In the days of smart technology, it’s common to have headphones that require power to function properly. Active headphones are actually relatively new on the market but have been continuously increasing in popularity with the rise of personal music/audio players.
How do headphones get power & why do they need power? Active headphones nearly always get their power from batteries (built-in or external) but can also receive power via special cable wires. Headphones need power for any of the following features: active noise cancellation; built-in amplifiers; wireless signal transmission, or electrostatic drivers.
In this article, we’ll talk more about the active components of headphones that would require power to function and few powering methods. To begin, we’ll describe the inherently passive nature of headphones.
Most Headphones Are Inherently Passive And Do Not Require Power
Allow me to preface this discussion by stating that most headphones actually do not require power to function.
This is because the vast majority of headphone drivers are dynamic (moving-coil dynamic to be more specific). Dynamic headphone transducers convert audio into sound via electromagnetic induction.
The design of the moving-coil is fairly simple and worth explaining here. Let’s have a look at a simplified cross-sectional diagram of a moving-coil headphone driver to better visualize:
The voice coil is electrically conductive (typically made of copper). Two lead wires are connected to the coil (one at each end) that effectively create a circuit between an audio output device and the headphone driver.
The coil is suspended in a cylindrical cutaway of an oddly-shaped magnet. It is attached to a movable diaphragm the allows it to be suspended.
As the audio signal (alternating current) passes through the coil, it causes a varying magnetic field within and around the coil. The induced magnetic field interacts with the permanent field supplied by the magnets.
This causes the suspended coil to move back and forth about resting position. As the coil moves, so too does the diaphragm. As the diaphragm moves, it pushes and pulls air around it and produces sound waves.
So the moving-coil dynamic driver effectively reproduces the audio signal as sound waves without requiring any external power.
To learn more about dynamic headphone drivers, check out my article What Are Dynamic Headphones And How Do They Work?
The Active Components Of Headphones
So if headphones are inherently passive, why would they ever require power? Well, there are several active components in modern headphone design that require power. They are as follows:
- Active noise-cancellation
- Built-in amplifier
- Wireless signal transmission (receiver)
- Electrostatic drivers
Let’s talk about each in more detail, shall we?
Active noise-cancellation is fairly common in headphones today. As the name suggests, it is a method of eliminating/reducing exterior noise so that the listener experiences the intended audio with better clarity.
Active noise-cancellation is complex but fairly easy to explain holistically. How does active noise-cancellation work?
Essentially, active noise-cancellation records the external sound around the headphones and incorporates the ‘noise signal’ in reverse-polarity in the headphone signal before it reaches the driver.
By recording the noise and then flipping the polarity of the recorded signal, deconstructive interference happens and effectively cancels out the extraneous noise. It may seem counter-intuitive to add more noise to the system to rid of noise, but science states that two identical signals perfect out-of-phase with one another will cancel each other out.
So that’s the theory, but what makes ANC active?
Well, the microphones typically used in ANC headphones are miniature electrets, which require some biasing voltage to run their impedance converters. Any microphone type used requires a preamp to apply gain and bring up the amplitude of the mic signal for use with the rest of the ANC circuit.
The mic signal must then pass through an all-pass stage to delay it and preserve unity gain while aligning it to be completely out-of-phase with the actually external noise.
Finally, there is a summing amp that combines the audio signal for the audio device with the audio signal from the noise-cancelling mic. It is critical for the summing amplifier to adjust the gain of the noise-cancelling signal so that it provides the best-possible noise cancellation.
So the active components in a typical ANC circuit are:
- Impedance converter and preamp of the microphone
- All-pass filter delay circuit
- Summing amplifier
The JVC HA-NC250 (link to compare prices on B&H Photo/Video and Amazon) is an example of a pair of wired active noise-cancelling headphones:
Note that, in addition to the active noise-cancellation, ANC headphones also provide passive noise-cancellation by providing a physical barrier between sound waves and the listener’s ears.
Headphone amps are usually designed as separate devices that headphone connect to. Built-in amplifiers are rare in headphones since they are typically bulky and heavy.
That being said, there are headphones on the market with built-in amplifiers. These amps act to improve the performance of the headphones while allowing the headphones to be connected directly to consumer-grade audio outputs (smartphones, laptops, tablets, etc.).
Built-in headphone amplifiers work the same as regular headphone amps, though they are typically designed to be lightweight. So how do headphone amplifiers work?
A headphone amplifier works by amplifying the voltage of an audio signal from an audio device (smartphone, laptop, turntables, etc.) to an adequate level for the headphones’ drivers. Once amplified, the audio signal will provide enough electrical energy for the headphone driver/transducer to produce sound waves at sufficient sound pressure levels.
The amplifier relies on external power to provide any gain to the signal.
Amplifiers also help to match the impedance of audio signals to certain headphone drivers. This is important for optimal headphone performance.
