YAMAHA B-50

Commentary

A stereo power amplifier equipped with ZDR (Zero Distortion Rule), inheriting the technology of the high-grade B-70.

The basic circuit configuration of the B-50 consists of an FET source follower stage, a cascode complimentary service push-pull differential-amplification input stage, a voltage amplification stage by a cascode complimentary service push-pull pre-drive stage, a 3-stage Darlington connection power amplification stage with a linear transfer circuit with a parallel push-pull stage equipped with newly developed ZDR, and an X-power supply.

We have developed and installed a ZDR consisting of a distortion detection circuit and an input addition circuit in order to completely eliminate the distortion generated at the final stage of the power amplifier.
The distortion detection circuit has a complete bridge configuration and detects only the distortion component. The distortion sensor with this bridge detects variations in the input / output linearity, including errors in the emitter resistance of the power transistor, operating point variations due to non-linearity of the transistor and junction temperature changes, and noise and disturbance from the power supply, all of which are detected as variations in the input / output linearity. In addition, the distortion of the speaker can be detected without being affected by the amplitude and phase of the voltage and current. In addition, the distortion generated by the back electromotive force from the speaker is also detected. Since the elements that make up this bridge are pure resistors, there is no distortion in this part, and the accuracy is extremely high and stable.
In the addition circuit, the voltage of the distortion component is added from the floating power supply to the power stage input part as a floating bridge circuit.
Since the series addition method is adopted for the signal source, the drive signal current from the voltage amplifier stage and the distortion correction current are kept independent, and accurate distortion cancellation operation is possible without adversely affecting each other.

The X power supply, which is also used for B-6, is used for the power supply part.
In this power supply circuit, when the DC output voltage tends to decrease according to the power consumed in the secondary side, the voltage comparison amplifier compares it with the reference voltage generated by the reference voltage generation circuit and detects it. The signal from the voltage comparison amplifier is fed back to the primary side at high speed through a photocoupler that combines a light emitting diode and a phototransistor, and the signal applied to the gate of the TRIAC inserted in the primary side is controlled to control the power input to the transformer.
The TRIAC element operates like a switch that turns ON/OFF in synchronization with the power supply frequency. In the conventional power supply circuit, the entire cycle of the power supply frequency is input to the transformer, while in the X power supply, the phase angle of energization is controlled as necessary and input to the transformer. This reduces the energization time to less than half of the conventional one. Moreover, since the output voltage does not fluctuate due to power consumption or input voltage fluctuations unlike in the normal power supply, the large power transmission equivalent to a huge power supply is possible.
As a result of the miniaturization, the impedance of the transformer can be kept low and the sense of volume in the low range is improved. Moreover, unlike switching regulators, which switch high power at high frequencies close to the audible band, there is no noise. Moreover, with conventional power supplies, when the load power increases, the output voltage drops significantly and the input voltage fluctuates directly. but with the X power supply, these fluctuations are kept to a minimum level.

Yamaha's original linear transfer circuit technology is used for the final stage.
In this circuit, the bias of each transistor in the parallel configuration is shifted and synthesized so that the transfer characteristic at the time of small signal is close to the square characteristic. By making the transfer characteristic at the time of small signal linear, the occurrence of crossover distortion is eliminated in principle and the bare characteristic is greatly improved.

Equipped with a PC limiter circuit that detects the collector loss (Pc) of the power transistor and limits the drive when the speaker is short-circuited or when a load of 4 Ω or less is connected.
There is also a built-in speaker protection circuit that uses a relay to disconnect the speaker from the amplifier when DC is input to the input terminal or when a DC component leaks to the speaker terminal due to a trouble. This protection circuit also serves as a muting circuit to prevent shock noise when the power switch is turned on.

Speaker A/B has an independent level controller.

Equipped with AC outlet.

The level meter uses an LED peak level meter with fast response speed.
This meter is equipped with a meter-off switch.

Model Rating

Type	Stereo power amplifier
Rated output	120W + 120W (8 ω, 20 Hz-20 kHz, 0.002%) 200W + 200W (4 Ω, Clipping Power)
Power Bandwidth (8 ω, 60W + 60W)	10 Hz ~ 100 kHz (Distortion 0.01%)
Input Sensitivity / Impedance	1.1V/25k Ω
Frequency characteristic	DC ~ 100 kHz -0.5 dB
Signal-to-noise ratio (IHF-A)	122dB
Total harmonic distortion factor (8 Ω, 100W)	0.0005%(20Hz) 0.0005%(1kHz) 0.001%(20kHz) 0.004%(50kHz) 0.01%(100kHz)
Intermodulation Distortion Factor (50 Hz : 7 kHz, 8 Ω)	0.002%(60W+60W)
Channel separation (Input Short)	100dB(20Hz) 95dB(1kHz) 70dB(20kHz)
Damping factor	200
Slew rate	200 V / μ sec
AC outlet	unswitched:200W max
Pwer	100 VAC, 50Hz/60Hz
Power consumption	200W
External dimensions	Width 435x Height 133x Depth 380 mm
Weight	11.8kg