We follow on from last month's theory and simple build with a more complex but much better amplifier.

We presented a lot of theory last month, and the build was quite small. This time, we're using a lot of that theory from last month, with a much bigger build and a little more theory to explain some new things we're introducing. This build is split in two, with a main amplifier, and a preamplifier, which is a more sensitive but less powerful amplifier used to boost weak signals before feeding them into a main amplifier. We'll make them on two halves of the same breadboard so you can use them separately or together as needed.

The main aim of this project, besides the learning experience of course, is to make a piece of useful test equipment. Having an amplifier on your workbench is very useful when you are making any form of noise-making circuit, or when just testing things like speakers or music players or the like that you may be trying to fix or just exploring the workings of. Because of that, at the end, we'll cover some options for making things a bit more reliable and permanent, but you'll need an adult to help because it involves soldering.

Note that in the parts lists, the breadboard, battery black and batteries, and speaker appear in both but you only need one of each. This is for people only building one or the other, so things don't get missed. On the subject of parts lists, two of the jumper wires listed are to connect to the speaker. If you're a Kids' Basics regular, you'll have this speaker made up already. Read the steps first before buying parts to know what you already have and what you need to get.

SOME HELPFUL HINTS

We encourage you to read all the way to the end of the article before you build. Not only will you then have a better feel for the overall picture as you build, but we sometimes discuss options or alternatives that you will need to have decided on. You will need some basic hand tools for most builds. Small long-nosed pliers and flush-cut side cutters meant for electronics are the main ones. Materials like tape or glue are mentioned in the steps, too. We always produce a tools materials list if you have to go shopping, but anything that is lying around in most homes is just stated in the steps.

As always with Kids' Basics, we avoid soldering to make the build more accessible to more people, but having an adult around can still be helpful. You won't need any particular skills besides being able to identify components at a basic level, and even then, we help as you go along. If, for example, you don't already know what a resistor is, you'll probably be able to work it out from the photos and description in each step.

We do provide a schematic or circuit diagram but this is just helpful if you already know how to read one. Don’t stress if you have never learned, but take the chance to compare the digital drawing of the breadboard layout (which we call a 'Fritzing' after the company that makes the software) to the schematic and see if you can work some things out. You can make this project from the Fritzing and photos alone. You might also like to check out our Breadboarding Basics from Issue 15.

The Preamplifier Build:

Parts Required:	ID	Jaycar
1 x Solderless Breadboard	-	PB8820
1 x Packet Breadboard Wire Links	-	PB8850
1 x 120Ω Resistor	R4	RR0550
2 x 2kΩ Resistor	R5, R6	RR0579
1 x 6.2kΩ Resistor	R3	RR0591
1 x 100kΩ Resistor	R1	RR0620
1 x 150kΩ Resistor	R2	RR0624
1 x 100nF Greencap capacitor	C1	RG5125
1 x 1µF Electrolytic Capacitor	C2	RE6032
1 x BC547 NPN Transistor	Q1	ZT2152
1 x BC557 PNP Transistor	Q2	ZT2164
1 x 4AA Battery Pack	-	PH9200
4 x AA Batteries	-	SB2425

Step 1:

Place the breadboard in front of you, with the outer red (+) rail facing away from you and the outer blue (-) rail closest to you. Install two wire links, one joining the two red (+) rails and one joining the two blue (-) rails.

STEP 2:

Insert a 100nF greencap capacitor, and then a 100kΩ resistor (BROWN BLACK BLACK ORANGE SPACE BROWN) between the upper red (+) rail and the right-hand leg of the capacitor. Finally, insert a wire link, carefully counting off the number of rows it covers.

Step 3:

Install a BC547 transistor with its flat side facing away from you and its middle (base) leg in the same row as the other end of the wire link. Add a 150kΩ resistor (BROWN GREEN BLACK ORANGE SPACE BROWN) between the base of the transistor and the upper blue (-) rail.

