Principle and design of high fidelity BTL amplifier This article introduces a BTL power amplifier circuit without debugging, high fidelity, and low cost, and can choose the final power amplifier integrated circuit according to its own situation. Due to its strong versatility, it brings great convenience to audio enthusiasts. IC1-IC4 can use a four op amp integrated circuit TL084, the frequency response of the differential amplifier is wide, the noise is low, and the transient index is high. In addition to adopting TDA2009, power integrated amplifiers can also use other similar integrated circuits without special requirements. The resistors used are all low-noise, precision metal film resistors, the purpose of which is to reduce the pre-stage noise, and at the same time, the signal amplitudes at points B and C should be equal. . The power of the transistor VT should be determined by the current required by the circuit, and there is sufficient capacity reserve in order to obtain good transient characteristics and sufficient dynamic range.
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Working principle High-fidelity BTL amplifier (TDA2009) 
. Only one circuit diagram of one channel is given here, the other channel is exactly the same. The audio signal is input from the A terminal of the circuit and amplified by the operational amplifier IC1 (the amplification factor is determined by R1 and R2). One channel is amplified inversely by IC2, and its gain is 1; , The gain is still 1, the essence is that IC3 and IC4 together form a positive-phase amplifier with a gain of 1, so the two audio signals that are equal and opposite in phase are obtained at the B end of IC2 and the C end of IC4. These two inverting audio signals are added to the ①and ⑤ pins of the dual audio power amplifier IC5 (TDA2009) through R9, C5 and R10, C6 respectively. These two input terminals are the in-phase input and the inverting input End, so after power amplification inside IC5, output from IC5's ⑩ foot and ⑧ foot, respectively, to push the speaker BL.
The list of components selection and debugging components is shown in the table below.
The working principle of the device circuit is shown in Figure 1. Numbering name model Quantity Numbering name model Quantity R1 resistance 1K 1 C11 Electrolytic capacitor 47u 1 R2, 20, 21 resistance 10K 3 C12, 13 Polyester capacitor 0.1u 2 R3, R9, R12 resistance 220K 3 C14 Electrolytic capacitor 10u 1 R4ï¼R7, 10, 11, 13, 16 resistance 20K 8 C15 Ceramic capacitor 0.01u 1 R8, 14, 15 resistance 100Ω 3 C16 Electrolytic capacitor 100u 1 R17, 18 resistance 10Ω 2 DW Zener diode 20V 1 R19 resistance 2.7K 1 VT Crystal transistor 2N5551 1 C1, 3, 4, 5, 6 Electrolytic capacitor 1u 5 IC1 Four op amp IC TL084 1 C2 Electrolytic capacitor 4.7u 1 IC2 Two-channel power amplifier IC TDA2009 1 C7, 8 Ceramic capacitor 1000P 2 BL speaker 4ï¼8Ω 1 C9, 10 Electrolytic capacitor 220u 2
Due to the versatility of this circuit design, any dual power amplifier integrated circuit output by OTL or OCL can be connected to the B and C ends of the differential amplifier to form a BTL amplifier. If the reader is interested, you can also insert the RC attenuation tone control circuit, which will receive better results. 
