One of my audiophile pals tipped me off to the Little Labs Monotor, and I'm glad he did! Little Labs is a pro sound company, but there's no reason why audiophiles wouldn't find a lot to like about their Monotor headphone amp. It's made in the US and lists for $600, but street price is $540, (£500, AU$899).
The Monotor's compact chassis houses two completely independent stereo amps, one for each pair of 6.3mm and 3.5mm headphone jacks, so you can play two headphones without forfeiting sound quality. Little Labs also claims you can run four headphones at a time with only a slight loss in sound quality. I tried playing four headphones and indeed, the sound was fine. I just wish the Monotor had separate volume controls for each headphone jack, but space on the front panel is very limited. The company is tight-lipped about the details of the amp's circuitry, other than to say it's a super-simple design. Its desktop-friendly size measures a scant 1.6 by 4.2 by 7.1 inches (40 by 106 by 180mm); the Monotor's external power supply is no puny wall wart, it's a 2.4-by-2.8-by-6 inch (63-by-72 -by-153mm) brick that looks like it means business.
This little amp doesn't have a built-in digital converter, which I prefer because converter tech changes all the time. I used Schiit Bifrost Multibit and BorderPatrol converters with the Monotor.
The first headphone I grabbed to try with the Monotor was a Hifiman HE400S, which remains my choice for best-sounding full-size headphons and runs $300/£239/AU$439. The Monotor was an ideal match and brought out the best in the HE400S' remarkable transparency, uninhibited dynamics, super-low distortion, clear treble and taut bass. With audiophile recordings like Austin Wintory's "The Banner Saga," I could hear super-quiet details in the mix. The Monotor may have been designed for the pro market, but it sounds like an audiophile headphone amp to me.
To check on the Monotor's compatibility with high-impedance headphones, I plugged in my 300 ohm Sennheiser HD 580s. No problem -- these 18-year-old 'phones' transparency and openness were truly exceptional. Jazzy hip hop band Too Many Zooz' "Subway Gawds" album kicked butt, and the tenor sax's growl made the hair on my arms stand up -- it's a great recording!
Since the Monotor also sports 3.5mm jacks, I plugged in a few in-ear headphones -- namely, the Campfire Lyra II and Beyerdynamic Xelento. The little amp was noise-free, and trust me on this, a lot of otherwise respectable headphone amps have too much internal noise and hum to be suitable for use with in-ear headphones (which are much more sensitive/efficient) than full-size 'phones. The Monotor had no problems, and if you've only played in-ear headphones with your smartphone you'll be amazed by how much better they are with a great amp like the Monotor. In-ears' bass power, definition, overall dynamics and treble air will all be much improved with the Monotor.
To finish up, I compared the Monotor with an Audeze Deckard headphone amp ($699,£599, AU$1,149). For these comparisons I used AudioQuest NightOwl headphones. The Deckard has a robust sound, with a warm midrange and controlled treble. Switching over to the Monotor lightened the tonal balance, and resolution improved. The music seemed livelier with country singer Alison Krauss' new "Windy City" album via the Monotor. I liked the Deckard's richer balance, but the Monotor seemed to liberate the music more.
While pro audio and high-end audio manufacturers aim for great sound, they sometimes seem at odds with each other. Not this time: the Little Labs Monotor will please the most discerning audiophiles, and I love that this amp has four headphone jacks. So if you have a lot of 'phones, you can plug 'em in and swap between four of them to your heart's content.