NOTE:  This is a very technical review, which deals with many technical terms.  Here are some definitions, so the body of the review can focus on the iFi GO Bar, rather than defining what everything is throughout.

DAC and AMP terms:
DAC: Digital to Analog Converter.  This is the process to convert digital audio bits (typically PCM) to the electrical analog signals that will eventually (after amplification) directly cause the headphone drivers to move.  This is also used as a generic term by the audiophile community for any device that takes sound out of a computer or other digital device and makes sound.  Also known as 'sound cards'.
Amplifier: A circuit that increases the voltage and current of the incoming signal and outputs a larger/more powerful one.
DAC chips: The specific chip used to do the DAC process.  There are many common models from ESS, AK, Cirrus Logic, Burr Brown, Wolfson, and others.
Crosstalk: The amount of interference between the channels.  An amp with high crosstalk means that you can hear (or there is a measured impact) on the right driver when only the left is getting a signal, and vice versa.  This is ideally avoided because eliminating crosstalk helps improve the clarity of the stereo image presented.
Digital filter: This is the means that a DAC uses to convert the digital signal into an analog waveform.  Every DAC designer has to choose which filter option to choose, as each method has tradeoffs, where one has to balance between impulse speed, ringing, phase coherency, or frequency response rolloff.
EQ or Equalizer: A means of adjusting the frequency response of the output to tune what you head to a desired type of sound.  This can be done in the digital (DSP) or using analog circuits.
DSP: Digital Signal Processing.  Some devices offer DSP effects, that manipulate a waveform in the digital realm to get a desired audio effect.
Analog EQ circuits: Adjusting the frequency balance via analog circuits rather than DSP.  Often inductors, resistors, and capacitors are used to shape signals in this way.  

Audio formats and related terms:
PCM: Pulse Code Modulation.  The most common method used to digitally represent analog signals.  Was popularized with the launch of the Audio CD; most modern audio formats since are based on an evolution of this concept.
DSD: Direct Stream Digital, a Sony-invented alternative to PCM that uses millions of 1s and 0s directly to determine if the next bit in a waveform should be higher or lower than the previous bit.  First used in the ill-fated SACD format, but digital audio files in this format can be purchased.
DSD128/DSD256: A higher bitrate version of DSD, which is sometimes called DSD64.
DXD: A hybrid combination of DSD and PCM.  Used mostly as an intermediary when mastering DSD, though a small percentage of audio files can be purchased in this format.

Headphone types and related terms:
Dynamic driver: Most common speaker type.  Looks like a round cone that moves in and out to make sounds.
Planar driver: A flat light diaphragm held in place by powerful magnets.  The signal passes through and it vibrates, making sound.
Electrostatic: Similar to a planar, except that instead of being held in place via magnets, it's held in place via high voltage electrically charged plates.
IEM: - In-Ear Monitor.  Common type of earbud that has a portion that goes directly into your ear canal.
Sensitivity: The amount of power needed to make the headphone get loud.  Typically rated in dB/mW (decibel per milliwatt).

Headphone connectivity types:
Single ended/Unbalanced: This is the common 3.5mm (or 6.5mm) stereo headphone jack plug.  The common plug type has three separate signals, tip (L+), ring (R+), and sleeve (common ground/return).
Balanced: This is often used in live music situations (via XLR cables), but is also becoming popular to use with headphones.  Instead of a single positive signal, there is also a negative inverted signal sent over the wire also.  In a headphone application, the L+ signal is seen on the positive terminal of the driver, and the L- inverted signal is seen on the negative terminal of the left driver.  (The same applies to the right side).  With no common return/ground, this often leads to greatly reduced crosstalk, and a push/pull effect on the drivers.  This means a doubling of the voltage swing, and if done correctly, a more controlled grip of the driver, leading to a clearer, higher-resolution sound.  (In theory.)  It must be noted that there is some controversy on the value of using a balanced signal with headphones.

Thank you iFi Audio for sending us the GO Bar USB DAC/amp to review!

I've been going down the audiophile rabbit hole for quite a while now, always settling on various levels of 'good enough'.  First it was a home theater system with a high end receiver.  Then that wasn't good enough, so I had to go with separates (separate amps and audio processor).  Then back at my desk, I upgraded my headphones several times to various dynamics and then planars.  I upgraded headphone amps a few times to make my existing headphones sound better, and I eventually got a pair of Koss electrostats that include their own amplification.  I also played the DAC game, though I was always budget limited on those to a certain extent.  (It's easier to justify spending more money on the home theater than desktop audio, since the entire family can enjoy it; when it's just me, I have to be much choosier.)  I felt like my setup was in a very good place, and indeed it was - but iFi just had to upset the apple cart!

