If your headphones sound dull, your guitar feels lifeless, or your home stereo sounds muddy, you may be dealing with an output impedance issue. Here's what you need to know about output impedance and why it can be such a drag on your stereo.
In order to understand output impedance, we need to understand some basic electrical engineering knowledge. When you transmit electricity from one place to another through wires or other conductive materials, not all of the energy can get through. Think of it like pouring hot water over your coffee grounds and filter in the morning; most of the water will pass through, but not all.
Therefore, you will lose some energy, usually in the form of heat. This is called resistance.
Next, you have to consider that there are limits to the amount of electricity you can impose into any material. It's like water flowing through a pipe; no matter how big or small, there comes a point where the water in the pipe has to flow out before more water can get in. This is called capacitance.
Continuing with the pipe analogy, water tends to flow in one direction. If you want to change this direction, it will take some time for the water to flow back. The same goes for electric current, called inductance, which is especially important for AC equipment.
Impedance is the sum of these factors, and there's some complicated math involved. Output impedance is the impedance at the "output" side of the system, such as a headphone jack or cable connection.
Let's go back to our pipeline analogy. Let's say you want to connect a well-functioning plumbing system with adequate water flow to another plumbing system. If you just weld a small pipe in there, it puts a huge amount of stress on the system and could rupture the pipe. In contrast, if you weld a larger pipe, you will only get a drop of water when you turn on the faucet.
In electronics, this manifests itself as either muddy sound, no sound at all from the speakers, or an overloaded system. This is why high-end audio systems often include amplifiers; they need a boost of power to properly match impedances.
Unless you custom design your own circuit, the heavy lifting has been done for you. Any device that relates to output impedance, such as an amplifier or a set of speakers, will have output impedance and input impedance as part of the overall specifications of the device. You can easily find these online or in your user manual. In most cases, cheaper devices such as earbuds will have lower impedance than expensive closed-cup headphones.
However, keep in mind that this needs to "match" across the entire device chain. For example, if you have an audio player, a cable, and a set of speakers, the player's output impedance should match the cable's input impedance, and the cable's output impedance should match the speaker's input impedance.
Ideally, the output impedance of the op amp should be 0. This means that the amplifier's output voltage is not affected by the current drawn by the external load.
If you increase the impedance of an amplifier designed to lower the speaker impedance, you will decrease the decibel output (dB) or volume.
