Presentation
This amp is very easy to make and very small size, operating with a single power source whose value can be between 4V and 12V (or 18V, as shown just below).It is based on the use of a type LM386 amplifier, capable alone to issue a power of a few hundred milliwatts to a load (HP) of 8 ohms, while consuming only a few mA at rest. Ideal for make a small portable battery operated amplifier. For an application in stereo, just to build this amp in two examplaires.
Different types of LM386
Before you begin, it seems worth noting that there are several types of LM386, whose output power and voltage are different. The table below outlines.| Circuit | Case | Supply voltage | Output Power (THD = 10%) |
| LM386N-1 | DIL08 (MDIP) | 4V to 12V | 325 mW / 8 ohms (6V power supply) |
| LM386N-3 | DIL08 (MDIP) | 4V to 12V | 700 mW / 8 ohms (9V power supply) |
| LM386N-4 (1) | DIL08 (MDIP) | 5V to 18V | 1 W / 32 ohms (16V power supply) |
| LM386M-1 | SOIC8 (CMS) | 4V to 12V | 325 mW / 8 ohms (6V power supply) |
| LM386MMX-1 (2) | Mini SOIC8 (CMS) | 4V to 12V | 325 mW / 8 ohms (6V power supply) |
Note (1) - The LM386-N4 is classified by some dealers as "preamp-amp", while the LM386-N3 is classified as "Amp".
Note (2) - The LM386MM-1 is obsolete and is replaced by the LM386MMX-1.
The LM386 LM386 quej'utilise are the N-3, they are standard DIL08 housing.
The diagram
The following diagram, based on the application note from National Semiconductor, reveals the simplicity promised above. It will then discuss the different values that can be given to components that revolve around the LM386. You'll be able to mount your portable little amp BF according to your own needs.
Mounting base (simplest)
The minimum installation comes down to wire the coupling capacitors C1 and C2 in addition to the LM386. All other components of the diagram may be omitted! This is called the circuit of the poor. And if the exit you will connect to the input of this circuit already has a coupling capacitor, you can even remove C1! Admit that in this case, there remains much to weld ... In this case, the gain of the LM386 is fixed by construction at 20 or 26 dB. For a higher gain, you will absolutely use pins 1 and 8, see below. Potentiometer
The potentiometer RV1 RV1, logarithmic type, used to take a more or less of the audio signal to be amplified. It is neither more nor less than a volume control. If your audio source already has a level control, you can do without this control, in retaining the coupling capacitor C1, the positive terminal will be directly connected to the input of the LM386 +.
R1 cells (Rx) / C3 (Cx) - Gain LM386
The two components R1 (Rx) and C3 (CX), connected in series between pins 1 and 8, are used to define a gain value between 20 and 200. In the absence of these components, the circuit gain is set internally to 20. The table below summarizes what to do depending on the desired gain.
| Desired voltage gain | Wiring required between pin 1 and 8 of the LM386 |
| 20 (26 dB) | Do not connect anything to pins 1 and 8 (leave them in the air) |
| 20-200 (26 dB to 46 dB) | Resistor R1 in series with capacitor C3 between pins 1 and 8 |
| 50 (34 dB) | R1 (Rx) and C3 = 1K2 (Cx) = 10uF |
| 200 (46 dB) | Single capacitor between pins 1 and 8 - C3 (Cx) and R1 = 10uF (Rx) = 0 |
Reminder: a gain of 20 dB corresponds to a voltage amplification of 10, and a gain of 6 dB corresponds to a voltage amplification of 2. Gain (20 dB + 6 dB) corresponds to a voltage amplification (10 x 2).
Note: It is also possible to vary the gain with a resistor placed between Pin 1 and ground. Knowing this, one can consider using the drain-source resistance of a field effect transistor (FET) for receiving a gain control from a DC voltage.
Cell C5 / R2
These two components, connected in series between pin 5 and ground, help maintain good stability of the amplifier at high frequency, offsetting the higher impedance of HP at these frequencies. It is strongly recommended to install these components, even if it is not mandatory.Capacitor C4 (Cy) - Bypass
This capacitor is optional and can improve the rate of rejection of food. With a typical value of 10uF, the rejection rate reaches 50dB.
Bass Boost
Because of their small size, small HP suffer from a lack of low apparent. By inserting a resistance of 10K with a 33n capacitor in series between pins 1 and 5 of the LM386, you accentuate the bass of about 5dB to 6dB, which will get the impression that "something more substantial ". This solution is no miracle with crisp "correct" low volume. However, it may give poorer results if the HP is really small and if you push the volume too much. You decide what you prefer (it is best to test).The printed circuit
Artwork
 | ||
| inverted |
Various remarks
- The LM386 is relatively sensitive to AM transmitters. When used with high gain, there is a risk of detection that can lead to hear a radio broadcast. If this happens, put a choke and a capacitor input to address them.
- The addition of a 470uF capacitor 100uF to the supply terminals (terminals 4 and 6) is recommended if the power supply cable is to long, or if you use a battery (the audio will be made better with the capacitor place when the battery starts to wear out).
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