LM 3875

Description: The National Semiconductor's Overture series of integrated circuit (IC) audio amplifiers gained recognition due to the success of the 47 Laboratory's Gaincard amplifier, which utilizes the LM3875 chip from National Semiconductor. The Gaincard's simple design, low-cost components, and excellent quality-to-price ratio contributed to its popularity among audio DIY enthusiasts. Its minimal parts count allowed for a compact and customizable setup, which is evident from numerous online resources dedicated to audio DIY and Gaincard clones, often referred to as "gainclones" or "chipamps." This article discusses a reliable low-power amplifier (30W) using the LM3875, originally designed for active speaker systems. The amplifier schematic features a standard non-inverting configuration, where the output signal aligns with the input signal. The chipamp's voltage gain is determined by resistors R3 and RF, while the input impedance is set by resistor R1. If a volume control potentiometer is present, R1 can be omitted, and the input impedance will depend on the potentiometer's resistance (typically ranging from 10 to 100kΩ). Capacitor C5 serves to limit the DC voltage gain to AvDC=1, mitigating potential DC offset voltage at the amplifier's output, which could adversely affect both the speakers and the amplifier. If there is no risk of DC offset, C5 may be omitted or shorted. Capacitors C1 to C4 filter ripple voltage from the rectifier, while resistor R4 at the amplifier's output enhances stability and assists in load distribution between parallel amplifiers. R4 can usually be omitted, with optimal performance determined experimentally. The power supply PCB is depicted in both pre-assembly and post-assembly images. The transformer specifications are P=100VA and Vsec=2x24V, providing adequate voltage for driving two channels of a 30-Watt active speaker system. The aluminum chassis serves as the heat sink, although improved cooling may be necessary for higher power output. This project is straightforward to assemble, requiring basic electronics knowledge, especially when using commercially available prefabricated PCBs. Proper internal wiring is crucial, starting with power supply and grounding wires, with particular attention to high-voltage wiring using well-insulated materials. All circuit grounds should converge at a single point (star ground) at the power supply PCB output to prevent ground loops, which can introduce noise and hum in audio amplifiers. Including a ground lift switch on the amplifier's rear can help mitigate hum from external ground loops by allowing the user to select the optimal switch position for reduced noise.

The Overture series amplifiers, specifically those utilizing the LM3875 chip, exemplify a practical approach to audio amplification with a focus on simplicity and performance. The non-inverting configuration ensures that the output signal remains in phase with the input, which is critical for maintaining audio fidelity. The design's reliance on a limited number of components not only minimizes costs but also facilitates ease of assembly, making it accessible for hobbyists and professionals alike.

The voltage gain configuration, dictated by resistors R3 and RF, allows for flexibility in adjusting amplification levels according to specific application needs. The careful selection of the input impedance via R1 or the volume control potentiometer ensures compatibility with a variety of audio sources, enhancing the versatility of the amplifier.

Capacitor C5’s role in mitigating DC offset is particularly significant in protecting both the amplifier and connected speakers from potential damage caused by unintended DC voltages. The inclusion of capacitors C1 to C4 for ripple voltage filtration is essential in ensuring a clean power supply to the amplifier, which directly influences the overall sound quality.

The output resistor R4, while optional, can enhance stability in certain configurations, particularly when multiple amplifiers are used in parallel. The design encourages experimentation, allowing users to optimize performance based on their specific setup and requirements.

The choice of a transformer with a power rating of 100VA and dual secondary windings of 24V ensures that the amplifier can deliver adequate power for a 30-Watt speaker system, while the aluminum chassis provides passive cooling. This design consideration is crucial, as overheating can lead to performance degradation and potential failure.

Overall, the Overture series and its implementation of the LM3875 chip represent a significant contribution to the field of audio amplification, offering a balance of performance, cost-effectiveness, and user-friendliness that continues to resonate with audio enthusiasts and DIY builders.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 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 exists, capacitor C5 can be omitted or shorted, as desired.

Capacitors C1 to C4 are used for the filtration of the ripple voltage from the rectifier. Resistor R4 at the output of the amplifier improves stability and, in parallel circuits, helps to split the load between two amplifiers. This resistor can also be omitted in most cases, which is best determined experimentally. The power supply PCB, both before and after assembly, is shown in Figures 6 and 7, respectively. The characteristics of the transformer used in this amplifier are P=100VA and Vsec=2x24V, providing a voltage sufficient to drive two channels of a 30-Watt active speaker system.

There are no dedicated heat sinks here because the entire aluminium chassis, which houses the amplifier, serves as a heat sink (Figure 7). However, if you decide that you wish to squeeze more power from the amplifier, you will surely need better cooling.

This project is simple to build and requires only basic knowledge of electronics, especially if prefabricated (commercially available) PCBs with silk screens are used. The most important thing is to make sure that all internal wiring is properly done. You may wish to start with the power supply and grounding wires first and pay extra special attention to the high-voltage(!) wiring use well insulated wires and insulation jackets and do not forget to ground the amplifier chassis (if it is metal).

All circuit grounds should meet at only one point (i. e. star ground) centered at the output of the power supply PCB (see the schematic below). Star-grounding will help you to avoid ground loops, which may lead to severe problems with hum and noise in audio amplifiers. It is not a bad idea to add a switch on the rear of the amplifier, whereby the star (i. e. audio) ground could be connected to or disconnected from the chassis (i. e. safety) ground, as needed. The switch (i. e. ground lift switch) may help you to fight hum caused by external ground loops (between different devices in the system) by simply selecting the switch position with less (or no) hum and noise.

To connect the output of the amplifier to the speaker output te