Audio Industry Updates

Headphone Jack and Plugs: Everything You Need to Know

The headphone jack is a family of electrical connectors that are typically used for analog audio signals. It is also known by other names like phone jack, audio jack, aux input, etc.

Evolution of the Headphone Jack

Who is jack?

The origin of the term “jack” can be traced back to 1874 when C.E. Scribner patented, what he calls, a “jack-knife” connector.

The earliest known jack was a ¼ inch (6.35mm) version and still has mainstream usages which we describe below.

Rounded to pointed tip profile
In the early development days, there were many different jack designs. The rounded tip design was particularly popular because it was compatible with different manufacturers. The rounded tip quickly gained traction as the de-facto tip profile for audio equipment.

But with the rise of stereo audio, a different tip profile was needed to prevent the old rounded tip from frying the circuit when inserted into incompatible equipment. A pointed tip jack will prevent a rounded tip plug from being inserted fully, hence solving the problem.

Anatomy of a Headphone Plug
Regardless of the plug size, all headphone plugs have conductors. Conductors are the contact points of the plugs that close the circuit.

A headphone plug has a minimum of 2 conductors and commonly up to 5.

If it has 3 conductors, it may be called a 3 conductor plug. Some manufacturer uses pole to replace conductor. Hence, it can also be called a 3 pole plug.

Identifying conductors.

Each conductor has a specific name:
Tip (T)
Ring (R)
Sleeve (S)
All plugs have at least a Tip and Sleeve. It is the number of Rings that differentiate them. If the plug only has one ring, it is a 3 conductor plug or a TRS connector. If it has two rings, it is a 4 conductor plug or a TRRS connector.

Don’t mistake the black band as a ring. It is an insulating band. The band separates the parts of the plug from being shorted together.

Nomenclature of the Headphone Plug:
2 conductor, 2 pole, TS
3 conductor, 3 pole, TRS
4 conductor, 4 pole, TRRS
5 conductor, 5 pole, TRRRS

Different Plug Connectors configuration

Depending on available conductors, manufacturers can choose to configure jack and plug in various ways. Both must be complementary to each other.

2 conductor plug (TS)

Available Connectors: Only the tip and sleeve connectors are available.

Connectors Connection:

Pin Function
1 Ground
2 Signal
The connection is fairly straightforward here. One connector is used to carry the audio signal while the other acts as a return path and ground.
The ground acts as a reference point for the signal but it also picks up interference noises like an antenna. The longer the cable, the higher chance the more noise will be picked up.

Common Usage: You can find TS connectors mainly with guitars, instruments, and applications that do not require a long cable connection.

3 conductor plug (TRS)

Available Connectors: Aside from the tip and sleeve, there is an additional ring connector with two insulating bands around it.

Connectors Connection:

Pin Unbalanced Mono Balanced Mono Unbalanced Stereo
1 Ground Ground Ground
2 Optional (Mic etc) Signal - (Cold) Right Audio Channel
3 Signal Signal + (Hot) Left Audio Channel
With the addition of another conductor “R”, we open up different possibilities such as supporting balanced mono signals and unbalanced stereo signals.
As seen in the “Unbalanced Mono” column above, the engineer can choose to make use of the additional conductor to carry a microphone signal instead. In this case, where the audio is unbalanced, we may also sometimes refer to this as a single-ended plug.

TRS still cannot carry a balanced stereo signal. For that, see TRRRS connector below.

Common Usage: Most common form of jack connectors. You see these on the end of most stock headphone cables.

4 conductor plug (TRRS)

Available Connectors: With a 4 conductor plug, we have two additional rings with three insulating bands.

Connectors Connection:

Pin Function
1 Microphone
2 Ground
3 Right Audio Channel
4 Left Audio Channel

The above connection configuration follows the Cellular Telecommunications and Internet Association (CTIA) standards. Such a connection format is also called the CTIA TRRS jack connector format.
Common Usage: It is the most commonly adopted standard for modern smartphones and gaming consoles where the cable supports a microphone and stereo audio connection at the same time.

5 conductor plug (TRRRS)

Available Connectors: For a 5 conductor headphone plug, we have 3 ring connectors.

TRRRS connectors are used to support balanced stereo signal. You will find that XLR connectors (3 pin pairs, 4 pin, and 5 pin) are also a common choice for stereo balanced headphones. XLR connectors have been the standard balanced connector in the professional audio market for decades.

