Engineer and maker Fernando Filippetti has designed a “revolutionary” pre-amp for three-pin electret microphones, promising a low value mixed with help for prime sound strain ranges (SPL) with out distortion — utilizing an op-amp in a cascade-style circuit.
“In my years as [an] engineering scholar I [had] a good friend, additionally [a] scholar of electronics, who has a dream of develop into a movie maker,” Filippetti explains by means of background. “He was a really good individual and good [at] math, however ultimately he fell off to the temptation [to] pursue [his] desires. As consequence of [his choices] in life, he confronted two penalties: he [is] in poor financial form; he wants costly {hardware}.”
A low-cost pre-amp circuit, designed like a transistor cascade, could make an electret microphone sing. (📷: Fernando Filippetti)
To assist his good friend out, Filippetti determined to look into methods to enhance the audio high quality — and particularly the sound strain degree (SPL) restrict, which controls the quantity at which distortion happens — in low-cost electret microphones, however with out breaking the financial institution.
“The design [chosen] is sort of a op-amp model of the well-known [transistor] cascade circuit, with the op-amp giving low impedance to 1 facet (near zero) and excessive voltage acquire, the identical we get with the high-side cascade transistor however a lot better,” Filippetti explains of the design he got here up with.
“One other enchancment is the addition of [an] output voltage limiter in a type of two an identical diodes in parallel with the suggestions resistor, which protects the entire system from leaving closed loop in case of an enter overload. Utilizing LEDs for this function supplies visible suggestions of the overloading situation. To additional scale back the distortion and make a negligible enhance in noise efficiency, using [an Analog Devices] AD797 [op-amp] is really useful.”
Each artificial and real-world testing confirmed a giant enchancment in audio high quality with the pre-amp. (📷: Fernando Filippetti)
In real-world testing, Filippetti’s pre-amp circuit confirmed a serious enchancment in SPL and “astonishing low distortion” for an electret microphone. “Pushing the mic to the restrict,” the maker notes, “offers a mushy, symmetrical saturation attribute suitable with transducer saturation.”
The complete undertaking write-up, with a hand-drawn schematic for anybody eager to attempt it out for themselves, is offered on Filippetti’s web site.
