FAQs

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If you have any questions or comments, please post a message in the HeadWize discussion forums.

The Headphone FAQs page is a distillation of conversations, readings and several years of headphone-related postings in various newsgroups. Throughout the FAQ, the HeadWize Headphone Guides are frequently cited references, which is appropriate since the Guides are based on the most common threads of newsgroups discussions. The Guides alone will answer over 80% of FAQs.

What this FAQ does not cover: The most frequently asked question is “What brand or model of headphones (or other equipment) do I buy?” While HeadWize does not recommend specific brands and models of equipment, HeadWize users may post reviews of headphones and related accessories in the HeadWize discussion forums. Another source for information is audio newsgroups such as rec.audio.opinion and rec.audio.pro. The second most frequently asked question is “Where can I find schematics for headphone amplifiers?” The Projects Library has plans for tube and solid state headphone amps (as well as acoustic simulators and other headphone stuff).

About HeadWize

Headphone Technologies

  • How do I choose which headphones to buy?
  • Where do I buy this brand/model of headphones?
  • How do dynamic (or electrostatic or electret or 4-channel, etc.) headphones work?
  • Why do headphones sound different from loudspeakers?
  • How do I evaluate the sound of headphones, if they sound different from normal hearing?
  • How do I evaluate headphones for PC games?
  • Is there a simple way to improve in-front localization in headphones?
  • What technical books about headphones do you recommend?

3D Sound and Surround-Sound in Headphones

  • How can stereo headphones sound more like loudspeakers?
  • Can I play surround sound recordings in headphones?
  • What is a binaural recording?
  • Can I play binaural recordings through loudspeakers?
  • What are other 3D headphone recording formats (holophonics, ambisonics)?

Headphone Amplifiers

  • How much power do headphones need?
  • Should I consider buying a headphone amplifier?
  • How can I drive headphones from a power amplifier?
  • Is an amplifier’s damping factor important to headphone performance?

Headphone Accessories

  • How do I wire a headphone plug/jack?
  • How do I make a volume control for headphones?
  • How do I add a balance control for headphones?
  • Is there a simple circuit to switch between two headphones?
  • Where do I get replacement earpads?
  • What is a crossfeed processor (or shuffler or auralization processor)?
  • Why can’t I splice/solder a Sony headphone cord to other headphones?
  • What is a spatial expander for headphones (and speakers) and how can I make one?
  • What is a “dreaming” circuit?
  • What is an “induction loop” wireless headphone?
  • How can I make a monaural FM wireless headphone?

Miscellaneous

  • Can I use the output of a preamplifier to drive headphones directly?
  • Can I drive two headphones from one headphone jack?
  • What is the best way to monitor and mix with headphones?
  • Can listening with headphones result in hearing damage?
  • Is it legal to wear headphones while driving or biking?
  • How do I choose headphones for computer sound cards?
  • Where can I find monaural/communications headphones?
  • Where can I find old-style high-impedance (2,000 ohms or more) headphones?
  • Where can I find water-resistant/underwater headphones?

ABOUT HEADWIZE

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HeadWize is a non-commercial headphone resource site started by Chu Moy (me) for the purpose of disseminating information about headphones and headphone listening (see the Announcements archive for the full story). HeadWize is not responsible for any consequences arising from the use of information on this site. Please read the legal notice. The site is organized into 5 sections. The Announcements page has the latest site and industry news. Announcements are shown for the last three entries, and the rest are archived. The Announcements page is updated about every two months (sooner if there is a special need).

The FAQs page (which you are now reading) provides answers to commonly-asked questions about headphones and related products and services. It is also is an excellent starting point for newcomers to HeadWize. The topics discussed in the FAQs are an overview of the subjects covered in the HeadWize Library. In fact, the answers often include hyperlinks to documents in the Library.

The HeadWize Library archives materials related to headphones and headphone listening. The Articles and Essays section includes technical as well as expository works. Expository works examine how headphones integrate into everyday living situations. They are informative, funny, sad, thought-provoking. Some have multimedia content.

The Projects and Technical Papers sections are for sound professionals and audio hobbyists alike. The Projects Library is for “do-it-yourselfers” with articles ranging from headphone tweaks to headphone amplifier construction plans and includes an inspirational DIY Showcase for various DIYer efforts. The majority of projects are aimed at DIYers who are short on time, and many are simple enough for DIY beginners. There is also a software section with downloadable applications that improve or enhance headphone listening in PC-based music players.

The Technical Papers Library features a range of topics and includes a patents section called the Design Showcase. Although the name “Technical Papers Library” suggests that the contents are for scientists and researchers, many of the articles are written in a style that is accessible to laypersons. On the other hand, the style of writing in patent documents in the Design Showcase can take a bit of effort to understand. The wealth of information in the patents usually makes the effort worthwhile.

The HeadWize Headphone Guides in the Articles section and the HeadWize Design Guides in the Projects and Technical Papers sections comprise the core holdings of the Library. The Guides contain hyperlinked references to each other and to other documents in the library for easy information retrieval.

