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Generally take Rs? RL (when RL = 8Ω, choose Rs = 0.39Ω, Rs / RL is about 5%, otherwise the power loss is large); take R? RL (generally R takes 220Ω). then:
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From the above formula, when the impedance RL of the speaker is large, the feedback voltage Uf is small, the gain of the amplifier is high, and the output voltage Uo becomes large; conversely, if the RL is small, Uf is large, the gain of the amplifier is small, and Uo is also small . This shows that the output voltage of the power amplifier will increase as the load impedance increases. This variable output load characteristic occurs the same as the tube power amplifier circuit. No wonder this power amplifier has the taste of a tube when reproducing the sound quality, and it is talked about by enthusiasts.
The current negative feedback circuit is not only used in integrated circuit power amplifiers such as LM1875, TDA1514A and TDA2030A, but also in discrete element transistor power amplifiers.
Second, regarding the DC circuit of the power amplifier, the standard circuit recommended by the power amplifier integrated circuit manufacturer was originally introduced. An electrolytic capacitor and a resistor were grounded in series at the inverting input of the IC to maintain the zero potential of the output midpoint, as shown in Figure 2. It is this electrolytic capacitor that exhibits a different capacitive reactance to the feedback signal with different frequencies, so that the feedback amount, phase shift and internal resistance of the power amplifier circuit will vary with frequency, so that the distortion of the power amplifier is significantly increased, and the bass will be audible. The intensity and depth of the dive are not enough, and the mid-to-high range is not transparent. In order to remove this capacitor which seriously affects the sound quality, and at the same time maintain the zero potential of the output point of the power amplifier, an active DC servo circuit is introduced, as shown in Figure 3. It is an integrating circuit with a low-pass filter. The signal output by the power amplifier circuit IC1 is filtered by the low-pass filter in the active DC servo circuit to remove the AC component, and only the DC component is left. This DC component is compared with the zero potential in IC2, so the slight change in the DC potential at the output of IC1 The quantity will be amplified by IC2 and added to the inverting input of IC1 through a resistor, thereby pulling the output back to zero potential. After using this circuit, the frequency response of the power amplifier can theoretically be extended to DC, the lower limit of the low frequency is no longer limited by the capacitor C and the signal phase shift and nonlinear distortion caused by the capacitor are eliminated, and the intensity of the low frequency on the hearing sense is greatly increased , The resolution of the midrange and treble is enhanced.
But looking back, there are two integrating capacitors in the active DC servo circuit, which will inevitably have a certain "feedback" effect on the audio output. Some experts even think that its impact on sound quality is not less than the DC blocking capacitor C mentioned above. In fact, the output offset voltage of some power amplifier integrated circuits with excellent performance is extremely low. For example, the LM1875 offset voltage is 2μV; the TDA1514A offset voltage is only 1mV, and the feedback capacitor can be completely removed without adding an active DC servo circuit, and the power amplifier directly becomes a DC amplifier. In the actual circuit, the LM1875 offset voltage is extremely low. The TDA1514A offset voltage is only found to be around 10mV. The long-term use of the circuit is stable, and the sound quality of the power amplifier is also very satisfactory. It is worth pointing out that the performance of this power amplifier depends on high-quality power supply and peripheral components.
The current negative feedback and power amplifier DC circuit are used in the LM1875 integrated amplifier circuit at the same time, the effect is better, because the LM1875 amplifier itself has a soft sound quality, closer to the taste of the tube, and the sound effect is very beautiful. The specific circuit is shown in Figure 4. Among them, R6 selects non-inductive resistance, and the rest of the requirements are the same as those of general power amplifier circuits, and will not be described in detail.
3. The transformer is the power of the transformer first. The rated power of the transformer should be more than twice the rated output power of the power amplifier. Otherwise, at high volume or in high dynamics, your power amplifier will be stretched, resulting in greater distortion.
Secondly, many articles endeavor to recommend toroidal transformers, and most imported power amplifiers do not use toroidal transformers, but use traditional square transformers, so it can be said that recommending the ring cow is only a domestic "heat" phenomenon. Indeed, the toroidal transformer has the advantages of low magnetic leakage, small magnetic resistance, small size, short line loss and low resistance. The author once made a ring cow, and each layer was insulated with cellophane and dipped in paint. It took a few days to say. Since it has been used for one year, it has no obvious difference in hearing at least from the ordinary EI type transformer. Some audiophiles have also discovered that there is an unpleasant "buzzing" ring cow. The EI iron core itself has a small gap on the magnetic circuit, which has a specific effect on electromagnetic conversion, and it is more convenient to make at home. As long as the quality is ensured in the process and material selection, that is, the chip quality should be high, the wire diameter should be thick and not thin, the rated power should be large, primary shielding, interlayer insulation, varnishing, and a layer of copper skin short circuit ring on the outer yoke of the transformer Measures such as reducing magnetic flux leakage completely meet the fever requirements.
? Yang Xiaolin, Jiangsu
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