• Date Posted:

    06 — 02 — 2017
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What should I look for in an amplifier? An introduction to headphone amps, part two

Welcome back to this mini-series of blogs looking at headphone amps. In part one, we explored the possible reasons why a headphone user might need an amp. This second part is for those who have decided that they do need an amp, and are now looking for tips on what features and specifications to consider before making a decision.

So, what should you look for in a headphone amplifier? There are four questions to answer:

  1. Are you looking for a headphone amplifier only, or an amplifier combined with a digital-to-analogue converter (DAC)?
  2. What types of amplifier technology are there?
  3. How will you use a headphone amplifier?
  4. Which headphones will you use an amplifier with?

1.    Are you looking for a headphone amplifier only, or an amplifier combined with a digital-to-analogue converter (DAC)?

Headphone amplifiers are solely responsible for the processing of analogue audio signals, but in the modern age of digital music and streaming services, there is another crucial component in audio setups – the digital-to-analogue converter, or DAC. DACs are responsible for taking a digital audio file, whether it be a downloaded MP3, lossless CD rip or data from a streaming service such as Spotify, and turning it into an analogue audio signal.

Like amplifiers, the quality of a DAC in non-audio specific devices (laptop, PCs, phones, tablets etc) is a low priority for manufacturers. This leads to many using cheaper, low quality DACs, or relying on all-purpose chips to handle the processing while also doing other tasks (known as system-on-chip or SOC). For this reason, many headphone amplifiers are paired with high quality, dedicated DAC chips to offer a complete audio processing solution for better sound quality than is available directly from a device.

While sound quality is by far the biggest benefit of using a DAC/amp compared to just an amplifier, there is a secondary benefit for those looking who want to listen to the highest quality digital audio files. Many integrated DACs will have an upper limit on the quality and size of file that they are able to process, as converting files which contain high amounts of data (such as High-Res Audio tracks) would require too much of the chip’s power which is needed for functions other than audio. This is a minor consideration for many users, but for those looking for the absolute best audio quality and a library of high fidelity digital audio files, it is worth considering.

2.     What’s the difference between amplifier technologies?

There are two types of headphone amplifier technology; tube amps and solid state amps.

Solid state amplifiers are the most common, using a mix of transistors and diodes to amplify audio signals. Solid state amps are the newer of the two technologies, and are generally favoured due to being cheaper, smaller, more reliable, and less likely to burn a hole in something. These are quite a few good reasons why, for the majority of users considering an amplifier, a product using solid state technology is the obvious choice.

Tube amplifiers use at least one light-bulb-like vacuum tube to process audio signals. This technology was commonplace until solid state amplifiers became more accessible in the late 1960s and early 1970s. Although archaic, this technology survives in current audio applications. Despite the technology's many downsides, some purists argue that tube amps offer a better tone or character of sound than solid state alternatives. Modern tube headphone amplifiers are most commonly designed for desktop use and feature a hybrid design, employing both solid state technology and vacuum tubes at different stages of the amplification process. Due to the high-power demands of technology, only a handful of companies produce portable tube amps with internal batteries.

3.     How will you use the headphone amplifier?

The next thing to consider is how will you actually use the amplifier; whether you need something that can be used for listening on the go, or whether you’re looking for something to add to a fixed desktop setup. Portable and desktop amps aren’t clearly defined categories, many amps are suitable for both purposes, but there are some things to look out for when making a decision.

Many desktop amplifiers do not contain an internal battery, and therefore require a constant power supply from a wall socket or outlet. These amps tend to be bulkier but they can offer more output power for demanding headphones than many of their portable counterparts.

At the other end of the scale, there are DAC/amps which do not contain an internal battery: these have the advantage of being ultra-compact. Rather than drawing power from a wall socket, they draw power directly from the audio source device, with both power and data being transferred via USB. This type of product almost always contains a DAC in addition to an amplifier, as power cannot be transferred without some kind of digital connection to the source device. In addition to their small size, these types of products have the additional advantages of being compatible with smartphones and feature on-board volume controls. The major disadvantage of the ultra-compact USB DAC/amps is that they can be limited in processing and output power compared to bigger alternatives.

