Impedance and sensitivity: do you need an amp?

Two numbers on a spec sheet, one in ohms and one in decibels, tell you whether a pair of headphones will get along with your phone, your audio interface or only a proper headphone amp. Here's how to read them and what they mean before you hit the buy button.

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Quick answer

What impedance actually means

Impedance is how hard a headphone pushes back against the signal flowing through it, measured in ohms (Ω). The signal coming out of your phone, interface or amp is basically a voltage that keeps changing, and that voltage has to push current through the voice coil to get the driver moving.

Higher impedance means more pushback, so less current flows for the same voltage. That doesn't make a high-impedance pair worse than a low one. It just changes what you need to plug it into, as it'll want more voltage to get loud.

Headphone impedance falls roughly into three groups.

Low impedance, under about 50 ohms, is built to run off the small, low-voltage outputs on phones, interfaces and USB-C dongles. The Audio-Technica M50x is 38 ohms, Beyerdynamic's DT 700 Pro X is 48, Sony's MDR-MV1 is 24, and nearly all in-ears sit down here too, like Sennheiser's IE 200 at 18 ohms. Some planars do as well, with HiFiMan's Edition XS at 18 ohms and the Audeze LCD-X at 20.

Mid impedance, about 50 to 150 ohms, needs a bit more voltage to get to normal listening levels. These do fine on a good interface (a Focusrite Scarlett or a UA Volt, say), on most decent dongles and of course on a desktop amp, while a weak phone output can leave them sounding quiet and a little pinched. Think of the Sony MDR-7506 at 63 ohms, the Focal Clear Mg at 55, the 80-ohm DT 770 Pro, the Sennheiser HD 560S at 120 and the HD 620S at 150.

High impedance, over about 150 ohms, really does want a proper amp. Run off a phone, they'll play, but the bass can go thin, the dynamics flatten out and you may run out of volume before you run out of song. The Sennheiser HD 600, HD 650 and HD 800 S are all 300 ohms, Beyerdynamic sells a 250-ohm DT 770 Pro, ZMF's Aeolus is 300, and Meze's new Arta planar comes in at 225.

So why make a high-impedance headphone at all? It's mostly a design choice. Winding the coil with lots of turns of fine wire ends up high-impedance, and some designers feel that buys them a lighter moving coil and cleaner sound. High-impedance cans also suit a lot of older studio gear, and you can hang several pairs off one amp in a studio or a broadcast truck without loading it down. If a pair is going to live on a real amp anyway, the trade costs nothing.

What sensitivity actually means

Sensitivity, sometimes called efficiency, is how loud a headphone gets for a given amount of input. You'll see it as dB per milliwatt (dB/mW) or dB per volt (dB/V), and it tells you how much juice you need to hit a comfortable volume.

Higher means louder for the same power. A pair at 110 dB/mW is a whole lot more efficient than one at 95, and here's how to think about the gap. Every 3 dB means twice the power, so 15 dB is about 30 times the power, and a jump of about 10 dB sounds roughly twice as loud to most ears.

Watch the units, because that's where spec sheets trip people up. Sennheiser quotes its open-backs per volt, so the HD 650's "103 dB" is 103 dB at 1 volt, which works out to roughly 98 dB/mW at 300 ohms. Per-volt numbers flatter high-impedance headphones, so it's worth converting before you compare one brand with another (subtract 10 times the log of 1,000 divided by the impedance, or just trust an online calculator).

Here's roughly how it breaks down.

High sensitivity, above about 105 dB/mW, gets loud off almost anything, even a feeble phone output. Most in-ears live here, and the Sony MDR-7506 (106 dB/mW) and Meze's 109 Pro (112) are over-ears that make the cut.

Moderate sensitivity, about 95 to 105 dB/mW, gets loud off a good source but can feel a bit underfed on a weak one. Most over-ears land here, like the M50x at 99, the 80-ohm DT 770 Pro at 96 and the LCD-X at 103.

