The simplest single differential OCL power amplifier circuit diagram

As the performance of modern power amplifiers continues to improve, the circuit structure is becoming more and more complex. This is the most troublesome problem for amateur producers, especially beginners. Here I will introduce to you a simplest power amplifier circuit to see how simplified it can be. , and what kind of performance can be achieved is also an interesting question.

1) Circuit principle and performance

(1)Circuit analysis

Figure 1 is the circuit diagram of this power amplifier. There are only about 20 components in the power amplifier, including transistors. At first glance, it looks like a schematic diagram, but it is indeed a practical power amplifier, and it can be used in relatively small quantities. Low harmonic distortion provides ≥50W (120W) output power to an 8Ω (4Ω) load. It uses a typical OCL circuit, but necessary improvements have been made to the design based on practical conditions during production.

The input stage BG1-2 uses a differential amplifier stage as usual, but it is slightly different from the common circuit in that it uses a PNP tube. Compared with using an NPN tube, the two tubes are easy to pair, have good consistency, and have lower noise. For simple circuit structures, this needs to be considered as much as possible.

The second stage BG3 is the main voltage amplifier stage, which provides most of the voltage gain but does not use the common "bootstrap" circuit. The use of "bootstrap" circuits in high-power amplifiers has little significance in increasing output power, and can eliminate an electrolytic capacitor that affects sound quality, and is beneficial to reducing components and simplifying the circuit. C2 is the phase compensation capacitor.

The final stage is a complementary output stage composed of BG4-7 in the simplest way. It has few components and no adjustments, so the use of a smaller power push tube BG4-5 is enough to meet the requirement of driving the final stage output to more than 100W. The setting of the final stage quiescent current is mainly to reduce the crossover distortion at low output power, usually 40-50mA. As for the crossover distortion at high output power, the impact is not obvious due to the "masking" effect. There is no thermal compensation for the quiescent current. As the temperature rises during operation, the quiescent current also increases accordingly, but no loss of control occurred during the trial. This can simplify the installation process and reduce debugging procedures. In addition, a slightly larger quiescent current can also reduce some crossover distortion at large outputs. C3 is used for high-frequency decoupling of the power supply.

The gain of this machine after adding the overall negative feedback is about 20 times (26dB), but it can also work well after canceling the overall negative feedback. The output waveform at full power is still symmetrical. No waveform distortion is observed with an oscilloscope and tested with a distortion meter. The harmonic distortion does not increase much (only about 0.2%) compared with adding negative feedback. It can be seen that the open-loop performance of this machine is good. The purpose of negative feedback is mainly to compensate for the discreteness of BG3 parameters and ensure the stability of the overall gain, rather than mainly to reduce distortion.

Since most preamplifier outputs have DC blocking capacitors, the DC blocking capacitors at the input of this unit can be omitted. When the front stage (or CD player) adopts a DC output design without output DC blocking capacitor, it can also be directly coupled with this amplifier. Just be careful that the output midpoint voltage of the unit does not exceed ±300mV after the front and rear stages are connected. It will not have any impact on circuit performance and operating reliability. Modern CDs can record down to a few Hz. If you do not want these ultra-low frequency signals to be lost during playback, it is necessary to consider the issue of direct coupling.

(2)Performance indicators

The circuit in Figure 1 looks very inconspicuous, but its performance is impressive. The measured performance of the prototype is as follows:

When BG6 and 7 choose different complementary high-power tubes, their rated output power (RMS):

2N3055/MJ2955 (35V, 8Ω) 50W,

MJ802/MJ4502 (35V, 4Ω) 75W

MJ802/MJ4502 (37.5V, 8Ω) 62W

MJ802/MJ4502 (37.5V, 4Ω) 112W

MJ802/MJ4502 (stabilized voltage 35V, 4Ω) 131W

Total harmonic distortion:

Rated output power 1kHz 0.35%

10W(RMS)1kHz 0.015%

Signal-to-noise ratio 115dB

Power bandwidth (-3dB) 100kHz

Frequency response (1W, ±1dB) 2Hz to 110kHz

Damping coefficient (8Ω) 90

It is not difficult to see that the circuit of this machine has strong versatility. As long as it is equipped with the corresponding output tube and power supply capacity, the output power of 50-100W can be obtained without changing the circuit. The circuit is so simple that it is almost impossible to reduce one component, but the performance is above average. Compared with some high-end amplifiers on the market, the sound quality of this amplifier is satisfactory.