Probably the biggest category of headphone amplifier on the market today are those which contain their own power source in the form of a rechargeable battery. All portable amp-only headphone amps require a rechargeable battery to operate. Many combined DAC/amps use a rechargeable battery for convenience and for use with as many devices as possible without draining the source device’s battery. The best products in the category combine the convenience of compact, portable USB DAC/amps with the power and processing ability of desktop devices with no compromise in performance or function. Though combined DAC/amps are increasingly popular, there is still a wide selection of amp-only battery-powered products available in a huge selection of shapes, sizes and abilities. The only drawback of this type of device is remembering to keep the product charged.

4.     Which headphones will you use the amplifier with?

The single most important technological factor to consider when looking for a headphone amplifier that works well with your ear phones is impedance matching. Amplifiers, like headphones, have an impedance rating. In amplifiers, output impedance is the combination of resistance, inductance, and capacitance to a current generated by a device’s outputs, i.e. how much power is resisted when pushed out via a headphone connection. To ensure good performance with your headphones of choice, the impedance of the amplifier should pair well (or ‘match’) with the impedance of the headphones.

Impedance matching with headphones and amps doesn’t actually mean finding ones with the same ohm rating, but there is a fairly simply way to calculate whether an amplifier and headphone will be compatible – the rule of eighths. Essentially, take the headphone’s impedance (usually on the product packaging or website listing) and divide it by eight; the number you’re left with is the approximate, ideal maximum output impedance that you should look for in an amplifier.

For example, a fairly common headphone impedance is 32 ohms. Dividing this by eight gives four, so the ideal amplifier for the headphone should have a max impedance of 4 ohms.

Headphone impedance Rule of eighths Max amp impedance
32 ohms 1/8 4 ohms

An amplifier with an output of impedance of around 2 ohms will work well with pretty much any headphone, as ear phones with impedance lower than 8 ohms are relatively uncommon and generally would not benefit from amplification.

There are three consequences of ignoring the rule of eighths and all relate to changing the sound of the headphone. Firstly, if an amplifier’s output impedance is significantly more than an eighth of the headphone’s impedance, the frequency response and sound of the headphone can change. This results in bigger mismatches and creates more variation from the headphone’s default sound signature. The way that a headphone responds to an amplifier with output impedance higher than one eighth of the headphone can be entirely erratic – different headphones will respond in different ways, but generally the results will be negative.

The second way that a mismatched headphone and amplifier can impact sound quality relates to the concept of damping. Damping is a solution to a potential problem that can occur with dynamic driver headphones. Dynamic drivers feature a moving voice coil which rapidly oscillates to produce sound, however controlling the degree of oscillation is required to ensure accurate sound reproduction; this is the function of damping. Damping can either be done physically, which often leads to compromised sound reproduction, or electrically, which is generally preferred as a more effective way of controlling sound reproduction without compromising on quality.

Electrical damping refers to the amplifier and headphone working together to deliver power in the right way as to maintain control of the moving elements of a dynamic driver; this ensures that there is no excess movement inside the headphone and that sound reproduction remains accurate. The bigger the difference between amplifier impedance and headphone impedance, the more electrical damping takes place. For example, an amplifier with an output impedance of 4 ohms driving a headphone with an impedance of 16 ohms is going to provide little-to-no electrical damping, which means less controlled audio reproduction. In terms of the audible effect, poor damping is especially noticeable in low frequency reproduction; bass can become boomy and the sound signature of a headphone overall warmer.

A third result of mismatched impedance is particularly relevant to headphones using balanced armature drivers, though it’s not exclusive to the technology and can be experienced in other low impedance in-ear headphones. Using a high output impedance amp with a low impedance IEM can sometimes result in an increase in unwanted noise, usually in the form of a low-volume hiss beneath the audio.

So there you have it, a list of things to consider if you’ve decided to pick up an amplifier for your headphones and some advice on what to look out for. Don’t feel daunted by the amount of information or the variables presented – it might seem pretty technical, but there’s not many major things that can go wrong with amp shopping, and there’s plenty more information available online.

Be sure to check out RHA's Dacamp L1 for high resolution audio anytime, anywhere.

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