Low sensitivity, under about 95 dB/mW, needs real amplification to get comfortably loud. Lots of planars sit here, like the Edition XS at 92 and HiFiMan's Susvara at a notoriously hungry 83.

The two numbers work together. A high-impedance pair with decent sensitivity, like the HD 650 (300 ohms, about 98 dB/mW), is far easier to drive than a low-impedance pair with poor sensitivity like the Susvara (60 ohms, 83 dB/mW), which wants a speaker amp or a very beefy headphone amp, and no dongle will cut it. Look at both, neither one tells the whole story on its own.

Do you actually need a headphone amp?

For most readers, no, you don't. The "you need an amp" line is true for a certain slice of audiophile headphones, but most headphones, plenty of pricey ones included, run just fine off a modern source.

The real question is whether your source can put out enough voltage and current to get your headphones to a safe listening level (about 75 to 85 dB) with some headroom left for loud drum hits and the like. For modern gear and modern headphones the answer is almost always yes, even from a phone with a USB-C dongle.

You'll want a dedicated amp in a few cases.

The first is if you bought high-impedance audiophile headphones on purpose. The HD 600, HD 650, HD 800 S and the 250-ohm DT 770 Pro were built to be driven properly, and without that you're short-changing them in a way you'll hear, as thinner bass and flatter dynamics, nothing as subtle as a bit of hi-fi magic. If one of these is on your list, I'd budget for the amp (or at least a strong dongle) before you buy.

The second is low-sensitivity planars. Some older ones and a few current flagships, the Susvara above all, need a lot of power. Most of today's planars, like the Edition XS and LCD-X, are far easier and run off an interface or a strong dongle, though a desktop amp gives them more room to breathe.

The third is electrostatics. These need their own electrostatic amp, called an energizer, to make the high bias voltage they run on, and a regular headphone amp simply won't drive them. My rundown of driver types explains how they work.

The last is a weak source. Some old laptops, cheap phones and vintage gear have puny headphone outputs that struggle with anything over 32 ohms. An amp will help there, though upgrading the source is usually the smarter fix.

And here's when you don't need one, whatever the forums tell you.

You don't need an amp for any wireless headphone. The Sony WH-1000XM6, the Bose QuietComfort Ultra (2nd Gen), the AirPods Max 2 and the Sennheiser Momentum 4 all have their own amps inside, and an outboard amp does nothing in wireless mode. When you plug in the cable, they're designed to get along with whatever you've got.

You don't need one for modern in-ears either. Universal IEMs like Shure's SE215, Sennheiser's IE 600 and Westone's Pro X series are made to run off phones and portable gear. Some high-end sets benefit a little from a cleaner source, but the difference is subtle, and a good USB-C dongle is plenty.

And you don't need one for low- or mid-impedance over-ears. The Sony MDR-7506, the M50x, the DT 700 Pro X and the Focal Clear Mg all run clean off an interface, a dongle or a decent phone output. A desktop amp might polish them a touch, but they don't need it to sound right.

The amp decision tree

If you'd like a quick way to work it out, run through these.

First, are your headphones wireless? If so, you don't need an outboard amp, and you can skip the rest of this section.

Second, what's the impedance?

Under 50 ohms, no amp needed, and a phone, laptop or interface all work fine.
From 50 to 150 ohms, an interface or a decent USB-C dongle is enough and a desktop amp is optional.
From 150 to 300 ohms, an interface might be borderline, and I'd recommend a desktop amp or a strong dongle, though it's not strictly required.
Above 300 ohms, you'll want a real headphone amp.

Third, what's the sensitivity?

Above 100 dB/mW, they're easy to drive even at mid impedance.
From 95 to 100 dB/mW is standard, and most decent sources cope.
Below 95 dB/mW, they'll likely need proper amplification whatever the impedance.

Fourth, what are you plugging into now?

