audio white noise generator

  
This is a circuit that generates white noise, rolled-off to drive earphones or a small speaker. White noise creates is a `rushing` sound, which sounds something like air rushing by your ear(s). White noise would be flat with frequency, and since this circuit rolls off within the audio range, I refer to it as `rolled-off` noise. White (or rolled-off) noise is said to be useful in helping people improve concentration, to make a `noise wall` to improve privacy, to fill empty spaces with sound so they don't feel so empty (done a lot in larger office buildings) and has been touted as a therapy for Tinnitus (ringing in the ears).
audio white noise generator - schematic

Q2 is a grounded emitter (inverting) amplifier with Q1 in its feedback path. Notice that Q1 is connected kind of "upside down" in that the emitter is positive with respect to its base, so that the junction avalanches, dropping about 5 to 6 volts across the junction. The purpose of Q2 is to provide just enough bias current to make the emitter-base junction of Q1 break down, and under this condition, there is a millivolt or two of broadband noise across the Q2 junction as well. Note that the collector of Q1 is not connected to anything. In the July 2003 version, the collector of Q1 was connected to the base of Q2, but I have since learned that some transistors exhibit negative impedance characteristics when connected in this manner, so the circuit has been revised to take the current signal from the base instead of the collector. A1 is a voltage follower that buffers the 50% of the battery voltage derived by the two 470k resistors connected to its non inverting input. The 50% voltage is used as a DC bias supply for the other three amplifiers on the chip. A2 and A3 provide a gain of 50X each, for a total gain of 2500X. The plot of open loop gain vs frequency for the LM324 shows the bandwidth intercepting the 50X gain point at about 5 kHz, so this filtering provides some "coloration" of the noise. Your choice of headphones or speakers will also color the signal. You can get back some of the bandwidth by putting a small...



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