Overview : 20W single-ended Pure Class A power amplifier circuit, the circuit is very simple, very little used components. Meet the "simple first" principle, the use of materials common, easy to imitate. Recently, a friend presented a 20W single-ended Pure Class A power amplifier circuit, the circuit is very simple, very little used components. Meet the "simple first" principle, the use of materials common, easy to imitate, to see a lot of enthusiasts interested in single-ended Pure Class A power amplifier, not exclusive, special writing this, with the majority of the exchange of audio enthusiasts. Schematic as shown below
Circuit theory and design ideas , the whole circuit can be divided into four parts:
Input stage: the core circuit is composed by two BC559 differential amplifier, 22K resistor to ground bias resistor for the transistor, its size also determines the amplifier’s input resistance. 8.2K resistor is common differential pair emitter resistor, determines the differential circuit common mode rejection ratio and the level of quiescent current. After the input stage to enlarge the current flows through the 1K resistor generated when the variable voltage signal directly to the next level. 1UF capacitor is the input capacitance of the whole machine, its size and material of the capacity of a great impact on sound quality. According to theoretical calculations, 1UF capacitor and the input resistance of 22K form a high-pass filter circuit, the low end of its transition frequency can be calculated: f = 1000 / (2 * 3.14 * 22 * 1) = 7.2HZ. (In the past the low-end frequency response of the amplifier when positioned in 20HZ or acceptable to. Now today’s popular digital audio sources, it seems a little high, low corner frequency set at 1HZ less is acceptable.) Because The importance of the capacitor, be sure to choose the audio quality of the imported special coupling capacitor, the capacitor in the domestic, the new brand is a trusted Texas, after the author and friends of the trial, the effect is satisfactory, but slightly larger some, in the design of the circuit board to consider whether the installation was under.
8.2K resistor determines the input stage transistors quiescent current can be estimated by the following formula (two values): VCC/8.2K = 20/8.2 = 2.4MA. Since the input stage transistors quiescent current has significant influence on the sound quality, you can adjust the size of the resistor to meet their own requirements. (Transistor quiescent current, high signal to noise ratio, but the sound quality dry, thin bass. If the current is larger, warmer sound, solid bass, but the transistor characteristic is reflected in the audio frequency noise and the sound will increase the current, the signal to noise ratio decreased. This machine is more appropriate to take 2.4MA.
Voltage amplifier stage: In order to simplify the circuit, the machine uses a transistor BD139, using common-emitter amplifier, also used the bootstrap circuit. The level of quiescent current can be estimated by the following formula: VCC / (1.5k +1.5 k) = 6.8MA. 100P small frequency compensation capacitor is used to do, the capacity to be as small as possible, if not high-frequency self-excited, you can not. (Of course, the existence of such a small capacitor on the sound quality are subtle regulatory role, specifically how to deal with, look at their own preference.
In order to ensure large-signal output amplitude characteristics and linear, without adding too many components, this machine uses a bootstrap circuit, the 100UF capacitor and two 1.5K resistors in the divider circuit. In the audio industry for more criticism of the bootstrap circuit, that it is a positive feedback, negative influence on the sound quality. As the circuit’s debut earlier, the design premise that "the simple first", may be considered here is not so comprehensive.
Output stage: the upper part of the schematic of the components of two MJE2955 formed and the surrounding single-ended Pure Class A amplifier, the lower half of the core to form two MJE2955 high current constant current source circuit. The constant current output stage is a static current. Estimated according to the following formula: 0.65/0.25 = 2.6A. (0.65V which is the silicon transistor emitter junction of the PN junction the forward voltage drop) of 0.25 by changing the size of resistor can adjust the output stage quiescent current power amplifier tube. This circuit, called for the 8 ohm load on the output 20W of Pure Class A, 2.6A This current is a little big, in fact, can be estimated with the formula: P = 2 * I * I * RL, such as the one about 1.1A to meet the design requirements, but we do not like the impedance of the speakers remain the same as pure resistance, sometimes at a specific frequency and extreme cases, the impedance may drop very low. In accordance with the circuit design using a 4 ohm load if the output 40W of Pure Class A is fully able to meet! This design only scientific attitude and the idea is worth serious study.
Speaker impedance compensation circuit: Because we use the speaker is inductive load, the load close to the pure resistance of the amplifier, the output of the amplifier are generally on the ground in series resistance and capacitance compensation circuit, the resistor value and standard speaker said considerable resistance and capacitance values for the 0.1UF-0.22UF, not repeat here.
Installation Note : The power part: As Pure Class A single-ended amplifier common mode rejection was low, and coupled with a large quiescent current of the machine, so the high power requirements, the best filter circuit using inductors, but After the production and application of inductive electromagnetic interference generated by the treatment are headache, the use of voltage regulator is a good choice, but cost and heat issues have come, or reluctantly discard it. Ultimately forced to choose a capacitor filter circuit, the transformer’s capacity to more than 1000W, the secondary voltage of four-lane and around the four 15-18V, the current capacity of 10A or more, between two series into two groups of dual voltage, respectively, about the sound supply Road, full-bridge rectifier current of 25A or more to choose, do not have too high a voltage 200V enough. Filter capacitor positive and negative power per channel on each side of not less than 22,000 UF, of course, the bigger the better, but pay attention to the best use of multiple small-capacity capacitor connected in parallel to achieve the required capacity, as to be parallel good high frequency characteristics of small-capacity capacitor polarity is necessary. While the cost has increased, but the effect is much better but, fortunately 25V electrolytic capacitor lower prices. Because "in a good amplifier, the power to half the cost!" I very much appreciate this view. If the dual mono design, from the transformer – rectifier filter circuit – amplifier – output, each is independent the better.
Production, testing : It is because of simple circuit, so the original sound quality is almost determined by the characteristics of the device. The figure marked the transistor, and now seems to have less fever, and the reader can under the current trend for substitution. Because high heat circuit require component reliability must be high, the resistance be selected 1/2W metal film resistors, capacitors consumes less used, be sure to use quality. As long as the quality of the original device and welding technology can guarantee the debugging machine is very simple, before the first 1K of the adjustable power resistor in the center of the future in power, adjust the resistance to ground potential so that the output can be as close to 0V the rest by the circuit and ensure that the original device. Maintain the load over half an hour to observe the temperature of the radiator is not too high, the other components without exception, re-measured output potential is not too large to be put into use. The amplifier components also need to aging, you may want to hear Liangsheng, also need to burn in a very long process.
This article is not related to the protection circuit, in order to protect your expensive speaker system, we strongly recommend installing a safe and reliable speaker protection circuit.