Step 4:

Place a 120Ω resistor (BROWN RED BLACK BLACK SPACE BROWN) between the left-hand (emitter) leg of the transistor and a spot to the left. Add a 10µF electrolytic capacitor with its negative (striped) lead to the upper blue (-) rail and its positive lead to the 120Ω resistor.

Step 5:

Insert a 2kΩ resistor (RED BLACK BLACK BROWN SPACE BROWN) between the left-hand (emitter) leg of the transistor and a space off to the right. Add a wire link between the right-hand (collector) leg and the next row to the right from the end of the 2kΩ resistor. Finally, place a 6.2kΩ resistor (BLUE RED BLACK BROWN SPACE BROWN) between the right-hand (collector) leg of the transistor and the upper red (+) rail.

Step 6:

Install a BC557 PNP transistor so that its flat side faces you, the left-hand (collector) leg goes to the 2kΩ resistor, the middle (base) leg goes to the wire link, and the right-hand (emitter) leg goes into a row to the right that so far has nothing in it. Add a wire link between the left-hand (collector) leg and a row off to the right.

Step 7:

Add a wire link between the right-hand (emitter) leg of the transistor and the upper red (+) rail. Place a 2kΩ resistor (RED BLACK BLACK BROWN SPACE BROWN) between the wire link from step 7 and the upper blue (-) rail, and a 1µF capacitor with its positive (unmarked) lead to the resistor and the negative (striped) lead in a row off to the right.

Step 8:

Temporarily connect a speaker with one wire to the negative (striped) leg of the 1µF capacitor and its other wire in the upper blue (-) rail. See the next build for connecting wires to the speaker. Connect the red wire from a 4xAA battery pack to the upper red (+) rail and the black wire to the lower blue (-) rail. Add four batteries. Take a spare jumper wire and plug it into the unused end of the 100nF greencap and touch the other end. You should hear a faint buzzing from the speaker. If not, disconnect the batteries and check all connections. When you're done, remove the batteries and disconnect the battery pack and speaker wires.

The Power Amplifier Build

Parts Required:	ID	Jaycar
1 x Solderless Breadboard	-	PB8820
1 x Packet Breadboard Wire Links	-	PB8850
4 x Plug-to-Plug Jumper Wires *	-	WC6024
1 x 100Ω Resistor	R3	RR0548
1 x 1kΩ Resistor	R4	RR0572
1 x 1.8kΩ Resistor	R2	RR0578
1 x 10kΩ Resistor	R1	RR0596
1 x 10µF Electrolytic Capacitor	C1	RE6066
1 x 100µF Electrolytic Capacitor	C2	RE6130
1 x BC327 PNP Transistor	Q3	ZT2110
1 x BC337 NPN Transistor	Q2	ZT2115
1 x BC547 NPN Transistor	Q1	ZT2152
1 x 1N4004 General Purpose Diode	D1	ZR1004
1 x Small Speaker	-	AS3000
1 x 4AA Battery Pack	-	PH9200
4 x AA Batteries	-	SB2425

Step 1:

If you have not built the preamplifier, place the breadboard in front of you, with the outer red (+) rail facing away from you and the outer blue (-) rail closest to you. Install two wire links, one joining the two red (+) rails and one joining the two blue (-) rails. If you have built the preamplifier, start with that board the same way.

Step 2:

Place a 100µF Electrolytic capacitor with its negative (striped) leg to the left. Add a 1kΩ resistor (BROWN BLACK BLACK BROWN SPACE BROWN) from the positive (unmarked) leg to the right, and a wire link between the negative leg and the next row to the right from where the resistor ends.

Step 3:

Install a BC337 NPN transistor with the flat side facing you, the middle (base) leg in the same row as the resistor, and the right-hand (emitter) leg in the same row as the wire link. Fit a new wire link between the left-hand (collector) leg and the lower red (+) rail.

