One of my favorite field tools is a basic analog
impedance meter. It can be used to troubleshoot a host
of sound system problems, either stand-alone or with
some additional gadgets. The TOA ZM-104 has been
around for many years and many experienced audio
people have one. Mine has served me well and has the
scars to prove it. It will be the example meter used in
this article, although in principle the tests can be con-
ducted using any similar device. Here is an overview
of the meter characteristics and some of the basic tests
that it can run.
Impedance
Electrical power sources produce voltage and cur-
rent. Electrical current between a source and load may
flow in one direction only (direct current or DC) or
in both directions (alternating current or AC). Imped-
ance is the opposition to the flow of electrical current.
It has two ingredients - resistance and reactance. Only
resistance need be considered in a DC circuit. Both
resistance and reactance must be considered in an AC
circuit. An ohm meter measures the opposition to DC,
which is why it is sometimes call a resistance meter.
The meter itself can serve as the current source. In
alternating current circuits, another form of opposition
is present. Reactance is the frequency-dependent oppo-
sition to the flow of current. It cannot be measured with
an ohm meter, because DC flows in only one direction.
An impedance meter generates an alternating current
which can be fed to the device-under-test (DUT from
here on).
Measuring Impedance
Several methods can be used to determine the imped-
ance of a load. The most straightforward way is to use
Ohm’s Law to determine impedance from the volt-
age across the load and the current through it. Figure
1 shows how to make the measurement with a volt-
ohmmeter (VOM). Impedance can also be measured
using a constant current source. This is the method that
the ZM-104 uses. The meter has a high output imped-
ance and acts like a constant current source (the current
through the load is independent of the load impedance).
The small voltage drop across the load is proportional
to the load impedance. This voltage produces the meter
deflection on the calibrated scale.
The ZM-104
Figure 2 shows the ZM-104. It’s a classic looking
device, with black chassis and analog meter move-
ment. There is an “off” button along with three range
switches, a zeroing button and a zeroing potentiometer.
The range switches are used to change the range of
the analog display by factors of ten. As with all analog
meters, the range setting that produces a mid-scale
reading should be used. The zeroing button temporar-
ily shorts the output terminals of the meter together,
allowing the zeroing pot to be used to “zero” the meter.
The meter probes can be shorted as an alternative to
using the zeroing switch. Note that the meter must be
re-zeroed if the range switch is changed. As with all
electrical or acoustical measurements, the measurer
should make an educated guess about what the read-
ing should be before the measurement is made. If
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Trade
Tools
of the
The Analog Impedance Meter
Fig. 1 - The “manual” method of determining impedance.
Z = V/A
From Syn-Aud-Con Newsletter Vol. 32 No.3
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