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Tool of the Trade: The Analog Impedance Meter

In this age of computers, DSP and wireless communications, don't overlook the time-proven fundamental tools of our trade.

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 conducted 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 current. 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).

Impedance 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 opposition 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 impedance of a load. The most straightforward way is to use Ohm’s Law to determine impedance from the voltage 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 impedance 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 movement. 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.

Fig. 2 – The TOA ZM-104 Impedance Meter.

As with all analog meters, the range setting that produces a mid-scale reading should be used. The zeroing button temporarily 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 reading should be before the measurement is made. If completely unsure, then start with the x1 button and work your way up until you get a mid-scale reading.

The meter’s output signal is a 2V peak square wave at 20mA (x1 range). The analog meter movement is great for finding intermittent problems.

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