Originally Web posted 8 July 1996.
Content last modified
Friday, 25 January 2008 .

Easy Test and Measurement of Audio Polarity

Sonic
Purity—Independent Broadcast Engineer— formerly of Albany,
CA

In the last
article, we explored the desirability of maintaining absolute polarity
(a.k.a. absolute phase) of audio signals from acoustic source to acoustic
reproduction, and the likelihood that such is not necessarily the case,
especially where those pesky head telephones[1]
are part of the process. While quite a bit of discussion ensued regarding lack
of standardization and optimum points in a system to correct polarity, there was
little mention of testing and verification of actual polarity at a given point
in a given system, nor relevant test equipment. Let us now fill that void…

First Generation: The Cosky Phaser

You may be disappointed to learn (as was i) that the human mouth (at least
the author’s) is an inconsistent generator of pressure waves, especially when
one does not wish to engage in destructive testing of ribbon mics and other
sensitive, delicate, high-precision transducers. Enter the Cosky Phaser
(described in the previous article, schematic shown in fig. 1)

Simplicity itself: the electrolytic cap. charges up to 9V. When the switch is
thrown, the stored charge is dumped into the speaker, whose cone pushes outward,
producing a pressure wave. Foam damping between the rear of the cone and the
frame damps oscillations quite effectively, improving wave shape. The transducer
under test is connected to a sufficiently sensitive noninverting oscilloscope
input according to marked polarity standards, and some of the dumped charge
triggers the ’scope via the BNC jack. Repeated throws of the switch (with a
sufficient charging interval) produce easily visible, repeatable pulses of known
polarity.

EIA-221-A-compliant microphones will produce a positive-going wavefront,
usually with an oscillatory decay, exact shape dependent upon the microphone and
its positioning with respect to the Cosky speaker (fig. 2). Tin-Can Standard
speakers and headphone (RS-331) elements will produce a negative-going wavefront
under identical conditions, typically with more oscillatory activity, with the
first few cycles not infrequently exceeding the amplitude of the initial drop
(or rise)(fig. 3).

As long as one is careful to focus in on the initial direction
(positive-going or negative-going) of the first cycle of the pulse, the results
are consistent and repeatable across a wide cross-section of transducers. The
Cosky Phaser is cheap, simple, and easy to make, but interfacing directly to
electronics is inelegant, and one’s hand does get awfully tired if there are a
number of devices to measure…

Second Generation: The Magic Polarity Tester

It didn’t take long to decide that the Age Of The IC
dictated a more sophisticated approach, and that the versatile, omnipresent 555
timer would be the IC of choice. The working hypothesis: use a 1% duty cycle
waveform, and find the most appropriate frequency to not only produce the
fastest repetitions consistent with full mechanical oscillation decay, but also
the most easily interpreted and visible ’scope waveform across the widest range
of transducers and electronics possible. Extended experimentation yielded a
circuit of only slightly greater complexity than the Cosky, but greatly improved
functionality.

In addition to the built-in speaker of the original design, a
transformer-isolated balanced output was added, level selectable for
compatibility with mic, low line (-10 dBV to 0 dBu) and high line (+8 dBu)
levels. The 600-600Ω transformer was the simplest path to ’scope/audio ground
isolation, and balanced vs. unbalanced “brain-free” interfacing (slam it in,
dial it up, and go!). 1/4" T-R-S was chosen
for the output connector both for durability and “automatic” balanced/unbalanced
“switching” via use of T-S (unbalanced) or T-R-S (balanced) plugs on the
cable.

Best of all, the entire affair is sufficiently non-critical as to provide the
perfect opportunity to use all those 20% carbon resistors, ungraded capacitors
and the like you’ve been stockpiling all these years for just the proper
occasion…. The only components i bought specifically for this project were the
enclosure (on sale), and the battery clamp! A rotary switch was scrounged from a
dead stereo receiver for the combination power/level/speaker/bal. out jack
switch. Look for former function selector switches for ample contacts in a
useful pattern, with the correct number of steps (5).

The PNP transistor inverts the negative-going (w.r.t. chassis) output of the 555 to a
positive-going trigger pulse. Although there is no reason one couldn’t omit this
stage, and merely trigger negative-going, i prefer to keep all outputs positive,
both acoustic and electrical, for consistency’s sake. The series resistances
were chosen empirically for best results (amplitude and waveshape) with the
speaker and transformer at hand, so you may wish to vary these for yours. No
doubt there’s a more elegant arrangement of flyback diodes, but i’m only making
one of these, not thousands, and i had a lot of homeless funky diodes…. The
470µF supply filter should help extend the usefulness of weak/hi-Z batteries,
though you can expect the trigger signal to "hang out" awhile after the unit is
turned off.

