What You Need to Know About Output Impedance: Key Insights for Engineers

If your headphones sound dull, your guitar feels lifeless, or your home stereo sounds muddy, you might be dealing with output impedance. Here’s what you need to know about output impedance and why it’s such a drag on your stereo.

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What Is Output Impedance?

To understand output impedance, we need to lay out some basic electrical engineering. When you transfer electricity from place to place via a wire or another conductive material, not all of the energy gets through. Think of it like pouring hot water over coffee grounds and a filter in the morning; most of the water gets through, but not all of it.

Thus, you’re going to lose some of the energy, often in the form of heat. This is called resistance.

Next, you have to consider there’s only so much electricity you can force into any material. It’s like running water through a pipe; big or small, there comes a point where the water in the pipe has to flow out before more can come in. This is called capacitance.

Sticking with the pipe analogy, the water tends to flow in one direction. If you want to change that direction, it’s going to take a little time for the water to flow backward. The same is true of electrical currents, called inductance, and it’s particularly important to alternating-current devices.

Impedance is the sum of these factors, which involves a little complicated math. Output impedance is simply how much impedance is at the “out” end of the system, like headphone jacks or cable connections.

Why Is Output Impedance Important?

Let’s go back to our pipe analogy. Let’s say you want to connect your well-running pipe system, with plenty of smooth water flow, to another pipe system. If you just weld a small pipe in there, it’ll put massive pressure on the system and possibly burst the pipe. Conversely, if you weld a bigger pipe in, you’ll get just a trickle of water when you turn on the faucet.

In electronics, this is either reflected by, for example, muddy sound or no sound at all from speakers, or whatever you plugged into the system overloading. This is why high-end audio systems often include an amplifier; they need the boost in energy to properly match impedance.

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Do I Need to Calculate Output Impedance Myself?

Unless you’re custom-engineering your own circuits, the heavy lifting has already been done for you. Any device where output impedance is relevant, such as an amplifier or a set of speakers, will have the output impedance and input impedance as part of the overall specifications of the device. You can easily find these online or in the user’s manual. In most cases, cheaper devices, like earbuds, will have lower impedance than expensive closed-cup headphones.

However, remember that this will need to “match” across the entire chain of devices. For example, if you have an audio player, a cable, and a set of speakers, the output impedance of the player should match the input impedance of the cable, and the output impedance of the cable should match the input impedance of the speakers.

FAQ

• What is the typical output impedance of an integrated circuit op-amp?
  Ideally, the output impedance of an op-amp should be 0. That means the output voltage of the amp is unaffected by the current drawn by an external load.
• How does increasing speaker impedance affect dB output?
  If you increase impedance on an amp intended for a lower speaker impedance, it will lower the decibel output (dB), or volume.

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