Gainclone amplifier circuit,stereo 40W using LM3875

This is gainclone amplifier circuit that sound quality is very good. We use a IC as main parts is

LM3875, produced by the National Semiconductor, IC is the highest power amplifier 56watts at 8 ohms speaker, As well as IC amplifier Gain Card that very famous.

General Features of the LM3875
IIt is less than 0.03% THD + N distortion in the audio frequency range 20 Hz-20KHz with DC voltage from +/- 20V to 40V. The IC include circuitry to block a lot, for example, prevent a spike or block the signal of the power circuit, short circuit protection and output over voltage protection circuit, out of voltage or overload. So it is almost a complete IC without additional circuitry, thus leading to yet more user-friendly and convenient.

It’s also a low noise IC by there is a S/N value or signal-to-noise ratio higher up than 95dB (min)

According to Figure 1 Appearance

and the inside circuit as Figure 2

The pins using  of LM3875
Figure 1 The pins using of LM3875

internal circuit in LM3875
Figure 2 the internal circuit in LM3875

How it works

In Figure 3 This project is non inverting amplifier circuit. Most devices within the IC almost all. There are some little further : the power supply, network at output speaker and fuse to protect the spike signal of external devices Each one has the following function.

the schematic diagram
Figure 3 the schematic diagram

The left-channel audio signal is connected to the input, to resistor R2 act as set the impedance of circuit is 33K through the coupling capacitor C2. The signal into the ultra sonic filter circuit, which include of capacitor C3 and resistor R3 and R1 To act to prevent the radio noise signal and attenuate RF signal is lower, before the signal will into pin 7 thus this have high gain up to 25 times. The gain derived from the input signal back to input through both resistors R5 and R6 is calculated by the equation (1 + R5 / R6).

The capacitor C5 is connected to R6 help lower frequency to -3dB at frequency about 10Hz. Sound signal is amplified and then come out the pin 3 of IC. Before leaving Speaker shall pass to the zobel network circuit that include of R4 and C4 To solve the problem with the load capacity and maintain stability at high frequency.

For the power supply Transformer reduced voltage AC power from high-voltage 220VAC into lower voltage at low voltage 24-0-24 VAC current 5A. This low voltage is sent to the bridge rectifier BD1 to approximately + 31V DC power adapter with a capacitor C13 and C14 filter the voltage smoothing, and The DC voltage at this time. Is used as a supply voltage to the IC power supply or amplifier circuit.

How to builds
This projects is easy to builds because to adjust anything and few parts as Figure 4 and Figure 5

PCB layout
Figure 4 the PCB layout

components layout
Figure 5 the components layout

This projects have the Frequency response and the total harmonic distortion : THD is lowest, as Figure 6 very good Frequency response.
very good Frequency response

You can buy this IC here

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  1. LevHyp May 19, 2015
  2. Salim Khan May 22, 2015

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