YAMAHA B-3
¥ 200,000 (around 1977)
Commentary
This is the third stereo power amplifier in the Yamaha SIT power amplifier series following the B-1/B-2.
The circuit configuration consists of a direct-coupled cascode bootstrap FET differential amplifier for the first stage, a current mirror load differential amplifier circuit for the pre-drive stage, a pure complimentary service symmetrical push-pull drive circuit for the drive stage, and a pure complimentary service OCL DC-amplifier circuit using SIT (vertical power FET) developed for B-3.
It can also be used as a monaural amplifier by switching between stereo and BTL.
The SIT (Static Induction Transistor) 2SK76A (Nch) and 2SJ26A (Pch) for B-3, which are used in the output-stage, are complimentary service versions of the world's first high-gain SIT 2SK-77 developed by Yamaha for the B-1.
In addition to the advantages of general FETs such as high input impedance and fast switching speed, the SIT for B-3 has fully examined various characteristics required as elements for B-3 circuits. In particular, the pair characteristics of Nch/Pch have been examined, and a unique manufacturing method and pair selection method have been devised and adopted. In addition, the chip size, impurity concentration and gate mesh structure of Pch/Nch devices have been fully examined, and pair characteristics in a wide operating range have been matched by strict control in the manufacturing process.
The developed SIT for B-3 has features such as good linearity of output characteristics, good start-up characteristics, high switching speed without carrier accumulation effect, high resistance to thermal breakdown due to voltage control element without local current concentration, and low thermal runaway due to negative temperature coefficient.
The first stage uses Yamaha's original low-noise, high-gm dual FET for cascode bootstrap differential amplification. This dual FET (2SK-100) combines the electrical and thermal characteristics of two low-noise, high-gm FETs in the same package. It ensures sufficient pairing for the first stage component of a DC amplifier. Since this dual FET is operated at a constant voltage (8 v) regardless of the input signal or output voltage, parameter changes due to temperature rise of the device are suppressed.
In addition, since this stage employs a cascode bootstrap circuit, there is no degradation degradation due to changes in signal source impedance. Combined with the adoption of a low-noise specification FET, an excellent value of -118dB input conversion noise is realized.
The second stage is a differential amplifier circuit for the current mirror load. The current mirror circuit is a kind of active load circuit with transistors. It has good frequency characteristics, little distortion, and very stable operation.
The voltage amplification stage of B-3 consists of a dual FET, a cascode bootstrap circuit and a two stage differential amplifier. Therefore, the temperature variation of the mid-voltage and the drift of the DC voltage with time, which is a problem of the DC amplifier, are very little, and the excellent stability is shown.
The drive stage employs a pure complimentary service symmetrical push-pull drive circuit using transistors.
In this symmetrical push-pull drive system, the Nch and Pch SITs of the power stage are each driven by a pure complimentary service push-pull circuit. Compared to the emitter-follower drive system, the drive impedance at ON and OFF times can be equally low.
The complimentary service stage consists of a pure complimentary service OCL circuit with SIT 2SK76A (Nch) and 2SJ26A (Pch) whose complimentary service characteristics are closely matched. This SIT has a drain loss of 100W and has good linearity and output characteristics in all output ranges. The SIT is mounted on a 2.5 mm thick copper plate and fixed to the heat sink on the side of the amplifier with low thermal resistance.
The SIT used for this B-3 has a rise characteristic curve close to the square characteristic, so by setting it to the optimum idling current, a composite transfer function with extremely good linearity is obtained, and an excellent output stage with almost no crossover distortion is realized, and the operation is class A up to around 1W.
The operation switch on the rear panel allows you to switch between stereo and BTL.
The BTL side operates as a 140W monaural amplifier.
The power supply circuit uses a 27,000 μ F/63Vx2 low-power etching large-capacity electrolytic capacitor that has been carefully selected for high-capacity transformer and sound quality.
This electrolytic capacitor was newly developed for the B-3. Its characteristics as a single unit are excellent in equivalent direct current resistance (ESR) and equivalent series inductance (ESL), and its distortion factor is 40 dB less than that of general products, making it suitable for audio equipment. In addition to improving the characteristics of the electrolytic capacitor, the impedance of the power supply in the high-frequency range is reduced by connecting in parallel a Yamaha original Meiler capacitor (10 μ F). In addition, the electrolytic capacitor is strictly shielded with magnetic material to prevent interference due to flux from the capacitor, and it is also thermally protected.
