In my experience, Furutech NCF tech is very good for increasing finer details, like in their plugs, connectors, outlets, and outlet covers. These NCF Clear Line devices (about $250 each, MSRP) are a little different and have their own unique signature to the effect they have on sound systems. They are marketed by Furutech primarily as an "AC optimizer" but that’s too generic to get an idea of their effect in practical usage. Short snippets from other reviews you might see, like "improves imaging" or "boosts resolution" lack context, in my view, because optimization is inherently about judicious usage based on what a system’s current state needs for improvement (aka tuning). So, here’s my thoughts on the effect you can likely expect from these things and how you can potentially use them to tune your sound.
These are small devices that plug into an AC socket. The important thing to "get" is that they work by passive damping, not by capacitors or more elaborate electronics. Damping has to do with reducing the amount of vibrational energy by dissipation (accelerating decay). They are both mechanically and electrically damping by implementing a combination different materials into one small device, including nano sized crystalline material, carbon fiber, nylon, metals, and an air chamber. The total result is a composite of the intrinsic resonances of the each of the materials combined, plus a characteristic of the NCF material that Furutech says "generates negative ions that eliminate static, and it converts thermal energy into far infrared." I think it really should be thought of as a device to reduce mostly higher frequencies through connection into the AC line. It appears that they engineered it to damp various high frequencies to different degrees.
So what does it sound like with in use? Here’s my best attempt to explain:
IF there’s already substantial unmitigated noise entering a sound system due to local EMF radiation, surface/air/systemic vibrational energy untreated by dissipation, noise polluted city power lines (and maybe also due to untreated room acoustics and sub-optimal building structure), THEN:
~The NCF Clear Line may help reduce some noise and get the top end to sound smoother and less grainy, as well as overall slightly better resolution, including imaging and soundstaging. Perhaps somewhat "tighter bass", but not more bass energy/amplitude.
~ I’ve found that in an all-digital streaming system, digital signal and power line noise mitigation (filtering) and vibration isolation is a critical part of getting high quality, natural, and smooth sound, as well as finer resolving power of high-res source material. I’m not sure that an NCF Clear Line would be the best choice near the more fundamental stages of system building, but maybe this device is a more affordable option to try out?
IF there’s not much high frequency noise affecting a systems sound because of effective inline treatments and/or a better acoustic and EMF environment, and the top end is already clear, and image and soundstage impressions are already highly resolved, THEN:
~ Overall, I would expect it to reduce the higher audible frequencies, shifting the emphasis from the higher frequencies into the higher mid-range frequencies. This is really how the device is tuned, in my experience, and this is a significant point.
~ If the top end is nicely energized and extended but not already smooth and precisely tuned, then the device can reduce sibilance, emphasize cymbals’ lower range giving them more presence and "meat", and give higher pitched vocals and stringed instruments more "body".
~ If there is already a very low noise floor, very good resolution, the top end is already nicely extended yet also smooth/liquid, and high-mids are present enough to meet your ideal sound, then these devices will probably immediately have a darkening effect and may reduce the soundstage dimensions and finer details that come from high frequency information. However, if you find some new tubes, cables, components, that you really like and have great extension, but you want a little shift from highs to high-mids or reduce sibilance in the 6khz-8khz range, for example, then it might me nice to have this device around for the optional tuning.
Important to note is that the effect of one device is subtle, and may or may not even be perceivable, depending on factors related to the individual system, acoustics, and the listener. There is an additive, approximately linearly increasing damping effect of using additional units simultaneously. So, perhaps there is a state positioned in between the two sound system conditions I mentioned above where, as more devices are added, one might first experience an increase in resolution by damping unwanted noise, and then as more are added, start to hear a reduction in the desirable top end detail by overdamping.
The strength of the effect of each device depends on where it’s placed:
1) in a wall outlet some distance from where components are plugged, vs.
2) in the same wall outlet as components or power conditioner are plugged into, vs.
3) in the same line from a power conditioner as a single component by using a splitter, vs.
4) an unused outlet in a power conditioner
So, one device used in one position can be like two devices used in two different, but less efficient positions.
Eventually, adding too many will darken the sound by overdamping. But hey, maybe there’s something about your acoustics that is irritating and that’s something what you might want? Most likely, 2 or 3 will be ideal for getting the right balance, or perhaps just one plugged right into an available outlet on a power conditioner. I’ve seen reports of users with several of these in play, still with no perceivable effect. Hence, the using term "optimizer" to describe this device really begs the full context of the conditions within which it’s being applied.
I’ve tried to be helpful and informative about this product by sharing my experience. I also chatted with someone else that had basically the same experience as I did as they added more of the devices to their system. So there ya go!