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 followed by two output transistors.
The audio signal is applied to the noninverting input of power opamp IC1 via socket K1 and capacitor C1. The supply current to the IC varies in accordance With the input signal. Consequently, there will be a similarly varying voltage drop across 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 across the resistors will be insufficient to switch on transistors T1 and T2. This means that outputs up to 2 W into 4 fl are provided entirely by the (Tamp. 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, the 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 \4fitted insulated on to a heat sink of about 2 k Use plenty of heat conducting paste.
The supply line should be protected by a 3.15 A fuse.
Technical data
Supply voltage 44 V
Maximum output 22 W 8 (ohm)
(for THD=0.1%) 40 W into 4 (ohm)
Harmonic distortion
1 kHz/8 (ohm)/11 W 0.012%
1 kHz/4 (ohm)/20 W 0.032%
20 kHz/8 (ohm)/11 W 0.074%
20 kHz/4(ohm)/20 W 0.2%
1 kHz/8 (ohm)/1 W 0.038%
1 kHz/4 (ohm)/1 W 0.044%
QUiescent current about 38 mA
Efficiency 8 (ohm) 62.5%
(rnaidrriurri load) 4 (ohm) 64%
Parts list :
Resistors:
R1-R4 = 100 k(ohm)
R5 = 8.2 k(ohm)
R6-R9 = 1.4 (ohm) 1%
R10 = 1 (ohm)
Capacitors:
C1 = 470 nF
C2 = 10 pF, 63 V,radial
C3 = 4.7 pF, 63 V, radial
C4, C5, C7 = 220 nF
C6 = 2200 pF, 50 V, radial
C8 = 100 pF, 50 V, radial
Semiconductors:
D1, D2 = 1N4001
T1 = BD712
T2 = BD 711
Integrated circuits
IC, = TDA2030
Miscellaneous:
K1 = audio socket
Heat sink 2 K W-1
Insulating washers, etc. for IC1, Ti, T2
