The broadband amplifier NE592 can be used also for building a simple selective audio amplifier. By means of the +6 V and +12 V supply voltages a DC coupling between the product detector and the AF amplifier is possible. The DC level at the differential inputs is about 4,5 V. The mixer drives the differential input of the NE592 - after suppression of the RF proportions by two 47 nF capacitors - with the remaining AF signal. RF blocking between the outputs is feasible with only one capacitor, but not between the single outputs and ground. But the latter is absolutly necessary due to the wide amplifier bandwidth. The AF stage offers a gain of 200 (46 dB) at 660 Hz center frequency.
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Headphones with Ri > 1 kOhm are mandatory due to the relatively high output resistance of the NE592 amp. 18 mA quietscent current isn't low, but it is insignificant in case of stationary operation and supply by a small ac-operated power supply. You can reduce the AF stage current consumtion down to 2 mA when with a dual op amp TL072/082 instead of the NE592 broadband amp. These consumer op amps have a lower output resistance, so that they are able to drive 2 x 32 ohms headphones connected in series with a 470 ohms resistor. When using a NE5532 no additional resistor is necessary, because the outputs have a less lower impedance. The transfer characteristic is the same as the one displayed for the first circuit (AF amp with NE592). Low distortion, CW selectivity, high gain, low noise and a high output voltage are the features of the following circuit. This circuit is seldom used despite its good characteristics. The latter is perhaps because of the fact that one must try different resistance values for R4 and R5 depending upon the quiescent current through IC1. This is ok for building a single device but not acceptable for a series product.
R1 10 kOhm
R2 8,2 kOhm
R3 1,8 kOhm
R4, 5 390 Ohm
R6 47 Ohm
R7 470 Ohm
R8 4,7 kOhm
C1,4 47 uF, electrolytic cap.
C2 10 uF
C3 2,2 uF
C5 150 pF
C6,7 100 uF, electrolytic cap.
C8 220 pF
P1 10 kOhm, linear
Dr1 33 mH
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