Why such a simple circuit has lower distortion and better sound quality, the author has not yet delved into this. It is initially speculated that it is due to the arrangement characteristics of the circuit structure that make the distortion of the front and rear stages compensate for each other. Therefore, the open loop of this machine Distortion is lower,

Lays the foundation for good sound quality. If this theory is correct, then in a simple power amplifier circuit, some attempts can be made to match the front and rear stage devices, which may be a way to reduce open-loop distortion. Interested readers may want to try it further.

2) Selection of main components

(1)Power supply

The power supply part of this machine adopts the same circuit formula when the output power is different. (See Figure 1), only the capacity of the power transformer and filter capacitor are different. Table 1 shows the recommended power supply capacity and filter capacitor size.

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The values ​​in the table refer to the power supply conditions that should be used when a single power amplifier is used. If it is used for a stereo power amplifier, the secondary current capacity and filter capacitance value of the power transformer should be at least doubled. For example, the secondary capacity of 050WX 2 power transformer should be 28VX 2/4.5-5A1100WX2, and the secondary capacity of power amplifier power transformer should be 28VX2/7-8A. If the current capacity is not increased accordingly, then when a power transformer is used to supply two power amplifiers to work at the same time, the output power will not reach the original rated value. Of course, it is best to use a separate power transformer and corresponding rectification and filtering for each power amplifier. circuit, which also helps reduce crosstalk between channels.

(2)Other devices

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

The fT of these two pairs of high-power tubes is about 2.5MHz. However, due to the large current and both switching power tubes, they have good performance when used as audio power amplifiers. In particular, 2N3055/MJ2955 is the preferred tube for the production of high-fidelity power amplifiers below 50W. It is a pair of "quasi-heater tubes" with a very high cost performance. The V ce o specification value of this tube is low, and is often greater than 100V in practice. There should be no problem when used at a voltage of about 40V. However, dealers often select and supply 60V or 100V in different grades according to Vceo. Do not purchase 60V applications at this time.

The Pcm of BG1-5 (BC557/546) is 0.5W, and the Icm is 100mA. BG3 requires Vceo≥50V, and BG4-5 requires Vceo≥100V . fT is ≥100MHz, hFE is 110~220. BC557 and BC546 are difficult to purchase. The corresponding high-frequency and low-power tubes can be selected according to the above requirements.

In order to improve open-loop linearity, differential tubes and complementary tubes should be used in pairs, which can also reduce debugging trouble and make it clearer. The pairing requirements are roughly as follows: The differential tube BGl-2 requires that the hFE difference at 0.5mA is <5%, and the VBE difference between the two tubes is <20mA. hFE is better, try to be ≥100.

The complementary push tube BG4-5 requires the two hFE tubes to be as close as possible within 1 to 80mA. If not, you can also select hFE basically the same at 60mA.

The final stage BG6-7 requires the two hFE tubes to be as close as possible within 100mA~3A (for 50W power amplifier) ​​and 100mA~4A (for 100W power amplifier). If not, you can also choose according to the hFE at 3A or 4A which is basically the same.

The main voltage amplifier tube BG3 requires hFE to be no less than 60 at 10~15mA. Of course, it would be better if the BG3 of the two channel power amplifiers can basically have the same hFE.

D1-2 is the switching diode 1N914, the maximum forward current is 75mA, and the forward voltage drop is about 1V at 10mA. Can be replaced with 1N4148.

R7-8 uses a dedicated 2~5W carburized resistor for the power amplifier, or it can be replaced with two 0.68Ω (1~2W) metal film resistors in parallel. The rest use 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 radiator should be made of profile material, and its thermal resistance (to the power amplifier) ​​should not be worse than 8~10~C/W. It is recommended to use a larger radiator as much as possible during production, because the life and reliability of the power amplifier tube have a great relationship with the tube junction temperature.

3) Installation and debugging

(1)Installation process

The components of the power amplifier part are all mounted on the printed board in Figure 2. It should be noted that Figure 2 shows the front component mounting surface. When making your own printed board, do not forget to copy it upside down. In addition, since the sizes of the components selected by each person may not be the same, it is necessary to try to arrange the actual objects on the drawing before making the printed board, and adjust the wiring spacing appropriately to ensure that the components are evenly arranged and beautiful. The following is the same method for making the power amplifier printing plate.

