Single differential simplest OCL amplifier-----simple OCL amplifier

(2)
The main specifications of the two complementary high-power tubes of other devices are shown in Table 2.
Table 2

_{The f T} of these two pairs of high-power tubes is about 2.5MHz, but because of the large current and the fact that they are both switching power tubes, they have good performance when used as audio power amplifiers. In particular, 2N3055/MJ2955 is the first choice for making high-fidelity power amplifiers below 50W, and is a pair of "quasi-fever tubes" with a very high cost performance. The Vceo specification value of this tube is relatively low, and is often greater than 100V in reality. There should be no problem using it under a voltage of 40V. However, dealers often select and supply it in grades according to Vceo of 60V or 100V. At this time, do not purchase it for use with 60V.
The Pcm of BG1-5 (BC557/546) is 0.5W, and the Icm is 100mA. Among them, BG3 requires Vceo≥50V, and BG4-5 requires Vceo≥100V. The f _T is ≥100MHz, and the h _FE is 110～220. BC557 and BC546 are difficult to purchase, and corresponding high-frequency low-power tubes can be selected according to the above requirements.
 To improve the open-loop linearity, differential tubes and complementary tubes should be used in pairs, which can also reduce debugging troubles and make it clear. The matching requirements are as follows: Differential tube BG1-2 requires that the h _FE difference at 0.5mA is less than 5%, and the V _BE difference between the two tubes is less than 20mA . The higher the h _FE , the better, try to ≥100. Complementary push tube BG4-5 requires that the h _FE
 of the two tubes is as close as possible within 1-80mA . If not, it can also be selected according to the basic consistency of h _FE at 60mA . The final stage BG6-7 requires that the h _FE  of the two tubes is as close as possible within 100mA-3A (for 50W power amplifier) ​​and 100mA-4A (for 100W power amplifier) . If not, it can also be selected according to the basic consistency of h _FE at 3A or 4A . The main voltage amplifier tube BG3 requires that the h _FE is not less than 60  at 10-15mA . Of course, it is better if the BG3 of the two-channel power amplifier can be basically consistent. _D1-2  is a switching diode 1N914, with a maximum forward current of 75mA and a forward voltage drop of about 1V at 10mA. It can be replaced with 1N4148.  R7-8 uses a 2-5W carburized resistor dedicated to the power amplifier, or it can be replaced with two 0.68Ω (1-2W) metal film resistors in parallel. The rest are all 0.5W metal film resistors. C1-3 uses CBB polyester non-inductive capacitors, C1 can also use general electrolytic capacitors, and C2-3 requires a withstand voltage of ≥100V.  The heat sink uses a profile, and its thermal resistance (to the corpse-path power amplifier) ​​is not inferior to 8-10~C/W. It is recommended to use a larger heat sink as much as possible in production, because the life and reliability of the power amplifier tube are closely related to the tube junction temperature.

3) Installation and debugging
(1) Installation process
 The components of the power amplifier are all installed on the printed board shown in Figure 2. It should be noted that Figure 2 shows the front component installation surface. When making a printed board, do not forget to copy it upside down. In addition, since the size of components selected by each person may not be the same, it is necessary to try to arrange the actual objects on the diagram before making the printed board, and adjust the wiring spacing appropriately to ensure that the components are arranged evenly and beautifully. The production of the power amplifier printed board in the following text is the same.
 The final stage BG6-7 is installed on the printed board through an L-shaped aluminum bracket (60×45×85mm). The installation requirements are shown in Figure 3a. It should be noted that a thermal insulation sheet should be placed between the tube shell and the L-shaped bracket. The tube shell is only connected to the corresponding copper foil on the printed board through the fixing screw A (preferably by welding), and the fastening screw B should not have any electrical contact with the copper foil of the printed board. The heat sink can be directly fastened to the L-shaped bracket, and the thermal contact surface is preferably coated with silicone oil.
 There are not many components in the power rectifier and filter part, which can be directly installed on the chassis with a bracket. Figure 3b is a wiring diagram of a channel, which only points out the following points:
① The "ground" line terminal on the printed board should be directly connected to the common connection point of the secondary center tap of the power transformer and the two filter electrolytic capacitors. This point can be directly welded to the metal chassis nearby or connected to the chassis with another thick wire to achieve "one-point" grounding. The ground terminal of the speaker is directly connected to the above-mentioned common grounding point without passing through the "ground" on the printed board.
② The signal input ground terminal and the power amplifier output ground terminal should be insulated from the metal chassis at the installation location.
⑧ The wiring of the power supply part should be short and thick. It is recommended to use Φ0.2mm×32 multi-strand plastic wire to connect. The speaker terminal is connected to the printed board and the common tapping point with a dedicated feeder for the sound box.

