Or, how to make a final LF with a handful of powerful components. The circuit uses a monolithic integrated from National, capable of delivering in the spikes, a power of at least 150 watts!
This seems impossible and yet it is indeed so: the tiny concentration of technology to 11-pin called LM3886 is capable of delivering a power of 50-60 watts continuous audio, with a fidelity that has nothing to envy amplifiers Trade (simply because many "compact" on the market, using this integrated!).
With this microcontroller from National, we achieved the ultimate power described in these pages. The possibilities are many. It can be used both in the field of hi-fi for sound diffusion (amplifiers megaphones, musical instruments, etc..). His
particularly small dimensions permit the construction of very compact devices, even if, as always in these cases must take account of food. On
4 ohms, with a dual supply of 28 volts, the amplifier delivers 60 watts. With the same diet, but into 8 ohms, its power will fall to 30 watts. In this case, increase the voltage to 35 volts for 50 watts. The maximum supply voltage that can support the integrated is 84 volts (42 V double).
To function, the integrated circuit needs very few other components, all liabilities. The heat sink deserves a break. Despite the progress, performance power amplifiers are still around 60 to 65%. However, if as in our case, the available power is very important, the amount of heat produced is also remarkable to prevent the amp does "grid", it is necessary to disperse this energy. Here comes in the radiator, whose dimensions should be proportional to the heat to be removed. In the case of mono amp 60 watts, the radiator has to dissipate about 25 watts of heat without producing any harmful thermal rise. To fulfill this task, the sink must have a thermal resistance between 1 and 1.5 ° C / W. An element having Such features can certainly not be small. We further deepen this aspect.
Study schema
Let us now diagram, which, as shown by the illustrations, very simple.
The LM3886, with a few passive components, is used as a non-inverting amplifier with dual power supply.
Pins 1 and 5 are connected to the positive branch, while pin 4 is connected to the negative branch. The audio signal to be amplified is applied to pin 10 of integrated circuit (input noncommutative) through trimmer R1 which allows to adjust the level. We have no plans to bypass capacitor for maximum performance.
However, if the signal to be amplified comprises a DC component, unfiltered in the previous stage, the use of the capacitor becomes essential. The capacitor C3
limit the bandwidth slightly, warns of the danger of self-oscillations and, more importantly, eliminates interference from electromagnetic nature of the electrical network (mainly noise generated by the lighting of lamps, appliances, etc.). . The gain
voltage of the amplifier depends on the ratio between resistors R3 and R4. With the values used in our circuit, the gain in the center of the strip is about 21.
This data is used to calculate the sensitivity of the circuit but first must obtain the rms value of the sine wave output at maximum power.
In the version with output impedance of 4 ohms, the effective value of the sinusoid is about 16 volts. It is obtained by the square root of P x R (P is the maximum output power and R value of the load). The gain of the circuit is 21, to obtain maximum power output, it is necessary to apply to the input of the amplifier, a signal of about 750 mV rms (16 V: 21).
Figure 1: Schematics of the internal integrated circuit, in the version with single power supply (left) and dual power supply (right). 
Figure 2: Pinout LM3886. LM3886, main characteristics and internal schemas.
The Integrated at National, used to make our amplifier is capable of delivering a power of 60 watts continuous on a 4 ohm load with a dual supply of 28 volts. With an 8 ohm load, maximum power is 50 watts (with a dual voltage of 35 volts). Other benefits of this circuit are quite respectable for these dimensions: bandwidth between 20 and 20 000 Hz, SNR better than 92 dB, distortion less than 0.03%. The integrated features many protections (against short-circuit output against overvoltage due to the inductive component of the charge against the excessive rise in temperature) that make it virtually indestructible, while allowing its use in many applications.
