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speakerguru
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Location: Green Hut, Tovil
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 Posted: Tue May 12, 2015 10:32 am Post subject: Ferrofluid in KEF tweeters |
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Extracted and edited from old posts and put here as a sticky
some info here http://ferrofluid.ferrotec.com/products/ferrofluid/audio/
KEF used to use the now obsolete APG512 which you won't find on this site. From memory it was a 75cp fluid. i.e. the fluid was used primarily to increase conduction of heat to the metalwork, not to provide damping. The base fluid is a synthetic oil and I guess everything dries/evaporates eventually.
In the APG L series, APG L17 would be the closest to the original low viscosity APG512. APG311 looks closer still and has a higher temperature rating.
APG S15N looks to be close to the original in viscosity. If Ferrotec are cautiously recommending it, then it is probably the most suitable for T33 use. The exposed part of the T33 vc is high temperature cured epoxy, aluminium and CA (superglue) adhesive. Not much risk of a bad reaction with an oil based fluid, I think, but no guarantees. |
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speakerguru
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Location: Green Hut, Tovil
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| Posted: Tue May 19, 2015 10:13 pm Post subject: |
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APG S15N was available from KAST Services
http://www.kastservices.co.uk/tannoy-spares
but no longer 
Last edited by speakerguru on Wed Dec 05, 2018 5:15 pm; edited 1 time in total |
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dublin78
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Location: Cheshire
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| Posted: Thu Mar 16, 2017 12:02 pm Post subject: |
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Hi
Sorry, I have looked around, but I cannot find any instructions as to how much ferro fluid to use in each tweeter (SP1191).
Any ideas?
Thanks |
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speakerguru
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| Posted: Fri Mar 17, 2017 1:13 pm Post subject: |
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| Sorry I don't remember exactly. It would be around 100ul (0.1ml). It was applied by means of a graduated glass tube syringe. The magnetic field pulls the fluid out of the tube as soon as you approach the magnet gap. |
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flumoxed
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Joined: 30 Mar 2015
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| Posted: Mon Oct 02, 2017 6:22 am Post subject: Ferrofluid in KEF tweeters |
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| Good HIFI VOF, NL sells ferrofluid formulated specially for speaker drivers. Price is about $20 for 1 CC (enough for 20 tweeters) compared to $6 for generic ferrofluid from Parts Express. |
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oldandintheway
Intermediate Contributor 25+
Joined: 16 Mar 2018
Posts: 32
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| Posted: Sun Apr 29, 2018 11:06 pm Post subject: |
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| So, is the gist of this that the generic ferrofluid from parts express would be no good for NT25? |
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speakerguru
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| Posted: Mon Apr 30, 2018 10:10 am Post subject: |
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| Impossible to say as they don't list the viscosity. |
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kevd
Junior Contributor 10+
Joined: 07 Dec 2019
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| Posted: Mon Dec 16, 2019 10:43 pm Post subject: Ferrotec alternative to APG512 |
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After some conversation with Ferrotec, it appears that due to the relative magnet strength the more critical parameter is the Gauss followed by viscosity. Therefore APGO77n is recommended as the best alternative with a gauss of 265/115cp. Just FYI,
Kevin |
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speakerguru
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Joined: 18 Nov 2005
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Location: Green Hut, Tovil
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| Posted: Sun Feb 23, 2020 11:08 pm Post subject: Re: Ferrotec alternative to APG512 |
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| kevd wrote: | After some conversation with Ferrotec, it appears that due to the relative magnet strength the more critical parameter is the Gauss followed by viscosity. Therefore APGO77n is recommended as the best alternative with a gauss of 265/115cp. Just FYI,
Kevin |
Not sure what Ferrotec meant by that. There was never any explanation in their literature of what happens to the fluid if the field flux density is significantly greater or less than the quotes "Gauss"
When KEF tweeters first acquired ferrofluid, the primary aim was to reduce the thermal resistance of the air gap whilst introducing as little mechanical damping as possible. A viscosity of ~75cp does this. |
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speakerguru
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mcandmar
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Joined: 20 Mar 2014
Posts: 7
Location: Dublin, Ireland
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| Posted: Thu Apr 22, 2021 2:44 pm Post subject: |
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Found the following on eBay with 80cp viscosity,
https://www.ebay.co.uk/itm/184459426391
1x Ferrofluid 0.5ml for audio tweeters / tweeters for all known manufacturers
Technical specifications
Basis: hydrocarbonic oil
Magnetization Ms: 300 Gauss (280 ~ 350)
Viscosity at 27 ° C: 80cp
Density at 20 ° C: 1.04g / ml (1.02 ~ 1.0
Temperature: -40 ° C - 280 ° C |
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speakerguru
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Joined: 18 Nov 2005
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Location: Green Hut, Tovil
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| Posted: Fri Nov 12, 2021 11:52 am Post subject: |
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| TheStan wrote: |
I'd be interested to hear from you, how I can know, how much ferrofluid I should put into the gap. I figured out that if I only put in a little bit of ferro fluid, it's possible that the upper part of the gap has ferrofluid, but the part below (so deeper into the magnet (gap)) doesn't have any.
