2. Solder all components of the preamplifier, where the resistors and transistors of the differential amplifier should be matched. It is best to use 1/2W resistors. The multi-turn potentiometer R12 should be turned to the maximum resistance position. Short-circuit the connection points between the collector of VT8 and the +42V power supply, and between the collector of VT9 and the -42V power supply.
3. Replace resistor R13 with a 100-ohm resistor in series with a 500-ohm potentiometer. Apply 55V power supply, adjust the potentiometer resistance so that the emitter (midpoint) potential of VT8 and VT9 is 0.00V. Solder off the 100-ohm resistor and the series potentiometer from the board, measure their resistance, and solder a resistor of the same value at R13.
4. Power off, disconnect the short-circuit connection between the collector of VT9 and the -42V power supply, and connect a DC ammeter. Power on, gradually decrease the resistance of potentiometer R12, and the ammeter reading will gradually increase. Adjust the current to 10mA. High current generator.
6. The adjustment steps for the output stage are generally the same as those for the preamplifier, except that the collector current of VT18 is set to about 20mA, and the collector current of VT20 is set to 50mA to 200mA (depending on personal preference and heat sink temperature).
7. It is best to use premium capacitors for the coupling capacitors in the signal path.
To improve the stability of the constant current source, VT4 uses a super transistor BC33740, whose value is generally between 300 and 400. The second stage amplification current is about 3 times that of the first stage. If a larger current is desired, resistor R10 can be appropriately reduced. The wiring for the preamplifier and output stage is... (Note: The original text seems to be cut off here, but I will translate as is) line mm.
To reduce the overall size of the unit, VT10, VT11, VT12, VT13, VT15, and VT16 are all soldered on the component side of the PCB. The leads of all medium and high power transistors that require heat dissipation (VT14, VT17, VT18, VT19, VT20) are bent 90 degrees, fixed to the heat sink (with insulating pads), and then soldered to the underside of the PCB. This way, the height of the output stage amplifier with all components soldered is only 15mm. The 320W toroidal transformer is fixed above the PCB with four 15mm high copper pillars. The overall dimensions of the unit are: 164mm × 126mm × 255mm (width × height × length).
The aluminum heat sink is 250mm × 120mm × 60mm, with a heat dissipation area of 30000mm². A total of 10 transistors from both channels (VT14, VT17, VT19, VT20) are fixed to the heat sink. To increase flexibility in construction, although the preamplifier and output stage are designed on a single PCB, the PCB can be cut at a distance of... mm to separate the preamplifier and output stage, making their placement more flexible.
To connect to high-end signal sources, this amplifier is equipped with both RCA input jacks and balanced input jacks (XLR jacks). Current transformers of the same model but from different manufacturers can be mixed... This is good news for users who have higher requirements for distortion and noise.
For audio enthusiasts, one unit can be used to form a Hi-Fi system, or three units can be used to form a 5.1-channel surround sound system. This design tries every means to reduce size, and three units placed together are only 492mm wide.