Figure 3: Dimensions of the LM3886. Technical
Power Output (4 Ω / 28 V) 60 W
Output (8 Ω / 35 V) 50 W peak instantaneous power
150 W
Bandwidth 20 to 20 000 Hz
Signal to Noise Ratio (1 W) 92 dB THD
0.03% Input Sensitivity 750 mV
Voltage (4 Ω) ± 28 V supply voltage
, (8 Ω) ± 35 V
Figure 4: Diagram of the amplifier 60 W. 
Figure 5: Diagram of location module 60 watts of power. 
Figure 6: PCB level 1. Iist
R1: 1O kΩ trimmer
R2: 1 kilohm
R3: 1 kilohm
A4: 22 kW
R5 R6
22 kW: 22 kW
R7: 2.7 Ω
R8: 10 Ω
C1: 47 uF 50 V
C2: 47 uF 50 V
C3: 220 pF ceram.
C4 10 uF 35 V tantalum
C5: 47 uF 50 V
C6: 47 uF ceram.
C7: 100 nF multilayer
U1: LM3886
L1: see text
S1: Switch
Various: terminal 5 holds
Terminal 2 Radiator taken
ML33 PCB ref. E51
View mounted amplifier. Circuit Analysis
The capacitor C4 and the network R5/C6, limit the gain of the amplifier compared to low and high frequencies.
In the first case, the impedance of C4 increases when the working frequency decreases. C4 is connected in series with R3, the voltage gain of the amplifier decreases in proportion, down by limiting the bandwidth of the circuit. And conversely, as regards the operation of the network R5/C6, connected in parallel to the resistance R4. In this case, when the frequency increases, the impedance of C6 decreases, thus lowering, also, the value of the resistor R4. In this way, the gain of high frequencies is reduced, limiting the bandwidth up.
We entrust to the S1 switch the mute function. when S1 is closed, the amplifier immediately becomes silent. The role of self-L1 is to limit the effects of the capacitive component of the load, which are felt in a special way to higher frequencies.
Now, before concluding the analysis of the circuit, we would like to briefly occupy the protections features the integrated indicating, above all, the presence of "under-voltage protection," whose role is to avoid the "coup" on the loudspeakers, as well as the ignition off.
The output is also protected against short circuits and overloads.
A particular stage acts by blocking the amplifiers whenever the current exceeds 11 amps.
The integrated circuit is protected against overheating by two floors.
The first is called "Advanced Protection" (Self Istantaneous Peak Temperature ° Ke), and its role is to intervene cons elevations thermal transistor amplifiers, while the second protection circuit operates on the entire integrated circuit by stopping the operation when the temperature exceeds 165 ° C.
The construction of this amplifier is very simple. As can be seen in the illustrations, all components are assembled on a circuit whose dimensions are 40 x 65 mm!
The only component to be achieved by the inductor L1 is YOURSELF. It consists of ten turns of enameled copper wire wound up in the air. The wire used shall have a diameter of about 1 millimeter, while than the inner diameter of the coil can be between 6 and 10 millimeters. To this end, a tail will drill through the case perfectly.
For maximum power, it is necessary to use a proper diet, which in the simplest version, can be composed of a transformer with center tap, a bridge and two electrolytic capacitors of 10,000 uF. The processor must be able to charge a power of 2 x 20 volts if the receiver is used at 4 ohms and 2 x 25 volts into 8 ohms. The transformer for the stereo version, must be able to provide 200 watts in the first case and 150 watt in the second. Toroidal transformers with these features are available from our advertisers.
The above considerations also apply to the heat sink.
If we want to work the circuit at maximum power, it is necessary to equip each module of a sink with a thermal resistance of 1 to 1.5 ° C / W. The radiator type ML33, which we used for testing (see photo), can operate with a maximum power of 15 to 20 watts. It is possible to fix without problem on the same heatsink, two or more modules, since the metal housing of the LM3886 is isolated from its circuit mail.
All that remains now is connect the amp to food, entry to amplify the signal and its output to the speakers.
If the assembly has been completed without error, the circuit will work immediately.
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