50 Watt Hi-Fi power amplifier circuit

SGS Thomson's integrated TDA7294 is a high-frequency acoustic power amplifier, with true high-precision specifications, suited for all relevant applications. Its main feature is much higher output power than is usual in amplifiers with similar distortion performance. According to its manufacturer, the output stage of this 15-pin integrated can give up to 100W. From the summary specifications listed above, plus the fact that the TDA7294 has reliable protection against short-circuiting the output and overheating, we conclude that it is remarkably complete. It should be noted, however, that the output power of 100W is rather excessive because it refers to the IEC output musical power, under 10% harmonic distortion, which of course is not the correct way to determine the output power for Hi-Fi. Besides, with balanced +/-40V supply and 4Ω load resistance, the integrated can overheat very quickly. For this purpose, the supply voltage is kept at the safest value of +/-30V, where it is effortlessly delivered 50W at 8Ω and 80W at 4Ω. These values are not insignificant, taking into account the reasonable disposal value of TDA7294.
50 Watt Hi-Fi power amplifier circuit - schematic

The TDA7294 circuit requires minimal external components. In order to keep the overall distortion low, the amplifier operates with a large percentage of negative feedback, with a gain of closed loop voltage of just 24dB.

The input signal is applied to pin 3 via C1 and the low pass filter R6-C10 that minimizes the risk of TIM degradation. R1 and R3 must be equal to minimize output voltage deflection, so the input resistance is practically equal to 10kΩ in our circuit. The low cut-off frequency is determined by the R1-C1 and R2-C2 lattices at approximately 16 Hz. The high cut-off frequency is about 100 KHz.

The amplifier can be mute with a positive voltage at pin 10 and put on hold with a positive voltage at pin 9. The squelch must always be done before the amplifier is switched to standby. By connecting these pins (9 and 10) permanently to the power supply via the resistors R5 and R4, we energize the amplifier as soon as it is energized. To avoid some noise during activation, the time constants R4-C4 and R5-C5 can be increased by always maintaining the R4-C4 time constant.

The (forced) use of large smoothing capacitors in the feed line causes delay in switching off the amplifier. If this is considered a drawback, you can add an external power supply detection network. This could be two rectifying diodes with two small electrolytes that will rectify the voltage of the secondary power transformer. The plaque has been predicted as additional welding pins near the squelch and standby inputs. Secondary positive voltage can be used to quickly control these inputs when the amplifier is turned off.

Because of the few components, the PCB can be designed very easy. The metal surface of the integrated communicates internally with the negative feed line, so to save the use of mica between the heatsink and the TDA7294 (but the silicone paste is needed), the heatsink is placed on the PCB and, of course, has the negative potential.

To choose the size of the heatsink we assumed a continuous output power of 50W at 8Ω. The same heatsink is enough for a music power of 80W at 4Ω. Thermal problems are unlikely to have, since the completed is automatically fused to 145°C and is waiting for 150°C.

The connection of the load to the board is made with three screwed wire contacts, which ensure a very good electrical connection. For symmetric feed, it is better to use a toroidal transformer, bridge 25A and two 10000μF electrolytic cells at 50V.

The TDA7294 is suitable for any high-fidelity application. Its small volume allows its use with a preamplifier or its integration into a loudspeaker to create a compact loudspeaker.

The measured deformation of the amplifier, as measured by the use of a spectrum analyzer, with 40 W at 8Ω and 80kHz bandwidth, climbs at high frequencies but does not exceed 0.04%. From low frequencies up to about 1 kHz, total harmonic distortion (THD + N) is below 0.002%, indicating excellent performance under the strictest requirements.

  1. Input sensitivity = 1.3V (50W, 8Ω)
  2. 10KΩ input impedance
  3. Bandwidth = 16Hz-100kHz
  4. Output voltage rise rate = 10V/μS
  5. Output voltage rise rate = 10V/μS
  6. Output power = 50W, 8Ω, 0.1% THD / 82W, 4Ω, 0.1% THD
  7. Signal to Noise Ratio = 105dBa, (1W, 8Ω)
  8. THD+N with 40W at 8Ω = 0.002% (1kHz) / <0.04% (20Hz-20kHz)



After you properly and carefully have connected your circuit with a audio signal generator, the heatsink on board, the power supply at  +/- 30V and the speaker at the output, you will heard a deafening sound coming out of the speaker so the amplifier works and works properly.

Then remove the terminals from the speaker and connect them to an oscilloscope for a more supervised study of the circuit. For the given input signal you should have a good output signal. The signal of 1KHz from the frequency generator (average acoustic tone frequency) and by changing the input width you will notice that up to 4Vpp the circuit works perfectly with a 6Vpp output signal. While at 5 Vpp input and beyond you will observe clipping at the output voltage. Also there shouldn't be any phase distortion in the range from 100Hz to 80kHz and up to 5Vpp.



According to the specifications of the 50W final amplifier with a single IC you get:

Practically a very good power amplifier that works perfectly up to 4Vpp inputs without amplitude, phase, frequency deformations and operates in the bandwidth of frequencies from 10Hz to 80kHz, ie in the audio frequency range. So this circuit is indeed a high-fidelity acoustic power amplifier, to be used in any Hi-Fi application.


List of Components

  • R1 = 150Ω
  • R2, R3, R5 = 10KΩ
  • R4 = 22KΩ
  • R6 = 680KΩ
  • C1 = 1.5nF / 63V Metallised Polyester
  • C2 = 2.7nF
  • C3, C4 = 100nF
  • C5, C6 = 10μF / 63V Electrolytic
  • C7, C8 = 22μF / 63V Electrolytic
  • C9, C10 = 2200μF / 25V Electrolytic
  • IC1 = TDA7294
  • Heatsink

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