5.4 The Universal Audio AmplifierWe already spoke about the universal amplifier in the text connected with pic.3.22. Pic.5.11 contains the diagram of another such device, where the transistor amplifier with BC547 is used as the pre-amplifier, instead of that with TLO71 IC. It can be used for practical check of all the earlier mentioned radio receivers. The LF signal is being taken from the detector in the HF part of the receiver to the hubs marked as In and Gnd (if the links aren’t too long the ordinary wires are used, otherwise - the microphone cable). On the third hub the DC voltage is outputted, which is used in some HF circuits for their operation (such as e.g. those on pics.3.24, 3.25, 3.29 etc.).
* The LED (and the appropriate resistor) are added if the amplifier is being supplied from the adapter connected to the household voltage installation. It can also be used if the amplifier is power supplied from the battery, but this is not recommendable, since its power consumption is fairly big, which significantly shortens the battery life.
* The amplifier can be put in a box of any kind, one of the possible solutions shown on pic.5.11-b.
* A very useful solution can be to place the adapter also in the box, with the ability to control its output voltage from few volts to 12 V. In that case, you have both the amplifier and adapter in the same box, which can be used for power supply and check-out of various electronic devices, and not just radio receivers.