The iFi GO Bar is an ultraportable USB DAC/headphone amplifier combo that tries to take whatever USB device you have - that includes modern phones, laptop and desktop computers, or even some game consoles like the Nintendo Switch - and vastly improve its headphone out.  I'm sure some of you are wondering why you would want (or need) this; the reality is that some headphones do a great job of revealing poor source quality, while others need more power to sound their best.  And with some, it's both!  Certain classes of headphones are known to be particularly power hungry, like planars and certain brands of high-impedance dynamics.  Others, like multi-driver IEMs (In-Ear Monitors), can be particularly sensitive to noisy sources, so unwanted artifacts like hiss or electronic noises can slip through.  This GO Bar is designed to solve these and other audio issues, promising some of the best sound quality available on a device this size.

It has to be said that while I own headphones as or more expensive than this device, outside of my home theater equipment, this is the most expensive DAC/amp I currently have access to.  Despite that, what I do have was hand-picked for quality, so I feel comparisons still make sense, and there is definitely a point of diminishing returns; finding the right point for you is half of the fun of this or any other hobby.

When producing a DAC, iFi had to choose how to deal with multiple aspects to make it sound its best.  Each part of the chain can cause signal/quality loss: the conversion from digital to analog signals, the amplification of the low-level analog signal, the electrical properties of how the signal is carried to the headphone driver(s), and of course the driver(s) themselves.  This iFi GO Bar attempts to deal with the first three: the DAC, the amplification, and the transport mechanism.

First is the digital to analog conversion itself.  There are many off the shelf DAC chips that do this, each with their own pros and cons.  The main families in the high-end space are the ESS Sabre chips, the AK4XXX models, the Cirrus Logic line, and other less commonly used ones like Burr Brown and Wolfson chips.  iFi used mostly Burr Browns for several years, but recently started using Cirrus Logic chips; as long as they are implemented properly, what DAC chip is used often has less impact on overall sound quality than the arguably more important implementation details and the analog audio chain.

That analog amplification part of any portable DAC/AMP like the GO Bar includes both signal integrity and amplification.  Signal integrity includes things like how much noise is introduced, or how much crosstalk there is.  This can be drastically reduced when using a balanced connection, which I'll get more into in a moment.  The other part of the analog chain is quite simply how much power you have on tap.  A lot of audio 'sins' can often be forgiven if the amplification is good enough simply because it's easier for us to hear dynamic swings and high volumes than tiny bits of distortion below a certain level, often called the level of audibility.  Of course having low noise and high power is even better!

The final piece that the iFi GO Bar addresses is the transport.  Of course, the classic 3.5mm headphone jack is present, and it works well enough.  In my testing I found iFi's implementation of the 3.5mm jack to be quite powerful, but it doesn't sound exceptionally better at lower volumes than some of my other high quality devices.  Some of them may even sound better.  This type of connector is often called unbalanced or single ended.  However, the 4.4mm balanced connector takes things to the next level on many headphones, at least with the iFi GO Bar.

First, it must be said that headphones must have independent conductors (wires) that go to each side in order to support a balanced connection.  In my case, I did most of my balanced testing with my Monoprice Monolith M1060, which I modded some a few years ago to help it sound more to my liking.  Thankfully, for the purposes of this review, each ear cup has a single 2.5mm jack on it, which connects directly to the driver inside.  I also have an older Fostex T50RP Mk3, which I also modded a bunch, and when the 3.5mm jack on it died a few years back, I removed that and replaced it with the same dual 2.5mm headphone jacks that my M1060 does; this means it uses the same cables, and also now supports a balanced connection.  These two headphones were used for the majority of my testing, though I also used my Etymotic ER4SR's with a balanced MMCX cable that doesn't fit very well, but it's cheap and works.  I didn't want to spend over $60 on a cable alone for this review, when a $20 model would work just fine for this testing.

So, does balanced mode work?  I give this a resounding yes.  The difference, on this DAC/amp, between balanced and unbalanced mode, is astounding.  I have used balanced cables in my home theater (via XLR connections), but with my headphones, I never felt a need.  But now that I've experienced it here, I can definitely see what all of the fuss is about.  At least with this implementation, the difference is indeed audible - and impressive.

It has to be said that while iFi and a significant portion of the audiophile headphone community agrees balanced connections are worthwhile, some famous and highly respected audio companies disagree and won't put them on their products.  Others point out that in some cases it can cost less to improve audio quality and power via a balanced connection than the equivalent improvement might cost when implemented in an unbalanced way.  Whatever the case is, I believe iFi really believes in balanced connections, and made sure to put engineering effort in place to make this product sound noticeably better when used via a balanced connection.