KENWOOD L-08C
¥ 180,000 (released in 1980)
Commentary
Stereo control amplifier capable of Σ connection with power amplifier.
The L-08C allows a Σ connection between the preamplifier and power amplifier.
In the conventional separate amplifier, there are various capacitances between the windings of the transformer. In the high-frequency region, the primary side and the secondary side are completely electrostatically coupled. In addition, the primary side of the preamplifier and the primary side of the power amplifier are in the same state as if they were connected through an AC cord. In addition, the home AC power supply, which has long indoor wiring, has a considerably high impedance. When the power amplifier drives the speaker, the voltage drop occurs and the secondary side voltage fluctuates. This electrostatic coupling and electromagnetic coupling due to voltage fluctuation create a current loop consisting of the power transformer of the power amplifier → the power transformer of the preamplifier → the ground of the preamplifier → the ground of the power amplifier → the power transformer of the power amplifier. The potential difference generated in the ground connection cable due to the current flowing in this loop enters the power amplifier and is amplified, and is input to the speaker with a distortion that cannot be ignored in the sense of hearing.
In the Sigma connection, the Sigma cable between the earth and the ground acts as an exclusive bypass for the power loop generated by mutual interference between the power transformers and does not affect the signal system. Therefore, when the pre-power and the power are connected, the potential between the earth and the ground is matched. This makes it possible to connect in accordance with the original purpose of the separate amplifier, enabling faithful signal transmission.
A newly developed multi-power supply + power supply current absorption circuit is used for the power supply.
In general amplifiers, a constant-voltage power supply is used to keep the output voltage constant against load fluctuations. However, a constant-voltage power supply with negative feedback is considered to be the same as a power amplifier in terms of circuit configuration. As a result, the output of the constant-voltage power supply and the amplifier are arranged in series between the power supply and the load. This goes against the simplification of the circuit and degrades the quality of sound, such as dynamic feeling and freshness.
To solve this problem, the L-08C employs a newly developed multi-power supply + power-supply current absorption circuit, eliminating the constant-voltage power supply with negative feedback. First, each amplifier has a dedicated power supply consisting of a separate transformer and a rectifying and smoothing circuit. Both the flat amplifier and the equalizer amplifier have a multi-power supply configuration consisting of independent power supplies on the left and right sides. Moreover, the flat amplifier and the equalizer amplifier are equipped with a newly developed power-supply current absorption circuit, so that no AC current flows through the ± B power supply circuit and ground circuit regardless of the signal magnitude.
As a result, even if a signal is input, the power and ground circuits are considered to be stationary, and the impedance of the power and ground circuits can be considered to be 0. Therefore, a constant voltage power supply and a bus ground line are basically unnecessary.
In the multi-power supply, each amplifier stage processes its own signal, so there is no mutual interference at all, and only pure signal current can be transmitted between stages.
The chassis has a non-magnetic structure to eliminate the influence of magnetic distortion.
Resin is used for the case and bottom plate, non-magnetic aluminum is used for the made of non-magnetic aluminum material, and inside parts that cannot be protected by iron are placed far away from the signal path.
It is equipped with an input indicator. When you select an input source, the words "Phono", "Tuner", "Tape" and "Aux" will appear as an illumination, and the FADER status will be indicated by bright red illumination.
The front panel is designed with a control pocket.
When the pocket is opened, the inner knob protrudes and becomes the same surface as the panel. In addition, the panel surface of the pocket is designed to be hidden by the bottom plate.
Equipped with a continuously variable frequency loudness control.
The cutoff frequency can be continuously variable from 30 Hz to 100 Hz, and 3 levels can be selected.
The equalizer amplifier section has a 3-stage differential DC configuration.
A constant current circuit is frequently used for the circuit and a newly developed power supply current absorption circuit is added to eliminate the influence of power supply and ground.
2-stage differential-output complimentary service is used for the MC head amplifier.
We have developed a new high-speed amplifier that takes the conventional high-speed amplifier one step forward and extends the open-loop gain beyond the audible band. By making the internal impedance and feedback constant, TIM distortion and switching distortion are reduced.
The flat amplifier section uses a two stage differential + differential output SEPP circuit using one chip dual FET which is hardly affected by drift. Since no AC current flows through the power supply and ground, the impedance can be regarded as 0 and is not affected by the power supply section.
A low-noise operational amplifier with large current capacity and low distortion factor is used for the headphone amplifier section, and the headphones are directly driven by the OCL / OTL circuit.
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Model Rating
| Type | Control amplifier | |||
| <Audio characteristics (Aux/Tape → Pre out, Phono → Pre out)> | ||||
| Total harmonic distortion factor |
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| Frequency characteristic | DC ~ 850 kHz -3dB | |||
| Phono RIAA deviation | 20 Hz to 20 kHz ± 0.2 dB | |||
| Signal-to-noise ratio | Phono MM:90dB(2.5mV) Phono MC:70dB(0.1mV) Tuner, Aux, Tape play : 106 dB |
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| Subsonic filter | 18 Hz, 6dB/oct. | |||
| Loudness control | 30 Hz to 100 Hz Continuously Variable + 3 db, + 6 db, + 9 db |
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| Rise time | 0.4 μ s (± 0.1 V, ± 1 V, ± 2.5 V, Vol max) | |||
| Input Sensitivity / Impedance | Phono MM : 2.5mV/50k Ω Phono MC : 0.1mV/100 Ω Tuner, Aux, Tape play : 150mV/25k Ω |
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| Phono maximum allowable input | MM : 320 mV (1 kHz, 0.0007% distortion rate) MC : 14 mv (1 khz, 0.0007% distortion rate) |
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| Output Level / Impedance | Tape rec PIN (1 kHz) : 150mV/420 Ω Pre out : 1V/0.01 Ω or less (Rated Output, Σ Cable 1.5m Tip) |
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| Maximum power output | 10V | |||
| Power Supply Unit and Others | ||||
| Power outlet | Power switch interlock : 2 systems (200W) Power switch not linked : 1 system (500W) |
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| Pwer | 100 VAC, 50Hz/60Hz | |||
| Rated power consumption | 50W (Electrical Appliance and Material Control Law) | |||
| External dimensions | Width 440x Height 74x Depth 387 mm | |||
| Weight | 5.3kg | |||
| Attachment | Two 1.5m Σ audio cables | |||