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40 W output amplifier

Although there are a number of hybrid output modules on the market, very few of them combine compactness with reasonable price and good performance. one of these few is SGS’s used in the present amplifier. The design of the amplifier is straightforward: a power opamp to the noninverting IC1 via socket K1 and capacitor C1. The supply current to the IC varies in accordance with the input signal. Consequently. there will be similarly varying voltage drop access resistors R6 R7 R8. and R9 since these are in the supply lines to the opamp .

as long as the current is lower than about 1 A. the voltage drop access the resistors will be insufficient to switch on transistors T1 and T2. This means that outputs  2 W into 4 Ω are provided entirely by the opamp. Once the output current exceeds a level of 1 A. the transistors are on and contribute to the power output.

when the input signal is small. there is no quiescent current through the transistor. but there is through the opamp. Crossover problems are thus obviated.  Since the IC also provides thermal compensation. stability of the operating point is ensured. The supply voltage may lie between 12 V and an absolute maximum of 44 V. Construction of the amplifier on the printed circuit board should be straightforward. The transistors, as well as the IC, must be fitted insulated on to a heat conducting paste.

The supply line should be protected by a 3.15 A fuse.

Technical data for “40 W output amplifier”

  • Supply voltage      44V
  • Maximum output 22 W into 8 Ω
  • (for THD = 0.1%)  40 W into 4 Ω
Harmonic distortion
  • 1 KHz / 8 Ω /11 W   0.012%
  • 1 KHz / 4 Ω /20 W   0.032%
  • 20 KHz / 8 Ω /11 W   0.074%
  • 20 KHz / 4 Ω /20 W   0.2%
  • 1 KHz / 8 Ω /1 W   0.038%
  • 1 KHz / 4 Ω /1 W   0.044%
  • Quiescent current about 38 mA
  • Efficiency 8 Ω 62.5%
  • (Maximum load) 4 Ω 64%
  • Parts list


  • R1-R2 =100kΩ
  • R5= 8.2 kΩ
  • R6-R9 =1.4 Ω 1%
  • R10 = 1 Ω


  • C1 =470 nF
  • C2 =10 μF, 63 V radial
  • C3 =4.7 μF, 63 V radial
  • C,C,C7 = 220  nF
  • C6=2200 μF, 50V ,radial
  • C8 =100μF ,50 V, radial


  • D1 ,D2 = 1N400
  • T1 =BD712
  • T2 = BD711

(Integrated circuits)

  • IC1 =TDA2030


  • K1 = audio socket
  • Heat sink 2 K W-1
  • Insulating washers, etc, for IC1, T1, T2


40 W output amplifier




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