Headphone amplifiers were almost un-heard of until the 1990s, though many receivers and integrated amplifiers had fairly good quality headphone jacks. As headphones have grown in popularity, and fewer people are purchasing classic stereos, headphone amplifiers have emerged as a product category.
The “headphone jack” on smart phones and laptops tends to have mediocre sound quality and doesn’t have enough power to drive audiophile grade headphones. For headphones with modest power requirements, I typically recommend upgrading the “headphone jack” with a good DAC that has an integrated headphone amplifier.
Headphone amplifiers make good sense when a headphone’s power requirement are more than what the source device is capable of producing, or when you are using a device which only does line-level outputs.
The very best amplifiers are nuetral when it comes to tone, low distortion and very responsive which typically means class A amplification with very little use of feedback circuits.
Today, I don’t have a dedicated headphone amplifier. I use a portable Chord Mojo DAC/amplifier to drive my Sennheiser HD800. There are better DACs and amplifiers, but I find the Mojo enjoyable to meet my needs. Most of the headphone amplifiers I have used are solid state because I don’t like the hassles of tubes and the temptation to constantly tinker by tube rolling.
I am a big fan of headphone amplifiers that are based on Kevin Gilmore’s designs which are free to people who build Gilmore amplifier. Headamp is the premium manufacturer of amplifiers based on Kevin’s designs. For several years I owned and used the Headamp GS-X mk2. I think it’s one of the finest amplifiers currently made. Headamp’s prices are high, but seem to reflect the actual cost of manufacturing rather than the typical audiophile profit margins. Headamp also sells the more reasonably priced Gilmore Lite Mk2. It’s also possible to find amplifiers such as the Headamp GS-1 and GS Lite which use Gilmore’s DynaLo circuit on head-fi.org for-sale forum.
There are a fair number of people who like the DIY (or professionally built) headphone amplifiers from amb.org which includes the M^3 which is a competitor to the GS Lite (dynalo), and the beta22 which is a competitor to the higher end/power Gilmore amplifiers. I commonly hear people say Gilmore designs are a bit more transparent and fast, while the AMB designs are a bit more forgiving and “fun”.
Commercially produced solid state amplifiers I would recommend checking out which generally increase in cost and sound quality: JDS Labs Objective2 amplifier which can often be found on MassDrop for less than $100, Schiit Magni, Matrix M-Stage, Schiit Asgard 2, Headamp Gilmore Lite Mk2, a variety of amplifiers made by Audio-GF, Bryston, and Auralic.
The Schiit Ragnarok is quite good. I liked Headamp GS-X mk2 more, but it was close. If the Ragnarok was able to drive my Martin Logan Aerius speakers I would have made the switch to simplify my electronic rack, but I found it wasn’t up to that job.
I have heard good things from people I trust about amplifiers from ECP Audio and but have no personal experience with them. I have read mixed reviews of the Questyle CMA800R, some people think it’s a very good value, and others like the Schiit Magni better. Cavalli Audio made a number of well regarded amplifiers, but the company has stopped making headphone amplifiers for consumers.
There are many people who like to use tube based amplifiers, especially with brighter or more forward headphones to tame the sound. Fairly inexpensive tube amplifiers which are a good value are made by Bottlehead, Little Dot, and Schiit. The top end tube amplifiers I would recommend are made by Eddie Current. I really enjoyed both the Super 7 and Balancing Act driving HD800. The amplifier I would love to hear is the discontinued ECP L-2 which has gone rave reviews from people who tend not to rave. Other manufacturers that have gotten good reviews, though I haven’t listened to them enough to have a strong opinion include Donald North Audio, Apex, Cavalli, and Woo Audio.
Some headphones perform best when paired with an abundance of power. Some people achieve this by using class A amplifiers designed for speakers such as those made by First Watt.
There are a number of good quality battery powered headphone amplifiers which let you use full size headphones with portable players. My favorite for sound quality is the Headamp Pico Power. Meier and Ray Samuels Audio make several good sounding portable amplifiers. FiiO and iBasso seems to make some of the best lower priced amplifiers. Generally I don’t think portable headphone amplifiers make a lot of sense. Rather I recommend people to use a decent quality portable DAC/player which has sufficiently powerful audio out for your headphones.