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NwAvGuy: Headphone & Amp Impedance

NwAvGuy: Headphone & Amp Impedance

2024-02-17 12:49:07

INTRO: The output Impedance of headphone sources is likely one of the most typical causes the identical headphones can sound completely different relying on what they’re plugged into. This necessary parameter is never specified by manufactures however could make an enormous distinction in sound high quality and headphone compatibility.

HEADPHONE IMPEDANCE MOVED: This text was once about each output impedance and headphone impedance. However, within the curiosity of shorter articles, I’ve break up them. So if you happen to’re in search of information headphones, please see:

THE SHORT VERSION: All you actually need to know is most headphones work finest when the output impedance is lower than 1/eighth the headphone impedance. So, for instance, with 32 ohm Grados the output impedance will be, at most, 32/8 = 4 ohms. The Etymotic HF5s are 16 ohms so the max output impedance is 16/8 = 2 ohms. If you wish to be assured a supply will work properly with nearly any headphone, merely ensure the output impedance is underneath 2 ohms.

WHY DOES OUTPUT IMPEDANCE MATTER? It issues for at the very least three causes:

  • The better the output impedance the better the voltage drop with decrease impedance hundreds. This drop will be massive to sufficient to stop driving low impedance headphones to sufficiently loud ranges. An actual world instance is the Behringer UCA202 with a 50 ohm output impedance. It struggles with some 16 – 32 ohm headphones.
  • Headphone impedance modifications with frequency. If the output impedance is far above zero this implies the voltage delivered to the headphones can even change with frequency. The better the output impedance, the better the frequency response deviations. Totally different headphones will work together in numerous, and usually unpredictable, methods with the supply. Typically these variations will be massive and plainly audible.
  • As output impedance will increase electrical damping is decreased. The bass efficiency of the headphones, as designed by the manufacture, could also be audibly compromised if there’s inadequate damping. The bass may change into extra “boomy” and fewer managed. The transient response turns into worse and the deep bass efficiency is compromised (the headphones will roll off sooner at low frequencies). A number of, resembling those that like a really heat “tube like” sound, may take pleasure in this kind of underneath damped bass. However it’s virtually at all times much less correct in comparison with utilizing a low impedance supply.

THE 1/eighth RULE: To attenuate all three of the above issues, it’s solely essential to preserve the output impedance lower than 1/eighth the headphone impedance. Or, put one other method, simply divide the headphone impedance by 8 to get the utmost output impedance with out potential audible degradation.

IS THERE A STANDARD FOR OUTPUT IMPEDANCE? The one customary I’m conscious of is IEC 61938 from 1996. It specifies an output impedance of 120 ohms. There are quite a few the explanation why that is customary is method out of information and a very unhealthy concept. In a Stereophile article about headphones, they stated of the 120 ohm customary:

“Whoever wrote that should dwell in a fantasy world.”

I’ve to agree with Stereophile. The 120 ohm customary may need been (barely!) tolerable earlier than the iPod and different moveable music sources turned immensely fashionable, but it surely’s not any extra. Most headphones are designed very otherwise immediately.

PSUEDO STANDARDS: A number of skilled gear has a 20 – 50 ohm headphone output impedance. I’m not conscious of any that follows the 120 ohm IEC customary. Shopper gear tends to be within the vary of 0 – 20 ohms and, apart from tube and sure different esoteric designs, most high-end audiophile headphone sources are properly underneath 2 ohms.

THE iPOD INFLUENCE: Because the 120 ohm customary was revealed in 1996, music gamers superior from lo-fi cassette tape and skipping moveable CD gamers to the huge iPod craze. Apple helped take top quality audio moveable and there are at the very least half a billion moveable digital gamers in circulation not together with telephones. Practically all moveable music/media gamers now run from a single cell Li-Ion battery. These batteries solely produce a bit over 3 volts which implies you usually get lower than 1 volt RMS of audio output driving typical headphones (typically a lot much less). In case you add 120 ohms to the output, and use typical moveable headphones (practically all of that are within the vary of 16 –32 ohms) the headphones often received’t play loud sufficient. And a lot of the battery energy is wasted as warmth within the 120 ohm resistor. Solely a small fraction of the facility makes it to the headphones. That’s a giant downside in moveable audio the place getting one of the best battery life from ever smaller gadgets is crucial. It’s far more environment friendly to ship all the facility to headphones.

