The Apollo has an interesting frequency response and is reminiscent of older Audeze models with a dark, laid-back character through the midrange. Combined with broadly elevated bass and treble ranges, this provides a clearly V-shaped signature. Where it differentiates itself is with regards to its warmer tonality invoked by a hump in the mid to upper-bass region. This gives the Apollo a thick and somewhat roomy character. The top-end is similarly broadly elevated which reintroduces an ample level of clarity and openness, especially in the context of its otherwise thicker and more laid-back presentation. Altogether, it is refreshing to see an unabashed truly V-shaped headphone these days as this style of tuning has fallen out of popularity. The Apollo also does so on a solid planar magnetic platform that helps to keep things focused and controlled.
The low-end immediately comes to the fore sitting just in front of the treble and clearly ahead of the midrange. It provides a robust foundation for the headphone’s thick, warm character. It doesn’t awe with huge pressure and dynamics due to a progressive sub-bass roll-off but delivers impressive drive and mid-bass impact in return. This means that though the headphones extend well into the perceptual, this isn’t a quality that stands out during listening. Rather, it is the plump, thick mid and upper-bass and the large, full note structure they create in turn. It isn’t an exceedingly bloated or fuzzy presentation due to the nature of its note presentation in addition to the wideband emphasis that doesn’t overly bias any particular frequency range. However, the headphone does sound somewhat tubby which simply cannot be avoided with this style of tuning.
Another quality to note is the lower-contrast nature of its transitions. The Apollo doesn’t employ the sizable upper-bass/lower-mid dip that other bass-orientated headphones usually do, and in turn, the midrange is more obviously coloured. I do find the driver to be well-controlled and offer both an assertive attack and agile decay, however, the note presentation is not quite in sufficient measure to completely clean up the presentation. Still, while this doesn’t make it into an especially fast sounding headphone due to the sheer level of fullness on display, the Apollo is far from a poorly defined or ill-separated headphone as a result. Mid-bass texture deserves special mention, and they are responsive enough to keep up with faster tracks too. While one should keep expectations in order regarding separation, the Apollo is certainly a headphone that will appeal to those wanting an especially rich and textured low-end.
I recall cringing at early audio reviews that bashed the midrange performance of a V-shaped headphone only on account of it being laid-back. In reality, you can have an excellent midrange even if it isn’t pushed into the listener’s face. In this instance, however, I do find myself agreeing with these sentiments as I do perceive the midrange as being the weakest aspect of the Apollo’s sound. That doesn’t necessarily make it a poor performer but does mean this won’t appeal to those valuing timbral accuracy and overall balance. The Apollo, as mentioned below, is a warm, roomy headphone with a rather laid-back midrange presentation. It essentially has a complete lack of head gain with a dip from 1-3kHz instead. While objectivist will light their torches, to me this isn’t blatantly incorrect but a matter of preference. For instance, fans of older Audeze headphones likely won’t find this too strange, likely enjoyable, and the voicing overall doesn’t strike as off even coming from other headphones. However, while the voicing isn’t overtly unnatural, they remain a highly coloured headphone relative to most competitors.
The tone is very warm, and this introduces a slight yet persistent stuffiness into their sound that is most noticeable on male vocals. In addition, the lower-midrange appears more present by comparison to its dipped centre and upper-midrange. This instigates its thick and roomy character and accordingly, separation, cleanliness and tonal accuracy are not the Apollo’s forte nor is small detail retrieval or layering. This does not mean it has no redeeming features. For instance, as treble ramps up after, vocal presence is adequate, and congestion and veil aren’t excessive as a result. Female vocals remain laid-back but have a far more transparent character than male vocals due to the quick ramp-up entering the treble. This also makes the Apolla a reasonably articulate headphone and, given its sizable warmth and body, this quality never manifests as thinness, rasp or sharpness, but permits an acceptable level of clarity and openness. While the Apollo is unorthodox in its methods, it may appeal to an audience wanting just that. This is a lush, full-bodied headphone with no pretence of balance or accuracy.
