YAMAHA A-7

Commentary

This pre-main amplifier was developed as the third product of the Yamaha X Amplifier series, which realizes absolute quality, high power, light weight and small size.

The main circuit configuration is a simple basic configuration consisting of an equalizer amplifier using the pure current servo amplifier method and a high-gain DC main amplifier. If necessary, a tone control db gain tone control or control function can be inserted. 0
Each unit amplifier receives a stable supply of power from the X power supply, and each circuit operates in the best condition to realize excellent characteristics.

The power supply is equipped with an X power supply (Phase Angle Control power supply). This X power supply has the same effect as the conventional giant power supply with a small and lightweight power supply, and achieves constant voltage up to the output stage of the main amplifier.
The mechanism of the X power supply is a precise feedback circuit from the secondary side of the power supply to the primary side. When the supply voltage on the secondary side decreases due to changes in the output signal, etc., it is detected by a voltage comparison amplifier. The information 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. This controls the operation of the TRIAC inserted in the primary side of the transformer.
The TRIAC is structured like a switch that repeatedly turns on and off in synchronization with the power supply frequency. This TRIAC controls the conduction phase angle of the voltage waveform that flows to the primary side of the transformer to keep the supply voltage exactly constant.
This provides a stable constant voltage power supply that is not affected by load fluctuations or primary side voltage fluctuations. In addition, by controlling the energization time (shortening), the loss at the iron core of the transformer can be reduced, and the number of coil windings can be reduced. Compared with conventional switching power supplies, the circuit is simple and reliability is improved. In addition, since high frequencies of 20 kHz to 80 kHz are not used, noise characteristics are excellent.

The equalizer amplifier section adopts the pure current servo amplifier method, which was newly developed based on the concept of "0 element sensitivity amplifier" which challenged the ideal of the amplifier to use the ideal element in the ideal environment. This pure current servo amplifier is a method in which no signal current flows into the power supply or ground line. The power line always flows with a constant current. Therefore, the structure is less affected by the power supply system such as L, C, R components of the power supply and wiring, non-linear due to changes in current, etc.
The circuit configuration of this equalizer amplifier consists of the first stage of low-noise dual-FET differential cascode bootstrap current mirror load → cascode amplification, pre-drive stage of constant current load → output stage of SEPP output. Combined with pure current servo amplifier system, excellent characteristics are obtained.

You can directly enjoy the MC head amplifier by switching the Phono switch on the front.
In terms of circuitry, the gain of the equalizer amplifier is switched.

The tone control circuit is a newly developed Yamaha tone control using a high-precision operational amplifier. The control curve is extremely precise and straightforward compared to the conventional one.
The tone control circuit is designed for a gain of 0 dB so that the gain of the amplifier as a whole does not decrease even if it is disconnected from the signal path. In addition, in order to minimize the deterioration of sound quality when the tone control and filters are operated, a flat amplifier with a high-precision operational amplifier is installed as a buffer with the main amplifier.
In this tone control circuit, the turnover frequency can be switched in two stages.

The main amplifier consists of a DC amplifier with differential input by low-noise dual FET + cascode bootstrap + active load first stage → differential + cascode amplification + pre-drive stage of current mirror load → 3-stage Darlington connection output stage. Since the input sensitivity is designed at 150 mv, the gain is approximately 16 db higher than that of a normal power amplifier.
In order to prevent center voltage drift, which is a particular problem in DC amplifiers, we have adopted a low-noise dual FET in which an FET with uniform electrical and temperature characteristics is packaged in one package at the first stage. In addition, we have achieved constant voltage at the output stage, which was impossible with conventional power supplies, so we can obtain high power with stability.
In order to reduce the switching distortion peculiar to class B amplifier, f_TElement with high linearity is adopted.

Continuous loudness is used for the loudness control.
This system is equipped with a dedicated slide volume that can be adjusted based on the actual listener's ear, and the situation can be freely controlled according to the volume, etc.

