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Low-Voltage Audio Products: Rail-to-Rail I/O & Balanced Amplifiers

This third article in a multi-part series discusses some of the challenges associated with using low-voltage audio information appliances.
This article is provided by Rane Corporation.

This is the third in a multi-part series. Additional segments are available here.

Single-Supply RRIO (Rail-to-Rail Input & Output) Audio Op Amps

Interestingly, the term “rail-to-rail” is a registered trademark of Nippon Motorola, Ltd. and can only be used by them; hence the rise and popularity of the expanded term ” RRIO.”

The marriage of analog and digital circuits created the need for RRIO op amps. Since digital circuits must run off low-voltage supplies (for speed and power reasons) the demand for analog chips to do the same was inevitable.

A RRIO op amp is just what the name implies, one where the maximum input and output levels equal the available power supply voltage. Up to the introduction of RRIO op amps, both input and output levels were restricted to something less (sometimes considerably less) than the supply. See Hahn for specific design detail.

Running an audio op amp off a single AA cell used to be a joke; today it is a reality. One incredible example is ON Semiconductor’s (formerly a division of Motorola) MC33501 called “SmartMOS”.

If you absolutely must run from one cell, this is a pretty good choice—and there aren’t many. Featuring a typical slew rate of 3 V/microsecond (min 1.8 V/microsecond), 10 kHz THD+N = 0.01% (although this degrades drastically for anything other than unity gain applications, e.g., it is 0.5% for a gain of x10), and a unity-gain-bandwidth of 3 MHz (min 2 MHz), this little guy is impressive, but its 1 kHz spot noise spec of 30 nV/per square-root-Hz (same as a 741 op amp), restricts its use to low gain circuits with large resistors (around 100k ohms ), otherwise it will degrade the S/N.

Once you increase your single 1.5-volt cell to something more useable, like 3-5 volts, then other audio-worthy single-supply RRIO ICs appear.

What looks like the low-noise low-voltage leader for the moment is Linear Technology’s LT1677 with a 1 kHz typical spot noise spec of 3.2 nV per square-root-Hz (max of 4.5 nV per square-root-Hz).

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Linea Research is a leading designer and manufacturer of high performance power amplifiers and DSP controllers for the entertainment, live sound, installed sound and commercial sound markets.