I haven't actually seen an actual midrange that uses ferrofluid before, but one thing must be true: the magnetic gap itself cannot be designed to be sealed against contact with ambient air (not and still make the driver be able to move in and out of the gap), so that means the ferrofluid WILL eventually dry out and THAT means the manufacturer must have made (or left) some provision for someone to replace it when it does. In fact, that may even be so simple that you could do that and save yourself the headache and expense of going through a technician - after all, if you were going to send the driver in for repair, you'd still have to remove it from the cabinet yourself, right?? We may as well take a look at it and see how it was constructed.
But before doing that, first a word about the right kind of ferrofluid. For lack of any formal introduction into ferrofluid types, you might imagine that the specific type needed for loudspeakers is presumably something on the order of a "high-" or "very-high-" grade of product and that only a very specialized manufacture of it will do. Luckily, for audiophiles this not the case. Well...we really don't want LOW-grade ferrofluid, but avoiding that is really all we have to worry about. All ferrofluid is, is simply pulverized iron filings suspended in a "carrier fluid". That carrier fluid is normally ordinary mineral oil, nothing more. All the carrier fluid has to do is allow the iron particles to be suspended uniformly throughout the oil on its own, which mineral oil does, and to help take away the minescule amount of heat in the voicecoil, allowing for better power handling and HF response. Mineral oil-based ff does all this just fine, there is no need to ever seek out any kind of 'wonder' formulations of other carrier-oil-based ff's that were dreamed up (in more recent years) for manufacturing, industrial or scientific uses - they offer absolutely no advantage to us. The amount of voicecoil heat we're talking about is so low with respect to the mineral oil that, honestly, it would be like insisting on only using premium gasoline in a 1972 VW Beetle...a big expense for zero gain. The LOW-grade ferrofluid, OTOH, is made and used primarily for educational purposes - schools (down to the elementary level), labs or even to be made at home for amusement's sake (you can google all that for a better understanding). This type substitutes ordinary Kero Syrup (a food product) instead of mineral oil because it is more kid- and environment-friendly...of course, it breaks down much sooner than mineral oil. Just avoid this type. Mineral oil-based ferrofluid is mineral oil-based ferrofluid, the brand or price is generally not critical.
Now back to the mid driver. If you can remove it from the cabinet, look at where the cone connects to the voicecoil and the voicecoil fits inside the gap. Push gently on the cone and see if you can see the dark brown ferrofluid 'rise' up, or bead up in the gap as the voicecoil travels along the in-stroke inside the gap. If you can begin to see the ferrofluid at any point along the length of voicecoil travel, then that's a good indication that the 'ferrofluid tank' is indeed full and there's no present need to add any more. This would be the same for your tweeters if they use ferrofluid too (but, never directly touch a titanium tweeter dome or it will be damaged). As for the mid, try examining the driver, looking for how the cone/voicecoil assembly was attached to the magnet assembly in such a way that they could both be separated from each other, by way of removing the right screws, so that nothing "glued" or otherwise permanently sealed need be violated. If the vc/cone assembly truly couldn't be separated without breaking something, then look for some kind of hole, or screw that is hiding a hole, and see if that is an access to a ferrofluid reservoir. If there is no such hole, then remember: if it can be put together, it can be taken apart, and also place some confidence in the fact that if it were put together indeed without any provision for someone like you to both clean out the reservoir and then add the needed amount, then that would in truth be rather shameful engineering (a product designed to fail) indeed - not at all something I would be expecting from a company like Hyperion, so take your time if you have to and perservere in uncovering the way to get the magnet assembly by itself and have direct unobstructed access to the magnetic gap. One word of caution: when you're at the point of having removed the requisite screws and are about to remove the vc/cone assembly from the magnet, you may find they are still stuck together even though all the screws are out. Be aware that these 2 items have been held together under high pressure for a long time indeed. It's ok that it may take a little force to separate them. Just make sure you dont't damage anything delicate the moment the surfaces are "popped" off each other - the exact moment of which may be unanticipated.
None of changing ferrofluid is necessarily all that difficult, really. The first time is a puzzle when you have to find out how to disassemble things a bit, but after that it gets to be a piece of cake, really.
If you have the magnet by itself with gap face-up in front of you and under good lighting, look inside the gap where the ff should be. If along the circumfrence of the gap the surface of the ff (in cross-section) describes a 'u-shape' between the surface of the flat steel on either side of the gap, then that is the correct ff amount and the vc, once inside the gap, should displace enough ff for it to become about even with the steel surface of the magnet assembly while the vc is at rest. If the ff level is too low, or there is no ff left at all, the best thing to to do while you have everything disassembled is to go ahead clean out the gap as much as you can, before adding more. For most gap sizes, all you need is a few, pressed paper business cards (not plastic). Just insert one into the gap and run it around inside a few times and let it begin to soak up the ff. Remember: ff stains everything it touches, even your skin, so it's a pita in that regard. But, use as many business cards as needed until the ff begins to stop soaking into them. Be sure to get out as much grunge with them as you can. All this is about 10 or 15 min. per gap, usually. Adding the right amount of ff is best done with an eye dropper you can get at hardware store. No need to squeeze it - the magnetic pull will draw it straight into the gap. Too much in the gap can easily be removed by placing a paper towel over the face of gap and bleeding off the right amount. That's it. Just reassemble.
AFA your tweeters go, you may need to separate the magnet from the vc and just look carefully into the gap and see if there is any ff in it.
If none of this your cuppa, you can always fall back on repair service, there are a slew of places that can do this for you.
Let me know if you need more info. Regards. John