That all being said, headphone manufacturers have adapted to the popularity of the low-level low-impedance headphone outputs of smartphones, laptops and tablets and many will work just fine plugging directly into these devices. However, high-quality headphones will often benefit from a headphone amp and some even have built-in amps to ensure they receive a custom-tailored benefit.
The Blue Sadie (link to check the price on Amazon) headphones have a built-in amplifier to ensure the best audio-quality even when connecting to low-level outputs like smartphones.
Wireless Signal Transmission (Receiver)
Wireless headphones are the relatively new craze in headphone technology. With Bluetooth and true wireless becoming popular in headphone design, more and more headphones are becoming active audio devices.
So how do wireless headphones work?
Wireless headphones work with wireless audio transmission. This transmission can be done via radio frequencies (RF) or infrared frequencies (IR) and requires a transmitter and a receiver.
The transmitter is connected to the audio source. It effectively encodes the audio signal into a wireless RF or IR signal and projects that signal outward.
The receiver, which is connected (built-into) the headphones, is programmed to receive this specific RF or IR signal and decode it to access the original audio signal. This signal is then sent to the headphone driver to be converted into sound waves.
This decoding requires power and the receiver often has an amplifier to boost the level of the decoded audio signal.
The Bang & Olufsen Beoplay H4 (link to compare prices on B&H Photo/Video and Amazon) headphones are a superb pair of Bluetooth wireless headphones.
Note that both analog audio and digital audio can be sent wirelessly. The wildly popular Bluetooth protocol, for example, encodes digital audio signals is radio frequencies centred around 2.45 GHz (between 2.400 GHz and 2.483.5 GHz, to be exact). In fact, it utilizes 79 specific frequencies in this range and a single connection change frequencies 1,600 times every second.
Electrostatic headphones utilize a completely different type of driver than the typical moving-coil dynamic driver mentioned earlier in this article. Rather than working on the principle electromagnetism, electrostatic headphones (as their name suggests) work on electrostatic principles.
The driver itself requires a biasing voltage that is supplied by an external power supply.
So how do electrostatic headphones work?
Electrostatic drivers work by having an electrically-charged diaphragm. An external power supply provides the diaphragm with the biasing voltage needed to hold its positive electric charge.
This diaphragm is placed closely between two perforated conductive metal stator plates that act as a sort of capacitor.
The audio signal is then boosted considerably (100 to 1000 V) and sent across two stator plates. At any given point, the stators will have equal but opposite electrical charges. As the audio signal varies the sign between the stators, the positively charges diaphragm is pushed and pulled which produces sound waves.
The audio signal is typically boosted in a separate amplifier but the diaphragm is still active.
Here is a simplified cross-sectional diagram of an electrostatic headphone driver:
STAX is an industry-leader in electrostatic headphones. Their STAX SR-007A MK2 (link to check the price on Amazon) model is an excellent example.
Stax is featured in My New Microphone’s Top 13 Best Headphone Brands In The World.
Note that electret materials can be used to quasi-permanently charge the diaphragms of electrostatic headphones. This does away with the need for an external biasing voltage, making the headphones “passive.” However, the headphones will still require a specially designed headphone amplifier, which is generally active.
Headphones Powering Methods
There are a few methods to supply the necessary power to active headphones. They include:
- Internal batteries
- External batteries
Internal batteries are often used in rechargeable wireless earphones and headphones. These batteries are designed into the headphones themselves so there’s no need to purchase external batteries. Rather, the earphones/headphones come with charging ports to recharge their batteries. Most modern headphones today have internal batteries.
The Sennheiser RS120 (link to check the price on Amazon) headphones are a great example of rechargeable headphones. Its charging port is also the transmitter.
The Bose QuietComfort Series 35 II (link to compare price on B&H Photo/Video and Amazon) are one of many headphone models that is rechargeable via USB.
The Bludio Hurricane (link to check the price on Amazon) earphones are one of the many examples of rechargeable (internal battery) Bluetooth earbuds on the market today.
Active noise-cancelling headphones are sometimes designed to accept disposable external batteries. This is the case with older models. It’s very rare that you’d find a new pair of active headphones that are not rechargeable.
Wired headphones can accept power through their cables. This is typically the case with electrostatic headphones that utilize special multi-pin connectors rather than the typical headphone plugs.
The aforementioned STAX SR-007A MK2 utilizes a custom-moulded 5-pin connector.
Who invented earbuds? The earliest version of acoustic earbuds dates back to 1891 when French inventor E. J. P. Mercadier created the bi-telephone, which improved upon telephone-receivers. However, American inventor, Nathaniel Baldwin is credited with inventing the first successful headphones in 1910.
How do wireless headphones work? Wireless headphones transmit audio signals encoded in either radio or infrared frequencies. A transmitter encodes the audio (analog or digital) into RF or IR and emits these signals through the air. The receiver (in the headphones) retrieves the RF or IR and decodes it back into audio for playback.