An iPhone with Apple's $9 USB-C-to-3.5mm adapter is fine for most easy-to-drive headphones. That little adapter has its own tiny DAC and amp inside, and it does well with in-ears and low-impedance over-ears.
A recent Android phone with a USB-C dongle is about the same story as the iPhone.
An audio interface (a Focusrite Scarlett, a UA Volt and so on) drives most headphones up to about 250 ohms well enough, and its quarter-inch headphone out is usually stronger than a phone's.
A stronger dongle DAC/amp from the likes of FiiO, AudioQuest or Moondrop gives you more push than the basic adapters and handles most headphones short of the toughest loads.
A desktop amp/DAC (Schiit's Magni stacks, JDS Labs' Atom gear, FiiO's K7) drives nearly anything with a normal headphone jack cleanly and is the right home for high-impedance audiophile cans.

Then match them up. If your headphones and source land in "needs an amp" territory, budget for one, and if they land in "works fine," put the money toward better headphones instead.

Amp categories explained

The amp market is split into a handful of groups that do different jobs. Prices move around a lot, so treat these ranges as ballpark.

At the bottom are USB-C dongle DAC/amps, from about $10 up to $100 or so. They plug into your phone or laptop and give you a headphone jack. Apple's $9 adapter is surprisingly capable for what it is, and the step-up dongles from FiiO, AudioQuest and others add more output, enough for most mid-impedance headphones, though usually not the hungriest audiophile loads.

Portable amps with a built-in DAC are a bit bigger, usually have their own battery and are made to ride along with a phone or laptop. FiiO's BTR series, iFi's xDSD Gryphon and Chord's Mojo 2 are well-known names, running from around $100 to several hundred. They put out a good deal more than a dongle, enough for most headphones short of the toughest, and they're handy if you take demanding cans on the road.

Desktop amp/DAC combos sit on your desk and hook to a computer over USB. Schiit's Magni and Modi stacks, JDS Labs' Atom stack, Topping's stacks and FiiO's K7 are common first buys, mostly in the low hundreds. They usually give you several outputs (3.5 mm, quarter-inch, sometimes balanced) and plenty of power, and for serious listening at home this is the sweet spot.

Audiophile desktop amps, roughly $500 to $3,000, add more refined circuits, and some use tubes for a particular flavor. Past this point the returns shrink fast, so the gain over a good $200 amp is real but subtle. I'd only go here for a demanding high-end pair like the HD 800 S or a power-hungry planar.

End-game amps run from a few thousand dollars to well past ten grand, from names like McIntosh, Cayin and assorted boutique builders. The differences from a good $1,000 amp are mostly a matter of taste, and I'd only look here if you already own flagship headphones and the headphones have stopped being the weak link.

Do you need a DAC too?

A DAC, or digital-to-analog converter, turns the ones and zeros of a digital file into the analog signal an amp and headphones can use. Every source you've ever used has one built in, from your laptop and interface to a cheap pair of Bluetooth earbuds.

So you've already got a DAC. The question is whether the one in your current setup is good enough, or whether an outboard one would really make things better.

Here's my honest read for most people.

Your phone setup is fine. Most phones have dropped the headphone jack (iPhones did it back with the iPhone 7), so the DAC is actually in the dongle you plug in, and by most accounts even Apple's $9 one measures pretty clean. For casual listening, good streaming and most headphones, the phone isn't what's holding you back.

Your audio interface is fine too. Modern Focusrite, Universal Audio and PreSonus interfaces have converters that measure past the point anyone can hear, so for mixing or general listening at home, the interface is not your bottleneck. It's built for exactly this.

Your laptop might be the weak link, though. Built-in laptop audio is all over the place, with some (recent MacBooks, for one) surprisingly good and some cheap Windows laptops pretty mediocre. If your laptop is your main source and you're not happy, this is the upgrade most worth making.

An outboard DAC earns its keep when you're running high-impedance headphones off a dedicated amp, when you're doing critical listening or mixing, when a side-by-side test shows you a real improvement, or when you're building a desk setup where every piece counts.

