Pros: Great Build Quality, High-Quality Cable, Happy Medium in Sound Signature and Sound Quality
Cons: Average Fit/Comfort, Time Coherence Issue of the Prior Art AX60 and Other Hybrid Designs Not Solved
Chinese/HK headphone manufacturer Astrotec is known for its value-priced products (beginning with their lauded AM90), and they've stuck to their guns by pricing the AX35 (and its near identical sibling AX30) aggressively at under $80. The list price for the AX35 in Mainland China is 399 RMB (~$65 USD), while the AX30 is priced at 299 RMB (~$49 USD). Current eBay prices for the AX35 hover around $70 USD. Considering the overall quality of build and acoustic design, the lower end of Astrotec's Hybrid Series certainly promises to deliver a lot for the coin.
It hasn't always been this promising, however. Months ago, I purchased Astrotec's flagship hybrid model, the AX60, and was sorely disappointed. During my listening sessions, I sensed a disconcerting disconnect between its ponderous, half-a-beat-late bass and its razor sharp treble. Despite my best efforts to appreciate the strengths that the AX60 brought to the table (i.e. immense build quality), I just did not think it was a very good product and I let Astrotec know about my opinions sans self-censorship. While my feedback was somewhat scathing, I set out to let them know exactly what they could improve on, and sent the AX60 on a world tour, where the reaction from other members, like my own comments, have been mixed.
Astrotec couldn't have been too happy to hear that the flagship product they spent years developing and patenting wasn't quite put together, but it seemed that they were determined to make hybrid IEM technology work --- I was promised that they'd do a lot better with their subsequent models. I had my doubts, however, as not even venerable AKG could get the hybrid modality quite right, even with a $1400 earphone in the K3003. Not phased, Astrotec sent me a sample of the AX35, perhaps in an effort to redeem themselves.
Fortunately this time, I believe they have.
Packaging, Accessories, Build Quality, Ergonomics & Comfort
The AX35 came in a small, self-contained box that proved to be drama-free even for any baby mamas. Inconsistent typeface and layout quibbles aside, the earphones were packaged well. Sandwiched between the various layers of foam padding were the earphones themselves, as well as an aluminium tin that held various accessories, such as a pair of ear hooks, and several sets of ear tips.
I'm not an ear hook type of person, so I never bothered to try them on. They seemed just like any other set of ear hooks on the market. It's good to know that they included a set for the people that desire using them, though.
As for the ear tips, three sets of single flange silicone tips have been provided in various sizes. I dislike them all. They don't seal very well in my ears, and don't feel very comfortable. Acoustically, they perform well, however, so if you have no problems with their comfort, then stock tips will be no problem at all.
The extra set of foam tips provided are truly great. They're a tad bit softer than the Comply T-400 tips they're designed to emulate, but they feel very nice.
The earphones themselves were strongly reminiscent of the AX60, whose outward build quality was arguably its strong suit. The brushed metal lines running along the body of the AX35 exude a luxurious, robust quality to the product. In the right light, the AX35 looks positively beautiful. At the very least, it is a handsome earphone that is just a little confused with which typeface to use.
While I didn't have the AX60 on hand to compare, it seemed that the finesse of build was slightly lesser with the AX35 --- understandable considering the large price difference --- but practical differences are minor.
Personally, I'm not a fan of the multi-colored (red and green) cable strands and would have rather Astrotec used opaque black sleeving on the cables of both the AX30 and the AX35, but functionally, they do work well --- I just wished Astrotec gave more regard to aesthetic design principles. I must once again mention that Astrotec can't make up its mind regarding which typeface to use. The aluminum tin uses the space-age font that is used in the AX60 as well, but the body of the AX35 reverts to the old wide-faced Helvetica style logo, while the Y-splitter labels the model number with a narrow-faced Helvetica style typeface. Design and style inconsistencies seem to be universal problems amongst Chinese companies.
In the same vein, as with the AX60, the silver anodize of the 1/8" inch plug and splitter still differs from that of the housing backplates, but I care less about these inconsistencies in a budget-level product. The fact that the AX35's build quality comes nigh close to that of Astrotec's flagship product is a positive sign.
As with the AX60, the housings will be hit or miss with respect to fit and comfort. While the AX35 will undoubtedly accommodate a greater variety of ears because it forgoes the awkward, squarish over-ear cuff of the AX60 for a more traditional barrel design, the sheer depth of the piston-like housings may cause some fit issues with certain segments of the population.
I, however, did not find the fit troublesome, and for the most part the majority of people will not find the AX35 a challenging fit.
In my own case, my smallish ears --- usually a fit liability --- may actually be a benefit because the weighty housings have more structural support within the concha. Conversely, people with larger ears and greater diameter ear canals may actually find the hefty torque weight to be a little overbearing over time.
