Protect Your Microphone Pre-Amp Against Overload

Typically, microphone pre-amplifiers for consumer electronics are designed in one of two ways. In Figure 1a, the microphone gain is set high to accommodate low-level signals and the output is attenuated as needed. It has an optimum transfer function (Fig. 2, "Optimum"), but it can be overloaded by loud signals. In Figure 1b, the microphone gain is adjusted to accommodate the actual signal level, which solves the signal-to-noise and headroom issues. The output, however, can't be completely turned off. Its transfer function is shown in Figure 2, "Gain trim only."

A "best of both worlds" solution can be accomplished using a dual log/antilog pot, such as the Panasonic EVJ-Y series, typically Digikey part number P2F0502-ND (www.digikey.com). The antilog portion serves as the gain trim, and the log portion is used as the output attenuator. In practice, the output attenuator only affects the lowest 20% of rotation. This effectively produces the output transfer curve of Figure 2, Optimum. Figure 3 shows a typical circuit based on the INA217 from Texas Instruments (www.ti.com).