Entry-level DACs worth knowing include Schiit's Modi, Topping's E30 II and iFi's Zen DAC, and Audio Science Review's bench tests have shown that this kind of affordable DAC can measure as clean as ones costing many times more. Spending a lot more usually buys features like balanced outputs or extra inputs, far more than any sound you could pick out.

What I'd do in your situation

For casual listening on wireless headphones, you're done. Don't give amps or DACs another thought, the headphones handle it all.

For casual listening on wired headphones from a phone, grab a decent USB-C dongle. Apple's $9 adapter is fine for easy loads like in-ears and the M50x, and a stronger dongle is worth it once you move to tougher headphones. Sennheiser's HD 600, for one, wants either an amp or a strong dongle that can swing a volt or two.

For working from home on closed-backs, your interface (if you have one) or your laptop is probably enough. A small USB DAC/amp makes things tidier more than it makes them sound dramatically better.

For mixing or mastering at home, use your interface. The headphone out on a modern interface is built for exactly this job, so you can skip a separate amp unless you've moved to high-impedance reference cans. My mixing and mastering headphone picks note which ones need the help.

If you're buying a $300-to-$500 audiophile pair (the HD 560S, the HD 600 or HD 650, or a 250-ohm DT 770 Pro), I'd set aside something like $150 to $300 for a desktop amp/DAC. A Schiit Magni and Modi stack or a JDS Labs Atom stack are the usual starter picks, and either one handles the HD 650, the toughest of that bunch, with authority.

For flagship audiophile headphones (the HD 800 S, the Focal Clear Mg, the Audeze LCD-X), I'd budget more for a proper amp and DAC. Past that point the returns drop off fast. Match the amp's power and the DAC's quality to what your headphones actually need instead of buying the priciest box on the shelf.

For premium in-ears, a good USB-C dongle DAC/amp is nearly always enough. Very sensitive IEMs can even pick up hiss from a noisy source, so a quiet output matters more than a powerful one, and a desktop amp for IEMs is rarely worth it unless you want one box for in-ears and over-ears both. My universal IEM picks cover the in-ears themselves.

Myths and audiophile marketing to ignore

"Higher impedance means better sound" gets thrown around a lot, and it's false. High impedance is a design choice, and it only pays off with a proper source. A 32-ohm pair on a good source can easily beat a 600-ohm pair run off something too weak to drive it, so impedance is just a spec.

"You need a $1,000 amp for a $500 headphone" is generally false. The "spend as much on the amp as on the headphones" rule is forum folklore with no engineering behind it. A good $200 desktop amp drives an HD 650 properly, and the $1,000 one mostly adds polish. I'd spend on the headphones and add a sensibly priced amp.

"Phones can't drive serious headphones" is false for most headphones. A modern phone with a good USB-C dongle has plenty for anything up to about 100 ohms and enough for a lot of headphones up to around 250. The "phones can't drive headphones" line was largely true 15 years ago and mostly isn't today.

"Balanced outputs are way better than single-ended" is mostly false in practice. Balanced amps do have some on-paper advantages, like lower crosstalk and more voltage on tap, but with good gear the audible difference is small for most people. It gets sold as a premium feature, and paying a lot extra for it when single-ended would do the job usually isn't worth it.

"All modern DACs sound the same" is partly true. Past a fairly low bar (roughly the $100-to-$150 entry-level DACs), measurements reach the point where nobody can hear a difference, and spending more mostly buys features and engineering pride. The claim's overstated, but there's a lot of truth in it.

"You need a tube amp for warm sound" is misleading. Tube amps can add a pattern of harmonic distortion some people hear as warmth. That's a coloring, and calling it an upgrade is a stretch. Whether you like that coloring is personal, and whether it's "better" than clean solid-state is up for debate. If you want neutral, solid-state is the safer bet, and if you love what tubes do to a jazz vocal, enjoy it.

Questions people ask about impedance and amps

Where do I find my headphones' impedance and sensitivity?

