Fit and Comfort

Seen above is the right channel of the Symphonium Helios installed into an artificial ear mold, I am using the included size M red bore tips here that best suited my ears. I have average-sized ears, and the ear mold above represents my own experiences well enough as a proxy. The front-on view here is a touch misleading, you might think it fits well well indeed, and it can do so if you have large ears. The triangular form factor with the rounded corners is intentional to fit inside the concha, but the issue is how large/thick the shells are. If you have small ears/ear canals, then these would not be a comfortable fit or even any sort of fit. I saw first hand, how these simply refused to comply with another person's ears to where I can only urge you to try different options before choosing. Perhaps Symphonium Audio could put out STL files for the shells so people could 3D print dummy copies and try them out prior to placing the order? Thankfully I was able to accommodate these for 2-3 hours at a time before I had to take them out and rest my ears. This is a good thing to do in general anyway, so I can't personally fault the Helios' design. Note how the steep angle of the cable connectors means those extending housings now are convenient to route the cable up and behind your ears, so that's a silver lining for what otherwise felt like a missed opportunity from this nicely engineered set. If you do get a good fit then the mass of the IEMs (~8 g each) is a non-issue given multiple support points both inside and outside, with the cable helping further, and that also provides excellent passive isolation with it.

Audio Performance

Audio Hardware

There are four balanced armature drivers used within the Symphonium Helios, and there is also a true 4-way crossover between them. This is to say that each individual driver unit handles certain specific frequencies in the rated 12-24 kHz range here, and that's about it as far as the company is willing to share. Oh, I suppose there are other high quality components in the circuitry too—likely capacitors—but the reviewer and fellow engineer in me wanted to know more! What drivers are these exactly? Where are the crossover points at? I do recommend going through this page for more info about the Helios, this also tells us the BA drivers are arranged so there is one for sub-bass, one for bass, one for the midrange, and one for the treble frequencies. In a weird way I begrudgingly respect this move because people would already have some pre-formed conceptions in their mind about what an all-BA set of IEMs can do. So this is Symphonium Audio and Subtonic saying don't worry about the driver type and driver count, just hear what they can do in the Helios. There is also a proprietary technology integrated inside the Helios called FLAT (Filtered Linear Attenuation Tuning), which is a nice way to describe complex filters working alongside the crossovers to allow for the Helios's sound signature to be unaffected by the output impedance of your source. This in itself has been executed to varying degrees of success before—64 Audio's LID comes to mind.

This is great news for those with perhaps not the cleanest of sources, and yet you should be aware that all this power comes at the price of, well, needing more power. With a rated sensitivity of 104 dB/Vrms and impedance of 8.5 Ω (hence making FLAT all the more relevant), the Symphonium Helios actually needs more power than any tested IEMs to date. See, 104 dB/Vrms here converts to just 83.3 dB/mW, meaning you need 47 mW to hit 100 dB transients and a whopping nearly half a watt for 110 dB transients. I personally listen to sound at lower volumes to begin with, so a good portable DAC/amp is plenty here. This is also where the Hidizs S9 Pro and/or Hiby R5 (Gen 2) DAP on the previous page came in, and it provided enough power to not hold the Symphonium Helios back. The lack of a 3.5 mm audio jack for most phones these days is another reason to consider at least a DAC/amp for portable use. If not on the go, space is less of an issue, but the relatively short cable might still be a handicap if connecting to a PC as the audio source.

Frequency Measurement and Listening

I will mention that I have a general preference for a warm-neutral signature emphasizing a slightly elevated bass and smooth treble range with detailed mids and good tonal separation. I also generally prefer instrumental music over vocals, with favored genres including jazz and classical music.


Our reproducible testing methodology begins with a calibrated IEC711 audio coupler/artificial ear IEMs can feed into enough for decent isolation. The audio coupler feeds into a USB sound card, which in turn connects to a laptop that has ARTA and REW running and the IEMs connected to the laptop through the Hidizs S9 Pro, after confirming it puts out a clean signal. I begin with an impulse measurement to test for signal fidelity, calibrate the sound card and channel output, account for floor noise, and finally test the frequency response of each channel separately. Octave smoothing is at the 1/12th setting, which nets a good balance of detail and noise not being identified as useful data. Also, the default tuning was used for testing, and no app-based settings were chosen unless specifically mentioned. Each sample of interest is tested thrice with separate mounts to account for any fit issues, and an average is taken of the three individual measurements for statistical accuracy. For IEMs, I am also using the appropriate ear mold fitted to the audio coupler for a separate test to compare how the IEMs fare when installed in a pinna geometry, and not just the audio coupler. The raw data is then exported from REW and plotted in OriginPro for ease of comparison.


The IEC711 is such that you can't really compare these results with most other test setups, especially those using a head and torso simulator (HATS). The raw dB numbers are also quite contingent on the set volume, gain levels, and sensitivity of the system. What is more useful information is how the left and right channels work across the rated frequency response in the Symphonium Helios, or at least the useful part. The left channel was separately tested from the right one, and colored differently for contrast. I did my best to ensure an identical fit for both inside the IEC711 orifice, so note how the two channels are basically identical throughout the entire frequency range—that tiny dip in the sub-bass for the right channel is not going to be heard in person. Indeed, this excellent channel matching continues well past the coupler resonance too, so much so that I have zero complaints here and I believe Symphonium Audio when it said the drivers and channels are tested extensively before being assembled and paired up. Measurements taken after 30 hours of testing, including with these playing a mix of various songs as well as white or pink noise and sine sweeps showed no differences. There was no perceived burn-in effect thus, and none was measurable either. The response with the anthropomorphic pinna in place also matches the ideal scenario in the coupler very well, which is effectively an indicator of how good the fit and seal was in the artificial ear.



