Headphone output impedance: why your amp's specs matter more than you think

Eight ohms. That single number, buried in an amplifier's specification sheet, can completely undo a $600 IEM. I got this wrong for years — running sensitive in-ears off a tube amp with a 120-ohm output impedance and wondering why the bass sounded thin and the highs were brittle. Turned out it wasn't the IEMs. It was basic circuit behaviour I should have caught from the start.

Output impedance is one of those specs that gets skipped over in almost every buying guide, yet it shapes the frequency response you actually hear more reliably than many of the things we obsess over. Let me explain the engineering clearly, and then we'll work through what it means in practice for every category of headphone you're likely to own.

What output impedance actually is

Every amplifier has a source impedance — the internal resistance (and reactance) that the output stage presents to whatever load you connect. In a perfect world it would be zero. In the real world it's somewhere between 0.1 ohms (a well-designed solid-state amp) and 120 ohms (some classic tube designs and a few portable amps that use resistors to limit short-circuit current).

The reason this matters is the voltage divider relationship between the amp's output impedance and your headphone's load impedance. When you connect a headphone to an amplifier, you get a simple resistive (and reactive) divider. The headphone's impedance and the amp's output impedance form a network, and the voltage that actually arrives at the driver is determined by both. If the amp's output impedance is negligible relative to the headphone's impedance, effectively all the voltage reaches the driver. If it's not negligible — and this is where people get into trouble — the amp's source impedance starts robbing voltage, and it does so unevenly across frequency.

That last part is the critical bit. A headphone's impedance is not flat across frequency. It varies with the resonance of the driver, the voice coil inductance, and the crossover network (if any). A typical dynamic driver will have an impedance peak at its resonant frequency, often well above the nominal impedance spec. If your amp has a non-trivial output impedance, the voltage delivered to the headphone will rise and fall as the headphone's impedance rises and falls — and you'll hear that as colouration in the frequency response.

The 1:8 damping factor rule

The commonly cited guideline — sometimes called the "1:8 rule" or the Damping Factor rule — says the amp's output impedance should be no more than one-eighth of the headphone's minimum impedance. So for a 32-ohm headphone, you want the amp's output impedance at or below 4 ohms. For a 16-ohm IEM, that means 2 ohms or less.

This isn't an arbitrary figure. It comes from loudspeaker amplifier practice — the same relationship between source impedance and load impedance that determines how well an amplifier controls driver motion (hence "damping factor"). For headphones the physics is the same; only the scale changes.

Where it gets interesting is that different headphone types have very different sensitivity to output impedance, and the reasons are worth understanding rather than just memorising a table.

Why IEMs are the most vulnerable

Modern IEMs — especially multi-driver balanced armature designs — are the most sensitive to output impedance mismatch by a significant margin. There are two reasons.

First, their nominal impedances are low. A typical BA IEM might be 8–18 ohms nominal. Apply the 1:8 rule and you need the amp's output impedance below 1–2 ohms. Many portable amplifiers, DAC/amp dongles, and phone outputs don't meet this. My old portable amp had a quoted output impedance of 10 ohms from its single-ended output — fine for a 300-ohm Sennheiser, but for a 16-ohm IEM it was rolling off the bass and lifting the mids noticeably.

Second, multi-driver BA IEMs have crossover networks built into the shell, and those crossovers interact with source impedance. The crossover's filter characteristics change depending on the source impedance driving it. You're not just getting a volume-divider effect; you're changing the crossover points themselves. A well-designed IEM crossover is tuned assuming a near-zero source impedance. Drive it from a 10-ohm source and you've detuned the crossover.

Single-dynamic-driver IEMs are somewhat more forgiving here — the Meze Audio ASTRU is a good example of a design that's clearly built around a controlled, low-reactance single driver — but you still want a low output impedance for best results with any IEM.

Planar magnetic headphones: largely immune

Planar magnetic headphones are, by contrast, almost completely immune to output impedance effects. The reason is their impedance curve. A planar's impedance is essentially flat — the thin film conductor has no resonant peak the way a dynamic driver does, and there's no voice coil inductance to create frequency-dependent reactance. Audeze's LCD series typically measures within a few ohms of their nominal impedance across the full audio band.

With a flat impedance curve, the voltage divider effect is constant across frequency. The amp may deliver slightly less voltage to the headphone if output impedance is high, but it does so uniformly — meaning the frequency response isn't altered, only the volume level. You can correct volume with the volume knob. You can't correct frequency response colouration in the same way.

This doesn't mean planars are easy to drive — many of them require substantial current that cheap dongles can't supply — but output impedance as a tonal colouration source is not your problem.

