What Is Output Impedance?


Output impedance is the amount of impedance between a preamp or amplifier’s output devices (usually transistors, but possibly a transformer or tube) and the actual output terminals of the component. This includes the internal impedance of the device itself.

To help fully understand this definition, let’s briefly recap the concept of impedance. Resistance is the degree to which something restricts the flow of DC electricity. Impedance is basically the same thing, but with AC instead of DC. Typically, the impedance of a component will change as the frequency of the electrical signal changes. For example, a small coil of wire will have nearly zero impedance at 1 Hz but high impedance at 100 kHz. A capacitor might have nearly infinite impedance at 1 Hz but almost no impedance at 100 kHz.

Why Do You Need Output Impedance?

So why would a component have an output impedance? For the most part, it’s to protect it against damage from short circuits.

Any output device is limited in the amount of electrical current it can handle. If the output of the device is shorted, it’s being asked to deliver a huge amount of current. For example, a 2.83-volt output signal will produce a current of 0.35 amps and 1 watt of power into a typical 8-ohm speaker. No problem there. But if a wire with 0.01 ohms impedance were connected across an amplifier’s output terminals, that same 2.83-volt output signal will produce a current of 282.7 amps and 800 watts of power. That’s far, far more than most output devices can deliver. Unless the amp has some sort of protection circuit or device, then the output device will overheat and will probably suffer permanent damage. And yeah, it could even catch fire.

With some amount of impedance built into the output, the component obviously has greater protection against short circuits, because the output impedance is always in the circuit. Say you have a headphone amp with an output impedance of 30 ohms, driving a pair of 32-ohm headphones, and you short the headphone cord by accidentally cutting it with a pair of scissors. You go from a total system impedance of 62 ohms down to a total impedance of maybe 30.01 ohms, which isn’t such a big deal. Certainly a lot less extreme than going from 8 ohms down to 0.01 ohms.

How Low Should Output Impedance Be?

A very general rule of thumb in audio is that you want the output impedance to be at least 10 times lower than the expected input impedance that it will feed. This way, the output impedance does not have a significant effect on the performance of the system. If the output impedance is much more than 10 times the input impedance that it will feed, you can get a few different problems.

With any audio electronics, a too-high output impedance can create filtering effects that cause weird frequency response anomalies, and also result in reduced power output. For more on these phenomena, check out my first and second articles about how speaker cables can affect sound quality.

With amplifiers, there’s an additional problem. When the amplifier moves the speaker cone forward or backward, the speaker’s suspension springs the cone back to its center position. This action generates voltage which is then thrown back at the amplifier. (This phenomenon is known as “back EMF” or reverse electromotive force.) If the amplifier’s output impedance is low enough, it will effectively short out that back EMF and act as a brake on the cone as it springs back. If the amplifier’s output impedance is too high, it won’t be able to stop the cone, and the cone will continue springing back and forth until friction stops is. This creates a ringing effect and makes notes linger after they were supposed to stop.

You can see this in the damping factor ratings of amplifiers. Damping factor is the expected average input impedance (8 ohms) divided by the output impedance of the amp. The higher the number, the better the damping factor.

Amplifier Output Impedance

Since we’re talking about amps, let’s start with that example, which is shown in the drawing above. Speaker impedances are typically rated 6 to 10 ohms, but it’s common for speakers to drop to 3 ohms impedance at certain frequencies, and even 2 ohms in some extreme cases. If you run two speakers in parallel, as custom installers often do when creating multiroom audio systems, that cuts the impedance in half, meaning a speaker that dips to 2 ohms at, say, 100 Hz now dips to 1 ohm at that frequency when it’s paired with another speaker of the same type. That’s an extreme case, of course, but amplifier designers have to account for such extreme cases or they could be facing a big pile of amps coming in for repair.

If we figure a minimum speaker impedance of 1 ohm, that means the amp should have an output impedance of no more than 0.1 ohm. Obviously, there’s no room to add enough resistance to this amp’s output to give the output devices any real protection.

Thus, the amplifier will have to employ some sort of protection circuit. That could be something that tracks the amp’s current output and disconnects the output if the current draw is too high. Or it could be as simple as a fuse or circuit breaker on the incoming AC power line or the rails of the power supply. These disconnect the power supply when the current draw is more than the amp can handle.

