AUDIO FREQUENCY AMPLIFIER SIMPLE ELECTRONIC PROJECTS COOKBOOK

HOW TO MAKE AUDIO FREQUENCY AMPLIFIER

1. PCB. The plain side is the component side and the soldered side is the track side. Figure 1 shows the track side full size. Make the PCB from the pattern given in Figure 1. Otherwise, build the circuit on a matrix board.

2. Three resistors. Locate the gold or silver band around the resistor, and turn the resistor until this band is to the right. There are three coloured bands at the left-hand end of the resistor. Find the resistor whose colours are YELLOW, VIOLET, RED, and look at the resistor colour code chart which you will find in Chapter 7. From this, you will see that YELLOW indicates the value 4, VIOLET the value 7, and RED the value 2. The first two colours represent real numbers, and the last value is the number of zeros (noughts) which go after the two numbers. So, the value is 47 with two zeros, i.e. 4700 ohms. In this way, the resistor coloured BROWN, GREY, BROWN has a value of 180 ohms, and the last one, BROWN, RED, GREEN, has a value of 1 200 000 ohms. The ohm (often written as the Greek letter omega ( )) is the unit of resistance. If you do not yet feel confident in identifying resistors by their colours, use the Resistor Colour Codes.

3. Four capacitors. The two small ‘beads’ are tantalum capacitors and will be marked 4.7 F or 4 7, with a ‘+’ above one lead. A tubular capacitor with wires coming from each end should be marked 220 F, with one end marked ‘+’ or ‘–’. This is called an axial capacitor because the wires lie on the axis of the cylinder. This is in contrast to the final capacitor, where both wires emerge from the same end. This is a radial capacitor, and will be marked 47 F. Again, one lead will be marked ‘+’ or ‘–’. Capacitors marked like this are said to be polarised, and it is vital that these are placed on the PCB the right way round, so take notice of those signs!

4. Two diodes. These are tiny glass cylinders with a band around one end, and may be marked 1N4148; this is their type number. Like polarised capacitors, they must be put on the PCB the correct way round!

5. Three transistors. One should be a BC548 (or a BC182), the other two should be BC558 (or BC212).

6. One volume control with internal switch.

7. One loudspeaker. This is quite fragile – don’t let anything press against the cone.

8. One PP3 battery clip with red and black leads.

Putting it together
Lay the PCB on a flat, clean surface with the track side downwards. It is
always useful to compare the layout with the circuit diagram, given here in
Figure 3. Although you can’t see it, the D-i-Y Radio sign should be at the
top. Compare the hole positions with those shown in Figure 2. Bend the
resistor wires at right angles to their bodies so that they fit cleanly into the
holes in the PCB. Push each resistor towards the board so that it lies flat on
the board. Then supporting each one, turn the board over and splay out the
wires just enough to prevent the resistor falling out. Then, solder each wire
to its pad on the PCB, and cut off the excess wire. When you have more
confidence, you can cut of the excess wire before soldering; it often makes
a tidier joint.

Now fit the four capacitors. Each must be connected the right way round,
so look at each component, match it up with the diagram of Figure 2, bend
its wires carefully and repeat the soldering process you performed with the
resistors, making sure that the components are close to the board and not up
on stilts! Fit the two diodes the correct way round, and solder then as
quickly as you can – they don’t like to be fried!
Mount the transistors about 5 mm above the PCB. Make sure the correct
transistors are in the correct places, and that the flats on the bodies match
up with those shown in Figure 2.

Mount the volume control so that the spindle comes out from the front of
the board. Use a piece of red insulated wire to the pad marked + on the PCB,
and a black piece to the pad marked –, and solder these to the tags on the
back of the control, as shown in Figure 4. Connect the two leads from the
battery clip to the other tags on the switch; Figure 4 will help you. Finally,
use two pieces of insulated wire about 100 mm long, twisted together, to
connect the loudspeaker to the PCB.