High-impedance dynamic headphones: the old calculation

The classic 300-ohm and 600-ohm dynamic headphones from Sennheiser and Beyerdynamic were designed in an era when amplifiers routinely had 50–150 ohm output impedances. The high nominal impedance meant that even a 120-ohm source only caused a modest deviation from flat. The 1:8 rule still applies — for a 300-ohm headphone you want no more than 37 ohms of source impedance — but the absolute values are much more forgiving than with IEMs.

There's a school of thought that certain tube amplifiers, with their higher output impedances, actually "tune" the sound of 300-ohm dynamics in ways some listeners prefer. This is technically accurate: the amp's output impedance interacts with the driver's impedance peak near resonance and slightly lifts bass. Whether that's a "feature" or a measurement artefact you've come to love is a conversation I'll leave open. The engineering answer is that you're hearing colouration, not accuracy. But I'll admit the colouration can be musically pleasant — just be clear-eyed about what it is.

How to find your amp's output impedance spec

This is where things get annoying. Many manufacturers don't publish this spec at all, which is a genuine problem. Others publish it for the headphone output specifically (good), or bury it in the general output specifications in a way that only applies to line-level outputs (useless).

For dedicated headphone amplifiers from reputable brands — the FiiO discrete Class A amp comes to mind, as does the Chord Electronics Mojo 2 — output impedance is usually specified and usually low (sub-1 ohm for a well-designed modern solid-state design). For the headphone socket on an integrated amplifier, a DAC, or a DAC/amp combo, it's far less consistently documented. The Chord Mojo 2 (check price) quotes below 0.075 ohms from its headphone output — that's effectively zero for any real-world load.

If the spec isn't published, the Audio Science Review database (asr.com) measures output impedance on most reviewed units as a matter of course. It's not a perfect resource for everything, but for this specific parameter it's invaluable. I check it regularly when I'm evaluating portable amps for IEM use.

Also worth knowing: balanced (4.4mm or XLR) headphone outputs often have different output impedances than single-ended (3.5mm) outputs from the same device. Sometimes better, sometimes the same. Always check both if you're considering a balanced cable upgrade.

Practical buying advice by use case

For IEM users, particularly anyone running multi-driver BAs or high-sensitivity single dynamics: output impedance below 1 ohm is not aspirational, it's the minimum. This rules out quite a lot of vintage kit and some surprisingly recent portable amplifiers. If you're running IEMs from a laptop headphone output, measure your luck with a dongle DAC/amp that publishes its spec — the Astell&Kern AK HC5, for example, is a dongle with a properly specified and measured output impedance suited to IEMs.

For planar magnetic headphones, output impedance is not your concern. Current delivery is. Make sure whatever you're using can maintain clean power into the low-to-moderate impedance and current draw of a planar. Many dedicated headphone amps handle this well; many phone outputs and cheap dongles don't, but for different reasons than output impedance mismatch.

For high-impedance dynamics like the Sennheiser HD 660S2 (check price) (150 ohms nominal), you have considerably more headroom. A 10–20 ohm source impedance won't cause dramatic colouration. That said, a properly damped solid-state output will still measure flatter — and if the rest of your chain is resolving enough, you'll hear the difference on transient control.

If you're building a more complete desktop rig and haven't read through our guide to DACs and network streamers, it's worth a look alongside this — the output impedance discussion there extends to line-level outputs feeding headphone amplifiers.

A note on multi-mode amplifiers

Some amplifiers switch between low-gain and high-gain modes, or offer impedance-switching to match different headphone types. The iFi Audio house style, for instance, includes selectable impedance settings on several of their portable units. These are genuinely useful — not marketing theatre. The switching network physically alters the output impedance to better suit high-impedance dynamics vs low-impedance IEMs. If you regularly switch between, say, a planar and a sensitive BA IEM, a switchable output impedance mode is worth looking for rather than assuming one setting will serve both well.

For context: a desktop rig with a quality DAC feeding a dedicated headphone amp is still the most reliable way to get this right, since you can choose each component to match your specific headphones. The Chord Mojo 2 feeding a clean low-output-impedance amplifier is a well-trodden path for good reason. If you're thinking about building that kind of setup, our DAC and streamer guide covers the source end, and the statement headphone systems article covers the amplification stage in more detail.

The short version is this: buy headphones first, check their impedance curve if you can find it, then verify the output impedance of whatever's driving them. Forty-five seconds with a spec sheet can save you from weeks of blaming your IEMs for a colouration that was never their fault.

— Theo Mensah, Digital, DACs & Streaming Editor