Picking a 45-100Ω speaker may eliminate the need for the series R, but might
as well use whatcha got. Plan on spending some focused time with a
current-limited D.C. supply verifying the actual polarity of the speaker you
plan to use: make certain the cone pushes out with the application of D.C., then
mark the speaker terminal connected to the positive D.C. supply lead + or red
(or whatever…) if it isn’t already. Make sure this terminal is the one going to
the switched +9V rail, and all will be well. Tedious and obvious as this may
seem, one doesn’t want to make an oversight here, lest one wishes an “opposite”
polarity box! It’s also worth spending a few minutes experimenting with the best
mechanical damping arrangement to get clean, non-ringy pulses out of the speaker
(this may require iterations of microphone selection and damping
experimentation).

The 2k transformer output load was empirically chosen for best rise time with
minimal overshoot, unloaded into the ’scope—adjust as necessary. Putting the
attenuator on the transformer output side yielded much more consistent waveshape
at the different amplitude steps.

The 32nF C2b frequency-determining capacitance was originally the only C2.
This odd value just happened to provide the best waveshapes for a wide variety
of transducers with the 5 cm speaker used, around 40 Hz. There was never a
problem interpreting results when a driver under test was connected directly to
the Magic Polarity Box (or via a verified noninverting power amp, in the case of
larger speakers), but measuring the waveshapes from some tweeters and midranges
in multi-way speaker systems through the stock crossovers proved difficult.
Hence, a frequency switch was added, along with C2a, as shown. 2.7nF yields
about 414 Hz, the range of the highest frequencies which still had acceptable
waveshape through the transformer. This has proven plenty high enough for any
h.f. transducer + crossover to date, and might also prove useful for some
extremely-limited-bandwidth electronics/media (though the author never
experienced deficiencies through circuitry at 35-40 Hz). The parallel
combination drops the low frequency to about 35 Hz, but no significant
differences in waveshapes were found.

I highly recommend enlisting any cheesy, spare microphone (which doesn’t ring
to the next county) for ease of testing speakers. Verify its polarity in the
usual way, and attach a BNC (or whatnot) for direct ’scope attachment &
guaranteed correct polarity every time (lest you left polarity inversion on…
buta liba!). The little speaker in the Magic Polarity Box is more than enough
for mics and headphone elements, and can be used with many speaker drivers, but
isn’t likely to put out enough to kick large woofers, especially back through a
passive crossover! Pump that line out into a power amp, verify electrical
polarity with the ’scope as the signal enters the speaker, then use your
reference cheesy mike to scope out each driver. Much faster!

Recorder/reproducers are straightforward electronic in/out affairs[2],
but keep in mind the pulse will be delayed by whatever delay is inherent in the
system, so it may lag substantially after the trigger pulse, and you may not
see it right away if you just wrapped up amp or transducer tests. Be sure to
trigger off your input channel and not the Box when looking at reproduced pulses
after recording has ceased.

The Magic Polarity Box has sufficient output to directly drive a piece of
wire acting as a flux loop for direct magnetic polarity testing pursuant to
SMPTE RP 134-1994. Select high frequency & maximum output. Hold the wire
parallel to the reproduce head gap(s), as close as possible, with the positive
end of the wire at the bottom of the head as you face it (when confused, think
Right-Hand Rule for clarification). ’Scope the deck’s output, and you’ve got
your repro. results, plus a known reference for record->play tests.

Complaints? I cannot believe how little regard was given to polarity on some
home audio equipment! Nakamichi 700 line in to line out: source inverts, tape
mon. doesn’t. Same at headphone out. 700II: both tape and source invert at the
line out, both noninverting at the h.p. out. This is one manufacturer and
virtually the same machine! Similar results were obtained with other brands. My
only other gripe is measuring polarity is now so much fun, i’m running out of
things to measure!

[1]
I never knew “headphones” was a contraction until reading an old FCC
Element 3 Study Guide.

[2]
if one is ignoring magnetic polarity, or assuming it is correct….

Polarity Matters
(in case you missed it)

World
O’ Radio/Recording/Audio The
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