The popular HeadWize Forums are opportunities for users to request and share information on headphone and audio-related topics. Currently, there are 7 categories: Headphone Reviews and RecommendationsHeadphone Electronics and AccessoriesDIY WorkshopAudio TechnologiesFor SaleGeneral Interest and Music Reviews. Users must register in order to post a new topic or a reply. Registration is free.

The Commercial Links and Educational Links pages list addresses, URLs and other contact information stored in the HeadWize database. Commercial Links refer to the manufacturers and dealers of headphones and related products and services. Educational Links are sites with information of interest to headphone listeners and DIYers. A simple SEARCH facility is available for both of these sections.

For the techies out there, the Links pages use a PHP hypertext preprocessor to interface with a mySQL database backend. The forums software is an custom application written in PHP and Perl and utilising a mySQL backend.

HEADPHONES AND HEADPHONE TECHNOLOGIES

How do I choose which headphones to buy?

Headphones for professional use will have a different set of selection criteria than those for home or portable use. There are so many types (home, studio, portable, dynamic, electrostatic, etc.) that many people find the selection process bewildering. More so than with loudspeakers, the sound of headphones is dependent on the listener. Since headphones are worn, other factors such as comfort play an important role in the selection process. A prospective buyer should regard recommendations as no more than suggestions. For information on the unique characteristics of headphone sound fields, see the FAQ regarding how to evaluate headphone sound quality. For more tips on evaluating headphones, see A Quick Guide To Headphones and Judging Headphones For Accuracy.

Where do I buy this brand/model of headphones?

HeadWize does not recommend retailers. The Commercial Links page includes a list of retailers, which is by no means exhaustive. As with any purchase of audio equipment, the advantages of mail order (lower prices) must be weighed against the disadvantages (no chance to audition before buying, the difficulty of obtaining customer service, etc.). If buying through mail order without any prior audition of the headphone model, make sure to confirm return privileges in case they turn out to be unsuitable.

How do dynamic (or electrostatic or electret or 4-channel or etc.) headphones work?

In the past few years, the available headphone technologies have both expanded and contracted. On the one hand, dynamic transducer headphones now dominate all headphone sales, with electrostatic headphones being the one remaining form of audiophile exotica. Competing transducer technologies, such as isodynamic and electret types, have virtually disappeared from the marketplace. On the other hand, there is great demand for noise-cancelling and surround-sound phones. New developments in headphone design may finally make virtual listening an affordable reality. For more information about how headphone technologies work, see A Quick Guide To Headphones and Technologies For Surround Sound Presentation in Headphones.

Why do headphones sound different from loudspeakers?

Sound from loudspeakers is acoustically contoured by the listener’s head and outer ears before reaching the eardrums. This interaction provides spatial cues (in the form of crossfeed and phase and amplitude shifts) which are important for perception of 3D sound. These spatial cues are called head-related transfer functions (HRTFs). Because headphones sit on top of the ears, the sound goes directly to the ear drums without any HRTF transformation. Lacking spatial cues, the brain images the sound field inside the listener’s head in a straight line between the ears. There are headphone designs and electronic processors that can make headphone listening more realistic. For more information about acoustic perception in headphones, see The Elements of Musical PerceptionA 3D Audio Primer and The Psychoacoustics of Headphone Listening.

How do I evaluate headphones, if they sound different from normal hearing?

Audiophile quality headphones are available for the price of “budget” bookshelf loudspeakers, and deserve the same attention to sound quality that is given to loudspeakers. Because headphones present a distorted perspective, listeners must take into account various psychoacoustic factors when judging sound quality. The good news is that unlike loudspeakers, headphones are not affected by room acoustics and do not require extensive weight-training to lift them. For an in-depth discussion about evaluating headphones, see A Quick Guide To Headphones and Judging Headphones For Accuracy.

How do I evaluate headphones for PC games?

State-of-the-art computer games have spectacular 3D sound effects. When evaluating headphones for PC games, the best approach to is to take a game demo disk or a recording of some samples of the 3D game sounds to the audio store to audition with headphones. The ability to convey the directionality of stationary and moving sound sources (front, back, up, down, left, right) is important in gaming headphones. In general, headphones that present a good sense of acoustic space with music will also perform well with games. If the headphones will only be used for gaming, some gamers will select headphones that sound especially good with sound effects (for example, a very strong bass response), although they might not be as suitable for general listening.

Is there a simple way to improve in-front localization in stereo headphones?

Here is a simple no-cost trick for getting a headphone soundfield to seem more like loudspeakers in front of the listener: with supra-aural headphones, rest the earcups slightly lower and forward on the ears. With circumaural headphones, try positioning them such that the earcups push the back of the ears forward a bit. If the back of the ear lobes are pushed too far forward, you may hear a “cupping” distortion. Experiment to obtain the best localization (don’t expect miracles, though).

The goal is to get the sound to enter the ears at an angle and thus engage more of the HRTFs of normal hearing. This technique works even better with a crossfeed processor (for more information about crossfeed processors, see FAQ on acoustic simulators for headphones). For true 3D sound in headphones, see the FAQ on headphone virtualizers.

What technical books about headphones do you recommend?