Incidentally, almost all tube power amplifiers use output transformers, and because output transformers are just coils of wire wrapped around a metal frame, they have substantial impedance of their own, sometimes as much as 0.5 ohm or even more. In fact, to simulate the sound of a tube amp in his Sunfire solid-state (transistor) amplifiers, famed designer Bob Carver added a “current mode” switch that placed a 1-ohm resistor in series with the output devices. Of course, this violated the 1-to-10 minimum ratio of output impedance to expected input impedance that we discussed above, and thus had a substantial effect on the frequency response of the connected speaker, but that’s what you get with many tube amps and it’s exactly what Carver wanted to simulate.

Preamp/Source Device Output Impedance

Brent Butterworth

With a preamp or source device (CD player, cable box, etc.), as shown in the drawing above, it’s a different situation. In this case, you don’t care about power or current. All you need to convey the audio signal is the voltage. Thus, the downstream device -- a power amplifier, in the case of a preamp, or a preamp, in the case of a source device – can have a high input impedance. Any current coming through the line is almost entirely blocked by that high input impedance, but the voltage gets through just fine.

For most power amps and preamps, an input impedance of 10 to 100 kilohms is common. Engineers can go higher, but they may get more noise that way. Incidentally, guitar amps typically have input impedances of 250 kilohms to 1 megohm, because electric guitar pickups usually have output impedances ranging from 3 to 10 kilohms.

Short circuits can be common with line-level circuits, because it’s so easy to accidentally rub the two naked conductors of an RCA plug against a piece of metal that shorts them out. Thus, output impedances of 100 ohms or more are common in preamps and source devices. I’ve seen a few exotic, high-end components with line-level output impedances as low as 2 ohms, but these will have either very heavy-duty output transistors or a protection circuit to prevent damage from shorts. In some cases, they may have a coupling capacitor at the output to block DC voltage and prevent output device burnout.

Phono preamps are a different subject entirely. While they typically have output impedances similar to those of a CD player, their input impedances are very different from those of a line-stage preamp. That's too much to go into here. Perhaps I'll dig into that subject in another article.

Headphone Amp Output Impedance

Brent Butterworth

The surge in popularity of headphones has brought the rather weird, non-standard system impedance arrangement of typical headphone amps to the spotlight. Unlike conventional amps, headphone amps come in a wide variety of output impedances. Really cheap headphone amps, like the ones built into most laptop computers, may have an output impedance as high as 75 or even 100 ohms, even though headphone impedance typically ranges from about 16 to 70 ohms.

It’s rare for a consumer to disconnect and reconnect speakers when an amp is running, and also rare for speaker cables to be damaged when an amp is running. But with headphones, these things happen all the time. People routinely connect or disconnect headphones when a headphone amp is running. Headphone cables are often damaged -- sometimes creating a short circuit -- while they’re in use. Of course, most headphone amps are cheap devices, which can make adding a decent protection circuit cost-prohibitive. So most manufacturers take the easier way out: They raise the output impedance of the amplifier by adding a resistor (or occasionally a capacitor).

As you can see in my headphone measurements (go down to the second graph), high output impedance can have a huge effect on the frequency response of a headphone. I measure the frequency response of a headphone first with a Musical Fidelity headphone amp that has a 5-ohm output impedance, then again with an extra 70 ohms of resistance added to create a total output impedance of 75 ohms.

The effect that a high output impedance will have varies with the impedance of the connected headphone, and especially with the change in the headphone’s impedance at different frequencies. Headphones that have large impedance swings -- as most in-ear models with balanced-armature drivers do -- will usually exhibit substantial changes in frequency response when you change from an amp with low output impedance to one with high output impedance. Often, a headphone that has a natural-sounding tonal balance when used with a low-impedance source will have a bassy, dull-sounding balance when used with a high-impedance source.

Fortunately, low output impedance is available in many high-end headphone amps (especially solid-state models), and even some of the little headphone amp chips built into devices such as iPhones. There’s usually no way to know for sure if a headphone is voiced for use with high or low output impedances, but I prefer to stick with low output impedance for the reasons cited earlier in this article.

I would prefer not to use headphones with huge impedance swings that would cause frequency response changes when used with headphone amps that have high output impedance (like the one in the laptop I’m typing this on). Unfortunately, though, I generally prefer the sound of a good balanced-armature in-ear headphone to one that uses dynamic drivers, so when I use these headphones with my laptop, I usually connect an external amp or USB headphone amp/DAC.