The IC is manufactured by Texas Instruments and can be expected to produce a good 60 watts of power output on a 8 Ohm speaker.
General Features of the LM3875
- The general features of the IC LM3875 can be understood from the following points:
- Total Harmonic Distortion (THD) less than 0.03% within trhe frequency range of 20Hz to 20kHz and supply range of +/-40V.
- Internal protection from voltage spikes, short circuit, and output overload conditions.
- Low Noise output specifically within 95dB
From the figure we can see that the input signal is connected to the resistor R2, which sets the input impedance of the amplifier at 33K along with the coupling capacitor C2.
The input stage comprises of an ultrasonic filter circuit stage made up C3, R3 and R1 which suppresses any entry of noise through a low RF attenuation.
After this the signal is allowed to pass to pin#7 of the IC enabling a 25 times gain for the signal. This gain is sustained by feeding it back to the input pinout via R5 and R6 and is calculated by the formula (1 + R5/R6)
A frequency cut of -3dB below 10Hz is achieved via C5 which may be seen attached with R5.
The amplified music input is delivered from pin#3 which is ultimately fed to the loudspeaker for the amplified sound generation, however before this can happen, the signal has to pass through a network of R4 and C4 which prevents the amplifier from getting unstable during full loads and thus ensures a continuous stable output under full volume.
The power supply employs a 24-0-24V 5 Amp transformer which is rectified by the indicated bridge rectifier module, and is filtered using C13 and C14.