Working principle high fidelity BTL amplifier (tda2009)

    Only the circuit diagram of one channel is given here, and the other channel is exactly the same. The audio signal is input from the a end of the circuit, amplified by the operational amplifier IC1 (the amplification factor is determined by R1 and R2), and then inverted amplified by IC2, with a gain of 1; The other way is amplified twice through IC3 and iC4, and the gain is still 1. The essence is that IC3 and iC4 together form a positive phase amplifier with a gain of 1. Therefore, two equal and opposite phase audio signals are obtained at the B end of IC2 and the C end of iC4. These two mutually inverted audio signals are respectively added to the ① and ⑤ pins of the dual audio power amplification integrated circuit IC5 (tda2009) through R9, C5, R10 and C6. These two inputs are in-phase input and inverted input. Therefore, after power amplification in IC5, they are output from the ⑩ and ⑧ pins of IC5 respectively to drive the speaker BL.

    Component selection and debugging

    See the following table for the list of components.

    The circuit working principle of the device is shown in Figure 1

No	Name	Model	Quantity	No	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 chip capacitor	0.01u	1
R8、14、15	resistance	100Ω	3	C16	Electrolytic capacitor	100u	1
R17、18	="9%"> 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	Quad op amp IC	TL084	1
C2	Electrolytic capacitor	4.7u	1	IC2	Dual channel power amplifier IC	TDA2009	1
C7、8	Ceramic chip capacitor	1000P	2	BL	speaker	4－8Ω	1
C9、10	Electrolytic capacitor	220u	2

    Ic1-ic4 can use a four op amp integrated circuit tl084. The differential amplifier has wide frequency response, low noise and high transient index. In addition to tda2009, the power integrated amplifier can also use other similar integrated circuits without special requirements. All the resistors used are low-noise, precision metal film resistors, which are designed to reduce the noise of the pre stage. At the same time, the signal amplitude of B and C should be equal. The pre stage coupling capacitor used should also be precision tantalum capacitor. The power of triode VT should depend on the current required by the circuit, and there should be enough capacity reserve to obtain good transient characteristics and sufficient dynamic range.

    Due to the versatility of this circuit design, any dual power amplifier IC with OTL or OCL output can be connected with the B and C ends of the differential amplifier to form a BTL amplifier. If readers are interested, they can also insert RC attenuation tone control circuit, which will get better results.

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