Application note of SF404 audio power amplifier drive circuit

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The SF404 audio power amplifier driver circuit is a product of Shanghai Radio 7 Factory. It can be equipped with a pair of power tubes and a small number of RC components. It can be an OTL or BTL power amplifier. The power amplifier composed of this circuit has a wide power supply voltage range and output. The power is large, the distortion rate is small, the adaptability is strong, and the adjustment is convenient. Since the SF404 power amplifier circuit can work at a deeper depth of feedback, there is no excessive requirement for the pairing of the external power tube and the small current characteristics of the circuit.
In addition, since the constant current source circuit is used inside the circuit, the quiescent current of the driving tube and the output tube is supplied with a stable offset of temperature compensation, so the reaction is not sensitive to changes in the power supply voltage or the ambient temperature. The circuit has good thermal stability and high reliability. SF404 can work normally within the 9~36V cigarette light voltage range. The circuit is widely used in a variety of desktop, floor-standing radios, tape recorders, small amplifiers, stereo multi-functional audio equipment and instruments, instruments to do audio power drivers.
The SF404 is available in a dual in-line 14-pin plastic package. Domestic similar products have SL404, XG404 and so on. The main electrical parameters of SF404 are as follows: open-loop voltage gain ≥70dB; closed-loop voltage gain: 40dB; distortion <2%; input impedance: 50kΩ; output noise voltage: 2mV; frequency response: 30Hz ~ 20Hz. SF404 for OTL power amplification The application example is shown in the figure. The figure includes the data in the arc where the load RL = 4 Ω is the selected value. W (680 Ω) potentiometer is to adjust the quiescent current of the power amplifier stage BG1, BG2, generally take 3 ~ 8mA. SF404 for OCL power amplification application examples as shown.