Interestingly, the cables are an irrefutable improvement over the ones on the AX60. They're softer, more pliable, less memory-prone, and less microphonic, but retains the same tangle-free predilection. These cables are therefore an incremental, but significant step-up over the cables on the AX60, and I would assume that the same improvements are being applied to the latest batches of the AX60, as well as other models in the Astrotec product portfolio. New age, thermoplastic elastomers (TPE) are coming into their own for cable sleeving, and they're definitely primed to replace traditional modalities of PVC, PTFE, and PE in the near future. The AX35's cables are a testament to TPE's potential.
These days, I find it a chore to describe a sound signature in discrete terms. Informally, I'll just say that the AX35 is neutral enough to give a balanced presentation to most music, but has enough salt and pepper as not to lose the average consumer. In other words, it's mildly V-shaped. The important part is that this mild V-shape is quite pleasing to the ears.
Perhaps I should go ahead and get the negative out of the way, because it'll later be qualified as inconsequential. Yes, the bass is the weak link in the AX35.
Sometimes, I really don't know what to say about "bass quality" when something is priced at $70. My gut instinct is to say that it's "not good enough", but compared to what? I guess you could say that the $99 HiFiMAN RE-400 possesses superior bass speed and extension, and could loosely be categorized as a direct competitor to the AX35 in sub-$100 price category. Anything else, I either have not yet heard, or costs several orders of magnitude more than the AX35, e.g. Sony MDR-EX1000, Tralucent 1Plus2.
Thus, I really shouldn't complain about the bass on the AX35 --- when taken alone, it performs admirably. Coupled with any source of negligible output Z (personally, I tested it with an amplifier with <0.3 ohm, not at liberty to reveal which model), it delivers with expectorant texture and fullness, while simultaneously not impacting the skull with the consistency of a 2 x 4. The bass response is, well, responsive. It's not really a neutral response, but it hits a nice happy medium. Most people would find the bass response quite pleasing. As always, bassheads don't apply --- they're never satisfied.
Take, for example, a K-pop electro/hip-hop track that includes a good bass mix, Taeyang's (of BIGBANG fame) Ringa Linga:
The bass mixed into this track is actually fairly subtle, but ever present. It exists as a gentle rumble, rather than a punchy beat sign. What does the AX35 do with the bass? Well, it lifts it to a level that would be satisfying for a good two out of three listeners (the remaining third listener would either complain that there's not enough, or too much bass), without obfuscating details of the midrange. I would harbor to say that this type of bass response would give listeners a entry taste to a "hi-fi" sound --- measured bass, with good detail levels.
Taken in concert with the midrange, however, and my opinion drops. Strictly speaking, the lower midrange, especially in the 300-400 Hz "mud" area, is a jumbled mis-timed syncopation of the two drivers, resulting in a "fuzziness" for vocal fundamentals. The best way to explain the bass is that it's a little slow on the attack phase. Take an impulse response measurement of the AX35, and you'll likely find that the impulse is not the clean step-like ideal that many hi-fi designers strive for. In practical terms, I find that it's hard to keep an accurate metronomic count of the beat with the AX35 --- only snare hits (whose signature sonic energy exists mostly in the upper midrange) seem to come down at precisely the right moment. In audiophile speak, "PRaT" feels a little off.
It's conceivable that Astrotec specifically tunes its dynamic drivers this way, but my personal philosophy is that if you're designing a hybrid system, ADSR traits should be well-matched between drivers, especially if the dynamic driver is seated behind the BA driver (thus introducing inherent time alignment issues). Thus, if the balanced armature driver possesses quick attack and decay, then the dynamic driver should be so as well, as not to emphasize any time domain incoherencies. It should be noted that even the $1200 AKG K3003 possesses the same issue, so coherence is a problem across all hybrid designs, but I feel that it's especially easy to hear on the Astrotec models.
Thankfully, the issue is much less pronounced with the AX35 as compared to the AX60. Whereas the AX60 introduced a noticeable time delay that was further made apparent with its strong mid-bass response, the AX35 is fairly mild in its bass response, and thus there's less intermingling of mis-timed response between the dynamic driver and the BA driver.
It's not all bad news, though. A palpable time delay in the lowest bass registers is absolutely fine, as it is able to somewhat simulate the characteristics of speakers in a room with omnidirectional bass fanning out and bouncing off the walls. The big issue is where the group delay is introduced. Under 60 Hz is usually the "ideal", but perhaps the low pass design of the AX35 hasn't been cut low enough to be perfect. Add a resistor in series, however, and it seems like the problem becomes mitigated, as the balanced armature's lower frequency response is dropped, as well as the dynamic driver's, minimizing driver overlap between frequencies. The downside to using a resistor in series, however, is that the dynamic driver becomes underdamped, resulting in poorer texture and impact.
