Getting Into Your Head: Headphone Amplifiers
By Mike Rivers
Headphones go back to the early 20th century but “cans” didn’t really take a role in music recording until Les Paul started overdubbing in the late 1940s. They became a necessity in studios of the 1970s that were built so dead that you couldn’t hear other players more than a couple of feet away, and now they’re expected as a matter of course.
We’ve all plugged into the headphone jack on a mixer, recorder, or computer but for the recording studio, stage, or even for armchair listening, many integral headphone jacks can’t provide enough volume with sufficiently low distortion. A headphone amplifier’s basic job is to drive one or more sets of headphones to an ample listening volume.
The headphone amplifiers commonly used in professional audio can be grouped into four families.
Simple amplifiers. Some simple headphone amplifiers are utilitarian, while others are expensive audiophile devices that, when used with high-quality headphones, make for a joyous listening experience.
Headphone distribution amps. Unless you only record one person at a time, you’ll want some type of headphone amplifier that offers more flexibility than a built-in headphone jack. A headphone distribution amplifier has a single (usually stereo) input and multiple sets of headphone jacks, generally with a volume control for each listener. The PreSonus HP4 and HP60 are good examples.
“More me” amplifiers. A “More Me” headphone amplifier has several inputs and several outputs and offers some mixing functions. Each listener can slightly customize the headphone mix, usually with “more me.”
Multi-channel remote-mixing systems. At the top of the heap is the multi-channel headphone mixing system, which puts a mixer within reach of each player, allowing musicians to fully customize what they hear.
With this in mind, let’s look at what goes into and comes out of a headphone amplifier.
A good headphone amplifier will deliver sufficient volume with low distortion—and distortion is the prime contributor to listener fatigue.
An amplifier’s maximum headphone volume is a function of its rated power output and the headphone’s impedance and efficiency. Most headphone amplifiers are rated at 50 to 200 mW per channel and provide adequate volume over a wide range of headphone impedance—generally from about 15 to 600Ω. For example, the PreSonus HP4 and HP60 headphone outputs are rated at 130 mW and 150 mW per channel, respectively, into a 60Ω load and are designed for headphones rated at 32 to 600Ω.
Gazintas and Gazoutas
Headphone amps provide conventional ¼” stereo headphone jacks, and some include mini jacks to accommodate ear buds.
Studio headphone amplifiers are usually fed from a line-level source, such as the stereo main or control-room output of a mixer or audio interface. When musicians request a custom headphone mix, auxiliary bus outputs are the usual source.
Most of today’s pro audio gear interfaces at the nominal +4 dBu operating level, though the output level of a computer’s built-in sound card is often lower. Most headphone amplifiers have sufficient gain to accommodate consumer sources, though the drummer may not be happy with his headphone volume when the reference track comes directly from a portable music player.
Mono or Stereo?
Should you use a stereo or a mono headphone mix? That seems obvious – stereo makes sense, since you have two ears and two earphones – but a mono (the same signal in both ears) headphone mix for tracking was the standard studio setup for more than 30 years.
A stereo distribution amplifier will provide everyone with the same mix; however, when only independent mixes will keep everyone happy, you may not have enough outputs available to feed everyone in stereo. This is a hardware limitation that you must consider when planning your studio or stage setup.
Few small-format analog mixers offer stereo auxiliary buses, though you can cobble up a stereo headphone mix using two (mono) auxiliary sends for left and right headphones. By sending a channel to two buses, you can adjust the panning by juggling the left and right bus send levels; it works but it’s not convenient.
Today’s digital consoles are a different story. Since their internal signal routing and control functions are derived through software, the ability to link a pair of buses for stereo operation is a fairly common feature. The PreSonus StudioLive™-series mixers’ implementation is typical: With Aux 1 and 2 buses “linked,” for a given channel, Aux 1 Send becomes the stereo auxiliary mix level while Aux 2 Send pans the channel signal between Aux Outputs 1 and 2.