HEADPHONE DESIGN: So what output impedance do headphone manufactures design for? As of 2009 properly over 220 million iPods had been offered. The iPod, and comparable moveable gamers, are the 800 pound gorillas within the headphone market. So, not surprisingly, most manufactures began designing many or all of their headphones to work properly with the iPod. Meaning they’re designed to work with an output impedance underneath 10 ohms. And better-end full measurement cans are most frequently designed for sources that observe the 1/eighth Rule or have a close to zero output impedance. I’m not conscious of any present audiophile headphones meant for dwelling use designed to the traditional 120 ohm customary.

THE BEST HEADPHONES ARE DESIGNED FOR THE BEST SOURCES: In case you do a fast survey of probably the most properly reviewed high-end headphone amps and DACs, they practically all have very low output impedances. Examples are merchandise from Grace Designs, Benchmark Media, HeadAmp, HeadRoom, Violectric, and so on. It solely stands to motive that almost all high-end headphones are designed to be at their finest with comparable merchandise. Among the most extremely regarded headphones have comparatively low impedances together with a number of fashions from Denon, AKG, Etymotic, Final Ears, Westone, HiFiMAN and Audeze. All of those, so far as I do know, had been designed for use with low (ideally close to zero) impedance sources. I’ve additionally had a Sennheiser consultant inform me they design their audiophile and moveable headphones for zero ohm sources.

THE FREQUENCY RESPONSE PROBLEM: If the output impedance is extra greater than 1/eighth the headphone impedance there will probably be variations within the frequency response. With some headphones, particularly balanced armature or multi driver designs, these variations will be reasonably excessive. Right here’s what 43 ohms of output impedance does to the Final Ears SuperFi 5’s frequency response—a complete, and really audible, variation of 12 dB:



10 OHM OUTPUT IMPEDANCE: Some may have a look at the above instance and assume it’s excessive with a 43 ohm supply. However loads of sources have round a ten ohm output impedance. Right here’s the identical headphones with a ten ohm supply—there’s nonetheless a really audible 6 dB of variation. This kind of curve creates weaker bass, a “obtrusive” midrange emphasis, muted excessive frequencies, and odd part traits as a result of sharp “notch” at 10 khz that may alter spatial perceptions:



FULL SIZE SENNHEISERS: Listed here are the total measurement, larger impedance, Sennheiser HD590 cans with the identical 10 ohm output impedance. Now the variation is simply a bit over 1 dB above 20 hz. Whereas 1 dB isn’t that a lot, it’s proper in probably the most “boomy” bass area which is the final place most need any kind of emphasis:



DAMPING EXPLAINED: Any dynamic driver, in a speaker or headphone, strikes forwards and backwards with the music. That’s the way it creates sound and so they all have transferring mass. The legal guidelines of physics say an object in movement tends to remain in movement. Damping is used to assist keep away from undesirable movement. With out going into too many particulars, if a speaker is under-damped, it retains transferring after it ought to have stopped. And if it’s over-damped (uncommon) its potential to precisely observe the sign is compromised—think about a speaker making an attempt to function submersed in maple syrup. There are solely two methods to damp a driver—mechanically and electrically.

BOUNCING CARS: Mechanical damping is very like the shock absorbers on a automotive. They add resistance so once you hit a bump the automotive doesn’t preserve bouncing up and down lengthy after the bump. However in addition they add harshness as a result of they cut back the suspension’s potential to precisely observe the street. They’re a compromise—gentle shocks give a softer however extra bouncy trip and stiff shocks management the bouncing higher however make the trip harsher. Mechanical damping is at all times a compromise.

ELECTRICAL IS BETTER: There’s a greater choice to regulate undesirable movement of headphone drivers and it’s known as electrical damping. The voice coil and magnet of the motive force work with the amplifier to regulate the movement of the motive force. This sort of damping has fewer detrimental unwanted effects and permits headphone designers to create headphones with much less distortion and higher sound. Identical to a automotive suspension that may higher observe the street, an optimally damped headphone driver can higher observe the audio sign. However, and that is the crucial half, electrical damping is simply efficient when the output impedance of the amplifier is far decrease than the impedance of the headphones. In case you plug 16 ohm headphones into an amp with a 50 ohm output impedance, there will probably be virtually no electrical damping. Meaning when the motive force is meant to cease transferring it won’t. The headphone is extra like a automotive with worn shock absorbers. If the 1/eighth Rule is adopted, nevertheless, there will probably be ample electrical damping.

