Fernando Filippetti's Clever Pre-Amp Gets the Best Performance Possible From Cheap Electret Mics

Engineer and maker Fernando Filippetti has designed a "revolutionary" pre-amp for three-pin electret microphones, promising a low cost combined with support for high sound pressure levels (SPL) without distortion — using an op-amp in a cascade-style circuit.

"In my years as [an] engineering student I [had] a friend, also [a] student of electronics, who has a dream of become a film maker," Filippetti explains by way of background. "He was a very smart person and good [at] math, but in the end he fell off to the temptation [to] pursue [his] dreams. As consequence of [his choices] in life, he faced two consequences: he [is] in poor economic shape; he needs expensive hardware."

To help his friend out, Filippetti decided to look into ways to improve the audio quality — and in particular the sound pressure level (SPL) limit, which controls the volume at which distortion occurs — in low-cost electret microphones, but without breaking the bank.

"The design [chosen] is like a op-amp version of the well known [transistor] cascade circuit, with the op-amp giving low impedance to one side (close to zero) and high voltage gain, the same we get with the high-side cascade transistor but much better," Filippetti explains of the design he came up with.

"Another improvement is the addition of [an] output voltage limiter in a form of two identical diodes in parallel with the feedback resistor, which protects the whole system from leaving closed loop in case of an input overload. Using LEDs for this purpose provides visual feedback of the overloading condition. To further reduce the distortion and make a negligible improve in noise performance, the use of [an Analog Devices] AD797 [op-amp] is recommended."

In real-world testing, Filippetti's pre-amp circuit showed a major improvement in SPL and "astonishing low distortion" for an electret microphone. "Pushing the mic to the limit," the maker notes, "gives a soft, symmetrical saturation characteristic compatible with transducer saturation."

The full project write-up, with a hand-drawn schematic for anyone wanting to try it out for themselves, is available on Filippetti's website.