Count your chickens before you hatch a new headphone system! With a mixer such as the StudioLive 16.4.2AI, which offers six auxiliary sends, linking in pairs gives you three stereo headphone feeds. A previously adequate mixer may come up short on sends when band members show up with their own headphones or in-ears and want their own stereo mixes. This also limits or eliminates your ability to feed outboard signal processors from auxiliary sends. In this case, you may be better off supplying mono mixes.
DAWs are usually capable of creating as many stereo or mono mixes as you want, but how many you can use for monitor outputs is limited by the number of independent hardware outputs your interface has. An 8-output interface will allow you to have four independent stereo mixes (of which one might be for the control room) though an interface with only one stereo output means everyone must share one mix. Know, too, how your interface counts its outputs, particularly if you’re still in the shopping phase. A brochure might tout separate headphone, control room, and auxiliary output jacks, perhaps with individual volume controls, but if they’re all fed from the same D/A converter, they all carry the same mix.
Headphone Amplifier Types
Now let’s have a look at some representative types of headphone amplifiers and how they might best be used. The models mentioned here are representative of various configurations you are likely to encounter. However, this isn’t a review, and I’m not recommending a specific model.
That said, PreSonus products are used as primary examples for obvious reasons.
The Headphone Distribution Amplifier
The PreSonus HP4 is typical of the basic stereo headphone distribution amplifier. Left and right input jacks branch out to four independent amplifiers and on to individual headphone jacks, each with its own volume control. This is about as uncomplicated as headphone amplifiers get and is also the least expensive.
With a basic distribution amplifier
(see Fig. 1), everyone hears the same mix, though each player can control their own headphone volume. Acoustic bands generally are comfortable playing to a well-balanced mix, so a distribution amplifier would be a good choice if that’s your style of music. Of course when it comes to individual players doing overdubs, you can send players whatever mix they want to hear with no squabbling about “the bass is too loud.”
When shopping, you’ll find a few variations and additions. The PreSonus HP4 has a Mono button that sends a mono input to both the left and right headphones. A Thru output sends the input on to another headphone amplifier for expansion in case you have a larger group.
Other differences are generally related to how the inputs are physically configured. For example, you might find a single stereo input jack rather than separate left and right jacks. This is convenient for distributing the headphone output of a mixer to several listeners but requires a special cable (an “insert” cable will work) to connect to separate left and right output jacks. You might find a second set of inputs so the musicians in the studio can listen to a custom mix while recording and, after a take, hear an evenly balanced mix.
While all distribution amplifiers have analog line-level inputs, some also have an S/PDIF digital input, and some have mini output jacks duplicating the standard ¼” jacks.
The Stereo Mix Plus (“More Me”) Headphone Amplifier
Here’s where things get interesting. There are a few different ways in which inputs are mixed in this type of headphone amplifier, and it’s necessary to dig into the specifications and descriptions, sort out the differences and similarities, and decide which one best suits your needs.
A block diagram is your best friend here, but when doing research for this article, I found that block diagrams of headphone amplifiers are scarce. Most manuals are available online from the manufacturer’s Web site but few include a block diagram, and detailed product reviews are rare. Although you should study the manual, often a hands-on trial is the best way to learn what a particular unit can do.
Generally, with this type of headphone amplifier, the headphone mix begins with a stereo mix that is sent to all of the headphone jacks, just like with the simple distribution amplifier. Each output has a volume control—but wait, there’s more! That’s where the differences between headphone amplifiers arise.
Figure 2 is a representative block diagram of a headphone amplifier similar to the PreSonus HP60. Although the HP60 is a six-channel amplifier (four or six outputs seem to be the most common), only three channels are shown here for clarity. The HP60 has inputs for two global (goes to all headphone outputs) stereo mixes, with either or both mixes available to each of the headphone channels. A balance (Mix) control for each headphone channel adjusts the blend of the two global inputs or, when fully counterclockwise or clockwise, selects one stereo input.
