A vintage headphone that sounds thin, quiet or dull is often perfectly healthy. It's just plugged into the wrong thing. Headphones from the 1960s to the 1990s were built for hi-fi receivers, studio desks and speaker outputs, not phones, and their impedance runs from a few ohms to 2,000. Before you buy, work out what will drive the pair you want. It's the cheapest mistake to avoid.
Two numbers, not one
Impedance, in ohms, is how hard the headphone resists the current an amp pushes through it. Low-impedance headphones run from about 16 to 32 ohms and high-impedance ones from about 100 to 600, with a few vintage models far above that. Impedance alone doesn't tell you how loud a pair will play. For that you need the sensitivity, how much sound it makes for a given input.
Makers quote sensitivity two ways, in decibels for one milliwatt (dB/mW) or in decibels for one volt (dB/V). The volt figure is the handier one, because a headphone output is really a voltage source. If you know how many volts your amp can swing and the headphone's dB per volt, you know roughly how loud it will go.
The rule of thumb that follows is simple. High-impedance headphones need voltage, and low-impedance headphones need current. A phone has very little voltage to give, so it suits a low-impedance, sensitive pair. A home receiver or a headphone amp has more voltage, so it suits the high-impedance classics.
Some real examples
The Grado SR60 sits at one end. It's a 32-ohm design that Grado aimed at Walkmans and portable CD players, and Stereophile's reviewer drove it to loud levels from a cheap Walkman. The Koss Porta Pro is similar, at 60 ohms and 101 dB per milliwatt. Either will play well from any phone.
The Sennheiser HD 565 Ovation shows why impedance isn't the whole story. It's 160 ohms, but it's also sensitive, at 108 dB for one volt, so a decent phone or laptop output gets it to a good volume. The Beyerdynamic DT 220 is similar. It's 400 ohms but rated at 102 dB per milliwatt, unusually high for the impedance, so a receiver's headphone jack drives it well.
The Beyerdynamic DT 880 of the 1980s is a 600-ohm design at about 94 dB per milliwatt. Beyerdynamic's own brochure rated it for 7.75 volts into 600 ohms at full output. The AKG K240 Monitor is also 600 ohms, with a modest 88 dB rating. Both want a real headphone amp.
Then there are the outliers. The original Sennheiser HD 414 of 1968 was a 2,000-ohm design sold with loudspeaker plugs, so you could connect it to an amp's speaker terminals alongside your speakers. Studios could hang a row of them off one power amp. Later HD 414s dropped to 600, 400 and finally 50 ohms, so the version matters. The AKG K1000 runs the other way, at a moderate 120 ohms but a very low 74 dB per milliwatt. AKG shipped it with a cable for wiring it straight to a power amp's speaker terminals, which tells you how much power it wants.
Output impedance, the hidden half
Your amp has an impedance too, called its output impedance. A widely used rule says it should be less than one eighth of the headphone's impedance. So a 32-ohm Grado wants a source of 4 ohms or less, while a 600-ohm Beyerdynamic is happy with almost anything under 75.
When the amp's output impedance is too high, two things happen. The bass gets looser, because the amp has less control over the driver. And the frequency response shifts, because a headphone's impedance changes with frequency, so the voltage it receives changes too. The result can be weak bass, a glaring midrange or dull treble, and it's easy to blame the headphone for it.
This is why old gear and old headphones often go well together. A standard from 1996 specified a 120-ohm headphone output, and many older receivers and tube amps have high output impedances. On those, a 300- or 600-ohm headphone sounds right, and a modern 32-ohm one can sound wrong. A modern headphone amp with an output impedance of a couple of ohms works with almost anything.
Plugs and speaker outputs
German headphones of the 1960s to 1980s often came with a 5-pin DIN plug, bare wires or loudspeaker plugs instead of a jack. Beyerdynamic sold the DT 48 with five different cable ends, and Sennheiser sold the HD 414 X with three. Check the plug in the listing photos, and budget for an adapter or a new plug if it isn't one your amp takes.
If you do run a headphone from speaker outputs, as the original HD 414 and the K1000 were sold to be, start with the volume all the way down. A power amp can deliver far more than a headphone needs, so turn it up slowly.
Electrostatics are different again
An electrostatic headphone has no impedance in the usual sense and never plugs into a headphone jack. It needs an energizer that supplies a high bias voltage and drives the diaphragm directly, or a transformer box run from a speaker amp. Stax made two bias standards that use different plugs, and the Koss ESP/950 works only with its own E/90. Our guide to Stax Pro and Normal bias covers matching one to an energizer.
A quick way to decide
- Plays from a phone. Low impedance, or high sensitivity in dB per volt. Grado's SR series, the Porta Pro and the HD 565 fall here.
- Wants a headphone amp. 250 ohms and up with ordinary sensitivity. Most of the Sennheiser, Beyerdynamic and AKG studio classics.
- Runs off a speaker amp. The AKG K1000, and the speaker-plug versions of the original 2,000-ohm HD 414 and HD 424 can work this way too.
- Needs an energizer. Every Stax and the Koss ESP/950.
Every model page on this site has a "Before you plug in" panel that puts each pair in one of these groups, and you can filter by them on the browse page. When a listing doesn't say which version it is, ask for a photo of the label with the impedance on it. For the other checks to run on any used pair, read how to check used headphones.