(2) Debugging
Before debugging, check whether the components, installation and welding are correct and reliable. Pay special attention to whether the polarity of diodes, transistors, and electrolytic capacitors is reversed, and whether the insulation between high-power tubes and heat dissipation brackets is good. After heating, check the power supply part separately. If there is no problem, then connect the amplifier for debugging. According to the load of the amplifier, it is divided into the following three steps:
① No-load debugging In order to reduce the possibility of instantaneous damage to the amplifier, do not connect the load first. After the power is turned on, touch the end tube with your hand. It should be slightly warm, or one tube is hotter and the other tube is cooler. As long as it is not hot and there is no other abnormality, you can safely measure the voltage at various places, the debugging point voltage and the static current. Use the DC voltage (200mV) range of the digital multimeter to measure the output midpoint voltage.
Generally, if it is below ±50mV, it can be considered normal. If the positive bias is too high, increase R2, otherwise reduce R2. As long as the differential tube is selected, it is usually easy to control within ±50m.
 Then test the voltage drop across R7 or R8. Since there is no load, the voltage drop on these two resistors is the same. The static current is 40~50mA, the corresponding voltage drop on R7 or R8 should reach 13~17mV, and the voltage between the bases of BG4-5 is about 2V. If there is no voltage drop on R7 or R8 or it is less than 13-17mV, you can test the voltage drop of D1 and D2 respectively, try to solder off the diode with smaller voltage drop (after disconnecting the power supply) and replace it with a diode with larger voltage drop and retest. If there is no corresponding diode, you can replace it with a 220Ω trimming resistor and fine-tune it until the voltage drop on R7 or R8 reaches 13-17mV. On the contrary, if the voltage drop on R7 or R8 is too large, you can use a 220Ω trimming resistor in parallel with D1 or D2 and fine-tune it until the voltage drop on R7 or R8 reaches 13-17mV, retest the midpoint voltage and adjust R2 so that the midpoint voltage is within 50mV.
② Pure resistance load debugging Connect 8～10Ω 1／4W resistor to the output of the power amplifier, and then measure the voltage drop across R7 and R8. At this time, since BG6 and 7 form their own independent DC circuits through this resistor, the voltage drops on R7 and R8 may be inconsistent. At this time, R2 can be adjusted to make the voltage drops on these two resistors consistent, and the midpoint voltage is close to 0V. If the voltage drops on R7 and R8 are the same but do not reach 13～17mV, D1 and D2 can be replaced or the above 2200 fine-tuning resistors can be fine-tuned 1～2 times. In short, the voltage drops on R7 and R8 should be the same and reach 13～17mV, and the midpoint voltage is generally adjusted.
⑧ Actual load debugging After the above debugging, the sound box can be connected for debugging. The feeder and its length connecting the sound box should be matched according to the actual use in the future as much as possible. After powering on, do not send a signal first, listen to whether there is obvious abnormal sound in the speaker, and touch the final tube shell with your hand. If there is abnormal sound in the speaker and the tube shell is hot, it means that the circuit is self-excited after connecting to the actual load. At this time, as shown in Figure 4, the R1, C1 series compensation network and the L1, R2 parallel compensation network can be connected to the output end of the power amplifier, and a bracket should be used to install it as close to the printed circuit board as possible. Generally, R1 and Cl are used first. If the self-excitation cannot be completely eliminated, L1 and R2 are added. L1 can be made of Φ1.0mm enameled wire wound 10 times on a 12mm frame and then cast. Generally, the self-excitation can be eliminated after the above measures are taken.
Finally, add a signal and listen to it for about 1 hour at a level slightly higher than the normal listening level (pay attention to the temperature of the final tube shell at any time), then re-measure the output midpoint voltage and quiescent current. Generally, the midpoint voltage is <±100mV, the quiescent current is <100mA, and there is no continuous increase. It can be considered that the debugging is completed. This step of debugging is best performed in the summer when the temperature is higher. If the machine is installed in winter, it is hoped that it will be re-tested in the summer.