SONY ESPRIT TA-N900
¥ 250,000 (around 1980)
Commentary
This monaural power amplifier was developed without any compromise, including elements, circuits, power supply, construction and even interface.
The power stage employs a double-diffused power MOS FET developed by Sony.
Typical power MOS FETs have high input impedance, fast response speed, and a wide safe operating range similar to typical FETs. Double-diffused power MOS FETs can withstand high voltage (210 V) and can handle large current (up to 10A), which is extremely advantageous as power devices for audio applications.
The TA-N900 uses this power MOS FET and employs a newly developed distortion reduction circuit that operates the power element in A-class operation in most regions while controlling the drive voltage so as to correct the linearity of the power stage.
As a result, the distortion factor of the power stage is improved by more than one order of magnitude compared with the conventional one. As a result, the NFB circuit from the power stage to the pre-stage can be eliminated, and the effect of reaction of TIM and speaker load is greatly reduced.
The the pre-stage circuit configuration of the power amplifier section, the first stage is an r differential input double cascode bootstrap circuit like a junction FET and a bipolar transistor to reduce high-frequency distortion due to non-linearity of the feedback capacitance of the FET. In addition, stability against temperature change and power supply rejection characteristics are improved by combination with the current mirror load of the cascode connection.
The second stage is a cascode-bootstrap inverted amplifier circuit of a bipolar transistor, and has good linearity and high power supply rejection characteristics similar to the first stage.
The final stage has an NFB loop from the complimentary service emitter follower SEPP outputs to the first stage. However, since the signal is small and the speaker load is isolated by the power stage, problems caused by TIMs and reactive loads can be almost ignored.
The gain of the pre-stage is approximately 27 dB, which is designed to drive the power stage powerfully.
The power stage is the SEPP output of the source follower with four power MOS FETs connected in parallel. The distortion reduction circuit is configured to drive the gate of the power MOS FET by adding the voltage of the gate voltage detection circuit to the output of the pre-stage, thereby correcting the linearity component of the output element.
A pulse locked power supply (PLPS) has been further improved for the power supply section.
A pulse-width-controlled voltage circuit is installed on each of the + side and - side of the power stage so that transient signals can be adequately handled. The output impedance in the low-frequency range is approximately 0.05 Ω, which is less than 1/10 of that when a conventional large transformer is used.
In addition, an independent tandem stabilizing power supply is used for the pre-stage to reduce interference between stages.
In the primary rectifier circuit of the power supply circuit, 100 VAC input is rectified to 140 VDC by a bridge connected diode, which uses a high-speed diode with excellent response characteristics and low switching noise.
In the power supply circuit, a 20-kHz square wave is generated by four high-power switching transistors and an oscillator, and power is supplied to the converter transformer. The transformer uses a ferrite core with low high-frequency loss because the frequency is as high as 20 kHz. Compared with commercial power supply (50Hz/60Hz), the number of turns of the winding can be reduced and internal impedance can be reduced.
In the secondary rectifier circuit, the voltage converted 20 kHz output is rectified to ± DC by a high-speed diode.
The pulse-width-controlled constant voltage circuit detects the ± DC voltage applied to the final stage of the power amplifier and controls the pulse width of 20 kHz to keep the output voltage ± and separately constant. In addition, a capacitor of 22,000 μ F is inserted into this output terminal so that it can handle transient signals.
In addition to the power stage, an independent tandem constant voltage circuit is provided for small signals (including pre-stage) to eliminate interference from the power stage.
A protection circuit is mounted to detect the DC voltage of the power amplifier output, the temperature of the power MOS FET, and load impedance. When the speaker or power amplifier becomes dangerous, the output is cut off to protect the speaker and the TA-N900.
It is equipped with a speaker impedance switch and is equipped with a speaker impedance selector switch so that each of 8 Ω / 4 Ω / 2 Ω speakers can be driven most efficiently and stably.
The voltage supply to the power stage of the pulse lock power supply is controlled so that the rated output is 200W at each position, so that it can cope with the difference of the speaker load.
When using a 16 Ω speaker, adjust it to the 8 Ω position, and the output at this time is 100W.
A high-efficiency cooling system with heat pipes and cross-flow fans has been adopted to realize a 200W large output amplifier despite being small and thin.
This heat pipe is an application of the heat treatment technology of a spacecraft, and has a thermal conductivity w several hundred times that of a metal rod of the same diameter. The crossflow fan uses a linear torque BSL (brush and slotless) motor used in Sony player systems and cassette decks to support a quiet and reliable cooling system. This two stage cooling structure allows output elements to be concentrated in one location, greatly reduces the wiring around the element that flows a large current, and reduces the effect of electromagnetic waves jumping to the front stage on sound quality.
In consideration of sound quality degradation due to interference between the front and rear stages in the same channel, the construction is thorough in order to connect the input to the output with the shortest distance and to arrange each part without interference.
In consideration of various cooling settings such as rack mounting, it is designed to intake air from the bottom and exhaust air to the back.
In order to improve linearity, each amplifier is operated in a region with good linearity, and if necessary, it is improved by cascode connection.
Since amplification elements with large input and feedback capacitances have high frequency characteristics, distortion factor characteristics, and power supply rejection characteristics, we use carefully selected parts to suppress these characteristics.
In order to solve the problem of occasional resonance and loss caused by applied voltage, film capacitors are wound and fixed quickly, and electrodes and films with mechanical strength and excellent electrical characteristics are used.
The large-capacity electrolytic capacitor used in the power supply section uses high-purity aluminum foil for both the anode and cathode. The oxide film, which is a derivative, uses the lowest chemical conversion method with low sound quality. In addition, a newly developed high-speed low-distortion type electrolytic solution is used. Magnetic materials are eliminated from the terminal material to plating. High-frequency impedance is easily increased. The high-frequency impedance is reduced to less than 1/2 compared to conventional capacitors.
The resistor has good temperature characteristics, allowable deviation, and distortion factor (10 kHz third harmonic), as well as excellent tone quality. In addition, is equipped with hand-made devices with sufficient allowance for allowable power, and series para-connection is used depending on the location.
In consideration of changes in sound quality due to wiring materials and connection code, we have adopted Class 1 oxygen-free copper (copper-purity of 99.99% or more) for printed circuit boards and wiring materials, which is less susceptible to sound quality.
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Model Rating
| Type | Monaural power amplifier | ||||||
| Circuit system |
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| Effective output (20 Hz to 20 kHz) | 200W (2 Ω ~ 8 Ω Load) | ||||||
| Harmonic distortion factor (at effective output) |
0.05% (8 Ω) 0.1% (4 Ω) 0.2% (2 Ω) |
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| Cross modulation distortion factor (60 Hz : 7 kHz = 4 : 1 Effective output) |
0.05% (8 Ω) 0.1% (4 Ω) 0.2% (2 Ω) |
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| Damping factor | 50 or More (1 kHz, 8 Ω) | ||||||
| Residual noise | 20 μ V or Less (8 Ω, A Network) | ||||||
| Signal-to-noise ratio | 120 dB or More (8 Ω closed circuit, A-network) | ||||||
| Frequency characteristic | DC ~ 100 khz + 0 -3dB (Direct) | ||||||
| Input Sensitivity / Impedance | 1.7V/50k Ω | ||||||
| Output terminal | Compatible with 2 Ω ~ 16 Ω speakers (switched by speaker impedance switch) | ||||||
| Power supply voltage | 100 VAC, 50Hz/60Hz | ||||||
| Power consumption | 200W | ||||||
| External dimensions | Width 480x Height 80x Depth 445 mm | ||||||
| Weight | 10.5kg |