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Sep 14 2020

TDA7294 Based Power Amplifier Circuit Design

I. Introduction

TDA7294 is a very innovative DMOS high-power integrated amplifier circuit launched by the famous European SGS-THOMSON STMicroelectronics to mainland China in the 1990s. It sweeps away the raw, cold, and hard tones of the previous  linear integrated power amplifiers and thick film integration, and is widely used in the HI-FI field: such as home theater, active speakers, etc. The design of this chip focuses on tone, and has the advantages of bipolar signal processing circuit and power MOS. It has the characteristics of high voltage resistance, low noise, low distortion, and very affinity for replaying sound; and has a silent standby function, short-circuit current and overheat protection functions to make its performance more perfect. This article will introduce several power amplifier circuit designs based on TDA7294.

tda7294

Catalog

I. Introduction

II.OCL Circuit

III. BTL Circuit

IV. Constant Current Power Amplifier

V. Hi-Fi Integrated Power Amplifier

VI. Active Subwoofer Amplifier Adds Standby Function

VII. High Fidelity Power Amplifier

VIII. Class AB Power Amplifier

IX. Two-channel Power Amplifier

X.100W Power Amplifier Circuit

II. OCL Circuit


The OCL circuit diagram is shown in Figure 1. This circuit is a dual-channel 70W power amplifier composed of two TDA7294. There are few external components and simple circuit. When the power supply voltage is ±35V, 70W continuous output power can be obtained on an 8 ohm load. It is very suitable for playback in an environment below 30 square meters. If the speaker impedance is less than 8 ohms, the power supply voltage should be reduced accordingly.

Figure 1 OCL Circuit Diagram

Figure 1 OCL Circuit Diagram

III. BTL Circuit

The BTL circuit is shown in Figure 2. It uses two TDA7294 bridges to form a BTL power amplifier circuit. The output power can reach more than 150W. It is suitable for places that require high power such as dance halls. 4 TDA7294 are required for stereo. When the power supply voltage is ±25V, a continuous output power of 150W can be obtained on an 8 ohm load. When the power supply is ±35V, a continuous output power of 180W can be obtained on a 16 ohm load. When use TDA7294 as BTL power amplifier, the load must not be less than 8 ohms.

Figure 2 BTL Circuit Diagram

Figure 2 BTL Circuit Diagram

IV. Constant Current Power Amplifier

 

This power amplifier circuit is somewhat different from the previous two structures. Its feedback circuit is current sampling, voltage summation and negative feedback. This kind of circuit structure is the constant current power amplifier that people often say. The specific analysis of the circuit will not be detailed, only the more prominent advantages compared with the traditional constant voltage power amplifier will be introduced.

(1) The output current of the power amplifier has nothing to do with the load impedance. Even if the load is short-circuited, it will not cause the amplifier to overheat.

(2) The output power increases with the increase of load impedance. Pushing the speaker load within a certain power reserve can ensure the bass strength and high frequency resolution of the original music signal.

(3) The force acting on the voice coil of the speaker only depends on the current. The use of fluid-controlled oscillation to promote the speaker must be faster than the voltage-controlled oscillation, so that the input and output impedance of the speaker vibration system can be easily matched.

The constant current power amplifier circuit is actually a controlled current source controlled by the input signal voltage. Its internal feedback circuit is current sampling, voltage summing negative feedback, and it has the characteristics of high input and output impedance. The input impedance is high, which is exactly what the previous stage constant voltage amplifier circuit needs, which is beneficial for the signal voltage to be sent to the input end of the power amplifier without loss. The high output impedance can reduce the shunt of the internal resistance to the signal, which is conducive to adding the output signal current to the load.

In Figure 3, the power supply voltage is selected as ±35V, and its magnification is determined by the ratio of the speaker to R6.

