TDA 7294 50W Audio Aplifier

The Type IC TDA7294 from SGS-Thomson is an integrated a.f. amplifier intended for use in all sorts of hi-f application. The TDA7294 is a monolithic integrated circuit in Multiwatt15 package, intended for use as audio
class AB amplifier in Hi-Fi field applications (Home Stereo, self powered loudspeakers, Topclass TV). Thanks to the wide voltage range and to the high out current capability it is able to supply the highest power into both 4W and 8W loads even in presence of poor supply regulation, with high Supply Voltage Rejection. The built in muting function with turn on delay simplifies the remote operation avoiding switching on-off noises. According to the manufacturer's data sheets, chip can deliver outputs of up to 100 watt. Considering other properties, such as low noise, low distortion and reliable short-circuit and thermal protection circuits as well, the chip is indeed an interesting one.

Moreover, with peak supply voltages of ±40 V and a load impedance of 4 ohm, the maximum dissipation of the IC TDA7294 will easily be exceeded. For these reasons, the supply in the present amplifier has been kept down to a safe ±30 V. At these voltages, the chip delivers, without any difficulty, 50 W into an 8 ohm load and 80 W into a 4 ohm load. These are still very respectable figures, particularly in view of the reasonable IC price.

The schematic of the amplifier in shows that the IC TDA7294 needs only a small number of external components. The input signal is applied to pin 3 via capacitor C1 and low-pass filter R6-C10. The filter improves the pulse response and flattens the frequency response. For maximum output offset, the values of R1 and R3 should be equal, so that the input impedance is 10k. The roll-off frequencies of R1-C1 and R2-C2 determine the lower bandwidth limit of the amplifier: with values a specified, this is about 16 Hz. The upper -3 dB point is at about 100 kHz.

The amplifier is muted by a relevant input to pin 10 and placed in the stand-by mode by a relevant signal at pin 9. Muting should always take place before the stand-by mode is selected. Connecting the mute and standby pins permanently to the supply line ensures that the amplifier comes on immediately the power is switched on. Any switch-on clicks may be eliminated by increasing time constants R3-C4 and R5-C5. If large-value electrolyte capacitors are used in the power supply, switching off will be rather slow.

It is best to build the amplifier on the printed-circuit board shown below. Insulating material between the heat sink and the IS is. The amplifier will have good performance with toroidal mains transformer, a 25A bridge rectifier and two 10.000uf,50 V electrolytic capacitors.

The measurements were carried out at an output power of 40 W into 8 ohm and a bandwidth of 80 kHz. As usual, the characteristic slopes upward at laugher frequencies, but the distortion does not exceed 0.04 per cent. In a large part of the a.f. range (up to about 1kHz), the total harmonic- distortion-plus-noise (THD+N) does not even rise above 0.02 per cent. This sort of performance is excellent for all but the most demanding applications.