A SPEAKER ANALOGY: Again within the day, earlier than my time, audio system had been largely pushed by amplifiers that used tubes as a substitute of transistors. Tubes are excessive impedance gadgets that function at excessive voltages so practically all tube amps use output transformers. With out going into all the small print, tube amps had extensively various output impedances that had been usually important and violated the 1/eighth Rule. Speaker manufactures couldn’t depend on amplifiers having a low sufficient impedance to supply a lot electrical damping. This compromised speaker design very like headphone design is compromised immediately if a headphone designer can’t depend on a low impedance supply for correct electrical damping.

ACOUSTIC SUSPENSION: Within the 1970’s the state of affairs modified as stable state amplifiers turned fashionable. Nearly all stable state amps simply cross the 1/eighth Rule. In actual fact, most cross a 1/fiftieth Rule—their output impedance is mostly under about 0.16 ohms—generally known as a damping issue of fifty. All of a sudden speaker manufactures had been free to design higher audio system that might make the most of these a lot decrease output impedances. And the primary actually good acoustic suspension sealed field audio system like the unique AR’s, Massive Advents, and so on. had been developed. That they had deeper and higher bass than any of their tube-powered predecessors may handle from an identical field measurement. It was a giant milestone in “hi-fi” to depend on a number of electrical damping from the amplifier. It’s too unhealthy many headphone sources are 40+ years behind.

WHAT OUTPUT IMPEDANCE DOES MY SOURCE HAVE? Some manufactures make it clear they attempt for a low output impedance (resembling Benchmark), whereas others specify the precise output impedance of their merchandise (resembling Behringer does with the UCA202 at 50 ohms). And most, sadly, preserve it a complete thriller. Some product opinions, resembling those on this weblog, embrace measurements of the output impedance because it’s crucial to the sound of the gadget with varied completely different headphones.


  • Headphone Safety – Extra highly effective sources with a low output impedance may be able to delivering an excessive amount of energy into low impedance headphones. To assist shield such headphones, some designers elevate the output impedance. It is a compromise to try to have the amp adapt to the load used. However it comes at a giant value with many headphones. A greater resolution is providing two acquire choices The low acquire setting can decrease the utmost output voltage when utilizing low impedance headphones. And, as well as, energetic present limiting can be utilized so the supply will robotically prohibit the utmost output into decrease impedance headphones even when the improper acquire setting is used. 
  • To Be Totally different – Some manufactures elevate the output impedance on objective claiming it makes their supply sound higher. Typically “completely different sells” because it’s a method to differentiate the sound of their product from their opponents. However, on this case, the actual “completely different sound” you get is solely depending on which headphones are used. With some it may be an enchancment and with others it’s extra seemingly a giant step backwards. The chances significantly favor degrading the sound.
  • It’s Low-cost – The next output impedance is a band-aid for a lot of cheap headphone sources. It’s an affordable method to obtain stability, a crude type of quick circuit safety, and it will probably enable utilizing an in any other case substandard op amp or output gadget that may be unable to drive 16 and even 32 ohm headphones instantly. By including some collection resistance to the output all these items get “mounted” with a $0.01 half. However a budget “repair” comes at a considerable value within the sound high quality with many headphones.

EXCEPTIONS TO THE RULE: There are just a few headphones supposedly designed for considerably larger output impedances. I do surprise if this may be extra fable than actuality today when it comes to audiophile and client headphones as I’m not conscious for any particular examples. However it’s actually attainable. In that case, utilizing these headphones on a low impedance supply may trigger under-damped bass efficiency and a special frequency response than the manufacture meant. This may clarify among the “synergy” claims when sure headphones are mated with a sure supply. However these “synergies” are solely subjective—one man’s “brilliant and detailed” is one other man’s “harsh”. The one method to get constant efficiency is to make use of a low impedance supply and observe the 1/eighth Rule.

A CHEAP TEST: In case you’re questioning in case your present supply is compromising the sound high quality due to an unknown output impedance, contemplate shopping for the $19 FiiO E5 amp. It has a close to zero ohm output impedance and has sufficient output for many many headphones underneath 100 ohms. If it clearly improves the sound, it’s seemingly your supply has an output impedance that’s too excessive.

BOTTOM LINE: Except you recognize your explicit headphones sound higher with a particular larger output impedance, it’s finest to at all times use a supply with an output impedance no larger than 1/eighth the impedance of your headphones. Or, to make it even easier, an output impedance of two ohms or much less.


