This audio line driver circuit is used to drive the final stage of amplifier. Integrated operational amplifiers are not always suitable for applications where a high signal level (Uo ≤ Vrms) is required for driving a relatively low impedance (Z=50-600 Ω). The amplifier described here is eminently suitable as a high dynamic range line driver of power buffer in public address systems and AF distribution amplifiers.
The input amplifier of the line drivers is formed by a low noise op-amp type OP-37 from PMI. This ensures the following technical spesification of the line driver: Uo = 70V Vpp max.; Io = 400mApp max.; Dtot = 0.01% at Uo=10 Vrms, ZL=50Ω and S/N ≥ 90dB.
Audio Line Driver Circuit Diagram
Regulators T1-T2 bring the supply voltage for the OP-37 down to ± 15 V. The complementary power output stage is formed by T3-T4. The amplifier has a standard negative feedback circuit R1-R2, which result in a voltage gain Av= -(R2/R1). A local feedback R3-R4 has been included to keep the output voltage of the Op-Amp within safe limits, while capacitors C1-C2 serve to improve the stability. It should be noted that the value of C1 and C2 depends on the construction of the line driver: typical values are 680 pF for C1 and 22 pF for C2. In a prototype of the circuit, neither capacitor was required for the frequency response to remain flat (±1 dB) up to 100 kHz.
Resistors RB should drop just enough voltage for T3 and T4 to start conducting (class A-B operation). The quiescent current of IC1 is about 3 mA, so that 150 ohm can be taken as asuitable starting value for RB. The quiescent current in the power output stage should be between 20 and 50 mA. Higher values of RB cause the quiescent, and hence the power dissipation, to increase, resulting in less distortion. The power output stage is not protected against thermal overloading, so that due dare should be taken in adjusting the quiescent current. This audio line driver circuit coupled to the power amplifier suitable medium to high.
