Thursday, 22 October 2009

Simple AF Amplifier

The Simple Audio Frequency (AF) Amplifier. An easy and useful introduction to home-build electronics, this project will take a low-level audio-frequency signal and boost it to 200 times its original strength. It has a level control to adjust the output for comfort, and will drive a pair of iPod / MP3 player headphones, or a small loudspeaker. It uses a 9V battery (PP3 / MN1604), and requires no setting-up or test equipment. To make it, you will need the following parts:
  1. LM386 audio amplifier device
  2. 10k logarithmic pot with switch
  3. 1uF 16V electrolytic capacitor
  4. 10uF 16V electrolytic capacitor
  5. 100uF 16V electrolytic capacitor
  6. Red 5mm LED
  7. 1K-ohm resistor (1/8 watt)
  8. 3.5mm stereo jack socket
  9. 2-off 4mm banana sockets
  10. Thin insulated wire (150mm)
  11. PP3 battery clip
  12. PP3 9V battery
  13. 100x55mm single-sided copper-clad board
An experienced constructor would find most (or all) of this in their junk-box, but a newcomer to the art will need to buy these things new. I plan to market a kit of parts for this project, along with all those which will follow in the weeks and months ahead. Two big-name suppliers of components are Farnell ( ) and RS Components ( ). Farnell may be the better bet for the non-corporate buyer, and they carry the stereo socket ( p/n 1280747 ), whilst RS do not. You will have to buy the sockets in multiples of five, as that is their packet quantity. No worries - you''l find uses for the other four in other projects and experiments. Another source of components is from old or broken equipment; see my article - for more information.

You will of course need tools, and solder. Soon, I'll be adding teach-in articles (including videos) on tools and techniques; but for now, unless you already have the skills and tools, ask someone who knows how before attempting to make this project. Stick around! I'll tell you how it's done. I've spent over thirty years in the electronics industry, in several varied jobs, and I've picked-up a lot of knowledge and skills down the years. I intend to pass it all on to you.

The panel drilling is very straightforward. There are nine holes, and you'll need 3mm, 5mm, 6mm, 8mm and 10mm drill bits. Drill all holes initially with the 3mm bit; this makes it much easier to start the larger drills, and this first drilling size is called a 'pilot hole'. Make sure you clamp the panel when drilling. If the drill bit snatches the panel, it'll whip round and catch your hand. A G-clamp with a piece of wood to spread the load makes a good makeshift clamp. Clean the raised burr from around the hole edges with a larger-sized drill bit (hand-held), and then buff-up the copper surface with scouring pad ready for soldering.

Mount the sockets and pot first, locating them in their holes and carefully tightening their nuts. With care, a pair of snipe-nosed pliers may be used to tighten the nut of the stereo socket. Bend pins 3 and 4 of the LM386 up, and then outward, level with the top surface of the plastic case. These will be soldered to the panel as 'ground' connections. Turn the LM386 over. With the writing-side down, you have the remaining six pins sticking upright like a dead insect. Connection to these pins is now easy, and they are clear of the panel. Bend the 10uF capacitor's leads around as in the layout picture, and solder the capacitor to pins 1 and 8 of the LM386. These pins are at the 'notch' end of the device. Now add the remaining components. The red LED is pushed through the panel, and it's cathode lead is soldered directly to the panel. This is different to the circuit diagram; the diagram shows the resistor in between the cathode of the LED and the panel. It will work either way round; this is a good example of the flexibility which can be employed in electronics. Things aren't always this easy or straightforward, but I haven't the space now. I'll expand on this and much more, later.

Connect a PP3, a pair of headphones, and switch on. A buzzing will be heard in the headphones if the input socket is touched. What can you amplify? Try a magnetic (moving-coil) microphone, or an electric guitar. Search the web for 'crystal set' radio designs; the output can be amplified by this project. If you want to use a computer-type microphone (electret), it will need a power supply. This can be done by connecting a 47k resistor between the input socket and the switched side of the power switch. This will put around two volts on the microphone.

This simple circuit is simpler than normal for an LM386-based amplifier. I've left out three components; and these may be added if the amplifier is unstable or noisy. They will not normally be necessary, but you should be aware of them. A 100uF capacitor can be added between pin 6 of the LM386 and the panel, with the + of the cap to pin 6. This 'decouples' the power supply, stopping spurious signals getting into the device via the power supply. It also helps to stabilise the voltage, acting as a reservoir. Two other devices normally used in circuits of this nature are a 10-ohm resistor and a 100nF capacitor in series, connected from pin 5 (the output) and the panel. This 'Zobel network' helps to cancel-out the reactance of the speaker's coil, but if headphones are used the inductive reactance is too small to be of consequence. If you hear popping or howling when using a speaker, add the Zobel components.

Next week, I'll offer up some skills. There's a lot to learn, and the basics need covering first. When I was trained as a wirer, many years ago, we were instructed to wire-up panels fitted with tagstips and terminals, and the joints were made such that they held together mechanically. The units we made had to work before they were soldered. The wires had to be laid straight, and the stripping accurate. Any deviation from the rigorous standards, and the instructor pushed a screwdriver through the wire, and politely asked us to 'do it again!'. I'm not that overbearing; but I will teach you all I know about wiring, fitting and electronics assembly. Valves, Transistors, ICs, panel wiring, PCBs, wirewrapping, soldering, stripboard, the lot.

Theory will be added into the mix as required. I will not be teaching Maxwell's Laws of Electromagnetism, but I will tell you why you don't need them.


Vinod said...
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Vinod said...
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