The final stage BG6-7 is installed on the printing plate 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 printing plate through fixing screw A (preferably welding). Tighten the screws. 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 best coated with silicone oil.

There are not many components in the power supply rectification and filtering part, and it can be installed directly on the chassis with brackets. Figure 3b is a schematic diagram of the wiring of one channel, and only points out the following points:

① The "ground" end of the printed board should be directly connected to the common contact 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 led to it with another thick wire. Other appropriate places are connected to the chassis, that is, "one point" grounding is achieved. The ground terminal of the speaker is directly connected to the above-mentioned public ground point without going through the "ground" on the printed board.

②The signal input ground terminal and the power amplifier output ground terminal should be insulated from the metal chassis at the installation location.

⑧The wiring of the power supply part should be short and thick. It is recommended to use Φ0.2mm×32 multi-strand plastic wire for connection. The speaker terminal is connected by a dedicated feeder for the speaker between the printed board and the public teasing location.

(2) Debugging

As usual, check the components before debugging. Whether the installation and welding are correct and reliable. Pay special attention to whether the polarity of diodes, transistors, and electrolytic capacitors are reversed, and whether the insulation between high-power tubes and heat dissipation brackets is good. After heating, check the power supply separately. If there is no problem, connect it to the power amplifier for debugging. According to the load condition of the power amplifier, proceed in the following three steps:

① No-load debugging In order to reduce the possibility of instantaneous damage to the power amplifier, do not connect the load first. After turning on the power, touch the final tube with your hand and it should be wide and slightly warm, or one tube is hotter and the other is cooler. As long as it is not hot and there are no other abnormalities, you can safely measure the voltage everywhere, the voltage at the debugging point and the static current. Use the DC voltage (200mV) range of a digital multimeter to measure the output midpoint voltage.

Generally, it can be considered normal if it is below ±50mV. If the deviation is too high, you can increase R2, otherwise reduce R2. As long as the differential tube is selected, it is usually easy to control it within 50m.

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Then test the voltage drop on both ends of R7 or R8. Since no load is connected, the voltage drop on the two resistors is the same. The quiescent 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 less than 13~17mV, you can test the voltage drops of D1 and D2 respectively. Try to solder off the diode with the smaller voltage drop (should be done after disconnecting the power supply) and replace it with a diode with a larger voltage drop. Retest, if there is no corresponding diode, a 220Ω trimming resistor can be used instead and trimmed 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. Repeat the midpoint voltage measurement and adjust R2 to make the midpoint voltage reach earth. Within 50mV.

② Connect the output of pure resistive load debugging power amplifier to 8~10Ω 1/4W resistor, and then measure the voltage drop at both ends of R7 and R8. At this time, because BG6 and 7 form independent DC circuits through this resistor, the voltage drop on R7 and R8 may If they are not consistent, you can adjust R2 again to make the voltage drops on the two resistors consistent, and the midpoint voltage will be close to 0V. If the voltage drop on R7 and R8 is the same but does not reach 13~17mV, you can replace D1 and D2 or fine-tune the above 2200 trimming resistor, repeat 1 to 2 times, in short, the voltage drop on R7 and R8 should be the same and reach 13~17mV. The midpoint voltage is generally adjusted well.

⑧ Actual load debugging can be connected to the speaker debugging after the above debugging. The feeder connecting the sound box and its length should be matched according to the actual usage in the future. After turning on the power, do not send a signal first, listen to see if there is any obvious abnormal sound from the speaker, and touch the final tube case with your hand. If there is abnormal sound in the speaker and the tube case is hot, it means that the circuit is self-excited after the actual load is connected. At this time, as shown in Figure 4, connect the R1 and C1 series compensation network and the L1 and R2 parallel compensation network to the output end of the power amplifier, and use a bracket to install it as close to the printed board as possible. Generally, use R1 and Cl first. If the self-excitation cannot be completely eliminated, add L1 and R2. L1 can be made by winding Φ1.0mm enameled wire 10 times around a 12mm frame. Generally, self-excitation can be eliminated by adopting the above measures.

Finally, add the signal and listen 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 all times) and 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 and this step of debugging is best carried out in summer when the temperature is high. If the machine is installed in winter, it is hoped to restart in summer. Test it.