The LM368 amplifier chip is a versatile no-nonsense amplifier that needs only a minimum of extra components and therefore is ideal to build if you only need about half a Watt for projects.
As there are many many circuits on the internet featuring the LM386 it seems a bit overdone to describe one in instructables, I agree, but I will also provide a PCB and that could help some people who have hesitated to start with this chip
The circuit is quite straightforward. The audio signal enters on JP1 and is fed to the + input of the LM386 (pin3) via a 10k variable resistor. The -input (pin 2) is connected to earth, which is necessary to be able to use higher gains.
The LM386 then does it's thing and feeds the amplified signal to the 8 ohm speaker via a 220 uF capacitor on pin 5. The 220uF capacitor together with an 8 Ohm speaker gives a low cut off of 90Hz (-3dB) (1/2πRC)
So far so good. There are a few extra components. The RC network between the output (pin 5) and the ground is to avoid oscillations. The RC network between pin 1 and pin 8 defines the gain. As shown, the gain is 50, but if you short the jumper the gain will be 200. Of course if you want the gain to be 200 permanently, an option is to replace R1 by a wire-bridge.
The data sheet explains as follows: With pins 1 and 8 open the internal 1.35 kΩ resistor sets the gain at 20 (26 dB). If a capacitor is put from pin 1 to 8, bypassing the 1.35 kΩ resistor, the gain will go up to 200 (46 dB). If a resistor is placed in series with the capacitor, the gain can be set to any value from 20 to 200. Gain control can also be done by capacitively coupling a resistor (or FET) from pin 1 to ground.
Additional external components can be placed in parallel with the internal feedback resistors to tailor the gain and frequency response for individual applications. For example, poor speaker bass response can be compensated by frequency shaping the feedback path. This is done with a series RC from pin 1 to 5 (paralleling the internal 15 kΩ resistor).
For 6 dB effective bass boost: R.15 kΩ, the lowest value for good stable operation is R = 10 kΩ if pin 8 is open. If pins 1 and 8 are bypassed then R as low as 2 kΩ can be used. If you want this, you have to alter the PCB or just solder the components between existing pins. (The new print design has a place for it)
1x 8 pin DIL foot
2x 10uF electrolytic capacitor
1x 1000 uF capacitor 16 Volt
1x 220 uF capacitor
1x 100nF capacitor
1x 47 nF capacitor
1x 10k variable resistor
1x 10 Ohm resistor
2x 1k resistor
1x 1k resistor (optional)
1x speaker 8 Ohm
connector for input
connector for powersupply (6-12 Volt)
(By the way, there are easier ways to build a small amplifier: use a TDA7052 1Watt, or use a TDA2822 for Stereo 2x 250mW)
Step 1: LM368 Amplifier: the PCB
The PCB is quite straightforward. There is room for a speaker with several options for the pin-out. If you intend to have the speaker seperate, that part of the print can be omitted and a connector soldered in (or just the wires).
A PCB design for direct toner transfer is added. This design is slightly different from the prototype shown in the pictures: there is a bit more room around the variable resistor and a small mistake in the connection of the LED was corrected