IMPEDANCE VS RESISTANCE: These two phrases are used considerably interchangeably, however technically there are some important variations. Electrical resistance is represented by the letter “R” and has the identical worth in any respect frequencies. Electrical impedance is extra advanced and its worth usually modifications with frequency. It’s represented by the letter “Z”. For the needs of this text, the unit of measure for each is Ohms.

See Also

OUTPUT IMPEDANCE DIAGRAM: The diagram under exhibits the impact of output impedance. The blue circle on the left above represents a “good supply”, the blue resistor (zig zag line) within the center represents the output impedance. And the resistor on the suitable represents the load impedance (the headphones). If the output impedance is just not zero, the voltage produced by the supply will probably be decreased when a load is linked. The upper the output impedance, the better the drop in voltage on the load. This drop is given by the method: Load Voltage = Supply Voltage * ( Zload / ( Zload + Zout) ). For extra info see Wikipedia Voltage Divider.

output impedance

VOLTAGE AND CURRENT: It’s necessary to have at the very least some understanding of voltage and present to know impedance and this text. Voltage is analogous to water strain (i.e. PSI) whereas present is analogous to the amount of water (i.e. gallons per minute). In case you let water run out of the tip of your backyard hose with nothing connected you get lots of move (present) and might fill a bucket shortly however the strain on the finish of the hose is close to zero. In case you put a small nozzle on the hose the strain (voltage) is far larger however quantity of water is decreased (it takes longer to fill the identical bucket). The 2 are usually inversely associated. Excessive strain often means low move and visa versa. The identical is true of voltage and present. The connection between voltage, present, and resistance (and for the needs of this text, impedance) is outlined by Ohm’s Law. Substitute Z for R.

WHERE DOES THE 1/eighth RULE COME FROM? The smallest audible distinction most can hear is about 1 dB. For the output impedance to create a -1 dB change, you have got antilog(-1/20) = 0.89. Utilizing the divider method from above, when the output impedance is 1/8 the load impedance you get 0.89 or a 1 dB drop. Headphone impedance can range by an element of 10 or extra over the audio band. The SuperFi 5 is rated at 21 ohms however varies from 10 ohms to 90 ohms. So the 1/8 Rule provides a max output impedance of two.6 ohms. Assuming a 1 volt supply we get:

  • Headphone Voltage at 21 Ohm Nominal Impedance = 21 (21+2.6) = 0.89 volts
  • Headphone Voltage at 10 Ohm Minimal Impedance = 10 (10+2.6) = 0.79 volts
  • Headphone Voltage at 90 Ohm Most Impedance = 90 (90+2.6) = 0.97 volts
  • Frequency Response Variation = 20*LOG(.97/.89) = 0.75 dB (underneath the 1 dB purpose)

MEASURING OUTPUT IMPEDANCE: As seen within the diagram above the output resistance kinds a voltage divider. By measuring the output voltage with no load, and with a recognized load, you’ll be able to calculate the output impedance. This online calculator makes it straightforward. The no load voltage is the “Enter Voltage”, R2 is the recognized load resistance (don’t use headphones), the Output Voltage is the loaded voltage. Click on Compute and R1 is the calculated output impedance. This may be achieved utilizing a 60 hz sine wave file (Audacity can create such a file), a Digital Multi Meter (DMM), and a 15 – 33 ohm resistor. Most DMMs are solely correct round 60 hz. Play the 60 hz sine wave file and regulate the amount for about 0.5 volts. Then connect the resistor and word the brand new voltage. For instance, 0.5 volts with no load, and 0.38 volts with a 33 ohm load provides an output impedance of about 10 ohms. The mathematics is: Zout = (Rload * (Vnoload – Vload)) / Vload

REACTIVE LOADS: Few headphones characterize a purely resistive load that’s fixed over the audio band. As an alternative, they’re reactive loads and characterize a complex impedance. Due to the capacitive and inductive parts in headphones their impedance modifications with frequency. For instance, right here is the Tremendous Fi 5’s impedance (yellow) and part (white). The impedance is simply 21 ohms under about 200 hz. Above 200 hz it climbs to almost 90 ohms at 1200 hz after which drops down under 10 ohms at 10 Khz:


FULL SIZE CANS: Some aren’t thinking about IEMs just like the Tremendous Fi 5’s so right here the impedance and part for the favored Sennheiser HD590. It nonetheless varies from about 95 ohms to almost 200 ohms—a spread of 2X:


