Yamaha's AI.A power amplifier and Denon's feedback-free dual super linear power amplifier are considered to be the circuits that "solved the dispute between Class A and
Class B ". The circuit is not complicated and the technical indicators are good, so they are favored by many enthusiasts and manufacturers. .
This article is about a ONE brand A930 parallel machine using ALA technology, but the listening experience is hard and tight, and the analysis ability is not
very good. So I decided to rub it in, and here are the steps to rub it:
(1) From the circuit analysis in Figure 6-19, there are two large capacitors
cciii, ciis) in the signal path that are set up to provide "suspended power supply" for the correction LC, and become The capacitive load of the front stage increases distortion, and this distortion cannot be corrected by the ALA circuit.
The field effect tube has a strong ability to carry capacitive loads, so first a pair of medium power tubes K214/J 77 are used to replace VT109 and
VT110 (2:SA1145/2SC2275), and a 470n Tada is connected in series with the VD101 and VD102 branches. Fine-tune the resistor and
adjust it so that the LC of VT109 VT110 is 12~18mA (the original LC is measured as 0.98mA), so that it can work in Class A state.
It should be noted that after modification, VT109 and VT110 need to be equipped with a small radiator.
(2) The impedance of the ALA correction circuit (R..7+R133) is high, and the original push tube uses a bipolar transistor. A heavy load
will affect the correction effect and also cause greater attenuation to the signal path. Therefore, t still uses K214/J 7 7 to replace VT112 and
VT113 (2SA1306/2SC3298), which can reduce the load of the front stage, reduce the loss, and suppress harmful reverse transmission. Adjust PR2
to increase the output stage (VT114, VT115) IC to 90~130mA. At this time, VT114 and VT115 will have some temperature rise. As long as
it is not hot, you don’t have to worry. However, you should pay close attention to the case temperature of VT112 and VT113. If it is too hot, It is necessary to replace the radiator with a larger one.
(3) The original machine uses half of the TL082 to calibrate the IC, and replaces it with the authentic dual op amp NE5532 when calibrating the machine. The external compensation capacitor of the op
amp is changed to 47pF (the original machine uses 330pF).
(4) Many electrolytic capacitors and non-polar capacitors of average quality are used in the signal channel and correction circuit of the original machine:
① The large electrolytic ciii and c115 in the signal path use two 470dUFl25V ELNA for AUdio+0, lr_eF/
63V red WIM or MKP substitution.
③ The front-stage negative feedback branch connection is slightly modified in Figure 6-20. For cl05, two ELNA ZZ OV F/16V are connected in series and then connected in parallel with
a WIMA 0.11 F/63V, 6.8VF capacitor using RIFA MKP.
③C109 anti-vibration capacitor is removed, and 2ZOpF silver mica capacitor is used for feedforward compensation on RlZO.
④ClOl, C102 C104. C122, C124. C125. C126 are all replaced by silver mica capacitors or imported ultra-small MKP
capacitors. Cl30 (0.lrzF) and C131 (o.22VF) are replaced with RIFA 100V capacitors .
⑤Replace all lLeF/50V small electrolytic capacitors in the circuit with ELNA or Panasonic gold-letter products.
(5) The power supply part is connected to the disassembled 25A/300V TDK filter. Readers can refer to the relevant information to make their own filter.
(6) After the circuit is rubbed, adjust RP1 so that the output DC voltage is oV. Connect the load and work for one hour before adjusting again. Once, pay attention to
the fact that all tubes, resistors, etc. should not smoke or be excessively hot. If self-excitation occurs,
you can try connecting a 100-330fl resistor to the base of VT112 and VT113. 6 As for
the AI.A calibrated trimmer resistor VR3, although there is one on the original machine drawing, it cannot be found on
the printed board Ordinary audiophiles do not have low-distortion signal sources, so
they can simply not adjust it. At this point, the motorcycle ride is basically over
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