It is no surprise that when balanced headphone connections grew in popularity, the standard XLR became a go-to solution.

————XLR5 Female (L) XLR5 Male (R)

Unlike TRS connectors, XLR connectors have pins instead of tips, rings, and sleeve. However, they work the same way. The XLR5 connector has 5 conductors just like a TRRRS connector.

Sony did make a TRRRS headphone jack – 4.4mm Pentaconn connector.

Penta means “5” and conn is short for connector.
Please be aware that the Pentaconn connectors are new and not adopted widely by the industry. You either have to get an adaptor to accommodate the 5 pole connector or purchase a compatible Sony amplifier.

Different sizes of headphone jacks/plugs

When we describe the size of the headphone jack or plugs, we are referring to the diameter of the connector.

Why does a headphone plug matter?

A headphone plug:

directly affects the audio signal it transmits.
indirectly affects the overall quality of sound.
tells us what the cable is capable or not capable of doing (mic, stereo support, etc).
Let us first understand two basic concepts with regards to the transmitted signal from the audio source to our headphones.

Mono VS Stereo Signal
Two types of signal can be transmitted from the audio source to the headphones

Monaural (Mono)
Stereophonic (Stereo)
A mono signal uses only 1 audio channel while a stereo signal uses two audio channels (left and right).

Stereo signal simulates “natural” hearing by creating the impression of sound coming from different directions. This is accomplished by the separate audio channels producing sound in two different speakers (or stereo headphones). You can call this the “surround-sound” effect.

As for mono signal, the sound reproduced is intended to be heard from one position.

Balanced VS Unbalanced Audio

For a more in-depth discussion on balanced and unbalanced audio, you can read “Balanced vs Unbalanced Audio Connections“
Balanced audio is all about interconnecting audio equipment and transmitting signals in a “balanced” manner.

To do that, we need a combination of an audio source (amplifier) that can produce balanced output and a cable that is capable of carrying that balanced output.
A balanced output has two signal phases (or a hot and cold signal) per channel. Each phase has an equal impedance relative to ground, hence the name balanced.

A balanced mono cable typically has at least three conductors (TRS) to carry the signal to the headphones. A balanced stereo cable has at least five conductors (TRRRS).

Advantage of Balanced Audio

The advantage of balanced connections over unbalanced connections is the canceling of noise interference via a technique named Common Mode Rejection (CMR).

Any noise interference that hits the two balanced phases in the cable is imprinted equally on them. The receiving equipment (headphones in our case) only cares about the difference between the phases.

So, interference that adds equally to both phases creates no difference between them and is canceled out when they are recombined by the amplifier. This canceling process of noise while preserving the original sound is known as CMR.

This means that balanced cable can run longer than unbalanced cable and operate in noisier environments because it cancels any noise interference.

So when is audio unbalanced?

When the requirements to meet balanced audio are not met, the audio is considered unbalanced.

For example:

The audio source is unable to produce a balanced output.
The cable is not capable of carrying a balanced signal (TS, single-ended TRS connectors).
Plug is the key
With the above two concepts, you should know that you can send 4 types of signal from the output (amplifier for example) via a cable to a pair of headphones.

Unbalanced mono
Balanced mono
Unbalanced stereo
Balanced stereo

Whether a signal can be fully supported or even supported at all depends on the number of conductors found on the plug.

Output Signal Jack Connector Balanced Audio Connection
Balanced Mono TS N
Balanced Mono TRS Y
Unbalanced Stereo TRS Y
Balanced Stereo TRS N
Balanced Stereo TRRRS Y

Is it confusing?
It can be hard to digest if you are reading all this for the first time. The part where stereo uses two channels and balanced audio needing two copies of the same signal with reverse polarity tripped me up pretty good, too.

An easy way to understand is to first acknowledge that we need conductors to send and carry signals.

The more signals we need to send, the more conductors we need.

For example, if we want to send a balanced stereo signal, it needs to send

one right channel (R+),
one flipped right channel (R-),
one left channel audio (L+),
one flipped left channel audio (L-).
In total, it needs 4 signal wire and one ground wire which only a TRRRS connector can provide.

Thus, if a jack with an inadequate connector is used (like a TRS connector), the headphone doesn’t receive the complete set of signals to do the CMR. Hence, the whole audio connection becomes unbalanced.