For example, one source might be a dry mix and the other the output of a reverb applied to that mix. With that setup, the listeners can, at their option, hear some reverb in their phones. One input could be a mix without vocals and the other a vocals-only mix so the listener can adjust the vocal level in his headphones. Another possibility for the two sources is a basic mix and a click. There’s a lot of flexibility if you’re creative.
In addition to those two global inputs, each headphone channel has a third input that is unique to that channel. This could be for “more me” or even a full custom mix for that player. A Mute button turns off the global mixes so they won’t get in the way of a custom mix, and a Mono button splits a mono external input source to both the left and right earphones.
Around back you’ll find the input jacks. The Left and Right jacks for each of the two global mixes are normalled: With nothing plugged into the Right jack, whatever is connected to the Left jack is automatically routed to both the left and right channels. This makes it easy to accommodate a mono source like a click track or a single instrument.
The Talkback mic input is a special feature of the HP60. When the amplifier is mounted in a rack in the control room, a front-panel button connects the Talkback mic to all headphone outputs. A rear-panel jack duplicates the Talkback switch for the engineer in the control room when the HP60 is located out in the studio with the musicians.
The stereo output of each headphone channel is a line-level copy of that channel’s mix, which can be used to feed a second headphone amplifier to provide that mix to additional listeners.
Other Common Designs
We don’t all work the same way in the studio, so some differences between headphone amplifiers may make one device more appropriate than others for your particular application or workflow. Although PreSonus’ HP4 and HP60 are excellent solutions, it’s instructive to consider some competing design ideas. In that spirit, let’s take a look at how Behringer and ART approach the design of a multi-input, multi-channel headphone amplifier.
The Behringer HA4700 and HA8000 share the same manual but there are significant differences between their inputs and outputs. Both begin with the “one input to all outputs” concept. The HA4700 has one main global stereo input plus a second stereo input labeled “Direct” that is mixed with the main input. This is where you might put the click when recording basic tracks.
Like the PreSonus HP60, the HA8000 has two sets of global inputs. But for each headphone output, each channel has a switch to select Main mix 1 or 2, rather than a blend control between the two mixes. In addition, the HA8000 has one Direct input jack (same name, different function!) for each headphone channel that, when connected, replaces (rather than adds to) the selected Main input.
The HA4700 has an Auxiliary stereo input for each channel, with a Mix knob that does double duty: It adjusts the left/right balance of the Main input when there’s nothing connected to the Auxiliary input, but when a plug is inserted into the Auxiliary jack, the Mix knob adjusts the Main/Auxiliary balance.
The HA4700 has a global Direct input (there’s that “Direct” word again!) that replaces the global main input for all headphone channels. It has bass and treble controls for each headphone output.
The HA8000 has a Mono button to combine left and right inputs of the channel’s Direct input to both headphones. The HA4700 can do the same trick but offers a bit more flexibility in that in addition to the stereo mix, you can listen to only the left or only the right channel in both phones. Essentially this gives you a second global input if your players don’t mind a mono headphone mix (some people prefer it).
Since each headphone channel of the HA8000 can be fed from either the Main or the Direct input, you can set up a combination of global (one-to-several) and custom (one-to-one) headphone mixes. The HA4700, on the other hand, can function as a “more me” headphone amplifier by adding a channel’s Auxiliary input to the Main mix. You’ll need to be creative to find “me” sources; direct outputs, auxiliary outputs, and subgroup buses are usually good places to look.
Another pair of kissin’ kin “more me” headphone amplifiers is the ART HeadAmp 6 and HeadAmp 6 Pro. These are more similar to the Behringer amplifiers than to the PreSonus, but there are subtle differences.
Both ARTs have a master Direct input jack that overrides the signal from the Main inputs. Like The HA4700, both have an Auxiliary input and Mix knob for each channel but Behringer and ART manage mono/stereo differently.
ART’s approach is to provide a left, right, and stereo switch for each headphone output, which puts the entire headphone mix (main + auxiliary) into mono. If you want a stereo headphone mix with the Auxiliary input centered, you’ll need to either send the same signal to both the left and right Auxiliary inputs (they’re on a single TRS jack) or jump the tip and ring terminals together at the plug going into the Auxiliary input jack. But in trade, you can feed one mono mix to the left input, a different mono mix to the right input, and the listener can put the phones in mono and select between the two mixes.