THE MATH: Earlier a graph was proven demonstrating about 12 dB of frequency response variation for the SuperFi 5’s pushed from a 43 ohm supply. If we take their rated impedance of 21 ohms because the reference degree, and assume a 1 volt supply, the voltage on the headphones will probably be given by:

  • Reference Stage: 21 / (43 + 21) = 0.33 V and we’ll name that 0 dB.
  • At their minimal impedance of about 9 ohms it’s 9 / (9 + 43) = 0.17 V = – 5.6 dB
  • At their most impedance of 90 ohms it’s 90 / (90 + 43) = 0.68 V = +6.2 dB
  • Whole Variation = 6.2 + 5.6 = 11.8 dB

DAMPING LEVELS: The damping of a headphone driver, as defined earlier, is both solely mechanical damping (Qms) or a mix {of electrical} (Qes) and mechanical damping. The full damping is named Qts. How these parameters work together at low frequencies is defined by Thiele Small modeling. Damping will be generalized into three classes:

  • Critically Damped (Qts = 0.7) – That is extensively thought-about superb because it offers the deepest bass extension with none frequency response variations or extreme “ringing” (uncontrolled driver movement). The bass from a critically damped driver is usually described as “tight”, “fast”, and “clear”. Q of 0.7 provides what most contemplate the perfect transient response.
  • Over-Damped (Qts < 0.7) – This retains even tighter management over the motive force however on the expense of much less deep bass (the response rolls off sooner). So manufactures hardly ever deliberately over damp their merchandise.
  • Beneath-Damped (Qts > 0.7) – This trades off some low bass extension for a peak at larger bass frequencies. The motive force can also be now not properly managed and reveals extreme “ringing” (i.e. it would not cease quickly sufficient when the audio sign stops). Beneath-damping creates frequency response variations, much less deep bass, poor transient response and an higher bass peak. Beneath-damping is an affordable method to supply the phantasm of extra bass on the expense of the standard of the bass. It is often utilized in low-cost headphones and audio system to supply “pretend bass”. Beneath –damped headphones/audio system are often described as having “boomy” or “sloppy” bass. In case your headphones had been designed for electrical damping, and you utilize them with a supply impedance better than 1/eighth their impedance, you’re going to get under-damped bass.

TYPES OF DAMPING: There are 3 ways to damp the motive force and management resonance:

  • Electrical Damping – That is recognized is Qes and it’s one thing like regenerative braking on on hybrid or electrical automotive. While you hit the brakes, the electrical motor slows the automotive by turning right into a generator and sending the power again to the battery. A driver in a headphone (or speaker) can do the identical factor. However because the output impedance of the amplifier goes up, the braking impact is significantly diminished—therefore the 1/eighth Rule.
  • Mechanical Damping – This is named Qms and, as defined earlier, it’s extra just like the shock absorbers on a automotive. As you add mechanical damping to a driver, it resists the musical sign driving it, and turns into extra non-linear. This will increase the distortion and degrades the sound high quality.
  • Enclosure Damping – The enclosure can present damping however this often requires both a sealed enclosure, one with a tuned port, or one with a managed restriction. Lots of the finest headphones, nevertheless, often are open backed. This largely eliminates the headphone designer’s choice of utilizing the enclosure to supply damping as is finished with audio system.

EAR CUP LOADING: For headphones that kind a reasonably constant seal, such a completely circumaural over the ear headphones with earpads that match snuggly towards the top, the designer can considerably depend on the “enclosure” fashioned by the ear cup to presumably present some damping. However the form of heads, ears, sort of hair, headphone positioning, eyeglasses and different components make it extremely variable. And this feature isn’t out there in any respect for all of the supra-aural (on the ear) headphones. Listed here are two graphs of the Sennheiser HD650 impedance. Observe the open air bass resonance peaks at about 530 ohms however drops to 500 ohms on a simulated head. This is because of damping offered by the ear cup enclosure and the ear pad.Sennheiser HD650 Impedance & Phase New Open Air

Sennheiser HD650 Impedance & Phase New Simulated Head

FINAL WORDS: Hopefully I’ve made it clear the one method to get constant efficiency between headphones and their supply is to observe the 1/eighth Rule. Whereas some might choose the sound utilizing a better output impedance, that’s very particular to every explicit headphone, the actual output impedance, and the individual’s personal subjective tastes. Ideally a brand new customary must be developed and manufactures must be inspired to design headphone sources with an output impedance under 2 ohms.

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