In the B-3, all power supplies before the drive stage are made constant voltage. In addition, in consideration of power supply noise around the voltage amplification stage, three series of diodes that can obtain constant voltage characteristics by tunnel effect are used for the reference voltage in the constant voltage power supply circuit to obtain the required voltage. This power supply improves temperature characteristics and low-noise characteristics by more than 20 dB compared with the one that uses a Zener diode for the reference voltage.
The bias circuit of the SIT employs an automatic stabilization bias circuit that automatically adjusts the bias voltage change caused by the fluctuation of the power supply voltage and keeps it constant. It also uses a diode matrix to prevent the rush current that flows into the SIT when the Power switch is turned on, thereby preventing damage to the SIT.
PDEquipped with a detection power limiter circuit that protects the final stage SIT. This circuit operates when the speaker output is short-circuited or in an overload state (output 50W or more at 2 Ω or less). It limits the signal that enters the drive stage to prevent overcurrent from flowing through the SIT.
It is equipped with a PC detection protection circuit. When a DC voltage above a certain level is generated in the speaker, a relay operates to disconnect the speaker from the amplifier. This circuit also functions as a muting circuit. In order to prevent shock noise generated when the power switch is turned on, the speaker output is cut off for a few seconds until the amplifier reaches a steady state.
The construction and wiring of the B-3 utilizes the know-how of Yamaha Amp. The ± B line wire from the power supply, which causes high-frequency separation and distortion, cancels out interference. In addition, the capacitor is connected to the speaker terminal with the shortest distance.
In addition, a copper plate of 2.5 mm thickness is used for the grounding wire around the capacitor through which a large current flows, a chassis with extremely low impedance is used for grounding between input and output, and a printed circuit board of 70 μ m thick, double the normal thickness.
The large-capacity capacitors, Meiler capacitors, and input coupling capacitors for AC operation used in the power supply section are made from Shinkai for both stand-alone and acoustic characteristics. In addition, speaker relays with gold-clad twin contacts and input pin jacks with gold-plated terminals are used.
Equipped with level control, input level can be changed from 0 dB to -20dB in 1 dB steps.
This level control uses a new attenuator that can be operated either from the front or from the rear.
The rear panel is equipped with an Input Mode switch that operates as a DC amplifier in the DC position, and as an AC amplifier in the AC position when one capacitor that has been examined for sound quality is input to the input.
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Model Rating
| Type | Stereo power amplifier | ||||
| Effective output (20 Hz to 20 kHz) |
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| Power bandwidth (8 Ω) | Normal : 5 Hz to 100 kHz or more (distortion 0.05%, -3dB) BTL : 5 Hz to 70 kHz (0.1% distortion, -3dB) |
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| Damping factor (1 kHz) | Normal:80 BTL:55 |
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| Harmonic distortion factor (8 Ω, 20 Hz to 20 kHz) | Normal : 0.007% or less (35W) BTL : 0.05% or less (70W) |
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| Intermodulation distortion factor (8 Ω, 50 hz : 7 khz = 4 : 1, 35W) | Normal : 0.007% or less BTL : not more than 0.03% |
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| Frequency Response (8 Ω, 1W) | Normal (DC) : DC ~ 100 kHz + 0 -1dB BTL (DC) : DC ~ 100 kHz + 0 -2dB Normal (AC) : 10 Hz + 0 -2dB to 100 kHz + 0 -1dB |
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| Input Sensitivity / Impedance | Normal : 1V/25k Ω BTL : 0.707V/25k Ω |
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| SN ratio (IHF-A network, input short) | 118 dB or more | ||||
| Residual noise (Volume max, input short) | 100 μ V or less | ||||
| Separation (Volume max Input Short, RL = 8 Ω Output ch70W) |
90 dB or More (1 kHz) 85 dB or More (20 kHz) |
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| Pwer | 100 VAC, 50Hz/60Hz | ||||
| Rated power consumption | 190W | ||||
| External dimensions | Width 200x Height 193x Depth 407 mm | ||||
| Weight | 16.2kg |