How To Choose an Op-Amp for an Audio Amplifier

by J.T. Barett

For decades, consumer and professional audio equipment has incorporated op-amps, components which accurately process electronic signals. Operational amplifier, or op-amps, have evolved significantly since their introduction in the 1940s; although some work well for audio, others are better suited to other electronic tasks. To upgrade an old amplifier or build a new one from scratch, you'll need to know how to select the right op-amp for the application.

Basics

An op-amp, or operational amplifier, is an integrated circuit which comes in an eight- or 16-pin package. Other than connections for power, the op-amp has two inputs and one output; one input inverts the incoming signal, converting positive to negative, and the other is non-inverting. The op-amp functions as a simple amplifier, boosting the voltage of weak signals. It forms the heart of many sophisticated audio functions, including mixers and filters. Although an op-amp increases a signal voltage, it does not produce large amounts of current, such as for driving a loudspeaker; audio amplifiers have power transistors specially designed for that purpose. An amplifier with good power, but with audible background hiss or only average high-frequency response, makes a good candidate for an op-amp upgrade. If your amplifier's sound quality is good but the unit suffers from other issues, new op-amps aren't the answer.

Upgrade and DIY

Audio boutiques offer upgrade kits that include high-performance replacement op-amps for amplifiers. These op-amps are compatible with the existing units but have better specifications. Other than for pre-1990s equipment, however, op-amp replacement seldom leads to better sound; modern amplifiers already have quality components. For hobbyists building their own amplifiers from scratch, component makers offer many inexpensive devices perfectly suited for audio.

Technical Details and Sound Quality

Op-amps, as sophisticated electronic devices, have several important operating parameters. The most basic, supply voltage, specifies the range of voltages on which the component runs. For example, an op-amp may run on a minimum of 5 volts to a maximum of 15 volts. You cannot exceed this voltage without risking damage to the part. Slew rate and power bandwidth measure the component's ability to handle high frequencies; the best audio op-amps have excellent high-frequency response, giving you clear, clean sound. Slew rate is measured in volts per unit time, such as 9 volts per microsecond. The power bandwidth is the highest frequency at which the op-amp can deliver output at full voltage. If an op-amp has a power bandwidth of 100 kHz, it can handle higher frequencies but at reduced output voltage. Good op-amps also have low noise or background hiss, which technicians measure with arcane units such as nanovolts per square-root Hertz. Don't let the units confuse you; devices with lower numbers have less noise.

Installation

Installing op-amps requires skill with electronic equipment and tools. To replace op-amps, unplug the amplifier and remove the chassis cover. In the easiest scenario, the amplifier has dual-inline package op-amps installed in sockets; you can pry the op-amps out with a flat-blade screwdriver or other small hand tool, then insert the new components into the sockets, paying careful attention to their orientation. DIP-style op-amps are the easiest to install, as their two parallel rows of pins fit neatly into the socket with little fuss. Metal "can"-type op-amps are sometimes socketed, but you have to maneuver the pins into the right socket holes. Soldered op-amps are the most difficult to replace; you must carefully desolder them before installing new ones. Because heat damages electronic components, work quickly. Unless you have experience building electronic circuits, consider hiring a technician to replace op-amps for you.

Recommended Op-Amps

The NE5534 and NE5532 op-amps have excellent performance at a very low cost. The NE5534 is a single op-amp device; the NE5532 combines two in one package. Other op-amps used in audio include the OPA2134, LME 49710 and LM6172, although these range from a few dollars to about $10 each. For beginner projects, older op-amp designs such as the LM741 and LM358 are very inexpensive and easy to use; however, their limited high-frequency response makes them unsuitable for audiophile or professional equipment.

Auditioning Op-Amps

In addition to op-amps, amplifiers contain dozens of components, each of which can make or break the listening experience. Because of the complexity of audio equipment, good sound becomes highly subjective. Where possible, try out an amplifier that has the op-amps in which you are interested and select a listening environment similar to your own. Have your favorite music on hand and listen critically to determine if you like the sound.

About the Author

Chicago native J.T. Barett has a Bachelor of Science in physics from Northeastern Illinois University and has been writing since 1991. He has contributed to "Foresight Update," a nanotechnology newsletter from the Foresight Institute. He also contributed to the book, "Nanotechnology: Molecular Speculations on Global Abundance."

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