Step 4:

Insert a wire link from the right-hand (emitter) leg of the BC337 then install a BC327 PNP transistor with its flat side away from you, and its left-hand (emitter) leg to the end of the wire link. Add another wire link from the middle leg (base) of the BC327 to the right.

Step 5:

Place a 1N4004 diode with its anode (unmarked end, A on the component diagram) to the middle (base) leg of the BC337 near the capacitor. Add a 100Ω resistor (BROWN BLACK BLACK BLACK SPACE BROWN) between the cathode (striped end, K on the component diagram) and the middle (base) leg of the BC327. You might need a small bit of tape to stop the resistor leg touching the end of the wire link it has to cross.

Step 6:

Install a BC547 NPN transistor with its flat side facing you and its left-hand (collector) leg to the end of the wire link from step 4. Add a 10kΩ resistor (BROWN BLACK BLACK RED SPACE BROWN) from the middle leg (base) of the BC547 to the left-hand (emitter) leg of the BC327. Finally, add a 10µF electrolytic capacitor with its positive (unmarked) end to the middle leg of the BC547 and its negative (striped) end to a spot on the right.

Step 7:

Insert a 1.8kΩ resistor (BROWN GREY BLACK BROWN SPACE BROWN) from the middle (base) leg of the BC547 to the lower blue (-) rail. Add two wire links, one from the right-hand (collector) leg of the BC327 to the lower blue (-) rail and the other from the right-hand (emitter) leg of the BC547 to the lower blue (-) rail.

Step 8:

Take two plug-to-something jumper wires and cut off the 'something' end before baring the wires. Twist each into a neat wire and thread it through the terminals of the speaker, before twisting them firmly but not too tight because they break easily. Finally, wrap the connections with tape to leave a speaker with two wires with pins on the end.

Step 9:

Plug one speaker wire into the lower red (+) rail and the other into the row with the negative lead of the 100µF capacitor and the 1kΩ resistor.

Insert the red wire of a 4xAA battery pack into the upper red (+) rail and the black wire into the lower blue (-) rail. Add four AA batteries to the battery pack.

Step 10:

Test the circuit by again touching your finger to the input, perhaps with the aid of a jumper wire. There will be a faint buzzing sound from the speaker. If not, you might have a fault but before you disconnect power and check connections, try connecting the preamplifier if you made it, or another music source like a phone. Some people's bodies don't create a strong enough signal to hear the buzz.

HOW IT WORKS

The theory is broken down into the two separate builds as we presented them above. You will benefit greatly from reading last month's 'How It Works' section, too, as we're not going to cover that theory again but it is relevant. The article, 'Kids' Basics: Amplified Antics' is available online for free if you're a month-to-month subscriber, one-off purchaser, or a website reader.

THE PREAMPLIFIER

The preamplifer's job is to boost a very small signal and make it big enough for a power amplifier to handle. All transistors have a voltage drop between the base and emitter, which is the voltage at which they begin to conduct current. To amplify a signal properly, we need to supply this voltage so that any further voltage applied causes current to flow from the base to the emitter. When that happens, a higher current flows from the collector to the emitter of Q1, a BC547 NPN transistor. The current from the input signal flows through capacitor C1, which makes sure that no DC voltage can get to the amplifier, only AC.

The bias voltage is set by the values of R1 and R2, which form a voltage divider. A voltage divider is two or more resistors which have a voltage applied across them. The voltage is divided across the resistors, based on their value. We know many Kids' Basics readers are young, but not all are. You may or may not have covered fractions and ratios at school, so if you haven't, or if you want a refresher, check out The Classroom Issue 51, where we used M&Ms to explore how they work. The voltage is divided across the resistors based on how much of the total resistance each resistor makes up. In this case, the 100kΩ resistor accounts for two fifths of the total, so it drops two fifths of the voltage across it. The 150kΩ resistor makes up three fifths of the total, and so drops three fifths of the voltage.

