Sometimes there is a need for a preamplifier to receive the output from a single audio input device such as a microphone and drive several audio power amplifiers. This could be done most easily with a shielded cable to each amplifier from the originating preamplifier. However, if this were done by simply paralleling the shielded cables and connecting them to the preamp, the result could be an oscillating preamp stage or degraded high-frequency response due to the heavy capacitive loading.
A simple solution to this problem is the three channel output distribution amplifier using a single TL084 as shown in Figure 3-30. The first stage is capacitively coupled with a 1.0- µF electrolytic capacitor. The inputs are at 1/2 VCC rail or 4.5 V. This makes it possible to use a single 9-V supply. A voltage gain of 10 (l MΩ /100 kΩ) is obtained
in the first stage, and the other three stages are connected as unity-gain voltage followers. Each output stage independently drives an amplifier through the 50-µF output capacitor to the 5.l-kΩ load resistor. As shown in the response curve (Figure 3-31), the response is flat from 10 Hz to 30 kHz. Sinewave distortion begins at about 0.45 Vpp input with 5.5 Vpp output at 1 kHz. The total supply current is about 9 rnA at a maximum input of 0.45 Vpp. The TL070 and TL080 family of op amps
operate in true c1ass-AB and therefore offer zero crossover distortion. The absence of crossover distortion can be noted in the scope photos (Figure 3-32). At a frequency of 1.0 kHz and with 3.0 Vpp output the total harmonic distortion is 0.14%.