By comparison to the midrange, the top-end is a breath of fresh air, impressing from both a tonal and technical point of view. It sits roughly on par with the bass and is impressively linear in its execution. Treble presence ramps up around 5kHz following the recessed upper-midrange and sustains evenly into the middle-treble region before a subtle lift in the upper treble. This gives it an even-handed, just slightly upper-harmonic biased voicing that manifests in the form of a slight “tizziness” and emphasis on shimmer. It has a keen note attack with good bite, crispness and definition in the lower treble that forms the foundation for a well-detailed, detail dense foreground. Instruments are just a touch thin but, otherwise, impress from a texture and body point of view. Accordingly, the timbre is quite natural, just slightly on the energetic side.
This contributes to an overall more engaging character that helps to round out the Apollo’s otherwise darker, thicker character. With an uptick of air and openness, the headphone upholds respectable if not outstanding headroom that does a lot to combat congestion. Extension performs at an average level in-class despite the headphone possessing a good amount of upper-treble on measurement. There isn’t a whole lot of information here meaning micro and background detail retrieval is about what you would expect from a midrange planar headphone. It serves more to enhance its energetic timbre and sense of vibrance in its foreground presentation. The Apollo’s top-end is in good execution and balance, being easy to enjoy and riding a tasteful line between engagement and accuracy.
As extension is limited, the Apollo doesn’t provide a huge, out of the head experience here, but rather a more intimate and focused approach. The stage dimensions are fairly modest, expanding just beyond the head and, therefore, being one of the more intimate open-back headphones I’ve tested. However, its proportions are especially well-rounded. The imaging performance draws most focus. The Apollo offers an impressively keen sense of directionality – more impressive given the lack of huge brightness or treble peaks. This gives the headphone a very multi-dimensional quality, the round stage aiding this impression.
While layering is harmed by the lack of midrange foreground/background contrast, I still found this to be a very involving headphone that places the listener in the centre of the stage, a quality that some may enjoy. Separation unsurprisingly leaves to be desired. The bass and midrange are both thick and warm with minimal air/ether surrounding each element. Note definition is quite good so the headphone doesn’t suffer from smear but does suffer on complex passages when taken as a whole. Treble, however, tells a different story as it is able to pace and separate well.
With a 97dB sensitivity and low 16-ohm impedance, the Apollo is an efficient headphone that doesn’t require much voltage to reach high listening volumes. At the same time, you will want a good amplifier with high current output due to the lower impedance. This isn’t so much of an issue in the modern-day where such can be found freely and cheaply.
Output Impedance Sensitivity
To test the impedance curve, I used 20-Ohm adaptors to artificially simulate a higher output impedance up to a 40-Ohm rating. Besides a volume drop, I didn’t notice a huge change, perhaps the headphones became a touch brighter and less bass-focused, however, not to a large degree. In turn, I would surmise that the Apollo has a relatively flat impedance curve at the impedances you’d be likely to see from modern sources. Being a low-impedance design, OTL tube amplifiers are not recommended.
The Apollo isn’t too demanding of the amplifier, able to achieve a dynamic and textured sound even from a dongle. Switching between my desktop stack with THX789 and the Astell & Kern Dual DAC, I was hearing a similar overall tonality. The desktop stack naturally provided a harder-hitting sub-bass with a bit more depth and power, but the dongle wasn’t far behind. Above, the midrange and treble were similarly defined. The desktop stack sounded slightly more spacious and the overall note definition was a step up. However, the Apollo is overall one of the easiest headphones to drive of those I’ve tested.
Suggested Pair Ups
True to Sendy’s design goals, the Apollo is efficient and easy to drive which adds to the value proposition as buyers shouldn’t feel the need to invest substantially in a source for these headphones. It isn’t sensitive to output impedance and can be driver nicely from lower-powered sources. Due to its coloured nature, the Apollo benefits from clean and well-defined sources such as the Hidizs S9 Pro and crop of NFCA and THX-based amplifiers. This will help to maximise separation and counterbalance its warmer nature. While some may prefer to further enhance its richness with a warmer source, this wasn’t to my personal preferences.