Are you beginning to get the idea that there’s more to a headphone amplifier than the number of headphones you can connect to it? Good!
Personal Monitoring Systems
The holy grail of headphone systems—at least it seems that way on first look—is a system that allows each listener to create their own headphone mix without stepping away from the mic stand or bugging the engineer or other band members.
A personal headphone-monitoring system is an elegant solution for the self-contained performing band that uses the same setup for each performance and wants to use in-ear monitors. In the studio, however, where setups can change daily (unless your studio is dedicated to recording your own music), setting up the headphone system for a session, patching inputs, and making labels for the control boxes so the musicians know which knob controls what can be quite time consuming. Flexibility has its price.
While the concept of sending several channels of audio out to individual monitor mixing stations isn’t new—the Oz Audio Q-Mix from the mid 1990s, later resurrected for a short time by Mackie, was one of the few early commercial products of this nature—digital technology has made these system more practical and less costly.
When talking about personal monitoring today, Aviom and Hear Back Technologies usually come to mind first because they were first on the scene with practical products. However, several other manufacturers have entered the game since.
On the surface, this seems like a fabulous concept, and in many cases, it is. While it frees the engineer from dealing with seemingly endless requests for changes to various headphone mixes, the musicians need to learn the basic concepts of mixing so they don’t dig themselves a hole, ending up with a less-than-satisfactory mix and not wanting to show their innocence by asking for help.
Components of a personal monitoring system.
There are two major components to such a system - an interface, or base station, to which the audio sources for the headphone mixes are connected, and the remote stations that have the mixing controls and headphone jack. The interface converts the incoming audio to a digital format and sends it out to the remote mix/headphone stations, usually over Cat5 Ethernet cable.
The system is configured much like computer network, using an Ethernet hub or switch to distribute audio to the mix stations. Depending on the architecture of the particular system, the actual mixing can either take place in the box at the listener’s position, or in the base station, with the remote box acting as a control surface.
The base station gets its inputs from a mixing console or a DAW equipped with a multichannel computer audio interface. Typical sources are direct outputs from the mic preamps, stereo subgroup mixes (stems), and dedicated sound sources such as a metronome, drum machine, or prerecorded backing tracks. Some remote mixing boxes have an input for the player’s mic or instrument (or both) that serves as the “more me” source.
Headphone mixes, particularly with in-ear phones, can be pretty dry, so some of these systems include a reverb processor as part of the mixing suite. Lacking that feature, you might include an ambient mic that can be added to the musicians’ mix or provide an overall reverb as one of the monitor mix channels.
The user interface
Because the controls at the mix station are operated by musicians who aren’t necessarily engineers or computer technicians, the user interface is usually quite straightforward. Each input source has a volume and pan control, though some go beyond that and offer EQ either on the overall mix or on individual channels.
The Movek myMix, perhaps the most fully featured (and, in my opinion, the geekiest) of all of the multichannel personal monitor systems, puts all of the controls in a tiny handheld box with a big knob, a touch screen, a few buttons, and the headphone jack. Mostly, though, form follows function, and at the player’s end there will be at least one knob for each channel, possibly switchable between level, pan, and other channel-strip-like functions.
iOS Makes the Scene
Many of today’s digital mixing consoles offer as many as a dozen auxiliary outputs that can be configured as stereo buses for headphone mixes. While time consuming, it’s straightforward on a console such as the PreSonus StudioLive to set up six individual stereo headphone mixes.
Recently, console manufacturers have developed ways to turn a mobile device, such as Apple’s iPad, running a custom application, into a remote controller for the console. Take the tablet out to where the players are, and you can tweak headphone mixes on the spot.
The next step, which came along pretty quickly, was an iPhone app that allows band members to log in to the network and adjust their own monitor mixes. While this was first conceived for live-sound applications, it’s also well suited to the studio.
