3AD30C gallstone amplifier

The system configuration of this machine: a newly made 30W semi-transistor amplifier drives the speakers to produce bass, while the mid-range and treble are produced by the speakers inside the radio, and the audio signals are provided by the cassette recorder and the radio.

The power amplifier circuit is shown in the figure. The audio voltage applied to both ends of the primary coils L1 and L4 of the input transformers B1 and B2 is added to the bases of the power amplifier tubes BG1 and BG2 respectively after being inverted by B1 and B2. After being amplified, it drives the speaker in the sound box to produce bass. The maximum output power of the power amplifier is 30W.

In order to obtain the ideal bass effect, the inductance of the primary winding of the input transformer must be large enough. It is difficult for a single transformer to meet the requirement. Therefore, two input transformers B1 and B2 are connected in series to increase the inductance, so that the bass frequency can be greatly expanded.

B1 and B2 are made by rewinding two identical 25W line transformers. All coils of the original transformer are removed, and 3300 turns of φ0.15mm enameled wire are wound flat on the original frame as the primary (L1 and L4 in the right figure); 96 turns of φ0.51mm enameled wire are wound on the secondary (L2, L3 and L5, L6 in the right figure).

B1 and B2 are connected, as shown in the right figure. The "tail" (the end with "・" in the figure) of B1's primary winding L1 is connected to the "head" of B2's primary winding L2, and the audio signal is input from the remaining two heads E and F.

The "tail" of secondary L2 is connected to the "head" of L5; the "head" of L2 (point M) is connected to the base of BG1; the "tail" of L3 is connected to the "head" of L6, and the "tail" of L6 (point P) is connected to the base of BG2.

Debugging: Connect the circuit as shown in the figure on the right, turn on the power supply, and adjust the resistance values ​​of R1 and R2 to make the quiescent current of BG1 and BG2 15mA.

Assembly: The left picture shows the audio amplifier circuit of the Panda brand 805 tube radio. Disconnect the circuit at the "X" and connect the primary E and F of B1 and B2 to the two breakpoints. At this time, the radio and the power amplifier form a two-way frequency amplification system: the high-frequency and medium-frequency frequency divider is composed of the primary winding Lo of the radio output transformer and the capacitor Co, which directly drives the speakers inside the machine to emit high and medium frequencies: the low-frequency part forms a voltage drop on the large inductors L1 and L2, which is inverted by B1 and B2, and amplified by BG1 and BG2 to drive the speakers to emit bass.

The electron tube 6H2 in the radio is the audio voltage amplifier stage; the electron tube 6H1 completes the treble and mid-range power amplification, and also completes the bass drive and phase inversion tasks; BG1 and BG2 complete the bass power amplification task.

After assembly, turn on the current, input the signal from the pickup hole CK, and you can listen to it.

Listening results: The high, mid and bass are well balanced, the music details are rich, the high-frequency resolution is high, the mid-range vocals are clear, and the bass is shocking. My friend was very satisfied after listening to it.

Component requirements: BG1 and BG2 (both green dots) are strictly paired, and each tube uses a 10cmx25cmx0.3cm aluminum plate for heat dissipation.

Power transformer B: Wound on El-type silicon steel sheet with tongue width of 26mm and thickness of 52mm, the primary is wound with 1210 turns of φ0.51mm enameled wire, the secondary is wound with 198 turns of φ2.0mm enameled wire, tapped in the center, power 100W.