Unfortunately, six does not divide by five neatly, so there is 2.4V across the first resistor, and 3.6V across the second. The voltage at the base of Q1 is therefore 3.6V, because voltage is a potential difference between a high amount of charge and a low amount of charge. In most circuits in Kids' Basics, the low charge is 0V at ground, so the 3.6V dropped across R2 is measured against that. However, the forward voltage of the transistor is around 0.7V, so it is well and truly conducting. There is a reason for this.

Current flows through resistor R3, which limits what can flow through the collector-emitter path of Q1. It can handle a maximum of about 100mA, but at 6V, there is a lot less than that flowing through resistor R3! As the current in the audio wave at the base of Q1 rises and falls, the charge on the capacitor rises and falls. So, when the wave is in its negative half cycle, C1 discharges back into the source, and the opposite plate charges via R1. This drops the voltage at the base of Q1 below the 3.6V mark, but not all the way to 0.7V. That means the current flowing in the collector-emitter path of Q1 will look like the same wave as at the input, but bigger (having a higher 'amplitude' and always above 0V even at the transistor output.

The emitter of Q1 is connected by 100Ω resistor R4 to the 10µF capacitor C2. The capacitor 'decouples' the transistor's current path, meaning it cannot amplify any of the DC current at the base of the transistor, which would be bad news. Current only flows across the transistor when it has somewhere to flow, and current can only flow into the capacitor when it is charging and discharging. Some current also flows through R5 and R6 to ground, but look closely at the diagram. The base of Q2 is connected to the junction of R3 and Q1's collector. Q2 is a PNP BC557, and current flows from the emitter to the base of a PNP transistor. So, when Q1 is conducting, there is a path for current to flow from Q2's emitter to the base, activating that transistor and causing it to conduct across its emitter-collector path.

Current then flows from there to the R5/R6 junction, with the current limited only by R6 and whatever current flows through R5 to the capacitor at certain stages. This is the main part of the amplifier and is where the amplification really happens. The gain, or the amount of amplification, is controlled by R5 and R4. The output is taken from the R5/R6 junction, via another capacitor for DC blocking and current isolation. The charge on this capacitor reflects whatever goes on at the R5/R6 junction, so the other side follows suit but in the opposite half of the cycle. This is the output that is used for the next stage.

THE POWER AMPLIFIER

It sounds odd to some people that we call this circuit a 'power' amplifier. Isn't that the kind of thing you have at a rock concert or in a home theatre set-up? Yes, they are, but this is too. The name comes from the job, not the size. Power amplifiers handle the main amplifying job in a system, while the pre-amplifier does the boosting of small signals into still small but bigger than before signals, to feed into a power amplifier. Sound signals from, say, a TV, come out at a strong enough level that the power amplifier can use them.

However, microphones, magnetic cartridges in turntables (record players) and other fine, sensitive devices, output a much smaller signal that needs to be pre amplified before going into the power amplifier. That's where the names come from. The reason is that building a power amplifier that is also that sensitive would leave it open to noise and interference, and also mean that stronger signals could not be turned down in volume far enough

This project uses the 'push-pull' set-up we mentioned last month. That part relates to Q2 and Q3, but we'll work left to right. On the subject of left to right, both the preamplifier and power amplifier schematics are drawn the way they are because inputs are usually on the right and outputs on the left in the drawing system we use. However, some other

systems arrange the components to fit the space better and put the inputs and outputs wherever they fit. That's why our power amplifier diagram looks a bit like a set of stairs.

In a similar way to the preamplifier circuit, Q1 amplifies the input signal based on the voltage divider formed by R1 and R2. The gain is a bit over 5, meaning the output at Q1's collector is about five times greater than the input at its base. It feels a bit odd to call the collector of an NPN transistor its output given how we normally use them in Kids' Basics, but it will make sense soon.

