I’m sure many associate IEM filters with some sort of a replaceable nozzle filter and a predictable selection of three pieces with a default sound, reduced bass sound, and enhanced bass sound. FLC decided to approach the filtering method in a completely different way. They split their filters into 3 groups: ULF (ultra low frequency, associated with sub-bass), LF (low frequency, associated with mid-bass), and MF/HF (mid and high frequencies associated with midrange and treble). Keep in mind, this is a hybrid design, thus ULF and LF will be focused on fine tuning the sound of Dynamic Driver, and MF/HF will be focused on fine tuning the sound of Dual BA drivers.
This filtering system is nearly the same as found in FLC8S, with the only exception of a nozzle filter. With 8S being a more traditional universal design, you unscrew and replace the nozzle part which controls a combined bore opening. Custom FLC8C has a 2-bore nozzle design with a dual rubber-plug filter to control MF/HF. Having the advantage of a clear shell, I can trace the path from a Dual BA going to one of the bores where the corresponding side of the dual plug has an actual filter while the Dynamic Driver (DD) goes to another bore and that side of a dual plug filter only varies in the opening width.
The actual filtering of DD is done through 2 vents in the shell using the corresponding ULF and LF filter plugs. The low frequency (LF) replaceable rubber plug goes into the vent located right across the DD. The sub-bass frequency replaceable plastic push pin (ULF) goes into the separate vent with a tube connected to the output of DD driver.
Each earpiece has 3 filters, one going into the nozzle and the other two going into the faceplate of the shell. Each filter is color coded and corresponds to the following:
ULF: clear – less ULF, gray – medium ULF, red – most ULF
LF: clear – less LF, gray – medium LF, black – most LF
MF/HF: clear – lower HF, gray – medium MF and medium HF, gold – most MF and medium HF, green – medium MF and most HF.
If you do the math, you’ll end up with 36 different sound combinations!!! I’m not aware of any other IEM that can pull off the same.
With so many unique sound combinations, how would you even describe the sound? Based on filter description where gray is considered to be a happy “medium”, I started with that setting as a baseline sound of FLC8C. With all 3 filters selected as gray, I hear a very resolving, reference quality, expanded sound with a balanced signature, excellent retrieval of details, and an impressive transparency. FLC8C has an excellent extension of the low end and high end (deep low end extension down to sub-bass, very textured, very articulate), punchy mid-bass, nicely balanced mids with just a perfect amount of body and excellent retrieval of details, sometime even down to a micro-detail level, and a high definition crisp airy treble.
Soundstage is definitely above the average, though I do hear a bit more width then depth. Layering and separation of instruments and vocals is very good, with a decent imaging that has a very convincing placement of instruments and vocals which actually improves as you switch to brighter high frequency filters.
Now, starting with this all-gray baseline, I will go into a brief description of each filter variation and its effect on the sound.
*** MF/HF variation
gray – gray – clear: treble and upper mids are a little rolled off, while low end and lower mids stay the same, sound becomes a little smoother and warmer.
gray – gray – gold: lower mids become a little bit thicker but not muddy, and as a result of this I hear treble a bit rolled off.
gray – gray – green: treble gets boosted and becomes a bit grainy, but you do hear an improvement in airiness.
*** LF variation
gray – clear – gray: noticeable reduction in mid-bass where it becomes more neutral and flat. It also thins out lower mids a bit, making sound more reference quality.
gray – black – gray: adds more mid-bass hump, the same speed, just a little bit of quantity boost, but I do hear more body in lower mids which now sound a lot thicker, making overall sound a bit less transparent.
*** ULF variation
clear – gray – gray: i do hear sub-bass slightly more rolled off, but it’s a rather subtle change, sub-bass is still extended but has a little less quantity.
red – gray – gray: beefs up sub-bass without too much exaggeration which also improves the impact of the mid-bass. While LF boost affects lower mids, ULF boost affects/improves the whole sub-/mid-bass in a very positive and controlled way.
*** favorite combos ***
red – gray – gold: my favorite low end setting with a very articulate and well controlled bass and a smooth yet still detailed top end.
red – gray – gray: the same as above, but with a more revealing and airy top end.
Step-by-step application of filters.
Comparison to other headphones.
This comparison was done using PAW Gold as a source, and FLC8C in its default filter config (except for 8S vs 8C comparison).
FLC8S vs FLC8C – even so we are dealing with the same 3-way hybrid design and nearly the same filtering system, the shell design and the nozzle filter difference should account for some changes. I used my favorite red-gray-gold filter setting on both for a comparison. The first difference I hear is 8C soundstage being a little wider, while 8S has a little more depth. The bass and the treble between these two sounds nearly identical, but I hear the difference in mids where 8C has more energy and a little better retrieval of details while 8S mids sound a little dryer and more withdrawn.
FLC8C vs DN2kJ – DN has a similar soundstage expansion, not as deep sub-bass extension, more neutral bass in comparison, leaner lower mids, brighter upper mids, detail retrieval is similar (though DN is more analytical), DN has slightly better treble extension with more airiness. Overall FLC8C is smoother with fuller body vs DN2kJ being leaner and more vivid.
FLC8C vs Primacy – similar resolution but Primacy by default has a more lifted low end with a heftier sub-bass and a stronger mid-bass, a little more body in lower mids, similar upper mids, and more rolled off treble. Very similar soundstage, though FLC is a little bit wider.
FLC8C vs A83 – FLC soundstage is a little wider. Mids and treble are very similar but the big difference is that A83 has more impact in the low end with deeper sub-bass and more mid-bass punch. In this comparison FLC is more balanced and has a little better resolution.
Regardless of Universal or Custom design, FLC8x is one very impressive pair of 3-way hybrid IEMs with an excellent resolution, great sound extension and soundstage expansion, and a very flexible sound tuning. Its triple-filter sound customization is what sets it apart from any other tunable monitor I have ever tested. You almost feel like a sound designer, going through different filter combinations, adjusting the sub-bass, mid-bass, mids, and treble to tailor it to your exact liking. It’s true that filtering pieces are very small to handle, but typically you shouldn’t be going back’n’forth with constant adjustments, though it’s tempting.
The biggest question in here is if Custom FLC8C worth nearly the double price of Universal FLC8S? The word “double” has quite a weight to it, until you stop and think about the actual price of the universal version and realize that if you take into account a premium cable and a premium aluminum storage case, you are only paying about extra $200 for an improved Custom shell design with a superior fit and isolation. I have a feeling many diehard fans of FLC8S will probably justify this price difference to turn this one of a kind 3-way hybrid IEM into one of a kind Custom fit earpiece. And for those who have been eyeballing FLC8x design, the FLC8S still has one great value.