Realistically, however, I don't know if my technical grievances truly apply to a budget earphone. The AX35 sounds relatively clean and controlled, and doesn't overdo it with the bass, something that too many budget-minded models are guilty of. I also wonder if that many people would actually notice the time coherence issue. While I manage to notice, both in the AX60 and in the AX35, the effect is somewhat subtle in the AX35. In real world listening, I don't really have a problem with the AX35. If you're a stickler for absolute technical performance, then avoid the AX35, but if you're not, then I highly doubt you'd have a problem with it.
If you've noticed that I sound wishy-washy about this issue, it's that I really don't know how to think. Should I apply strict standards of acoustic performance even to budget-minded IEMs? If so, too many IEMs would fail miserably. Then, should I grade on a curve? If so, then the AX35 acquits itself nicely, and it finds itself near the top of the heap when it comes to the bass.
As mentioned previously, the time coherence issue prevents the AX35 from being ideally transparent, but timing issues aside, the midrange is actually quite pleasing.
Warmth and fullness are conveyed well, measured for a happy medium. Imaging is one of the better aspects of the AX35, as the Knowles ED balanced armature driver that drives the mids and highs is highly accurate and able to convey good transient response. The AX35 therefore manages to be one of the more detailed and transparent earphones in this price range.
Treble presence is mostly neutral, with some canal-related resonances that may end up bother a small portion of the population, but should not be an issue for most. Extension seems to do decently well.
With a 64 ohm serial resistance adapter, the AX35 improves further in imaging and treble accuracy. As mentioned before, the lower midrange drops out a bit with added resistance, so vocals lose a bit of body, but overall, the sound improves for the better.
Sensitivity, Source Pairing & "Synergy"
At 8 ohms (at 1 kHz), the AX35 has likely had its drivers hooked up in parallel, which drops its total DC resistance. The AX35, therefore, is a very sensitive earphone. I barely have to turn up the volume. However, with a serial resistance adapter, its sensitivity drops significantly.
So what should the AX35 be paired with? Should a serial resistance adapter be added to the chain? It depends on what you want out of the sound of the AX35. Do you want more midrange body and solid impact to your AX35? Then look for a warm source with low output impedance, e.g. FiiO X3. Do you want less bass impact, more clarity, and a greater sense of imaging? Try adding some in-line resistance.
The AX35 seems to be polarity aligned, so adding in-line resistance will electrically underdamp both the BA driver and the dynamic driver. As mentioned in previous sections, this leads to a linear suppression of bass driver's whole response, as well as linear suppression of the lower half of the BA driver's response, resulting in a cleaner, leaner, more transparent lower midrange, at the expense of some bass texture.
After some trial and error, I arrived at a portable setup that consisted of an amplifier with 10 ohms of output impedance (Firestone Audio Fireye HD, which is based around the TPA6120A2), an in-line resistance adapter with 64 ohms of series resistance, from a Sony NWZ-F886. This setup really helps normalize the response of the AX35 into something that is acceptably neutral while simultaneously minimizing the time coherence issue between the two drivers.
However, this setup is not for everyone. It really depends on what you want out of the earphone.
In many ways, I find the Astrotec AX35 to be a very good product. It's very well-built --- well beyond most peoples' expectations for a $70 earphone. It has a balanced sound signature that doesn't go overboard in any one aspect --- erring on the side of neutral, allowing it to be versatile across multiple music genres. Its ergonomics are reasonable without sacrificing aesthetics.
It's not perfect, however.
Like most other Chinese companies, Astrotec has yet to figure out the significance of a consistent corporate identity across product lines. Space-age typeface here, Helvetica there, Arial elsewhere, and Proxima Nova on the box do not instill in people, especially casual customers new to Chinese products, any kind of consumer confidence.
Perhaps more importantly, there are inherent technical difficulties to overcome with a hybrid dynamic/BA design that require precise time-alignment and decay phase coherence. While it's arguable that the average listener would care, the head-fi community nevertheless houses a subset of very picky listeners that may very well find some aspects of the hybrid sound "off". The AX35 will not allay those peoples' concerns. Astrotec seems to be banking on the market appeal of the hybrid driver paradigm and hoping technical concerns go by the wayside.
Yet, while It's important for potential customers of the AX35 to recognize its limitations, it is, after all, a ~$70 earphone and Astrotec has surely had to make budget-based concessions on both dynamic driver development and the overall developmental cycle. When these factors are taken into consideration, the AX35 should be seen as a great accomplishment for Astrotec given its target price envelope.
Therefore, I believe that Astrotec has delivered a product that is well-worth its asking price, and I hope they continue to release products that bring a lot of value to the consumer.