As with the preamplifier, there is a capacitor on the input. This one is a 10µF electrolytic, and you can ditch the 1µF on the output of the preamp if you like, but it's probably easier to just leave them there. The circuit will work either way. As an aside, you can increase the volume of the circuit by reducing the value of R1, down as far as 4.7kΩ. However, we recommend working in increments, changing the 10kΩ R1 for a 9.1kΩ first, then an 8.2kΩ, then 7.5kΩ, and so on.

The reason we call the collector of Q1 the output is that this is where we connect the components that control the main amplifier stage. Current flows via the speaker and the 1kΩ resistor R4 to the base of Q2, an NPN BC337. Current can flow into the base of this transistor and out its emitter, but also through D1 and R3. The two of these together give a voltage drop of around 1.3V between the base of Q2, and the base of Q3, a PNP BC327. That difference helps deal with the crossover distortion we discussed last month.

The arrangement is important because if things were arranged the other way around, the two transistors would conduct at the same time, and short-circuit. It's hard to see without a video, but this way, the direction of the current in the negative and positive halves of the cycle means there is no short-circuit. That's why push-pull amplifiers are usually made this way.

So, the voltage at Q1's collector rises and falls because as it conducts, it shorts the collector to ground, reducing the voltage there. When it stops conducting, the voltage rises again as it is supplied through the path covered above. Note that it is not a hard on/off. But a gradual on/off in a wave shape. Back to Q2. As the voltage at Q1's collector rises (as the current through it reduces in response to the input wave at the base being in its downward slope), current can flow through Q2's base because the short-circuit to ground disappears. Current is limited by the 1k R4. That allows current to flow from the supply rail, through Q2's collector-emitter path, and to the positive terminal of C2, a 100µF capacitor. It now charges, which causes the opposite plate to discharge. These current paths relate to the input wave, so the speaker current reflects that too.

When Q1 begins to conduct and the voltage at its collector begins to fall, there comes a point where Q2 cuts out because the voltage at its base has fallen below its 0.7V forward voltage. At this point, Q3 starts to conduct, because its base can discharge through Q1. Remember, in PNP transistors, the current flows from the emitter, to ground. It gets to ground via the conducting Q1's collector-emitter path. This current discharges the capacitor, because current is no longer flowing through Q2 to charge it.

That means the other plate charges up, via the current from the speaker, which moves accordingly. The capacitor is big enough that this process never fully finishes before the input wave reverses the cycle and Q2 takes over, so no DC is ever involved. However, this system does allow more current than any of the previous designs, and so it can drive the speaker with significant volume. The BC337 and BC327 transistors are called a 'complimentary pair', and this term is also used to name the amplifier type. Q2 and Q3 are both rated to at least 500mA, whereas the BC547 and BC557 that did the job in the preamplifer are limited to 100mA.

USING THE CIRCUITS

The circuits can be used independently or chained together so that the preamplifier feeds the amplifier. It is unlikely that you would need the preamplifier on its own unless you were using it with, say, a home theatre amplifier to add your own microphone or boost a turntable signal. Most of the time you will use it with this amplifier. However, using the amplifier on its own is common on the workbench. To use them together, just use a jumper wire or wire link to connect the output positive on the preamplifier to the input positive on the amplifier.

You don't need to connect the input and output grounds because those are just the ground rail anyway. On some other designs, the ground for the input or output is not necessarily the supply rail ground. If using them independently, have a look at the Fritzing Diagram above each build, because the input and output connections are marked there. The same applies for the input when using them together, but the other connections are taken care of.

When using the circuits to test an audio project, use plug-to-plug jumper wires as probes. Connect a dark coloured one to one of the blue (-) rails, and use it to connect to the ground of the circuit you are probing. Connect a lighter coloured one to the input at either the 100nF greencap for the preamplifier if needed, or the 10µF electrolytic capacitor if using just the power amplifier.

