LM3886 dynamic feedback amplifier-the charm of sound quality can not be stopped!

I believe that most audiophiles know that the power amplifier has negative feedback. Its purpose is to: ① overcome the effect of nonlinear speaker impedance on the feedback loop (transient distortion); ② improve the stability of the amplifier circuit; ③ reduce nonlinear distortion; ④ suppress Disturb and suppress the noise of the transistor caused by the thermal motion of the carrier; ⑤ Broaden the frequency band: the upper limit frequency of the amplifier circuit is increased, and the lower resistance frequency is reduced.

The power amplifiers on the market no matter whether they are Class A or Class A or B, voltage-type negative feedback is used more, and current negative feedback is less. The output voltage of the voltage-type negative feedback amplifier is only related to the amplification factor, and it is related to the impedance of the speaker. Irrelevant, this makes the amplifier unable to overcome both nonlinear distortion and transient distortion. Using current negative feedback can solve this shortcoming: use the linear element resistance to sample the current flowing through the speaker voice coil and feed it back to the power amplifier input terminal, so that the amplifier drives the load in a fixed current mode, and the speaker can be very sound when it is oscillated by current control. Solve the problem that the nonlinear distortion and transient distortion in the power amplifier can not be taken into account.

In recent years, audio technology has developed rapidly, and some well-known domestic audio manufacturers have developed another feedback technology-dynamic feedback technology (MFB). This is a feedback technology with both current feedback and voltage feedback: when ZO (horn load) becomes smaller, voltage feedback is used, and when ZO changes continuously, both current feedback and voltage feedback are used. Compared with voltage and current, dynamic feedback technology is a control method with super performance, which makes the sound quality of the power amplifier more attractive, such as high signal-to-noise ratio, warm and elastic sound, clear and transparent midrange, strong low frequency dynamics , Powerful bursting, accurate positioning of vocals and violin sound, high resolution, sounds that are audible and not annoying.

Figure 1 is a power amplifier circuit with dynamic feedback using the LM3886TF of National Semiconductor (NS), which is composed of current negative feedback plus voltage negative feedback. It also includes the speaker system in the feedback network to control the speaker. To a positive effect, it can effectively offset the distortion caused by the vibration of the speaker, so that the bounce force of the music is obviously enhanced, and the sound of the rigid flat sound field becomes lively and angry, the sense of space is enhanced, and the width and depth of the sound field are Significant improvement.

In order to solve the digital music source (such as CD, VCD, DVD, etc.) digital music is hard and hairy, the circuit in Figure 1 has a softening effect on digital sound, can soften the sound of the digital sound source "hair" sound, To make hard sound music, this function is that the voltage negative feedback in the circuit of Figure 1 is adjusted correspondingly than the conventional one, and the voltage gain is adjusted from low to high, so that the voltage negative feedback is changed from deep to shallow. At the same time, the amount of current negative feedback has also been adjusted in proportion. For these purposes, RF1 (choose between 1-3K) and RF2 (choose between 1-10K) are used. In the figure, cf1 and cf2 are high-frequency AC gain compensation networks, and their role is to reduce digital sound (for 60-80KHz and above) High frequency overtones are compressed or attenuated). IC2 in the figure is the role of DC servo.

The purpose of component selection: good circuit design and the use of high-quality components can maximize the effect! ①Signal coupling uses Japanese famous brand capacitors, its performance is very high quality; ②Resistance adopts imported metal film low noise precision (± 1%) five-color ring resistance, its noise is very low; ③Promise capacitor uses Japanese music sound metalized non-inductive CBB Capacitor; ④ The circuit board uses silver-plated glass fiber epoxy board (the benefits of silver plating: to prevent oxidation, reduce the resistance of solder joints, and help to improve the analytical power of small signals).

Assembly amplifier application: The circuit in Figure 1 is a mono channel without power supply. The advantage of this is that it improves the separation of the left and right channels and the signal-to-noise ratio, and the power supply can be far away when assembling the amplifier combination. , So that you can make a good amplifier. At this time, a suitable solution for AV and Hi-Fi is recommended: virtual Dolby surround sound processing circuit QS7779 → BBE circuit (BA3884) → tone circuit (M65831 class A pre-stage / LM4610N) → two LM3886TF amplifiers → speaker protection board In addition, add 300W dual 18V + dual 12V ring cow and 6A power board. ▲ When assembling, please pay attention to the grounding method and the input and output grounds must be separated to the power ground to be connected. ▲ The above schemes are actually selected and deleted according to hobbies and applications. ▲ The simplest and good effect is: BBE circuit (BA3884) → both sides of the power amplifier LM3886TF → speaker protection board. Plus 300W dual 18V + dual 12V ring cattle and 6A power board.

For enthusiasts who want to assemble the power amplifier, the dynamic feedback power amplifier circuit of Figure 1 is a very ideal power amplifier, plus BBE, virtual surround, karaoke class A pre-stage, etc. is a high-quality power amplifier. And increase the fever experience, feel the intimacy of their assembly! A sense of success!

I won't say much about the beautiful playback audition of the dynamic feedback power amplifier. Please let the enthusiasts experience it yourself! That sound quality charm is really great!

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