If you think the only purpose of headphone amplifier is to make a sound louder, you will be in for a big surprise to discover how much it can shape the sound and contribute to improvements in details, soundstage, etc. It is true that some headphones require more power to drive them, but I would like to focus in my review on the contributing factor of the sound improvement while using amplifier connected to a portable audio source. With E11k being my first portable headphone amplifier, I don't have too many references for comparison, though prior to E11k I have used E18 as an amplifier. Here is my impression of this latest product from FiiO.
Actually, I should start first by mentioning this is an updated version of FiiO popular budget amplifier, E11. One impressive thing about FiiO, beside constantly working on new products, they also revisit their older models to improve the design and to update the components. That's exactly what happened with E11 model where the design was updated from inside out.
Arrived in a small box (as it turned out, my review unit had an older packaging where 2nd batch will be similar to E10k), beside E11k there was also a charging usb cable, a basic 3.5mm audio cable, a pair of silicone bands, 6 pieces of rubber feet, instruction manual, and warranty card. The unit itself is very compact, measuring about 91mm x 56mm x 13.5mm, with a solid aluminum alloy body that feels very smooth in your hand and also relatively lightweight at only 92g. The shape reminds me of a drinking flask due to it's rounded toward the edges surface and volume potentiometer knob between two ramp up pieces at the top. Next to the volume, you have toggle switches for Low/High Gain and Bass boost on/off. The power turns on by turning the dial of the knob past zero where on the opposite side you will find a blue led indicator next to micro-usb charging port. On the same side as micro-usb port, you also have headphone output and aux input (3.5mm ports) located symmetrically on each side of micro-usb port.
I do have to mention it was a bit awkward to see Headphone and Line In ports on the opposite side of the volume knob, and perhaps for some this will affect a connection of audio cables and the logistics of placement to have access to volume control. At the same time, it really doesn't require to use two fingers to turn the volume knob, and I was able to use just my thumb. As a matter of fact, I tried my X5 with HS6 kit (designed for a larger E12 amp) using a single rubber band to hold down E11k, and found it to be working quite well. Also, want to mention that I really like the audio cable from HS6 kit, something you can get separately as L16 short cable from FiiO. Furthermore, I know some will notice a shape of E11k to be not perfectly flat, but next to a flat surface of a DAP (even using X5 without a silicone skin for the purpose of HS6 kit) - it felt sturdy without wobbling. For an extra security, you can use included rubber feet that just stick on to the body of E11k. Personally, I think E11k will make a PERFECT pair up companion for the upcoming X1 DAP.
Don't let the small size of this amplifier fool you since it actually hosts a hefty 1400 mAh battery (non replaceable) which can provide over 16 hours of continuous play. I have been using E11k in a low gain setting for almost two weeks now and have a feeling it can probably stretch pass that limit. And even if you run out of juice, you can continue using amplifier while charging it up. Gain toggle provides a decent boost in volume to assist in driving demanding headphones, and the extra power actually helped to bring up more details even in less power hungry IEMs. I also found Bass boost to be surgically clean, where flipping that toggle provided a more controlled enhancement of sub-bass quantity and some mid-bass punch without spilling into mids and muddying the rest of the spectrum. In contrast, E18 bass boost wasn't as tight, and bleeded a bit into lower mids.
So how good does this amp sounds and how much improvement are you going to get over your built-in amp? There is no simple short answer to this question, and that is a beauty of amplifiers where it will depend on it's pair up with your source. One thing I found for sure: the sound will always improve, but to what degree you can never predict so it was a journey to try different hardware sources as well as using connection to Line Out (LO) or Headphone Out (HO) to find a better combination. Typically, you want to drive your external amp from LO, but it never hurts to experiment. At the current moment my two main DAP sources are X5 and AP100, and for example I found E11k connected to HO pairs up better with AP100 rather than X5. But once I switched to using LO port, I found a combo of X5 + E11k to show more improvement over AP100 + E11k. As a matter of fact, while I prefer AP100 built in amplifier over X5 built in amplifier (by comparison of direct HO connection and while AP100 is in 24b/192k oversampling mode), X5 actually sounds much better with E11k from LO. Also, comparing the sound between X5 with direct headphone connection vs LO with E11k, I found the sound to get wider and deeper (soundstage), overall tighter, brighter with more details while still being smooth, and a significant bass improvement (deeper and punchier). A performance of X5 paired up with E18 as amplifier connected to LO was inferior in sound quality to E11k where E18 colored the sound to be darker, less transparent, and with less details in comparison to E11k.
Overall, I found that amplifier adds another variable to a complicated equation of sound where you already have an array of file formats, different hardware sound sources, and various headphones. There could be numerous combinations and it's a never ending journey. One thing I discovered for sure, if you are serious about sound quality - amplifier is a must have addition to any setup. With E11k being a very portable and a very capable amp, I found it to provide a noticeable improvement to X5 DAP when comparing direct HO connection vs LO w/E11k. It has just a perfect balance of clarity and transparency with a touch of warmth without adding too much color or distortion when paired up with a capable DAP that has Line Out. For use with smartphones or supported tablets, I would still recommend E18 which goes straight from digital domain to internal DAC/amp of E18. For everything else, E11k is an excellent portable amplifier on a budget!
Here are the pictures.