This paper introduces a BTL power amplifier circuit that does not require debugging, high fidelity and low cost, and can select the final stage power amplifier integrated circuit according to its own situation. Because of its versatility, it is very convenient for audio enthusiasts.
Working principle high fidelity BTL amplifier (TDA2009) 
. Only the circuit diagram of one of the channels is given here, and the other channel is identical. The audio signal is input from the A terminal of the circuit, amplified by the operational amplifier IC1 (the amplification factor is determined by R1 and R2), and the gain is 1 by the IC2 for inverting amplification; the other path is inverted by IC3 and IC4 twice. The gain is still 1, which is essentially that IC3 and IC4 together form a positive-phase amplifier with a gain of 1, so that two equally-equal and opposite-phase audio signals are obtained at the B terminal of IC2 and the C terminal of IC4. The two mutually inverted audio signals are applied to the 1st and 5th pins of the dual audio power amplifier integrated circuit IC5 (TDA2009) through R9, C5 and R10, C6, respectively. These two inputs are non-inverting input and inverting input. Therefore, after power amplification is performed inside IC5, it is output from pins 10 and 8 of IC5, respectively, and the speaker BL is pushed.
Component selection and commissioning <br> The list of 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 |
| R4-R7, 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 triode | 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 | 4-8Ω | 1 |
| C9, 10 | Electrolytic capacitor | 220u | 2 |  | 　 | 　 | 　 |
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IC1-IC4 can use a quad op amp integrated circuit TL084, the differential amplifier has wide frequency response, low noise and high transient index. In addition to the TDA2009, the power integrated amplifier can also be used with other similar integrated circuits without special requirements. All the resistors used are low-noise, precision metal film resistors, the purpose of which is to reduce the pre-stage noise, and the signal amplitudes of the two points B and C should be equal. The pre-stage coupling capacitor used should also be a precision tantalum capacitor. . The power of the triode VT should be based on the current required by the circuit and have sufficient capacity reserves to achieve good transient characteristics and sufficient dynamic range.
Due to the versatility of this circuit design, any dual power amplifier integrated circuit of OTL or OCL output can be connected with both ends of B and C of the differential amplifier to form a BTL amplifier. If you are interested, you can also insert the RC attenuation tone control circuit, which will get better results.
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