The best technical reference is The Loudspeaker and Headphone Handbook by John Borwick (1994, 2nd ed). Even though the coverage of headphones is a fraction of that for loudspeakers, no other book (at least none that this writer has seen) is as comprehensive. The current 2nd edition was first published in 1989, and some sections, such as the one on acoustic simulators, need revision. Also missing is any discussion of IEC standards for headphones. Nevertheless, this book is highly recommended. There is a 3rd edition published in 2001. I have not seen this edition yet.

The Art of Sound Reproduction by John Watkinson (1998) is an excellent reference about audio technology generally. Unfortunately, the author devotes just a few pages to headphones.

3D SOUND AND SURROUND-SOUND IN HEADPHONES

How can stereo headphones sound more like loudspeakers?

The sound field of most headphones forms in a straight line between the ears, because headphone listening lacks the head-related and visual cues which aid in spatial location. Acoustic simulators make up a broad category of products that electronically restore these cues to make headphones sound more like loudspeakers.

There are many types of stereo acoustic simulators, each with different circuitry. Simulators can try to recreate the complex sound field projected by loudspeakers, use psychoacoustic effects to fool the listener or a combination of both. The simplest are called crossfeed processors and spatial expanders. Crossfeed processors are analogue circuits that add inter-aural crosstalk to headphone sound fields (crosstalk exists when sounds that reach one ear are also partially heard in the other ear). They range in complexity from passive networks to multi-stage opamp filters that reshape and time delay the crossfeed. Spatial expanders emphasize ambience information in the stereo signals for a more spacious sound.

Crossfeed filtered sound is more focused and may have a sense of in-front depth. Spatially-expanded sound is more diffuse with an echo-like characteristic. Neither will produce a believable illusion of listening to loudspeakers, although they can make headphone listening more pleasant. The best way to get loudspeaker-like sound is to use a headphone virtualizer such Dolby Headphone, WOW from SRS Labs or Natural Headphone from Spatializer Labs. Virtualizers have more sophisticated electronics to produce the most realistic and convincing 3D sound fields in headphones. The performance of a virtualizer can be listener-dependent, because everyone’s hearing characteristics are slightly different.

Headphone virtualizers can create the illusion of listening to a multi-channel loudspeaker setup, and for that reason, they are popular in DVD players (and especially software DVD players). There are stereo virtualizer plugins (these accept a basic two-channel input) that work with popular software audio players such as Winamp and the Windows Media Player. Because low-cost stereo virtualizer chips have only recently hit the market, OEMs have announced that they will be incorporated into general consumer electronics products (such as portable players) in late 2001 or early 2002.

Headphone virtualizers can be found in PC sound cards. Be careful to distinguish between virtualizers for for headphones and for loudspeakers (virtualizers for loudspeakers generate surround sound from stereo loudspeakers). For more information about acoustic simulation, see the FAQs on playing surround sound recordings in headphones and on spatial expanders, A Quick Guide To Headphone Accessories and Technologies For Surround Sound Presentation in Headphones and The Psychoacoustics of Headphone Listening.

For more information about 3D hearing, see The Elements of Musical Perception and A 3D Audio PrimerVirtual Audio For Headphones.

Can I play surround-sound recordings in headphones?

Yes. Once the surround recording is decoded into its discrete channels (e.g., 4-channel or 5.1 channel surround), the audio can be listened to with multi-channel headphones; or with regular stereo headphones and a virtualizer. Multi-channel headphones have 4 or more transducers arranged to mimic a surround loudspeaker setup. However, because the transducers are positioned very close to the listener’s ears (resulting in a loss of spatial cues), most multi-channel headphones will still require additional electronic processing to achieve realistic 3D sound.

Virtualizers are sophisticated types of acoustic simulators that can make headphones seem like listening to loudspeakers. They work by creating a virtual multi-channel speaker setup inside headphones. There are numerous surround virtualizer technologies (such as Dolby Headphone, Spatializer Natural Headphone and SRS Lab’s WOW processing) that are features of video and audio players – especially software video and audio players. PowerDVD and WinDVD are two examples of software DVD players with Dolby Headphone. How well a virtualizer images depends on the technology and how well it can generate the spatial cues that a listener needs to hear in 3D. For example, if a virtualizer cannot simulate head movement, some listeners will hear a flat sound field regardless of any processing done to the signal.

Be careful to distinguish between acoustic simulation for for headphones and for loudspeakers (acoustic simulation for loudspeakers generates surround sound from stereo loudspeakers). For more information about acoustic simulation and virtualizers, see the FAQ regarding acoustic simulators, A Quick Guide To HeadphonesA Quick Guide To Headphone Accessories and Technologies For Surround Sound Presentation in Headphones.

What is a binaural recording?

Regular stereo recordings are meant to be played back through loudspeakers. They do not image correctly in headphones which lack the head-related transfer functions (HRTFs) that infuse sound with spatial cues (see FAQ on why headphones sound different from loudspeakers). Binaural recordings are recorded with a pair of microphones positioned near the artificial ears of a dummy head. When heard through headphones, they image with great realism, because the HRTFs of the dummy head provide the spatial cues. How well they image depends on how closely the shape of the dummy head resembles the listener’s head and on the positioning of the microphones. Some binaural recordings can be played back over loudspeakers with a slight degradation of the sound field. For more information about binaural recordings, see Taking Sound In Another Direction. HeadWize also has project plans for building a binaural headset to make binaural recordings.