It is equipped with a DISC switch. When this switch is pressed, regardless of the position of the built-in selector,) regardless of the position of the built-in selector, and each selector switch jumps.

Equipped with a phono selector, impedance selection and MC position selection can be made with the Phono1 circuit.

Equipped with a Rec out selector, you can select the signal to Rec out regardless of the program being played.

There is a Main Direct switch. When this switch is pressed, the tone control, continuous loudness, high filter and subsonic filter are removed from the signal path and can be directly input to the main amplifier.

It is equipped with a Listening Level Monitor, and you can check the peak value of the output when listening by the LED on the slide volume. If you set it to the allowable input of the speaker beforehand, you can check it by the LED when the volume is raised too much.

Equipped with 2 speaker outputs, Balance, Mode, Phones, self-illuminated power switch, etc.

For higher sound quality, we have adopted a low-noise dual FET at the first stage of the equalizer, a mica capacitor for phase correction, a Mylar capacitor for coupling, a Mylar capacitor inserted in parallel with the power supply Chemi-Con, a one chip dual FET at the first stage of the main amplifier, and a Phono terminal with gold plating.

Model Rating

Type	Stereo pre-main amplifier
Effective output (20 Hz to 20 kHz, 0.007%)	120W + 120W (8 Ω)
Power Bandwidth (60W, 0.002%)	10 Hz ~ 50 kHz (8 Ω)
Damping factor	55 (8 Ω, 1 kHz)
Input Sensitivity / Impedance	Phono MM : 2.5mV/100k Ω, 47k Ω, 33k Ω, 100 Ω Phono MC : 160 μ V/100 Ω Tuner, Aux, Tape : 150mV/47k Ω
Maximum allowable input	MM:250mV MC:15mV
Output Level / Impedance	150mV/600 Ω
Headphone Output / Impedance	0.89V/8 Ω (0.01%)
Frequency characteristic	Tuner, Aux, Tape : 20 Hz to 20 kHz + 0 -2dB
RIAA deviation	MM : + / - 0.2 dB MC : + / - 0.3 dB
Total harmonic distortion rate (20 Hz to 20 kHz)	Phono MM → Rec out (10 V) : 0.003% Phono MC → Rec out (7 v) : 0.006% Tuner → SP out (60W) : 0.005%
Cross modulation distortion factor	Tuner, Aux, Tape (120W, 8 Ω) : 0.002% Main Direct on (1W, 8 Ω) : 0.01%
SN ratio (IHF-A network, input short)	Phono MM (2.5 mv standard) : 87 dB Phono MC (250 μ V standard) : 71 dB Tuner, Aux, Tape : 103 dB
Input Reduced Noise (IHF-A Network)	Phono MM:-139dBV Phono MC:-143dBV
Channel Separation (1 kHz, Vol-30dB, 5.1 kHz)	Phono MM:70dB Phono MC:70dB Aux, Tape : 70 dB
Tone control characteristics	Turnover frequency Bass : 125 Hz, 500 Hz Treble : 2.5 kHz, 8 kHz Maximum variable width Bass : ± 10 dB (20 Hz, 500 Hz) Treble : ± 9.5 dB (20 kHz, 2.5 kHz)
Filter characteristics	Subsonic Filter : 15 Hz, 12dB/oct. High Filter : 10 kHz, 12dB/oct.
Maximum compensation for continuous loudness control	20 dB (1 kHz, based on sound correction curve)
Slew rate	200 V / μ sec
Attachment mechanism	Disk switch Main direct switch Phono selector Rec Out Selector Mode switch Reaming level monitor Speaker jacks (A, B, A + B)
AC outlet	Interlocking : 2 systems (100W) Non-linked : 1 system (100W)
Pwer	100 VAC, 50Hz/60Hz
Rated power consumption	215W
External dimensions	Width 435x Height 133x Depth 365 mm
Weight	10.3kg