Description
This unit 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
mic)
and a switch. Current drain is low, since the circuit only uses one
dual opamp. There is no need for a high quality device, and a 1458
is all that is needed.
Circuit Diagram
The first stage is the oscillator itself. 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 don't understand why it is not
more common.
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 didn't 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 log pot. Maximum output into a typical microphone input will be
about 100mV (unloaded oscillator output on mine was 140mV).
Not much to it - the whole circuit can be built on a small piece
of veroboard, and the battery, pot and XLR connector will take up far
more room than the oscillator. There is no LED indicator for power, as
this would draw more current than the circuit. To prevent accidentally
turning it on, a slide switch is suggested. They are a pig to mount
compared to a toggle switch, but are much less easily bumped. If you
can get a pot with a switch, this would be even better, but these are
now hard to get - especially as linear.
Rod Elliott (ESP)
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