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LUXMAN MQ68Custome
¥ 108,000 (Released in November 1978)
Commentary
This is a non-feedback tube type stereo power amplifier that does not rely on NFB and pursues bare characteristics from the bottom.
With NFB switching, it is possible to apply a light NFB of 16 dB.
The output tube uses a 3-pole tube 50CA10 for power amplification, which is used in 38 series pre-main amplifiers and 60 series power amplifiers.
This ball has a large plate loss of 30W and can produce 30W of power with push-pull operation. It also has a low internal resistance and the speaker is driven by a constant voltage, which helps improve the bare characteristics of the power stage.
The new OY series OY15-6KHP is used for the output transformer.
This transformer was newly designed to bring out the performance of 50CA10. Compared with the conventional OY type, the power loss is reduced by half by increasing the diameter of the wire used. Moreover, by taking advantage of the wide optimum load range of the 3-pole output tube, the secondary side winding is unified, and frequency characteristics, phase characteristics, and AC balance are improved.
In order to produce a non-feedback amplifier that can be used for audio applications without using NFB, the operating point of the driver stage is made variable, and each unit is carefully adjusted to cancel out the distortion of the power stage and reduce the distortion of the amplifier as a whole. Thus, the naked characteristics are improved.
The driver circuit is based on a cathode-coupled Mulard type. The first stage uses 3-pole tube connection 6267 with sufficiently low internal resistance, and the phase inversion stage uses Lux-brand bi-3-pole tube 6240G developed for high-voltage drives.
An attenuator is inserted in the input circuit to eliminate the gain difference (gain difference) that occurs when NFB is switched.
We have also studied the phase correction circuit required for NFB amplifiers to improve the distortion factor characteristics in the high-frequency range.
The integration type phase correction circuit inserted in the secondary side of the output transformer has little effect on the high-frequency characteristics, works effectively under no load, and ensures high stability from no load to full load.
A differential-type phase correction circuit consisting of an NFB resistor and a capacitor connected in parallel to the resistor has problems in high-frequency characteristics and stability depending on the capacitance of the capacitor. However, in the case of MQ68C, a high-frequency choke of several μ H is inserted to limit the amount of NFB in the ultra-high-frequency range, and a capacitor of appropriate capacitance is selected to improve balanced stability and characteristics.
A power transformer with excellent regulation is used for the power supply part, and the power supply to the output stage is a bridge rectification system to reduce voltage fluctuations.
The smoothing circuit combines a choke coil with low DC resistance and a block capacitor (220 μ Fx2) to keep the source impedance low and provide a stable power supply circuit.
Model Rating
| Type | Bulb type stereo power amplifier | ||||
| Continuous effective output |
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| Total harmonic distortion factor |
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| Frequency characteristic | At NFB16dB : 10 Hz to 60 kHz -1dB or less No feedback : 20 Hz ~ 30 kHz -1dB or less |
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| Input sensitivity | 880mV | ||||
| Input impedance | NFB16dB : 100k Ω Non-feedback : 50k Ω |
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| SN ratio (input short IHF-A correction) |
NFB at 16 dB : 105 dB or more No feedback : 95 dB or more |
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| Attachment | NFB selector switch Independent input level set for left and right channels |
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| Pwer | 100 VAC, 50Hz/60Hz | ||||
| Power consumption | 160W (Electrical Appliance and Material Control Law) | ||||
| External dimensions | Width 410x Height 183x Depth 160 mm | ||||
| Weight | 13.8kg |