Can I play binaural recordings through loudspeakers?

Yes. Most modern binaural recordings are compatible with loudspeakers with little or no degradation in sound quality. The compatibility results from the placement of the microphones on the dummy head. For headphone-only binaural recordings, the mikes are placed over the ears and facing outward. In the case of loudspeaker-compatible recordings, the mikes are positioned near the ears and facing forward. A few listeners feel that the headphone-only recordings image better. A transaural processor can filter out interaural crossfeed in a binaural recording for improved imaging through loudspeakers. Standalone transaural processors are rare, but a headphone spatial expander or any surround effects decoder that uses inverted crossfeed (“ambience enhancement”) might have similar beneficial effects. Schematics for spatial expanders can be found in the Headphone Accessories FAQs. Build a binaural microphone headset to make binaural recordings.

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Also, try playing binaural recordings in surround systems using a surround decoder for stereo recordings (which work by directing ambience information to the rear channels). The Dynaco-Hafler passive surround circuit is an inexpensive and effective configuration. More sophisticated surround decoders will probably have ambience-based decoding as a default option (for example, Dolby Labs recommends leaving its ProLogic decoders on when playing regular stereo recordings for a surround-like effect).

What are other 3D headphone recording formats (holophonics, ambisonics)?

Besides binaural, no other headphone-specific recording format has met with any success in the marketplace. However, these technologies are important, because they may be able to improve the playback of standard stereo and surround recordings in headphones. Holophonics and ambisonics are probably the best known of these alternative recording formats. Holophonics appears to be a variation of binaural recording done with a specially constructed dummy head and the use of a reference tone during the recording process to enhance spatial cues. The audio community continues to debate the merits of holophonics over standard binaural recording.

Ambisonics (also called B-Format) is a technology for recording and manipulating sound fields. Four microphones with figure 8 pickup patterns record sound on the x, y, z and w (omnidirectional) axes. On playback, a decoder transforms these 4 directions of sound into any number of representations (2-channel stereo, 4-channel surround, 6-channel surround, etc.). For headphone listening, the decoder outputs a 2-channel signal which is then binaurally filtered with spatial cues. Unlike other surround format decoders, an ambisonic decoder can simulate head movement by rotating the x, y and z components prior to binaural filtering. Therefore, the binaural filter can be static with a single EQ-modified HRTF to simulate head movement. An ambisonic decoder can store many more HRTF patterns than a dynamic HRTF decoder, and so has the potential of being compatible with more listeners.

For more information about ambisonics, see Creation Manipulation And Playback Of sound fields. For more information about headphone surround, see Technologies For Surround Sound Presentation in Headphones.

HEADPHONE AMPLIFIERS

How much power do headphones need?

Modern dynamic headphones are very efficient (over 90dB SPL per milliWatt) and will reach full volume with just a few milliWatts of input. Headphone amplifiers are typically rated between 10mW and 100mW. For more information, see Understanding Headphone Power Requirements.

Should I consider buying a headphone amplifier?

This question has two points of view: the consumer and the audio professional. Home audio equipment such as receivers, preamplifiers and integrated amplifiers, have headphone jacks. All portable stereos have headphone jacks. Despite the availability of these facilities, a separate headphone amplifier may offer better sound for a number of practical reasons. For example, portable stereos limit the output of headphone circuits to conserve battery power, so they may not be able to drive headphones cleanly at loud levels. The headphone circuitry in home audio components may be distorted or noisy, because the manufacturer designed it as an afterthought.

Audio engineers work on a larger scale – driving numerous headphones at the same time. A power amplifier can be configured as a distribution amplifier (see FAQ regarding how to drive headphones from a power amplifier), but will not have the features and flexibility of a commercial distribution amplifier. A variation of the basic distribution amplifier is the headphone mixer, which integrates a full mixing console with a multiple output headphone amp. For more information about headphone amplifiers, see A Quick Guide To Headphone Accessories.

How can I drive headphones from a power amplifier?

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Small power amplifiers (20 – 40W per channel) can be configured as distribution amplifiers for driving 1 to 10 or more headphones simultaneously. The simplest setup is to place a current-limiting 100 ohm, 2W resistor at the L and R speaker outputs and then connect the headphones to the resistors (see diagram above). Add another set of 100 ohm resistors per additional headphone. Each resistor forms a voltage divider with the impedance of the headphone transducer. Thus, if the headphone has a 50 ohm impedance, the voltage across a transducer will be:

Vout = Vin(Z(phones)/(100 + Z(phones))) = 50/150(Vin) ~ .33(Vin)

Headphones will reach full volume with only a few milliwatts of input. If the volume is too high, increase the value of the resistor. When driving many headphones from one amplifier, the total impedance of the resistor array should not fall below the rated load of the amplifier. For example, two pairs of headphones with 8 ohm impedances and 100 ohm current-limiting resistors will present a load of about 50 ohms per channel to the amplifier. If the headphones are different models (e.g., they have different impedances and sensitivity specs), they will be playing at different volume levels.

