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 Elliott Sound Products Project 50 

Microphone Circuit Test Oscillator
Rod Elliott (ESP)

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So, a reader sends me an e-mail, and says ....

Say, I am looking for a tone generator schematic.  Specifically, one that is mounted on an "XLR" style plug, to test mic lines.  These are great when installing multiple mic lines, to sort out which one is which; you can test the lines with only one person, too!  Do you have one?  It has got to be cheaper than buying one, and they can't be that difficult to build, can they?  Thanks!

Well, it turns out I don't have one, but I could see the circuit in my head as I wrote the reply.  Using my trusty opamp test board (see Project 41), I whipped it up in about 10 minutes.  And here it is ....


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 an LM358 is all that is needed. You can also use a 1458 (or 4558 which is somewhat better but still cheap). These will all work fine for this application, but the LM358 is low power, and guarantee that the circuit will still function if the battery starts to go flat.

Figure 1
Figure 1 - Mic Circuit Test Oscillator

The first stage is the oscillator itself.  This is a simple opamp based three stage phase shift oscillator - a circuit that was remarkably uncommon when this project was first published. At the time, I had never seen it used elsewhere.  I designed it for another project a few years ago, and I never understand at the time why it was not more common. Phase shift oscillators using valves and transistors have been used in their millions, but not so for opamps. Fast forward 16 years or so and the situation is very different.

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 low pass 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.

NOTE Make sure that you do not connect any of the internal circuit to the case.  As shown, the unit will be quite happy on phantom powered (48V) mic lines - this will not be true if you connect to the case.


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Copyright Notice. This article, including but not limited to all text and diagrams, is the intellectual property of Rod Elliott, and is Copyright © 2000. Reproduction or re-publication by any means whatsoever, whether electronic, mechanical or electro- mechanical, is strictly prohibited under International Copyright laws. The author (Rod Elliott) grants the reader the right to use this information for personal use only, and further allows that one (1) copy may be made for reference while constructing the project. Commercial use is prohibited without express written authorisation from Rod Elliott.
Page Created and Copyright (c) 24 Jan 2000