Here is the average frequency response for both channels of the Symphonium Helios plotted against my personal target taken from VSG.squig.link, which also gives you an idea of my personal preferences, to better correlate any possible biases. The tuning of a set of headphones or earphones does not have to match my target, as long as it is tuned with some direction, makes sense, and is executed well. After all, no one set will appeal to everyone, and having different options is what makes this hobby so interesting and hard to quantify. Symphonium Audio certainly has a lot of marketing spiel in its website as it pertains to the Helios, but the main thing that struck to me were the driver allocations as well as a deliberate tuning based off the Harman in-ear target. This is why I have a second graph above, showing how this unit compares to it too.

A quick look above confirms that the Harman target adherence bit is valid enough, and the company does also say the higher frequencies (3-10 kHz) were tuned more conservatively, to avoid fatigue and allow for a more natural presentation. What a great move this turned out to be, because the treble response alone is something I typically see in IEMs that are much more expensive, and in the TOTL class. Another reason behind the larger shells here is because Symphonium Audio had to go with larger tubes, which in turn assists with the excellent treble extension noticed here. I will also mention that the resonance peak here is not set to 8 kHz, because doing so would be only possible from a shallow/loose fit that is not representative of my experiences—you can still see it in the link above. Instead, I see it appear at 8500 Hz on my IEC711 coupler and found it closer to 7.5-8 kHz in my ears. It's also certainly not as peaky there, but I suspect some of the filters are playing a role here too. Also, do try the red bore tips here as I found them far more pleasant and smooth in what could otherwise be construed as harsh or even sharp. I ended up with plenty of range for complex orchestral pieces where instrument fundamentals play harmoniously with second/third order harmonics, and the Helios is excellent for classical music and contemporary jazz. Piano keys and string instruments in particular are a delight here, and the instrument timbre comes off distinct and non-BA like too.

The "doesn't sound like BA" part rings all the more true with the bass response here whereby the Helios has two of the four BA drivers dedicated to the lower frequencies. Notice the ~10 dB rise from 200 Hz going down, with the BA driver handling the sub-bass effectively acting as a subwoofer akin to what we saw on the Truthear Zero recently with a dynamic driver doing the role there. This is the best bass response I've heard out of any all-BA set outside of some that cost 4-6x as much, and I'd say the bass impact and detail rivals most premium hybrid IEMs too. It's a delicate balance of slam, quality, detail and quantity to where I can see many purchasing the Helios just for this. That being said, the dip past 200 Hz can make the mid-bass somewhat lacking and dry, so much so that it is one of the few weaknesses here in what is shaping up to be a fascinating and positive review. Poorly mixed classic rock music in particular can make you yank these out of the ears very quickly.

Indeed, I'd say this "something is missing" thought popped up further in the mids too. I saw another review talk about how perhaps adhering to the Harman target wasn't ideal here given the increased bass leading to perhaps recessed vocals, and I would have preferred to see a touch warmer tuning in the lower mids too. It's not easily solved by EQ either, since I never got that fine balance the bass response did, and found the Helios not delivering as well on vocals as it does most instruments throughout the frequency response. It also doesn't help that the Helios is extremely detailed, so you pick up on these nuances easier. Female vocals were well reproduced to my HRTF, but I can see where they might come off slightly hot, or nasally to others. In short, I'd pick the Helios to be an all rounder more than specific vocal-first genres. The drivers themselves are technically very proficient though, and resolving power is not the least of it. Indeed, I thought the Symphonium Helios with its presentation also benefited from having a wider and taller soundstage than most IEMs on the market too, and imaging is quite precise in a wide cone up front in addition to around the ears itself.

Comparisons

I've reviewed multiple of these so-called kilobuck IEMs in the past, and no doubt more will follow. There are many which cost more than the Symphonium Helios, including the three seen above in the form of the 64 Audio Duo (USA, hybrid), the Effect Audio Axiom (Singapore, hybrid), and the the Lime Ears Pneuma (Poland, hybrid). There are also many tribrids in this price range in addition to a few other all-BA sets including the 64 Audio U18t, and they all go with more drivers than the four in the Symphonium Helios. Would you believe it then that I'd certainly pick the Helios over these three and also the $3000 U18t? It's extremely detailed, fun to listen to even with the bass being more than I'd personally prefer, and has the best treble reproduction of any set reviewed here. The tuning of the other sets is also not as pleasant or well-executed in some cases, and I am happy to see a relatively new company being successful and innovative while also upstaging the establishment.


Things get more serious when I chose three of my favorite sets to date, two of which cost slightly less than the Helios in the form of the DUNU ZEN PRO (Taiwan, single DD) and the ThieAudio Monarch MK.2 (China, tribrid). and then there's the ThieAudio V16 Divinity that costs more but adopts the all-BA driver configuration with a whopping 16 BA drivers per side. I only had a few hours with the Monarch MK.2 so take these impressions with a grain of salt, but I thought its sub-woofer implementation was weak and the bass there wasn't as strong as many fans of the IEM thought. The Helios is also more resolving than the Monarch MK.2 if I say so myself, although the fit issues might be a concern. This is all the more apparent with the smaller DUNU ZEN PRO that showcases how good a single DD set can be, and I'd take it over the Helios for the low and mid frequencies myself. But then it loses big time to the Helios in the higher frequencies in addition to once again not being as detailed. The ThieAudio V16 Divinity is an extremely close fight though, and both sets happen to be on the larger side of average too. I'd argue that the V16's bass is more fun but still comes off BA-like compared to the magical bass on the Helios, and the Helios is a touch more resolving with better instrument separation and layering too. The V16 on the other hand handles the mids better and most people will find forward-facing vocals there. I could honestly go either way and be happy, but the Helios does come in ~2/3rds the cost so I suppose it wins in that regard.