How to Measure Output Impedance with a Multimeter

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Did you know that impedance is very much the same as electrical resistance? Both forces oppose the electricity flow through a wire, but resistance takes place in DC or direct current cables, while impedance takes place in AC or alternating current cables.

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Compared to resistance, impedance is influenced by the electrical signal carried by the frequency. Impedance is also referred to as Z. 

There’s no doubt that impedance is out of those complicated subjects when we talk about speakers, amplifiers, and much all other audio devices. Have you ever thought about what the ohm rating means on your amp and how it influences its performance in a system? Lucky for you because this post got you covered. 

Keep in mind that audio amplifiers have outputs and inputs which have impedance values. Speaker, line and mic inputs have specific impedances to function accurately as loads for inputted devices. Amp outputs always have rated impedance to help balance the nominal impedance of speakers even though the real impedance is infrequently specified. 

In today’s article, we will determine impedance, what output impedance is, and then how you can test it using a multimeter. 

The relationship between audio and impedance

Audio is determined as an electrical representation of sound. Meanwhile, the sound is determined as mechanical wave energy between 20 hertz and 20,000 hertz. Like all other audio devices dealing with audio signals, amplifiers have inherent impedances on them. 

Unlike headphones and mics, amplifiers have both outputs and inputs, and hence they have output and input impedances. 

The impedance values of amplifiers are measured as nominal values as well. Nonetheless, you will find an impedance graph to present the valleys and dips in the actual impedance of the amplifier across its response. 

What the heck is impedance?

In case you didn’t know, electrical impedance is the measurement of the system’s opposition to AC when a voltage is applied. It can be thought of as an AC resistance in a way. 

Impedance has an actual DC resistance component and an imaginary reactance component. It has both phase and magnitude, not to mention it’s frequency-dependent as well. Take note that impedance is measured in ohms and is renowned in equations by the letter Z. 

As stated earlier, audio devices have both outputs and inputs, and that amplifiers have both. That’s because amplifiers are created to intensify an inputted signal. They also take a lower-level signal at their inputs, distribute gain, as well as output a more powerful version of that signal. 

Hence, both output and input impedance values are essential to understand along with audio amplifiers. Such values will indicate how the amplifier will do when different devices are linked at the inputs and when different devices are linked at their output, such as processors, headphones, and speakers. 

When it comes to impedance, the device receiving a signal at its input is the load, especially when connecting audio devices together. Further, the device outputting signal is considered as the source. 

But what makes the output the source? Bear in mind that an amplifier takes a signal at its input and then outputs a much stronger version of that specific signal. The amplifier’s signal is attached to another device accepting that amplified output signal.

In short, the output of the amplifier serves as a source, and the connected device functions as the load. 

What is output impedance?

Output impedance is all about the ability of the device to give unrestricted power or current when passing a musical signal. It computes the quantity of restriction or holds that signal back. It’s essential to realize output impedance as it’s relative to the input impedance of whatever the amp is currently driving.

To make it short, the output impedance of a DAC is relevant if you’re considering what the output is going to feed into. 

The simplest way to determine what output impedance indicates is to visualize a resistor in series of how low the output impedance is. That’s because there is no such thing as the best or perfect output or source. Further, the best output would have zero output impedance. That indicates the value of that legendary resistor is zero and hence would have no impact on the musical signal passing through it.  

How do you measure output impedance? 

The best way for you to measure the output impedance of your amplifier is through a digital multimeter. In case you didn’t know yet, a digital multimeter sends out a small DC current to measure resistance. You see, impedance is a quality of AC circuits, but that will not measure impedance directly. 

For instance, you can easily determine between a 4-ohm and an 8-ohm speaker that way. 

Now, how do you measure output impedance through a multimeter? Below are the steps you need to do:

  1. Get your digital multimeter and set it to ohms mode. 
  2. Measure accurately the resistance of the load resistor that you end up with.
  3. You might have ten percent tolerance resistors accessible, so an accurate measurement might make a difference to the calculations. 

Is the voltage output amplitude of the generator exactly half of what it is when the load is removed? It means the output impedance is equal to the load resistance. You are completely done if you have minimized the output to roughly half and you only require an approximate measurement. 

Final Thoughts

Always bear in mind that these impedances play an integral role at the amplifier’s interfaces. They dictate how the power signals or voltage signals are transmitted from either an amplifier to a transducer, from an amplifier to another amplifier, or from a source to a preamplifier. 

You will find two criteria that are often utilized to set the impedances: the efficiency or the transferred power. Normally, a high efficiency is chosen for voltage amplifiers. It can be accomplished by setting the input impedance of stage n+1 higher than the output impedance of stage n. 

Through that configuration, much of the power is created at the n+1 stage despite the increasing global resistance, rather than dissipated by heat loss in the output.

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