386instruct Amplifier 1

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Little Gem Amplifier

This project is based on the Little Gem amp (http://runoffgroove.com/littlegem.html). Its a -watt amplifier similar to abunch of tiny practice guitar amps; it uses a single integrated circuit and just a handful of components. The version wellbuild has a single gain control; this is like the drive control on a guitar amp, turning it up will give a dirtier, more distortedsound.

Well use this as an introduction to building circuits on a breadboard, as well as introducing a couple new kinds ofcomponents and schematic symbols. Well also be using this amp to listen to the oscillators and stuff well start building nexsession.

Heres the schematic for the Little Gem; it uses a couple of new symbols:

Ground: the down-pointing striped trianglesrepresent the ground connection, which justmeans theyre all connected together. The input,output, and power supply (-9v) all have an

assumed connection to ground that is not shownin this schematic.

Integrated Circuit: in this case, its the trianglelabeled 386. It has 8 legs for connections, andthe numbers next to the connections representthose legs. (pin 7 is not used, so its not shown).

Electrolytic capacitors: the 100uf and 220ufcaps have little plus signs on one side, whichmeans they are polarized and have to be put inright way around.

Integrated circuits are just little building blocks that have more complex circuitry inside them. The ones well

be using come in small rectangular packages with 8, 14, or 16 leads. This package is called a DIP,meaning dual inline pins. The LM386 has 8 pins:

IC pins are numbered in order, counter-clockwisefrom the little dot in one corner. Occasionally thedot is missing, in which case the little half-moonnotch will indicate which end is up.

The line diagram shows how the inner circuit isarranged (different for various ICs). The labelsgive information about the pins In this case, Vson pin 6 indicates the power supply (Voltage)connection; pin 4 is ground.
http://runoffgroove.com/littlegem.htmlhttp://runoffgroove.com/littlegem.html


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Preliminary: how breadboards work:

Breadboards have holes connectedtogether in patterns; you stick components inthe holes to connect them together

Each set of 5 vertical holes (red line in thispic) are connected together; so if you stickone lead in hole A1, and another in E1,theyre connected.

The channel in the middle is notconnected; so E1 and F1 are notconnected together. That channel is thesame width as most integrated circuits, sowell usually place ICs straddling that channelso you can make separate connections toeach side.

The outside horizontal buss lines (green)are also connected together; a lot of times,youll use one of those lines for power and theother for ground. Some breadboards have 2busses on each side, which can be handy--

just remember which one is ground and which

is power.

Step 1: Place the 386 op-amp. Pin numbers start to the left of the dot/halfmoon on one end, and gocounterclockwise:


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Step 2: Add the ground connections, including the 100uf power filtering capacitor. The cap is polarized; the sidemarked with a minus sign goes to ground; the other side goes to pin 6 of the IC:

Step 3: add the input capacitor (.01uf; may be marked 103); it is not polarized:

Step 4: add the gain control (connections to your 5k pot.):


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Step 5: add the 220uf output cap; watch the polarity (negative side is the output to your speaker; positive to pin 5of the LM386):

Step 6: add the output bridging resistor/cap.:

Thats basically it. Test it out:

Connect a contact mike/piezo element (little brass disk) to the input: red wire to the In connection on the .01uf cap;black wire to ground.

Connect the gain pot if you havent already.

Connect a speaker: one side to the 220uf cap, the other side to ground.

Connect power via the 9v battery clip: black wire to ground, red wire to the +9v connection (IC pin 6).

Tapping on the contact mike should produce sound in the speaker.

Coolness. If it doesnt work at first, first make sure that none of the components, or the battery, is getting hot--if it is, youhave a short someplace, meaning theres a direct connection between power and ground. Disconnect the batter and findthe short.If nothings hot and its still not working just start tracing your connections one at a time; I usually start with the powerconnection, then ground, then with the input and work my way from left to right. My most frequent mistake is just mis-counting the breadboard holes and thinking I had connected something when I hadnt.


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Step 2: Building the same circuit on Perfboard:

You could leave your amp on the breadboard; well use it to amplify the oscillators and stuff well start building next session.Breadboards circuits are a little delicate, especially as we unpack/pack each week, and you might want to free up some ofthat space for more fun stuff. One solution would be to transfer your amp to perfboard. You have a small perfboard in your

kit; basically you stick the components through the holes and solder them together on the back. Heres the layout:

This is a view from the top side of the board. Components go on top, and the wires are soldered on the bottom.

Use the IC socket from your kit (ICs are a little heat sensitive, so its safer to solder the socket and plug the IC inafter). Start by soldering pin #7 of the socket (its not used otherwise) just to hold it into place.

Next, lay down a piece of wire to use for the ground connection strip a piece of the thin wire, and just solder itdown at either end of the perfboard

Add the rest of the circuit in basically the same order as we did on the breadboard: ground connections, 100uf cap,input etc. Make each connection one at a time and then solder; double-check your work as you go.

For the ground connection to pins 3 & 4 youll need a short piece of wire; most of the others youll be able to justbend the component lead into place and solder it down.

Remember that the picture is from the top--when you turn over the board, everything will be kind of mirror-imaged.Go slow.

We will need some kind of little amp starting next week. If this seems like too much trouble at this point, other alternativeswould be:

A powered computer monitor speaker from the thrift store. If it has a wall-wart power adapter, its powered.

Any of the tiny practice amps (like the Smokey amp) from a music store.

http://www.radioshack.com/product/index.jsp?productId=2062620

Out to speaker
http://www.radioshack.com/product/index.jsp?productId=2062620http://www.radioshack.com/product/index.jsp?productId=2062620

386instruct Amplifier 1

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