Start with the maker's own product page, Sennheiser, Sony, Beyerdynamic and so on, and look for the specs or tech data section. Impedance is in ohms, and sensitivity is in dB/mW or dB/V, so check which one they're using before you compare. RTINGS also publishes its own measurements for hundreds of headphones. Amazon listings can be incomplete or just plain wrong, so for a pricey purchase I'd always check the maker's site.

Will an amp make my $50 headphones sound better?

Probably not by much. Cheap headphones are held back by their drivers, tuning and build far more than by a lack of power. The same $200 spent on better headphones will do a whole lot more than an amp would, so I'd upgrade the headphones first and only add an amp once they're no longer the weak link.

Can an amp damage my headphones?

It can if you crank a powerful amp way past sane levels, since most passive headphones have nothing inside to stop a driver from being pushed too hard. At normal volumes it's not a worry. The bigger risk is to your ears. A strong amp gives you plenty of rope to damage your hearing long before the drivers give up. Start with the volume down and bring it up slowly.

What about "synergy" between certain amps and headphones?

It's real but overblown. Some pairings work better thanks to impedance matching, output power or tonal balance, but the differences between competent pairings are usually subtle, and "synergy" sometimes gets used to justify spending a lot when a cheaper option would do just as well. I'd trust sources that publish real data, like RTINGS and Audio Science Review, over talk of magical pairings.

USB-powered amp, or one with its own power supply?

For most people USB power is fine. Modern USB, especially USB-C with power delivery, is clean enough for a desktop amp, and while a self-powered amp with a proper transformer is better in theory, the difference under about $500 is small. Dedicated power starts to matter on high-end systems, tube amps that need their own voltages, and setups where you're chasing the last sliver of performance.

Separate amp and DAC, or a combo unit?

For a first setup, a combo (amp and DAC in one box) is fine and saves money, and FiiO's K7 and similar units do both jobs well. As you move up, separates make more sense, letting you upgrade one without replacing the other. Under about $400 total I'd go combo, and past $500 separates start to pull ahead.

Does the cable change how easy my headphones are to drive?

At normal lengths, say 3 to 10 feet, no. The cable's resistance is tiny next to the headphones' own impedance, so a few extra feet don't change what the amp sees. It can start to matter on very long runs (30 feet and up) or with very low-impedance in-ears, but for almost everybody, the cable is about durability and convenience.

Does impedance matter for wireless headphones?

Not in wireless mode. The amp is built in and matched to the drivers, so the spec only comes into play if you plug in a cable, and wireless pairs are designed to be easy to drive when you do.

My bottom line

"Do I need a headphone amp?" has a simpler answer than the forums make it sound.

Most people don't. If your headphones are wireless, the answer is flat-out no. If they're under 100 ohms, which most modern headphones are, a decent USB-C dongle or your interface is plenty, and if you're mixing or mastering, your interface's headphone out is built for the job.

Some people do. If you're buying high-impedance audiophile headphones (the HD 600, HD 650, HD 800 S or a 250-ohm DT 770 Pro) or a low-sensitivity planar, a dedicated amp gets you better results you can measure and hear. Set aside something like $150 to $300 for a starter desktop amp/DAC along with the headphones.

A few people need specialist gear. Electrostatics need their own electrostatic amps, and a few power-hungry planars like the Susvara want something closer to a speaker amp. Those are edge cases, and the forums are a good place to dig in when you get there.

So if you're just wondering whether you need an amp, probably not. Buy the headphones that fit how you listen, use them with the source you've got, and only add an amp if the headphones really call for it. The most common mistake new audiophiles make, I think, is spending on amps before their headphones are good enough for an amp to matter.

The whole chain (headphones, amp, DAC, cables, files) is a fun rabbit hole once you're hooked on the hobby, but you don't need any of it to enjoy great headphones. A Sony WH-1000XM6 sounds great straight off your phone, an HD 560S sounds great off your interface, and the 63-ohm MDR-7506s that show up on every truck I've worked out of need no amp at all, with 106 dB/mW of sensitivity to spare.

Impedance and sensitivity, and whether you need an amp.