YAMAHA C-2a
¥ 170,000 (released in November 1978)
Commentary
As an improved version of C-2, this control amplifier uses newly developed parts such as low-noise High-gm Dual FET.
The basic circuit configuration consists of a low-noise head amplifier for the MC cartridge, a low-noise equalizer amplifier with an assembled current-noise reduction circuit, and a high-precision tone control amplifier. All amplifier circuits are balanced push-pull in all stages to minimize distortion in principle, and a DC amplifier configuration.
The main improvements from C-2 to C-2a include improved performance and additional functions. First, all characteristics such as distortion factor, S / N ratio, frequency characteristics and transient characteristics have been greatly improved. In addition, Phono Selector and Rec out Selector have been added, continuously variable tone control has been adopted. In addition, all parts examined for sound quality have been fully adopted.
A newly developed low-noise Highgm Dual FET (2SK101) is used in the first stage of the equalizer amplifier section. The 2SK101 has excellent low-noise characteristics with low 1/f noise in particular, and has a high gm of 120m mho (Typ.). It is an Nch silicon junction FET with matching electrical and thermal pair characteristics of a pair FET.
The circuit configuration consists of a differential circuit using a Dual FET with low noise and high gm, and a new cascode bootstrap circuit that is effective for improving DC drift, which is a problem in DC amplifiers, preventing distortion due to an increase in signal source impedance when a cartridge is mounted, improving frequency characteristics, and reducing current noise. In addition, an automatically balanced active broad that is effective for reducing distortion is arranged.
The pre-drive stage of the equalizer amplifier is composed of a current mirror differential push-pull circuit, and the output-stage is a two stage emitter-follower complimentary service push-pull circuit using a transistor with high fT and sufficient collector loss.
The two stage emitter-follower complimentary service push-pull circuit two stage emitter-follower push-pull circuit provides a low impedance of 100 Ω and a sufficient voltage with low distortion.
A high-precision large-capacity metal film resistor and a newly developed high-precision polypropylene film temperature-compensated capacitor are used in the RIAA element that determines the accuracy of the equalizer to achieve high accuracy.
A current noise reduction circuit is installed to reduce the current noise generated by the load resistance of the cartridge.
In this circuit, the load resistance value is set to a large value to reduce current noise, and in order to prevent adverse effects on characteristics by increasing the load resistance, several times higher resistance is used, and the desired load resistance is obtained by the current noise reduction circuit.
This reduces the current density of the input conversion noise because the load resistance is a fraction of the used resistance, resulting in a load resistance with less noise.
The MC head amplifier section has a new cascode complimentary service push-pull circuit DC amplifier configuration in which four low-noise transistors with a low rbb (base resistance) and a large hfe are connected in parallel to each of NPN and PNP in order to reduce voltage noise, and one transistor is connected in a cascode. Excellent low-noise characteristics and low-distortion factor characteristics are realized because low-noise transistors are connected in parallel to form a complimentary service push-pull circuit.
The load resistance and load capacity of the Phono1 circuit of C-2a can be set according to the cartridge to be used. The load resistance can be switched to 3 levels by the Phono Selector, and the load capacity can be connected to the Cartridge Load terminal on the rear panel.
The 100 Ω position is exclusively for high-power MC type cartridges.
The circuit design of the tone control amplifier is almost the same as that of the equalizer amplifier and is a mid-point defeat NF type tone control circuit.
One chip Dual FET with excellent thermal balance is adopted in the first stage, and the cascode bootstrap circuit is assembled to reduce the drift of the midpoint voltage which is important for DC amplifiers. The cascode bootstrap circuit is also used to reduce the distortion factor due to the variation of the source impedance when the volume is manipulated.
The pre-driver stage is a differential cascode current mirror output of the low impedance drive by the emitter follower to prevent the loop frequency characteristics from being degraded by the mirror effect of the feedback capacitance of the element.
The complimentary service stage employs a pure complimentary service OCL circuit using transistors with high fT output characteristics to keep the output impedance sufficiently low.
The tone amplifier is designed so that the element with the time constant completely deviates from the signal path at the midpoint of the tone control volume, and operates as a broadband flat buffer amplifier.
This is a DC tone control circuit in which no capacitor other than polypropylene film capacitor for frequency characteristic setting is inserted even when tone control is used.
The time constant element for tone control uses the same polypropylene film capacitor (accuracy 1%, temperature characteristic 50 ppm / ℃) and metal film resistor as the RIAA element, and the newly developed high-accuracy conductive plastic volume with special curve is used for the control volume.