Figure 3 Constant current power amplifier circuit diagram

Figure 3 Constant current power amplifier circuit diagram

V. Hi-Fi Integrated Power Amplifier

 

The famous European SGS-THOMSON STMicroelectronics has launched a Hi-Fi high-power DMOS integrated amplifier circuit TDA7294. The circuit is shown in Figure 4. It integrates the best design of modern power amplifier circuit, combines the advantages of bipolar signal processing circuit and power MOS, has the characteristics of low noise and low distortion; standby and mute circuit completely eliminates the impact noise caused by power on and off, and eliminates speaker protection circuit overheating, short-circuit current protection and other functions make its performance more excellent. This device is suitable for home theater and Hi-Fi amplifiers. The main parameters are: VS (power supply voltage) ±10~±40V (maximum voltage without signal ±50V); Io (peak output current) 10A; Po (RMS continuous output power) 70W when vs=±35V 8Ω; vs= 70W when ±27V 4Ω; (effective value of music output power) 100W when VS=±38V 8Ω; 100W when VS=±29V 4Ω.

Figure 4 Hi-Fi Integrated Power Amplifier Circuit Diagram

Figure 4 Hi-Fi Integrated Power Amplifier Circuit Diagram

The closed-loop gain of the circuit in Figure 4 is 30dB. Increasing R3 can increase the gain, and vice versa, but the amplifier gain should be ≥24dB. The amplifier has the best performance when R1=R3. R7, C4 and R5+R6, C3 determine the standby and mute time constants. The larger the value, the longer the time.

When the control terminal is connected to low potential ground, it is mute and standby; when the control terminal is connected to VS, because (R5+R6)> R7, pin ⑩ rises to a higher potential than pin ⑨, and turns to a low potential first when shutting down, which makes the standby and shutdown processes go on in a silent state, ensuring that the amplifier is turned on and off without noise.

Figure 5 BTL Power amplifier finished board

Figure 5 BTL Power amplifier finished board

For high-power professional applications fileds such as dance halls, you can choose the BTL power amplifier finished board shown in Figure 5. Both TDA7294 are equipped with their own professional radiators. When vs=±25V8Ω, the maximum continuous output power reaches 150W; when ±35V 16Ω, The maximum continuous output power reaches 170W.

We used TDA7294 standard application circuit and Marantz PM80 and YAMAHAA-592 to make a listening comparison. The former is a mid-priced Hi-Fi machine with a Class A, Class A and B status switch, and the latter is a 439.16 dollars class with Ac -3 input AV power amplifier, audio source is music fax E60CD, speaker is Tanner No. 5.

It turns out that the sound orientation of TDA7294 has a distinctive European style, soft, mellow, delicate, and full of bouncing feeling. It is similar to the Marantz PM-80 in Class A and B status, but the sound field of PM-80 is deeper when working in pure Class A. Compared with YAMAHA A-592, the difference is larger. The low frequency of A-592 seems to be slightly improved. It sounds powerful, but it is harder and the lines are blurry. TDA7294 is sweet and natural, with higher resolution, really like landscape painting done with splashes of ink and fine-brush flowers and birds (a technique of chinese ink-painting), each has its infinite charm.

 

VI.Active Subwoofer Amplifier Adds Standby Function

 

This circuit is an improvement on the audio circuit using the integrated circuit TDA7294. The circuit diagram is shown in Figure 6. TDA729410 pin has a mute function. When the external DC provides high level, the integrated block is in the working state; when the low level is applied, the integrated block is in the cut-off state. At this time, the circuit consumes little power and IC1 14 pin has no output, that is, "standby "form. The general circuit is to provide a high level to pin 10 to make it in the conducting state, in fact, the development of this pin function can meet some special work requirements. This circuit is based on this to increase the standby function of the active subwoofer power amplifier, and its circuit is reliable and responsive.

Figure 6 Active subwoofer amplifier adds standby function circuit diagram

Figure 6 Active subwoofer amplifier adds standby function circuit diagram

VII. High Fidelity Power Amplifier

 

The circuit is shown in Figure 7. The drive stage adopts TDA7294. The internal drive stage and output stage of the chip use field effect tubes, which are powered by ±40V, and the output power can reach 70W (RL=8Ω; THD=0.005%). It has a delicate tone and an excellent sense of hearing. Power output VT1, VT2 adopts Shanken high-power pair tube 2SA1394, 2SC3858.