The current-limiting resistors will increase the output impedance of the amplifier as seen by the headphones. Whether the higher impedance affects the sound of the headphones depends on the design of the headphones. The IEC 61938 international standard recommends that headphones should expect a 120 ohm source (5V rms max.) – regardless of the headphone’s own impedance. If the manufacturer followed this standard, the 100 ohm resistor will not affect sound. See also the FAQ regarding the value of damping factor measurements for headphone amplifiers.

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A slightly more elaborate scheme is to use two resistors as a voltage divider across each channel. Here, the output impedance of the amplifier is lower (essentially the resistance of the two resistors in parallel) and the output voltage is less dependent on the impedance of the headphones. Although the effective output impedance is lower, the amplifier itself sees the voltage divider as a series load (which is good). Begin with R2 = 10 ohms (1/2W) and scale R2 up or down to adjust volume. For more information about distribution amplifiers, see A Quick Guide To Headphone Accessories.

Is an amplifier’s damping factor important to headphone performance?

With loudspeakers, the lower the amplifier’s output impedance, the higher the damping factor into the rated load. Damping factor is given as the ratio of loudspeaker impedance to the amplifier’s output impedance. As the theory goes, the higher the damping factor, the better the amplifier’s ability to control a loudspeaker’s low frequency response (when the motional reactance of the system is at maximum), because the low output impedance of the amplifier allows any back-emf generated by the loudspeaker to be absorbed by the amplifier. That theory has been discharged by members of the audio community as unsubstantiated.

However, even if the theory were correct for loudspeakers, its applicability to headphones is suspect. John Woodgate, a contributor to The Loudspeaker and Headphone Handbook (1988), had the following to say about the effect of damping factor on headphone performance:

Headphone transducers are resistance-controlled, not mass-controlled like loudspeaker drivers above the main resonance. In any case ‘damping factor’ is largely nonsense – most of the resistance in the circuit is the voice-coil resistance and reducing the amplifier source impedance to infinitesimal proportions has an exactly corresponding effect on damping – infinitesimal.

However, the source impedance affects the *frequency response* of a loudspeaker because the motional impedance varies with frequency, and thus so does the voltage drop across the source impedance. This means that the source impedance (including the cable) should be less than about one-twentieth (not one two-hundredth or less!) of the rated impedance of the loudspeaker, to give a *worst-possible change* in frequency response from true voltage-drive of 0.5 dB.

The motional impedance of headphone transducers varies very little (or should vary very little – someone can always do it wrong!) with frequency, so the source impedance can be high with no ill effect.

The IEC 61938 international standard specifies that headphones should be driven by a 120 ohm source – regardless of the impedance of the headphones themselves. If the headphones were designed to this standard, then an amplifier’s high output impedance should have little effect on the sound of the headphones. In general, headphones with a flat impedance curve over the audio range will not be affected by high output impedance. For example, in May 1995, Stereo Review published a review of the Grado SR125 headphones. The impedance curve of the SR125s, which have a nominal impedance of 32 ohms, varied from 31 to 36 ohms over the entire 20Hz to 20kHz spectrum. Not all headphones may be as well behaved as the Grados, but neither do they usually have the roller-coaster impedance runs of a loudspeaker. Tube amplifiers (with their high output impedances), it should be noted, have very low damping factors.

HEADPHONE ACCESSORIES

How do I wire a headphone plug or jack?

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The diagram above illustrates how to wire a standard stereo headphone plug. The tip is the left channel, the ring is the right channel, and the sleeve is the ground. Use an ohmmeter or continuity tester to determine the channel designations of the solder logs. With a headphone jack, insert a headphone plug with known wiring scheme and use an ohmmeter or continuity tester to match the jack connections to the plug. Note: Headphone jacks are sold in open-circuit and closed-circuit versions. The open-circuit jack is general purpose and has 3 pins for the left and right channels and ground. The closed-circuit jack has extra connections for an external circuit, such as a speaker which is disconnected when a plug is inserted.

How do I make a volume control for headphones?

The best way to control headphone volume is with the amplifier’s own volume control. However, the following two schemes will also work, but may affect the sound of some headphones:

1. Put a resistor or two resistors in series with the output to form a voltage divider. A voltage divider has the disadvantage of being a fixed attentuator. For more details about this type of setup, see the FAQ regarding how to use a power amplifier to drive headphones.

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2. A potentiometer can provide variable attenuation. Choose a low value (8 to 200 ohms), dual audio taper pot. Very high resistance pots will increase the amplifier’s output impedance to the point where the pot’s performance as an attenuator will be compromised. Also, select pots with a power rating of 2W or more. A current-limiting resistor before the pot can help prevent hearing and headphone damage. This circuit is simply an adjustable voltage divider.

For information about headphone volume control products, see A Quick Guide To Headphone Accessories.

How do I add a balance control for headphones?

