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"Smokey & The LM386" 

Ok, so in this first chapter we I will introduce the undisputed king the DIY amp building scene ... The LM386 power amp IC. We will then build the simplest circuit it is possible to build with this particular chip, The smokey amp circuit, and explore all the ways you can add to and improve the circuit. The Smokey circuit is the perfect first build as it is so easy that even a total newb should be able to tackle it in well under an hour without any headaches which will allow you to focus on the really fun bit, building a cool box to put it in. I'll also show you a load of ways you can improve the circuit and add extra features to it and along the way, show you how to read and design a Veroboard (or Stripboard) layout. This opens the door to a million other projects you can have a go at should you set the bug. 


 


 

 

 

 

 

'The 386 Chip' 

So this is the LM386 also known as the JRC386  or NJM386. It is a  solid state power amp IC or Integrated Circuit.(An IC is basically a whole circuit, shrunk down fit inside a chip, in this case, a JFET power amplifier circuit).

They come in three flavours (or used to). The  ...LM386-1 (1watt)    (LM386-1 (3/4watt)   LM386-4 (1/3 Watt). The JRC chips are the same but are labeled a/c/d rather than 1 / 3 / 4) Obviously, go for the highest power version you can although the difference in volume is a lot less than you might imagine [unfortunately both the JRC and LM 1watt chips seem to have been discontinued although you may still be able to buy old stock - April 2020]

 The problem with most solid state power amps is that they sound like pebble dashed dog shit unless you run them completely clean, so the only way to make a decent sounding guitar amp is to use a complicated pre-amp circuit to shape the sound and create the distortion ... and then use the power amp section to cleanly amplify the distorted pre-amp signal. This is maybe a bit too complicated for your average first time amp builder to tackle.

 

                

 

Enter the LM386  ... The DIY guitar amp builders knight in shining armour. The LM386 chip is dirt cheap 1 watt power amp chip designed to be used in shitty battery powered clock radios or Children's toys. Anything that requires a low cost, low power consumption amplifier where musical fidelity is not a critical factor. Now while the LM386 chip might be a bargain basement solution to audio amplification, it is absolutely amazing for the DIY guitar amp builder because while most power amp IC's clip like a bag of spanners falling down a mountain  ... The LM386 chip overdrives like an absolute fucking champ.  

 

Chip NameMin VoltageMax VoltageMinimum Output PowerTypical Output PowerLoad Impedance
LM386N-14 volts12 volts250 mW325 mW8 ohms
LM386N-34 volts12 volts500 mW700 mW8 ohms
LM386N-45 volts18 volts700 mW1000 mW32 ohms

The beauty of the 386 chip is that it overloads gracefully like the power amp section of tube amp or at the very least a decent overdrive pedal. This is awesome for 2 reasons, 1) you can really crank the volume and milk every last drop from that 1 watt of power without it sounding like shit and 2) you don't need any complicated pre-amp circuit to create the clipping, you can just overload the input and the chip will take care of the rest and that's exactly what the smokey amp does. 

 

 

 

 

 

 

  

'The Smokey '

 

 

 

So if you have never heard of a 'Smokey' this is what it is. A really simple JRC386 powered amp circuit housed in a cigarette packet. The smokey amp uses a grand total of 3 components ( the chip plus 2 x 47uf capacitors) along with an In and Out Jack, a battery snap and a speaker.

 

 

 

 

 

The 386 chip has 2 inputs, an inverting input on pin 2 and a non-inverting input on pin 3. I'll explain the difference between these two later, but for now all you need to know is whichever input is not being used, needs connection to 'Ground' which basically means the NEGATIVE terminal of the battery. The little upside down Christmas trees indicate 'Ground'. So wherever you see that symbol (sometimes just a plain upside down triangle is used) you need to connect those wires to the negative (-) battery terminal one way or another doesn't matter how it gets there or where in the circuit it is connected as long as all the ground points are connected together. 


So what we have on this circuit is ......

  • Pin 2 (inverting input) is not used, so needs connecting to Ground
  • Pin 3 is the Guitar signal input pin
  • Pin 4 is the V- or Ground pin ... so obviously needs connecting to Ground
  • Pin 5 is the Output and needs connecting to either the speaker or Speaker Jack via a capacitor
  • Pin 6 is the V+ or power pin so is connected to the positive (+) terminal of the battery
  • Pin 7 is the bypass pin which passes some frequencies to ground via a capacitor in order to stabilize the circuit (which I'll explain in more detail later)
  • Pins 1 & 8 form the Gain loop. If these 2 pins are left unconnected the amp operates at minimum x20 Gain (Gain is how much the signal is amplified or increased). If the 2 pins are Joined together the amp operates at maximum x200 gain. 
Now you might think 200x gain would be 10 times louder than x20 gain but it in reality it doesn't really work like that for 2 reasons. 1) Volume doesn't increase in a linear manner (A 100 watt amp is not twice as loud as a 50 watt amp for example), and 2) The actual output volume is capped by the supply voltage which in this case is 9 volts (EXPLAIN HEADROOM AND VOLTAGE SWINGS) so what happens is the amplitude of the signal is greater than the chip is capable of outputting because of the supply voltage so the tops and bottoms of the guitar signal get clipped off and that's what creates the distortion or overdrive sound which is the 386 chips party piece.

 

 

 

 So what does the schematic look like if we build ... 

 

 

 

 

 If you where to build it on a Breadboard it would look like this. (If you don't know what a breadboard is, don't worry about it just yet. I'll talk you through all that in the next section.) If you have a breadboard you could build it like this first to test the circuit but if you don't, it's really not worth buying one for this. The circuit is so simple it's virtually impossible to fuck up so lets get straight on to building it on a piece of  Stripboard ........