The circuit principle is as follows: The signal is input to the TDA7294 non-inverting input pin ③ through C1 and R1. R7 and R3, C3, C4 of IC pin ② form a negative feedback network, the closed loop gain of this amplifier is about 34 times. The ⑨ and ⑩ pins are the standby and mute terminals respectively. Since the RC network time constant of the ⑩ pin is larger than that of the ⑨ pin, the switching machines are all performed under mute satge, avoiding the switching impact sound, and C7 is a bootstrap capacitor.

Figure 7 High-fidelity power amplifier promoted by TDA7294

Figure 7 High-fidelity power amplifier promoted by TDA7294

Production points:

(1) Insulating mica sheets should be added between the metal cap and the heat sink of TDA7294 (the metal cap is connected to the pin ⑧).

(2) The power transformer uses ring-shaped 300W double 20V, four 50V/10000μF filter capacitors, two 50V/100μF, and two 100V/0.1μF. The power supply part should be tested separately, first without connecting the power amplifier, measure whether the positive and negative output voltage of the power supply are symmetrical, the error should be within 0.6V.

(3) When testing the machine, for safety reasons, you should first use a lower voltage test (such as ±25V) without adding a signal, and measure the DC voltage of the output terminal to the ground. Normally, it should be within 20mV.

(4) R8, R9, R10, D1 form the final bias circuit. This bias makes the output tubes VT1 and VT2 not cut off during operation, so the quiescent current can be small (about 5mA).

(5) The power tube should be strictly matched (within 3%) and genuine products should be selected. The output resistance R14 is a 5W non-inductive type, and the inductor L is formed by tightly winding 10 turns on R14 with a diameter of 1.5mm enameled wire. TDA7294 uses 60mmTimes, 85mmTimes, 20mm 12-slot heat sink, and the output pair tube needs a professional heat sink. The sections with high current on the printed board need to be tin-rolled, which is extremely beneficial for the transparency and strength of the sound.

Figure 8 Power amplifier PCB promoted by TDA7294

Figure 8 Power amplifier PCB promoted by TDA7294

VIII. Class AB Power Amplifier

 

TDA7294 integrated circuit can be used as a high-fidelity audio class AB power amplifier. It can drive 4 ohm or 8 ohm speakers, and when connected to an 8 ohm speaker, it will provide 50 watts of output power and 0.1% THD.

Figure 9 Class AB power amplifier circuit diagram

Figure 9 Class AB power amplifier circuit diagram

 

You must install a large enough radiator for TDA7294. Pin 10 is a mute input, and pin 9 provides a standby mode. Mute should always occur when selecting standby mode. The IC has internal thermal protection, which causes mute reduction at 145°C, and the amplifier enters standby at 150°C. The TDA7294 integrated circuit heat sink is internally connected to the negative power rail. If the module is installed in a grounded metal enclosure, then the IC must be insulated from the heat sink. If not, the negative power rail will be shorted to ground.

IX. Two-channel Power Amplifier

 

TDA7294 high-power integrated circuit IC is specially designed for assembling high-performance audio amplifiers. Two TDA7294 pieces can be used to make a powerful dual-channel high-fidelity power amplifier. The circuit principle is shown in Figure 10. The overall circuit is composed of two TDA7294 core components, and the periphery includes some resistors and capacitors.

The circuits of the two channels are almost identical in design. They all use the standard circuit officially released by TDA7294, connected to a non-inverting amplifier circuit, with a voltage gain of 30.5dB, an output power of up to 70W per channel, and a dual power supply symmetrical power supply voltage ± 35V. Among them, Cl5 and Cl6 are input coupling capacitors. 0.47μF is used in the original circuit. Here, if you increase it to 1μF, you can improve the low-frequency response of the circuit. It is recommended to use polypropylene special audio capacitors, such as WIMA's MKT4 series capacitors, it can greatly improve the sound resolution.