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Balance controls are non-existent on most headphone amplifiers and those minimalist preamps. This circuit is one way to wire a set of balance-volume controls for headphones. The volume control should be a dual pot (audio taper). The balance control is a single pot (linear taper). The pot values shown are the smallest that are commonly available, but the circuit would probably work better if the values were lower. If the volume control is not needed, the headphones can be connected directly to the balance control. As the values of the input resistors are decreased, the balance effect will move towards the extremes of the balance control and the overall volume may be higher.

Is there a simple circuit for switching between two headphones?

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Yes, a DPDT switch is all that’s needed. The circuit above uses each side of the switch to toggle an audio channel between two headphones.

Where do I get replacement earpads?

Radio Shack sells inexpensive foam earpad replacements for generic lightweight headphones and earbuds (RS numbers: 33-374, 33-375 and 33-376). Acoustic foam manufacturers are a potential source for earpads. For example, Aurelex Acoustics sells replacement cushions for the popular Sennheiser HD414 in many different colors. When the headphones are inexpensive, ordering replacement earpads from the manufacturer can cost as much as the headphones themselves. Generic earpads are not available for the other headphones types, so replacements must be purchased from the manufacturer. The Commercial Links page includes a list of the website URLs, postal addresses and other contact information of headphone manufacturers.

What is a “crossfeed processor” or “shuffler” or “virtualizer”?

See the FAQ on acoustic simulators, A Quick Guide To Headphone Accessories and Technologies For Surround Sound Presentation in Headphones. For information on the differences between 3D sound processing for stereo loudspeakers and stereo headphones, see Virtual Audio For Headphones.

Why can’t I splice/solder/repair headphone cords from Sony (and some other brands)?

The wires in Sony headphone cords are insulated with a lacquer coating, which must be removed to make an electrical connection. Most low-wattage soldering irons are NOT hot enough to vaporize the coating. Here are some options:

  • Dip the wires in alcohol and ignite with a match. The ignited alcohol should burn off the insulation. (Note: I have received email that merely heating the wires with a match or cigarette lighter (without any alcohol) works equally well.)
  • Put the wires on an aspirin tablet and apply a soldering iron to the wire and aspirin until the aspirin melts. Aspirin is an acid that should dissolve the insulation.
  • Try a high voltage soldering iron if available which will burn off the insulation while soldering
  • Use a pocket knife to gently scrape off the insulation
  • Dip the wires with a lacquer solvent until the coating dissolves

With any method involving a solvent, make sure there is adequate ventilation and avoid inhaling any vapors released by the process. After burning or dissolving the insulation, wipe the wires clean of any residue before soldering.

What is a spatial expander for headphones (and speakers) and how can I make one?

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A spatial expander varies the amount of ambience information in a recording for a more spacious sound. Spatial expanders are NOT acoustic simulators. Unlike acoustic simulators, a spatial expander does not strive to generate a more realistic sound field in headphones, but can produce a pleasant sound effect. The schematics above describe a passive and active spatial expander. Both versions can be used with headphones, but the active version can be put between a preamp and amplifier to expand the soundstage of loudspeakers.

In the active version, the spatial effects are accomplished by mixing into each channel an inverted crossfeed signal from the opposite channel to vary the presence of the mono signal. When the MIX controls are at maximum, the mono portion of the stereo signal is entirely eliminated. In headphones, the effect widens the soundstage until it ALMOST seems to externalize. The tradeoff is reduced focus and loss of bass, because bass signals tend to be monaural.

The passive version can only be used with headphones. It offers the entire range of effects that the active one has, except that you must use two controls. The SOUNDSTAGE control varies the broadband crossfeed, while the AMBIENCE control varies the amount of L-R ambience information. The AMBIENCE control inverts crossfeed when adjusted so that the voltage across the pot is opposite in polarity to the signal across the 1K resistors with respect to the headphone transducer.

What is a “dreaming” plug?

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Named for its remarkable effect with a recording called “The Dreaming,” a “dreaming” plug wires the headphones to playback L-R ambience information in a recording and is a variation of the Hafler passive surround decoder circuit. It is a simple way of eliminating vocals, for example, because any sounds that are common to both channels are canceled out. The amplifier will see half of the normal load, which should not be a problem, but listen for distortion or other signs of electronic stress. Instead of rewiring the headphone cable (which is difficult to do and even harder to undo), add an SPST switch to the headphone jack to disconnect the ground. And see the spatial expander circuits above which feature an adjustable ambience control.

What is an “induction loop” wireless headphone?

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Audio induction systems are most commonly used as wireless communication in theaters and by hearing-aid wearers. An audio amplifier drives a coil of wire that is placed around the auditorium or room and broadcasts the audio signal in the form of a fluctuating magnetic field. An induction loop receiver (also called a T-coil in hearing aids) operates on the principle of mutual induction. It contains a wire coil that picks up the magnetic field and converts it back into audio. Induction systems are usually monaural, unless there is additional circuitry to encode and decode a stereo signal.

The diagram above shows a simple induction system that can be the basis for a monaural wireless headphone system for home use. The transmission coil is 4 turns or more of insulated wire installed around a perimeter of a room – the more turns, the stronger the transmitted signal. The loop should have a DC impedance of at least 8 ohms (or add a resistor in series to reach 8 ohms). The amplifier can be as small as 5-10W. The receiver, which is used inside the area of transmission loop, develops a 1-2mV signal across its internal coil from the fluctuating magnetic field. Q1 provides pre-amplification to drive a headphone amplifier. The Tone control varies the amount of gain in the high frequencies. C1 and C2 block AC hum (C2 also blocks DC to the headphone input). C3 functions as a low pass filter to prevent oscillation.

