LM 3875

Posted on Feb 7, 2014

The National Semiconductor`s Overture series of integrated circuit (IC) audio amplifiers owes its rise to `fame and glory` to a relatively big success of the 47 Laboratory`s Gaincard amplifier, based on the LM3875 chip from the National Semiconductor. Due to the simplicity of the Gaincard design and construction, availability of (inexpensive) part

LM 3875
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s, as well as excellent value for money in terms of its quality and power, it soon became very popular and highly regarded among audio DIY hobbyists (do-it-yourselfers or DIYers). An extremely low parts count also allowed for a very compact and customizable design, as evidenced by numerous websites, fora and blogs dedicated to audio DIY and Gaincard clones (aptly named `gainclones` or `chipamps`). This article focuses on a tried and true low-power amplifier (30W) with the IC LM3875, originally intended for the design and testing of active speaker systems. The amplifier schematic is shown in Figure 1. It is a standard non-inverting circuit, where the output signal is in phase with the input signal. The voltage gain of the chipamp is determined by resistors R3 and RF, according to the following equation: The input impedance of the amplifier is determined by resistor R1. However, if your amplifier has a volume control potentiometer, as this one does, the resistor can be omitted. The input impedance of the amplifier is then determined by the value of the potentiometer resistance (normally ranging from 10 to 100k ©). Capacitor C5 is used to reduce the gain of the DC voltage amplifier to AvDC=1. This reduces the risk of DC offset voltage appearing at the output of the amplifier, which may have an adverse affect on the operation of the speakers, as well as the amplifier. However, if you are absolutely sure that no such risk...

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