Equipped with a subsonic filter.
The power supply circuit uses an OI core transformer, which is small but highly regulated and has little leakage flux, and an electrolytic capacitor of the low-noise series for audio with little distortion.
In addition, the cascode bootstrap active load is assembled into a low-drift comparison amplifier with dual transistors, and a low-noise avalanche effect diode is used as a diode for generating the reference voltage. Thus, a high-response constant-voltage power supply circuit with low distortion factor and low noise is constructed.
In addition, the power supply for the MC head amplifier, which is easily affected by power supply fluctuations, is further reduced in impedance by the local power supply.
The C-2a uses high-quality parts such as a low-impedance 70 μ - thick printed circuit board + pure copper bus earth, IO core transformer, Yamaha's original audio coupling capacitor, high-precision conductive plastic volume, new mechanism rotary switch, metal film resistor and polypropylene film capacitor. It also uses gold-plated cutting pin jacks and connectors to improve reliability.
Equipped with -20dB audio muting circuit.
The Rec out selector allows you to select the signal to Rec out regardless of the program source being played by the amplifier.
Relay circuits are installed in Rec out and Pre out1/2 to prevent shock noise when the power is turned ON / OFF.
It has an AC outlet.
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Model Rating
| Type | Control amplifier | ||||
| Input Sensitivity / Impedance | Phono1 mm : 2.5mV/47k Ω, 68k Ω, 100k Ω Phono1 MC : 100 μ V/50 Ω Phono2 mm : 2.5mV/47k Ω Aux, Tuner, Tape PB1/2 : 150mV/47k Ω |
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| Cartridge load | 100k Ω, 68k Ω, 47k Ω, 100 Ω | ||||
| Maximum allowable input | Phono1 MC : 10 mV or more (20 kHz, 0.03%) Phono1/2 MM : 350 mV or more (1 kHz, 0.01%) Aux, Tuner, Tape PB1/2 : 30 v or more (Volume-34dB) |
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| Rated Output / Impedance / Maximum Output | Preout 1 / 2 : 2V/250 Ω / 15 V or more Rec out1/2 : 150mV/100 Ω / 20 V or more |
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| Frequency characteristic | Phono1/2 MM : 20 Hz to 20 kHz 0 ± 0.2 dB (RIAA) Phono1 MC : 20 Hz to 20 kHz 0 ± 0.3 dB (RIAA) Aux, Tuner, Tape PB1/2 : 10 Hz to 100 kHz + 0 -0.2 dB |
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| Total harmonic distortion rate (20 Hz to 20 kHz) | Phono1/2 mm : not more than 0.003% Phono1 MC : not more than 0.01% Aux, Tuner, Tape PB1/2 : 0.003% or less |
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| Harmonic distortion rate (20 Hz to 20 kHz Sum of 2-10 degrees in HP-IB) |
Phono MM → Rec out (1.5 V) : 0.0007% or less Aux, Tuner, Tape1/2 → Pre out : 0.0007% or less |
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| Cross modulation distortion factor | Aux, Tuner, Tape PB1/2 : 0.003% or less (10 V output) | ||||
| Signal-to-noise ratio (IHF-A network) | Phono1/2 MM : 92 dB or more Phono1 MC : 78 dB or more Aux, Tuner, Tape PB1/2 : 103 dB or more |
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| Residual noise | 0.03 μ V or less | ||||
| Channel separation (1 kHz, Volume max) |
Phono1/2 MM : 90 dB or more Phono1 MC : 78 dB or more Aux, Tuner, Tape PB1/2 : 90 dB or more |
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| Function separation (1 kHz, Volume max) |
Phono1/2 mm → Tuner : 90 dB or more Tuner → Phono1/2 MM : 95 dB or more Aux → Tuner : 90 dB or more |
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| Tone control characteristics |
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| Subsonic filter | 15 Hz, 12dB/oct. | ||||
| Audio Muting | -20dB | ||||
| Semiconductor used | FET : 5 pcs Semiconductors : 101 units Zener diode : 10 units Diode : 32 pcs |
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| AC outlet | switched:400Wmax unswitched:400Wmax |
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| Pwer | 100 VAC, 50Hz/60Hz | ||||
| Power consumption | 36W | ||||
| External dimensions | Width 435x Height 72x Depth 320 mm | ||||
| Weight | 7.9kg |