How can I make a FM monaural wireless headphone?

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A FM wireless headphone system consists of a FM transmitter and a FM receiver with a headphone amplifier. The FM receiver can be any portable FM radio with a headphone jack. There are commercial low-power FM transmitters suitable for personal use, but the schematic above shows a simple one-transistor transmitter that will accept input from most line-level audio sources such as a CD player or television. The circuit lacks a multiplexer to encode a stereo signal, which is instead converted to monaural through a summer at the input.

The antenna can be a standard telescoping radio antenna or a 4-15″ piece of wire attached to the second turn of the inductor. So equipped and with a 6V power source, the transmitter will cover a range of about 150 feet. To adjust the transmitter, tune the receiver to a clear frequency and vary the trimmer capacitor until the signal is heard. The transmission frequency can be pushed into the VHF (TV) band by reducing the number of turns (2-3) on the inductor.

MISCELLANEOUS

Can I use the output of a preamplifier to drive headphones directly?

Maybe. Preamplifier outputs are not designed to drive headphones. They may have very high output impedance and not have enough current capability to drive headphones adequately. Preamps with buffered outputs will probably work. There is a risk that the headphones will draw too much current and damage the preamp’s output stage. For more information, see the Audio Cables section of A Quick Guide To Headphone Accessories.

Can I drive two headphones from one headphone jack?

A standard Y cable (stereo headphone plug to dual headphone jacks) will double the headphone outputs. Headphone outputs on home and professional audio equipment can probably drive two headphones to full volume. If the headphones are not the same model, the volume levels will be different. However, headphone outputs on portable stereos often have barely enough power for one set of phones, let alone two. This arrangement will present the amplifier with half of the normal load (assuming that the two headphones are the same model), which most amps should be able to handle. Distortion or inadequate volume are indications that the amplifier is being stressed. For more information about headphone adapter cables, see A Quick Guide To Headphone Accessories.

What is the best way to monitor and mix with headphones?

Performers gain many benefits by monitoring with headphones: true stereo mix, clearer sound and hearing conservation among them. But this question always starts a lengthy debate with recording engineers as to whether headphones should be used in mixing and how they should be used. The quality of a mix done with headphones depends on how well the engineer is able to correlate headphone sound fields with loudspeaker sound fields. A spatial processor can make the job easier by simulating an acoustic space. For an in-depth look at this subject, see The Art of Monitoring and Mixing With Headphones.

Can listening with headphones result in hearing damage?

Listening to any loud sounds can result in hearing damage – so the answer is yes. Headphone listening has a greater potential for causing hearing damage, because the transducers are positioned very close to the eardrums. For information about how to listen with headphones safely, see Preventing Hearing Damage When Listening With Headphones.

Is it legal to wear headphones while driving or biking?

In many places, the answer is no. In other places, the answer is yes or maybe (for example, some laws permit wearing headphones, if one earcup remains off the ear). If an accident occurs, the party wearing headphones may have trouble collecting insurance payments. The only way to know for sure is to check the local laws. Laws banning the use of headphones while driving or biking assume that headphones will block out important background sounds such as horns and sirens and that such use is unsafe. For an opposing view about headphone laws, see A Bicyclist’s Sense of Hearing: How Important?.

What are the best computer sound cards for headphone listening?

Sad to say, most computer sound cards have unremarkable sound (often noisy, with an uneven frequency response and easily overloaded). They are built to be utilitarian and cost effective, and such design goals not always compatible with high fidelity. To make matters worse, it is rare (if not impossible) to find a computer store with its selection of sound cards installed and ready for auditioning. Trying out different sound cards on one’s own PC is an exercise in frustration – it is difficult enough to configure a machine for one sound card, never mind a couple of them. Computers with the sound circuitry built into the motherboard may not be able to accept add-in boards. Professional sound magazines are publishing sound card reviews, and consumer audio magazines are beginning to evaluate them as serious audio equipment as well.

A discussion of criteria for selecting sound cards is beyond the scope of this FAQ, but here is a short list of suggestions. While the heart of sound card performance typically lies in the onboard D/A and A/D converters, the ability to drive headphones is dependent on the audio amplifier. Most consumer sound cards have a separate audio input (often used to amplify the audio output of the CD-ROM) which is a good means of testing the audio amplifier. In general, if a sound card has both speaker and line outputs, the speaker output is likely to be noisier than the line output (any hiss and hum will be more noticeable in headphones) . Unfortunately, the line output will probably have inadequate current drive for all but the most efficient headphones (look for models with sensitivity ratings of 100dB or higher). If available, compare the sound card’s audio specifications with those of good headphone amplifiers. To improve sound quality or for more volume, consider adding a headphone amplifier to the line outputs. Some computers radiate RF frequencies that can cause a headphone amplifier to hum, so try to select one with good RF shielding.

