Microphone Circuit Test Oscillator

This circuit would be mounted in a small plastic or preferably metal box, with a 9V battery, level control, a male XLR connector (same as on a microphone) and a switch. Current drain is low, since the circuit only uses one dual op amp. There is no need for a high quality device, and a 1458 is all that is needed. The first stage is the oscillator i
Microphone Circuit Test Oscillator - schematic

tself. This is a simple three stage phase shift oscillator - a circuit that is remarkably uncommon - which is to say I have never seen it used elsewhere. I designed it for another project a few years ago, and I dont understand why it is not in any op amp application notes. Maybe I invented a new circuit. If you want to tune it, you can use a 50k pot instead of R1. I suggest that if tuned, set it to A-440 Hz. Frequency stability is not wonderful, and it changes by a few Hertz as the battery discharges, but this is unlikely to cause problems - it is a test oscillator, not a tuning standard. As shown, frequency will be about 430Hz, depending on the accuracy of the capacitors. The phase shift network (R1-C1, R2-C2 and R3-C3) serves two purposes. First (and for an oscillator, most importantly), it shifts the phase of the output signal so the feedback is positive, causing oscillation. Secondly, since it is a three stage filter, it attenuates the signal and filters the output square wave so the signal at pin 2 is a reasonable sine wave. Distortion (if you really care) is about 3% or so - I didnt measure it this time, but I recall having done so before. The second stage is the output buffer, and the signal is simply split to supply the two mic leads. The metal case should be connected to pin 1 (earth) on the XLR connector. The output level control must be a linear type, as the circuit loading will create a good approximation to a...

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