VOLUME CONTROL

There is no volume control on the designs as-is, and in some situations, that will be annoying. Thankfully, it's easy enough to add. While this connection will in theory work on the preamplifier, it is less common as most input devices are very faint as-is. We'll show it anyway in case you have a situation where that would be useful. However, most people will want the volume control on the input to the amplifier. In either case, this is done by taking the input to one terminal of a potentiometer, connecting the other terminal to ground, and using the wiper to connect to the input of the amplifier. That way, you have a variable voltage divider and the amplifier gets whatever amount of the input signal that you get. The only challenging bit might be fitting it on the board! You'll probably need some jumper wires here.

ADDING AN ELECTRET MICROPHONE

You can add an electret microphone to your preamplifier. An electret microphone is also called a condenser microphone but the two are actually different. There are very few genuine condenser microphones on the domestic market now. Condenser is an old name for a capacitor and you can get some quality microphones that use condenser pickups, but we mean band or studio microphones, not the little ones makers use, which are actually called microphone inserts.

An electret microphone insert does kind of work capacitively. It has a moving diaphragm which moves in response to sound, and has a permanent electrical charge built into it. This electric charge controls the gate terminal of a Field Effect Transistor, or FET. FETs are voltage-driven, not current-driven like the bipolar transistors we used in the amplifiers. The closer the electric charge is to the gate, the more an effect is has on the amount of current that can flow from the source to the drain of the FET, which is the equivalent of the collector and emitter in a bipolar transistor.

However, that current must come from somewhere, so the electret insert needs a supply voltage. This is often called

a bias voltage, but that is incorrect and dates from real condenser microphones (which do use a bias voltage). It is just a supply voltage for an electret but the term continues to be used and really, it doesn't matter anyway. The electret has two pins. One is connected to the case of the insert by tracks on the PCB at the base, and is the ground.

This is connected to the ground rail of the supply, or the ground of the amplifier input if using a different type of preamplifier. The input is taken from the other pin, which is isolated from the case. However, a resistor is also needed between here and the positive supply rail. Usually, this is a 10kΩ resistor. If you want full details, check out the electronic stethoscope project from Issue 67.

WHERE TO NEXT?

We mentioned at the start that these circuits will perform better as workbench tools if they are made permanent. Breadboards are not the most reliable things at the best of times. The spring contacts inside them are imperfect, and there is a lot of electronic noise and stray capacitance from the way they function.

The best pathway is to make them permanent by soldering the components on a board. Not that long ago, that would have meant perforated strip board, often called Veroboard after the first main brand of the product, or doughnut board, which is just pads which you solder your own tracks between. Both are painful.

With veroboard, you have to cut tracks and solder wire links between rows all over the place to make the circuit. With a doughnut board, you use quite a bit of solder and it's not as easy as it should be to actually get it to bridge between pads to make the tracks. In both cases, the layout changes from the breadboard.

Thankfully, times have changed. Now, you can buy solder equivalents of the regular prototyping breadboards we use in Kids' Basics. The only differences between a breadboard and this solder version shown is that the supply rails are one space closer to the rows, and there are two centre rails instead of an unfilled gap like the breadboard. Some other brands have slightly different hole counts, alignments, or rail spacing. We have drawn on this one with a permanent marker to show you where the rails are and where the rows are. Photographing the copper side is nearly impossible because it is so shiny and the colour is too similar to the board material!

With these boards, your circuit can be placed on the solder board in exactly the same way as the breadboard, and you can even use the same wire links to make the connections! You just solder the links instead of using spring contacts.

The downside of this is that, unless you already have soldering skills and equipment, you'll need an adult to help you. Unless you are an adult reader (and we know we have many adult readers using Kids' Basics to help them begin in electronics), please ask your adults first before trying to solder. At over 220°C minimum, there is a significant risk of serious burns if you touch the wrong thing or even if the iron slides around under the tension of its power cord.