedance (100W, 1 kHz) | 1V/27k Ω | ||||||
| Intermodulation distortion factor (50W, 50 Hz : 7 kHz) | 0.001% (4 Ω / 8 Ω) | ||||||
| Frequency Response (8 Ω, 1W) |
|
||||||
| Residual Noise (IHF-A, Input Short, 8 Ω) | 20 μ V or less | ||||||
| Signal-to-noise ratio | 123dB | ||||||
| Rise time | 0.3 μ sec | ||||||
| Slew rate | 600 V / μ sec | ||||||
| Damping factor | 160(1kHz) | ||||||
| Semiconductor used | Transistor : 165 units Diode : 89 pcs FET : 2 Zener diode : 26 units |
||||||
| Power supply voltage | 100 VAC, 50Hz/60Hz | ||||||
| Rated power consumption | 370W | ||||||
| External dimensions | Width 271x Height 230x Depth 488 mm | ||||||
| Weight | 18.4kg |