R3 and R4 are input resistances, which determine the input impedance of TDA7294 in the in-phase amplification state. Here, 22kΩ is relatively moderate. Too large a value can reduce the burden on the front-end signal source, but it may affect the stability of TDA7294 and make the output midpoint voltage drift increased, too small value will affect the response ability to low frequency.

The theoretical value of the feedback resistors R7 and R8 should be equal to the input resistors R3 and R4, which can ensure the bias current balance of the TDA7294 input differential circuit and reduce signal distortion. The feedback ground resistance R5, R6 cooperates with R7, R8 to set the circuit gain. Here, the feedback DC blocking capacitors Cl3 and Cl4 are used to form AC negative feedback, inhibit DC voltage output, and protect the speaker.

Figure 10 Two-channel power amplifier circuit diagram

Figure 10 Two-channel power amplifier circuit diagram

TDA7294 has a start mute function, and cooperates with an external circuit to achieve a non-impact sound effect when the power is turned on and off. R9, R1O, R11, Rl2, Cl7, Cl8 and VD5, VD6 in the schematic diagram form an external mute control circuit. Delay the energization of pins 9 and 10 of TDA7294 to achieve the function of soft start.

The power supply rectifier filter circuit is also very simple. The full bridge rectifier circuit is composed of 4 European speed rectifier diodes VD1-VD4. Capacitors C1 and C2 are the main filter capacitors. Large-capacity and high-current audio filter capacitors are required, such as ELNA's FORAUDIO series or BHCAerovox industrial grade capacitors.

Design

PCB is generally a bottleneck in amateur production, so that many excellent schematic diagrams have not been transformed into finished PCBs that can be actually assembled. Here, the popular Protel99se design software is used to draw the printed circuit board against the schematic diagram 10, as shown in Figure 11. The 2mm thick FR-4 board-based double-sided PCB is used, and the copper foil is thickened to 70mm, which is suitable for high current power amplifiers.The whole PCB trace adopts one-point grounding method, which effectively eliminates ground wire interference noise. In some high-current traces, tin plating is also adopted to increase the additional current carrying capacity. The power input and power output structure uses high-current screw terminals to ensure sufficient over-current capability and durability of repeated wiring.

Figure 11 Printed circuit board

Figure 11 Printed circuit board

X.100W Power Amplifier Circuit

Figure 12 100W power amplifier circuit diagram composed of TDA7294

Figure 12 100W power amplifier circuit diagram composed of TDA7294

Figure 12 is a 100W power amplifier composed of a monolithic audio power amplifier integrated circuit TDA7294. TDA7294 includes pre-op amplifier, final power amplifier, temperature protection, short circuit protection, mute control and other circuits. The final stage adopts bipolar DMOS power transistor, which has the characteristics of high output power, bandwidth, low distortion, and good versatility.The integrated circuit also has perfect anti-overload, anti-short circuit and temperature protection circuit functions. When the chip temperature is too high, it automatically cuts off the audio signal to protect the chip from burning

The power amplifier circuit composed of TDA7294 has the characteristics of simple peripheral circuit and easy production. The circuit input impedance is 20k, the input sensitivity is 750mV, the voltage gain is 32dB, the power supply voltage range is ±(25~40)V, and the quiescent current is 50mA. When the load impedance is 8Ω, the output power is 100W; when the load impedance is 4Ω, the output power can reach 180W.

In actual production, TDA7294 should be equipped with enough heat sinks. The supporting power circuit should have sufficient capacity. If you need to increase the circuit voltage gain, you can appropriately change the ratio of R3 to R2, voltage gain A=201g(R3/R2)(dB). However, it is not advisable to one-sidedly pursue the voltage gain of this stage. Excessive voltage gain can easily cause circuit self-excitation. The solution is to increase the voltage gain of the pre-stage.

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