LUXMAN MQ70

Commentary

Tube type stereo power amplifier using 5-pole output tube 6CA7.

The output tube uses 6CA7, a 5-pole tube that is well established in the audio world.
Although the 6CA7 is relatively small and has a smart shape, it can be used for a large output power of 100W in push-pull operation. In the MQ70, the 6CA7 is an ultra-linear push-pull output circuit, allowing sufficient margin to obtain output power of 45W/45W for high durability.

The output transformer uses the OY15 to 6.5 KHF developed in Shinkai for the MQ70.
In order to improve the performance of the MQ70 without losing the efficiency of the output tube, the output transformer is equipped with an SG tap for ultra linear connection and a third winding for cathode NF. By using these methods together, the internal resistance of the output tube is sufficiently low, the speaker is driven at constant voltage, and the bare characteristics of the output stage is improved.
Power loss is reduced by half compared with the conventional OY15 type by increasing the diameter of the output transformer wire. In addition, the secondary side winding is unified in order to greatly improve frequency characteristics, phase characteristics and AC balance.

The power supply circuit uses a large power transformer 9F380 with excellent regulation, and the power supply supplied to the output circuit is bridge rectification system to minimize voltage fluctuation.
In addition, the smoothing circuit is connected in parallel with a choke coil #4705 with low DC resistance and a high-voltage block capacitor 47 μ Fx3. By combining two of these, the power supply impedance is sufficiently low and the power supply circuit with low voltage fluctuation is completed.
In addition, the making the negative power supply supplied to the differential circuit constant voltage, the operation of the first stage is stabilized and the directly coupled drive stage is stabilized.

The drive circuit consists of a 2-stage differential amplifier circuit.
The first stage of the MQ70 is equipped with a differential circuit using both 12AX7 units. This circuit is highly stable and resistant to changes in temperature and power supply voltage, and can produce output with 180 ° phase difference. Therefore, it can also function as phase inversion.
The drive stage directly connected to the first stage uses an ultra-linear connection and a cathode NF for the output stage, which requires a large drive voltage. Therefore, a high voltage driver tube 6240G of the Lux brand is used for the drive stage, and a differential amplifier circuit configuration is adopted.
In such a configuration in which the phase inversion stage of differential amplification is placed in the first stage and the input signal is added to one grid and the negative feedback input is added to the other grid, the balance of the push-pull operation is automatically corrected, and the distortion factor characteristics are further improved.

The phase correction circuit has also been studied to improve the distortion factor characteristics in the high frequency range.
The integrating phase correction circuit inserted in the secondary side of the output transformer has little influence on the high-frequency characteristics when a speaker is connected to the speaker terminal. It works effectively when there is no load. It ensures high stability from no load to full load.
The differential-phase compensation circuit consists of an NF resistor and a capacitor connected in parallel to the resistor. In the MQ70, a high-frequency choke of several μ H is inserted into this circuit to limit the amount of NF in the ultra-high frequency range and to select a capacitor of appropriate capacity to ensure balanced and high stability.

Equipped with independent input level setting mechanism for left and right channels.

Model Rating

Type	Bulb type stereo power amplifier
Continuous execution output	45W/45W (4 Ω, 8 Ω, 25 Hz to 30 kHz) 30W/30W (16 Ω, 25 Hz ~ 30 kHz)
Total harmonic distortion factor	0.1% or Less (8 Ω, 1 kHz, 10W) 0.5% or Less (8 Ω, 1 kHz, 45W)
Frequency characteristic	10 Hz to 40 kHz -1dB
Input sensitivity	650mV
Input impedance	80k Ω
Signal-to-noise ratio	110 dB or More (IHF-A Correction, Input Short Circuit)
Damping factor	15 (8 Ω, 1 kHz)
Residual noise	0.3 mV or less
Power consumption	190W (Electrical Appliance and Material Control Law) 260W (8 Ω, both channels simultaneously, maximum output)
External dimensions	Width 405x Height 165x Depth 240 mm
Weight	17kg