Software-based acoustic simulators and audio equalizers, such as the Graphic Equalizer Pro from Anwida Soft, require sound cards with quality AD/DA converters that support full duplexing and have low latency times. Multimedia cards now commonly have built-in acoustic simulation algorithms that support 3D audio playback in headphones. Again, the quality of the DA/AD converters and the audio amplifiers will determine sound quality. For more information about headphones and headphone specifications, see A Quick Guide To Headphones. For more information about headphone amplifiers and equalizers, see A Quick Guide To Headphone Accessories. For information on the differences between 3D sound processing for stereo loudspeakers and stereo headphones, see Virtual Audio For Headphones.

Where can I find monaural/communications headphones?

Off-the-shelf mono headphones are more likely to be found in office and radio supply stores than audio outlets. Often sold as communications headphones (also referred to as headsets), they are used in business equipment such as telephone and speech recognition systems, as well as amateur and broadcast radio. Unlike stereo headphones, communications headsets are designed for intelligibility of speech and may have a limited frequency response (between 300 and 3kHz). Although the audio signal is monaural, they may have one or two earpieces (“binaural” headsets feed the mono signal to both ears) and accessories such as keypads and microphones or voice tubes. They are available in a variety of headphone technologies such as cordless and noise cancellation and wear styles such as in-ear and supra-aural earpieces, under-chin headbands.

The Commercial Links page lists communications headphone makers and retailers. However, be forewarned that communications headphones tend to be expensive (at least more so than average stereo sets). If that is the case and an integrated microphone or other headset accessory is not needed, consider buying stereo headphones and using a mono adapter (such as Radio Shack part no. 274-368) to split the signal to both earpieces. Adapters are not a good idea if the cord will be subject to constant tugging and there is no way to anchor the cord to prevent the adapter from being pulled loose. Also, since stereo headphones have an extended frequency response, a low pass filter can improve speech intelligibility. For more information about mono headphones, see A Quick Guide To Headphone Accessories.

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Where can I find old-style high-impedance (2,000 ohms or more) headphones?

By modern standards, the term “high impedance” when applied to headphones usually means anything between 100 and 600 ohms, but in the not too distant past, headsets of 2,000 ohms or more were in common use. These old-style hi-Z headsets have steel or mica diaphragms and are very efficient, achieving high volume with only millivolts of input. They are especially prized by Xtal radio owners, many of whom prefer to build their sets with “authentic” parts (Xtal or “crystal” radios do not use any transistors or batteries).

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Antique and surplus electronics stores may carry old stock – brands such as Western Electric and Baldwin. The Xtal Set Society, which is dedicated to crystal radio building, occasionally has antique headsets in inventory. Antique Electronic Supply sells new hi-Z headsets. DIYers can try making a pair of ultra-high impedance headphones by using two old telephone receivers. Modern low impedance headphones (choose models with very high sensitivity) coupled to an audio output transformer (such as Radio Shack #273-1380) as an impedance converter, may also work. For more information, see High Impedance Headsets for Crystal Radios.

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Where can I find water-resistant/underwater headphones?

Several companies, such as Sony and Panasonic, make water-resistant (“sports-style”) headphones which can survive the occasional splash of water or brief submersion. Water-resistant headphones are not waterproof and prolonged exposure will damage them. The lightweight sports-style electronics (mostly radios and some media players) are worn with headbands, neckbands, arm-bands or wrist-bands (like wrist-watches) or are sometimes integrated with the headphones (all-in-one). Water-resistant headphones are either in-ear types (earbuds or vertical) or supra-aural (now in the popular neckband style). Water-resistant supra-aural headphones have hard rubber-like ear cushions. Swimmers and other water-sports enthusiasts will often use water-resistant headphones with regular portable stereos packed in protective plastic bags with water-tight seals such as ZipLoc bags or sealed with waterproof tape.

True underwater headphones (also called submersible headphones) are completely waterproof. They are available in closed-back or in-ear designs that are specially calibrated to compensate for hearing differences underwater. The closed-back versions can often be found at dealers of underwater metal detectors. Common features include heavy-duty construction and cords, large ear-cushions that provide superior coupling and isolation from ambient noise, and standard underwater-equipment bulkhead connectors. They are generally monaural and not high fidelity – with a frequency response to about 3kHz at best. The transducers and the headphone frame should be both waterproof and corrosion-proof.

Waterproof in-ear headphones (canal-type headphones) are sold in some specialty sports equipment outlets. The transducers are embedded in waterproof earplugs instead the typical foam surround. Speedo (of swim gear fame) makes this product with a waterproof radio (worn around head or wrist) or a waterproof belt pack for use with regular portable stereos. While fine for use in swimming pools, they should not be worn in deep water. Jim Lee writes:

These systems are often used by lap swimmers to play music or motivational tapes while they work out. I am a little concerned that someone might assume that these devices are not only waterproof but suitable for diving or even snorkeling. Not so. Plugs that completely block the ear canal can cause serious injury because they create a space that cannot be pressure equalized. Any hearing devices suitable for diving would have to allow the relatively free flow of water into and out of the outer ear canal.

c. 1998, 1999, 2000, 2001 Chu Moy.