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Build a
Microphone Polarity Tester

by Ethan Winer

This article is from the May 1997
issue of Recording
magazine

Many things can go wrong in a modern
recording studio, but few are as difficult to track down as reversed microphone
polarity. When a microphone is placed in front of a sound source, a positive
pressure on its diaphragm caused by forward-moving air results in a positive
voltage on one of its output pins and a negative voltage on the other. But which
pin is positive? These days most microphones have XLR output connectors, with
Pin 2 used for the positive voltage output and Pin 3 for the negative. But you
can't always count on this, especially if the microphone is an older model, or
was ever repaired or otherwise rewired.

When a single microphone is used to capture
an instrument or voice, the absolute polarity of its output doesn't really
matter. But it is common to use two microphones, especially with larger
instruments such as a piano or maybe a small string section. If you're using a
pair of identical microphones it isn't likely they'll be wired differently.
However, many engineers like to use two different types of microphones, for
example when recording an acoustic guitar or perhaps a cello. One common
technique is to place one of the mikes relatively close to the instrument and
the other several feet away. Another method places both microphones close to the
sound source, but with each aimed at different places on the
instrument.

When two microphones are used to record a
single sound source it is imperative that they respond to a forward pressure on
their diaphragms in the same way. If the polarity of one of the mikes is
reversed, the tone will become very thin and lack bass when the two outputs are
mixed together. Even if the mikes are panned hard left and right for a stereo
effect, if their polarity is not consistent the sound will appear to be
unfocused and vague, and difficult to localize in the stereo sound
field.

It's a simple matter to determine the
absolute polarity of most studio equipment using an oscilloscope. By feeding a
pulse wave into the device and monitoring the input and output signals
simultaneously, you can quickly see if the polarity is the same or reversed.
Unfortunately, there is no easy way to know the absolute output polarity of a
microphone. To solve this problem I designed the microphone polarity tester
described here. This handy gadget is comprised of two components: a "pulser"
that sends a forward-moving pulse through a small loudspeaker, and a detector
circuit that determines which of the microphone's output pins presents a
positive voltage in response to that test pulse. You simply aim the microphone
at the tester's built-in speaker, and press the Test button. One of the LED
indicators corresponding to Pin 2 or Pin 3 then lights up to indicate which
output pin is positive.

Battery operation makes the tester portable
and nearly pocket size, and you can optionally add a pair of 22-1/2 volt camera
batteries to provide phantom power for condenser microphones. These batteries
are about the size of a standard AA cell, and they will last for years since
they are used for only five or ten seconds at a time. Without the extra
batteries you will need an outboard power supply to test condenser mikes that
require phantom power.

HOW IT
WORKS

When the push-button switch is pressed, a
brief voltage pulse is applied to a small loudspeaker which is used as the
"sender." By connecting the speaker's Plus terminal (marked with a red dot or
"+" symbol) as shown in the schematic in Figure 2, the speaker diaphragm will
jump forward creating an increase in pressure in front of the mike being tested.
The microphone's output pins are routed to a detector circuit that remembers
which pin went positive first. It's not sufficient to merely read which pin goes
positive, because all mechanical devices continue to vibrate, or ring, after the
application of a single pulse. Since a certain amount of ringing is inevitable
in all loudspeakers and microphones--even very expensive models--the mike under
test will respond to the speaker's pulse by outputting several complete voltage
cycles. Thus, the tester must determine not only which output pin went
positive, but which went positive first. A digital "lock-out" circuit disables
the second LED once the first one has lit.

CONSTRUCTION

All of the parts used in this project are
inexpensive and readily available, and I have provided Radio Shack part numbers
for everything. If you already have a supply of common resistors, capacitors and
other assorted components, the total parts cost should less than $25. Because
the circuitry is fairly simple (see Figure 1), I hand-wired the various parts on
a piece of perforated construction board. Each component's wires are pushed
through the pre-drilled holes, and those wires are then routed to and wrapped
around the other component leads according to the schematic. This is shown in
the accompanying photograph.

You may want to buy a slightly larger case
than the one specified, especially if you haven't built many electronic projects
before. The small case I used requires that you pack the various components
fairly tightly. Further, if you decide to add the optional phantom power
batteries as shown in Figure 3, you will have to use a larger enclosure. You
should attach the speaker to the plastic case using a thin bead of contact (not
rubber) cement applied to both the speaker's gasket and the inside of the case.
Be sure to let the cement dry for at least five minutes before pushing the
pieces together. And, of course, you must drill 1/8th inch sound holes in the
case before mounting the speaker!

CHECKOUT AND
USE

When the power switch is first turned on, one
of the LEDs will light up for a few seconds and then go out after the capacitors
charge and the voltages have stabilized. If you have a phantom supply connected
or are using batteries for that, it will take a bit longer for the LED to go
out. Plug in the microphone to be tested and aim it at the tester's speaker
getting as close as necessary to obtain an indication. Most dynamic microphones
must be within an inch or two of the speaker; many condenser models have a
higher output level and don't need to be that close. Be assured, however, that
the tester will never give a wrong answer (assuming you built it correctly),
since neither LED will light if the mike isn't close enough.

PARTS
LIST

Plastic case, 4-3/4" x 2-9/16" x 1-9/16"; RS#
270-222
Perforated circuit board, 0.1" grid spacing; RS# 276-1395
2-1/4"
miniature speaker; RS# 40-246
9-volt rectangular battery; RS# 23-553

9-volt battery clip with wires; RS# 270-324
XLR connector, 3-pin female; RS#
274-013
Push-button switch, momentary SPST; RS# 275-618
Miniature toggle
switch; RS# 275-634
2 pcs. 470 ohm 1/4 watt 5% resistor; RS# 271-1317
2
pcs. 1K 1/4 watt 5% resistor; RS# 271-1321
1 pc. 10K 1/4 watt 5% resistor;
RS# 271-1335
1 pc. 270K 1/4 watt 5% resistor; RS# RSU-11345337
2 pcs. 470K
1/4 watt 5% resistor; RS# 271-1354
2 pcs. 10M 1/4 watt 5% resistor; RS#
271-1365
2 pcs. 0.1 uF. mylar capacitor; RS# 272-1069
2 pcs. 10 uF.
50-volt non-polar electrolytic capacitor; RS# 272-999
3 pcs. 22 uF. 16-volt
electrolytic capacitor; RS# RSU-11296860
2 pcs. 1N914 silicon diodes; RS#
276-1122
2 pcs. standard size (1/4 inch) LED; RS# 276-066
2 pcs. LED
mounting clip; RS# 276-079
1 pc. 1458 dual op-amp; RS# 276-038
1 pc. 4011
CMOS quad gate; RS# 276-2411

For optional phantom
powering:

2 pcs. 22-1/2 volt battery; RS# 23-510
2
pcs. 6.8K resistor; RS# RSU-11345030
Use toggle switch RS# 275-636 instead of
275-634

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by Ethan Winer. All rights reserved.