Only if I fill up a bit more it starts to descend (I guess), but it also starts to overflow on the top.
Do you know what exactly is supposed to get cooled by the fluid? Is it the magnet, or the voice coil? And how much of the voice coil should be emerged in ferro fluid? just a bit, so that there's no oil on the copper wire, or so much, that the entire copper winding is submerged? Or something in between?
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Firstly, it's the vc that needs to be kept cool. Cooling increases the power handling and prevents the insulation burning and ultimately the vc wire fusing. The thermal conductivity of ferrofluid is much greater than air so for a given voltage the temperature rise of the vc is reduced. The resistivity of copper increases with temperature giving rise to compression of the audio at high levels. FF reduces audio compression. Heat goes from the vc to the magnet steelwork. Cooling of the magnet would be needed for long term high power operation. This is not normally considered necessary for domestic hi-fi speakers.
The magnet gap should "only just" be filled with ff. If 100ul does not completely go right around the gap is does not matter. Do not overfill the gap. The vc length is less than the gap but takes up at least a third of the gap volume, so, if the gap is "only just" full, there will be sufficient coverage as the vc is inserted and displaces the ff.
Excess ff does not assist cooling; it makes a mess. It can increase the moving mass of the tweeter moving parts, thereby reducing the sensitivity. Ff over the top will contaminate the top plate surface which should be spotless to receive the adhesive tape which holds the diaphragm assembly in place.
Last edited by speakerguru on Thu Oct 06, 2022 2:02 pm; edited 1 time in total |
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TheStan
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| Posted: Mon Nov 29, 2021 10:02 pm Post subject: |
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| Thanks for your message. So it's okay if not the entire voice coil wire is covered in oil, but just a bit? Or hardly at all? |
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Lee in Montreal
Senior Contributor 200+
Joined: 22 Aug 2009
Posts: 231
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| Posted: Wed Oct 05, 2022 9:15 am Post subject: |
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I am totally mystified. Currently redoing 2 sets of 104/2 T33s.
How is 100 micro liter of ferrofluid (0.1 ml / 0.1 cc) enough when the gap shall be filled at around 75% capacity, and that I tested a 5ml / 5cc capacity with alcohol ?
Please let me know what I do / think wrong...
_________________
Current line up
2 sets basic Kef 104/2
2 sets bi-wire Kef 104/2
Plenty of KEF components for a few fun builds |
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speakerguru
Über Contributor 1000+
Joined: 18 Nov 2005
Posts: 1192
Location: Green Hut, Tovil
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| Posted: Thu Oct 06, 2022 2:14 pm Post subject: |
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| Lee in Montreal wrote: | | How is 100 micro liter of ferrofluid (0.1 ml / 0.1 cc) enough when the gap shall be filled at around 75% capacity, and that I tested a 5ml / 5cc capacity with alcohol ? |
I am totally mystified too. What do you mean "that I tested a 5ml / 5cc capacity with alcohol"??? Guesswork required - here goes:
Non-magnetic alcohol would be filling the whole of the internal magnet assembly void, from the backplate to the top plate (axially) and from the external diameter of the pole piece to the internal diameter (radially) of the magnet. That could well be 5ml. The ff should and will only fill the magnet GAP not the whole void. i.e. the space between the internal diameter of the top plate and the pole piece and only to the depth/thickness of the top plate.
Did you not read two posts back where I say:
"The magnet gap should "only just" be filled with ff. If 100ul does not completely go right around the gap is does not matter. Do not overfill the gap. The vc length is less than the gap but takes up at least a third of the gap volume, so, if the gap is "only just" full, there will be sufficient coverage as the vc is inserted and displaces the ff"? |
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