The low-frequency amplifier is assembled on eight silicon transistors, according to a very common scheme on the Internet, known as the “Class A Ultra-Linear Amplifier”. I repeated the scheme, assembled mainly from domestic components. In the first stage, I used transistors (KT501I on the T1 circuit; 2 pcs.), Then (KT608B on the T2 circuit; 2 pcs.), In the output stage I used (KT808A on the T3-T4 circuit; 4 pcs.), The number is indicated for the stereo version. Dual Channel Circuit Board divorced in the Layout 6 program. All elements are placed on printed circuit board, except for powerful KT808A and rectifier diodes KD202V. To avoid the use of insulating spacers, the output transistors are mounted on separate aluminum heat sinks. The rectifier is made according to the bridge circuit on KD202V diodes, which are also installed on small radiators (there are no heat sinks in the photo).

The smoothing electrolytic capacitors of the rectifier have a total capacity of more than 10,000 microfarads. If you use a diode bridge, for example KBU810, then it can be placed on the PCB in the place intended for it, and preferably with a small cooling plate attached, (it is convenient to use a bridge with a hole to attach the heat sink). For forced cooling, you can also use a fan that will blow on elements that have a large heat dissipation. The circuit board also provides a place for installing a five-amp LM338T voltage regulator in a TO-220 case with a strapping of several additional elements and a place for a cooling radiator for it. If the stabilizer is not needed, then these elements can not be mounted, but then on the board you need to install one jumper between

Tracks are in and out contacts. LM338T chips (see figure). On the circuit diagram another version of the stabilizer is shown. To suppress the self-excitation of the amplifier, a correction circuit recommended in various publications is installed between the T3 emitter and the negative wire, consisting of a series-connected MLT-2 resistor with a resistance of 10 ohms and a capacitor with a capacitance of 0.1 μF. Power step-down transformer with a power of 90 W., secondary winding, made with a wire with a diameter of 1 mm., output AC voltage 22 volts. The photo shows two options.

ULF with and without a stabilizer, also other transistors are installed in one of them, T1 - KT3107B, T2 - KT961B, T3-T4 the same KT808A, see photo. UMZCH was tested with a homemade two-way acoustic system, which consists of a broadband speaker 4GD-35 (8 GDSH-1) frequency range 63 - 12500 Hz, and a high-frequency speaker 3GD-31 (5 GDV-1-8) frequency range 2800 - 20000 Hz. Inside there is a filter for the tweeter, consisting of an 8 ohm resistor and a 2 microfarad capacitor connected in series. (see fig.). The acoustic labyrinth-type enclosure is made of 16 mm chipboard sheets, to eliminate extraneous overtones of chatter that may appear from resonance, I pasted over the walls of the enclosure with sound-absorbing material inside, I used embossed foam rubber, the dimensions of each speaker are: height 1000 mm, width 270 mm, depth 300 mm .

AC impedance is about 5 ohms. The oscilloscope screen shows a signal with a frequency of 1000 Hz. and a voltage of 0.7 volts supplied from the generator audio frequencies to the input of the amplifier and, accordingly, the output signal at maximum volume with an equivalent load connected instead of acoustics, a PEV resistor with a resistance of 5 ohms and a power of 7.5 watts. UMZCH test results: Output power is about 6.5 watts. on the channel, there is a slight background, the sound is pleasant, I want to listen. The audio signal was fed from the line output of the Sony DVP-NS308 player. The amplifier worked for a long time (more than 1 hour) at a power slightly above average and showed good result, the only drawback is the heating of the weekend

transistors. I measured the temperature with a multimeter, attached a thermocouple close to the bottom of the KT808A, the tester showed 65 degrees during operation, at room temperature 25. I did not hear a big difference when playing between both versions of the assembly, but with the stabilizer the background was noticeably reduced. The setup is simple and has been described many times. If the installation is correct and there are no errors, turn on the amplifier and set the trimmer resistor R1 on the emitter of the transistor T3 to a voltage equal to half the power source (I got 13.5 V., with an input of 27 V.) Next, turn off the power, solder the wire going to the T3 collector . and connect the ammeter to the gap, then turn on the power again and look at the readings of the device, this is the quiescent current of the output transistors, by changing the resistance of the resistor R6 we select it according to the table.