YAMAHA B-4
¥ 138,000 (around 1978)
Commentary
Stereo power amplifier equipped with newly developed Ro (output impedance) control.
The basic circuit configuration of B-4 consists of a low-noise dual-FET cascode bootstrap differential-amplifier circuit at the first stage, a cascode connected current-mirror push-pull differential-amplifier circuit at the pre-drive stage, and a 3-stage Darlington connected pure complimentary service parallel push-pull OCL with High-fT transistor at the output-stage.
In addition, the B-4 is equipped with an Ro control that can change the output impedance in the range of + 1 Ω to -1 Ω, and the operation switch on the front panel allows switching between B-class operation and A-class operation.
The first stage consists of a low-noise, high-gm dual FET source follower and a balanced output differential amplifier circuit with cascode-connected dual transistors. The second stage consists of a push-pull differential amplifier circuit with cascode-connected current mirror circuit.
The dual FET used here has the electrical and thermal characteristics of two low-noise, high-gm FETs in the same package is packaged in the same package. It has sufficient pair characteristics for the first stage of a DC amplifier. In this stage, the cascode bootstrap circuit is used, so that the increase in distortion factor due to the change in source impedance is minimized.
The current mirror circuit used in the pre-drive stage provides sufficient gain as a voltage amplifier stage, and the two transistors function as a kind of push-pull circuit to cancel out even order harmonic distortion.
The temperature change of the midpoint voltage and the drift of the DC voltage with time, which are especially problematic in the DC amplifier, are minimized by the adoption of the dual FET and the cascode bootstrap circuit, etc., and the voltage amplification stage with sufficiently stable high slew rate and low distortion factor is realized.
The power amplifier circuit is a pure complimentary service parallel push-pull OCL power amplifier circuit with 3-stage Darlington connection composed of High Speed High-fT (gain band) transistors. By constructing 3-stage High-fT transistors with well-matched pair characteristics, sufficient power gain can be obtained over a wide band. Each stage is designed to ensure rapid discharge of stored charge and stability under no load.
In order to maximize the performance of the High-Speed and High-fT transistors, a series of diodes and resistors are connected between E and E of the transistor in the drive stage. The resistor value is set to a sufficiently low value of 33 Ω. This reduces the charge discharge impedance in the base region, resulting in a quick rise and fall amplifier.
The power amplifier stage can be switched between class B operation and class A operation at the same volume level by switching the operation switch. This system uses Yamaha's original bias circuit. When the power supply voltage of the power amplifier stage is switched, the change is automatically detected and the bias current is switched.
The operation is V of each transistor at class B bias.BEThe constant voltage bias, which has no temperature characteristic during class A operation, is automatically added to class B operation. This ensures temperature stability of idling current, which is especially important for amplifiers that can switch between class A and class B operation, and provides excellent low distortion over a wide range.
Equipped with an Ro control (output impedance control) circuit that can continuously vary the output resistance of the amplifier over + 1 Ω ~ 0 ~ -1 Ω.
This circuit detects the output current of the amplifier with a high-precision metal clad resistor, inverts the phase with an inverting buffer amplifier, and optionally adds the current signal and NFB-PFB to the NFB circuit of the power amplifier to control the output impedance.
This allows you to set the damping values that are appropriate for the practical use of various speaker systems, so that you can draw out the capabilities of the speaker system.
The power supply circuit uses two completely independent power supply systems, two large-capacity transformers and an audio low-magnification etching large-capacity 15,000 μ F (75 V) electrolytic capacitor that has been carefully selected for sound quality. In addition, to compensate for the limitation of electrolytic capacitors, a Meiler capacitor is connected in parallel to suppress the rise in the power supply impedance in the high-frequency range. In addition, by strictly shielding with magnetic material, interference of flux between capacitors is prevented and thermal protection is achieved.
In addition, by utilizing the know-how obtained up to now, the current loop is reduced and the inductance in the loop is reduced, resulting in a circuit with excellent transient response with very low power supply impedance until the later stage.
In addition, the pre-drive stage is also an independent local constant voltage power supply on the left and right sides, reducing the influence of other stages.
A speaker protection circuit and an output transistor protection circuit are mounted as protection circuits.
The speaker protection circuit automatically detects a DC voltage that is harmful to the speakers and separates the speakers from the amplifier by means of a speaker protection relay. This circuit also functions as a muting circuit. To protect the speakers from the shock noise that occurs when the power switch is turned on, the speaker protection circuit separates the speakers for a few seconds before the amplifier returns to normal operation.
The output transistor protection circuit is a Pc limiter circuit that detects the output signal voltage and the power Tr emitter current and calculates it to limit the collector loss of the power Tr under overload conditions such as a short circuit of the speaker output. This protection circuit is designed to operate with a load resistance of 2 Ω or less.
The B-4 uses a symmetrical two monaural construction to reduce cross-talk between channels. It also reduces the current loop that occurs in the amplifier as a whole, preventing the internal flux of the amplifier from being affected by the magnetic characteristics of the chassis and causing a distorted reaction in the current line. A 1.5 mm thick copper plate is used to ground the power stage through which a large current flows to reduce impedance.
In order to further improve the sound quality, parts such as a large-capacity low-magnification etching electrolytic capacitor used for the power supply, a Meiler film capacitor, a styrene capacitor, a high-precision metal-clad resistor with less current distortion, a non-inductive wound emitter resistor, and a speaker coil with less DC resistance using 2.3 mm copper wire are used.
A level controller is mounted on the rear panel.
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Model Rating
| Type | Stereo power amplifier | ||||
| Rated Output (10 Hz ~ 30 kHz, Distortion 0.007%) | ClassB : 120W + 120W (8 Ω) ClassA : 30W + 30W (8 Ω) |
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| Power bandwidth (8 Ω) | ClassB : 10 Hz to 100 kHz (0.02% distortion, 60W) ClassA : 10 Hz to 100 kHz (0.008% distortion, 15W) |
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| Input Sensitivity / Impedance | 1V/25k Ω (8 Ω, 100W) | ||||
| Frequency Response (8 Ω, 1W) |
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| Total harmonic distortion factor (8 Ω) |
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| Intermodulation distortion factor (8 Ω) | Class B (60W) : 0.002% or less Class A (15W) : 0.002% or less |
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| Output impedance control | Variable impedance range (1 kHz) : + 1 Ω ~ 0 Ω ~ -1 Ω Variable frequency band : 10 Hz to 20 kHz |
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| Separation | 80 dB or More (1 kHz) 70 dB or More (20 kHz) |
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| Residual Noise (IHF-A) | -87dBV | ||||
| Fuse used | T5A250V | ||||
| AC outlet | unswitched:100W | ||||
| Pwer | 100 VAC, 50Hz/60Hz | ||||
| Rated power consumption | 330W | ||||
| External dimensions | Width 435x Height 145.5x Depth 381 mm | ||||
| Weight | 21kg |