Since many remote-controllable consoles also serve as a DAW audio interface, recorded tracks are available for playback through the console at the touch of a button, taking the same route to the headphones as when the track was recorded. In addition, nearly all digital consoles allow you to save a snapshot of their settings, so it’s easy to recall the headphone mix from the previous session when it’s overdub time.
Setting up such a system requires a very basic knowledge of computer networking as well as an understanding of how to set permissions so the drummer doesn’t accidentally boost the snare in the lead singer’s headphones.
The StudioLive/QMix solution
PreSonus’ free QMix™ was one of the early console monitor-mix apps for iPhone/iPod touch. Designed to wirelessly control the aux-send mixes of PreSonus’ first-generation StudioLive-series digital mixers, QMix enables each musician to manage one monitor mix. (QMix-AI for second-generation StudioLive AI-series mixers has essentially the same feature set, look, and feel as QMix.)
With QMix or QMix-AI, the musicians select the mixer channels they want to control in their personal monitor mixes and identify their own channels as “Me.” Then they can use the Wheel of Me, a virtual thumbwheel that adjusts the volume of their “Me” channels in relation to the rest of the monitor mix. Each musician can have as much “Me” as they want; if they push “Me” levels past unity gain, QMix simply reduces the level of the other mix channels.
The StudioLive/QMix system uses a set of permissions to control access, along the way solving the problem of musicians who are not capable of building a good monitor mix. For each iOS device on the network, the engineer can allow complete access to a monitor mix, or can deny full access, set up the musician’s basic monitor mix for them, and just give access to the Wheel of Me. The engineer also can lock out an iOS device from the network entirely to prevent hacking—not a likely problem in the studio but important for live performance.
Some systems of this type require some knowledge of networking and IP addresses. Not so with QMix; just log your iOS device into the network and QMix automatically searches the network and finds any networked StudioLive mixers.
Of course, the StudioLive aux outputs are line level, so you still need some type of headphone amp to drive the phones, but you don’t need to spend big bucks for a dedicated personal headphone-monitoring system.
The computer connection
Typically an outboard computer is connected to the console and provides the remote control functions. The computer connects to a Wi-Fi router, which provides a communication link to the remote mobile devices. The mobile devices control the computer software, which in turn controls the console. QMix works this way.
However, StudioLive AI-series mixers can connect directly to a wireless router, without requiring a computer, so QMix-AI devices can simply join the network and control the assigned (in the permissions) aux sends.
Many bands are jumping onto this concept since it became affordable. With equipment costs being spread among band members (typically they buy their own headphones and iGizmo), the band’s investment is limited to the console, router, and simple, no-frills headphone amplifiers.
There’s a risk of obsolescence, though, since the console maker must constantly update the remote software to support new devices and operating systems. Eventually someone will get left behind. Such is the cost of progress.
The Hi-Fi Headphone Amplifier
To bring us back to where we started, there are single input/output headphone amplifiers that are designed to provide plenty of volume at very low distortion to one pair of headphones. This family includes straightforward, high-quality stereo amplifiers; tube amplifiers for audiophiles; and amps that incorporate a D/A converter to go straight from a digital source to the headphones.
The Focusrite VRM box is an interesting variation. In addition to being a decent headphone amplifier driven from a USB port, it offers simulation of a variety of listening environments for checking a mix using headphones.
The SPL Phonitor mastering amplifier goes well beyond sending the left input to the left ear and right input to the right ear, incorporating DSP modeling of the loudspeaker listening environment, with adjustments for cross-blending of the two channels to simulate the placement of the speakers relative to the listener. Of course you’ll need some very fine headphones to get the most value from its $2,000 cost. It’s not something you’d use on stage or in the studio but you might use it in a mastering lab.
Sit Back and Listen
So there you have it, a short overview of the various things that we call a “headphone amp.” You can see that there are many variations and each has its purpose. Choose the right type for your job, and you’ll monitor happily.