diy modification on drivers to lower mechanical losses

[capslock]

While it is still not clear whether Rms as a whole is important for precise bass sound (http://www.diyaudio.com/forums/showthread.php?s=&postid=60555), I have found a couple of references on the subject of unfavorable air flow conditions. A modern woofer will have any or all of the following features to improve venting and hence lower turbulence and air compression:
a) phase plug
b) magnet venting holes
c) venting holes in the semi-sealed volume between the centring diaphraghm (is this the correct word?) and the cage

I wonder if it is possible to implement this on vintage drivers that are still very good except for venting.

b) will probably destroy the speaker

a) should be doable. One would have to cut a big circular hole in the dustcap and then stick some sort of metal column through the hole and glue it to the magnet

c) should also be possible if the cage is dye cast aluminum. Maybe one must use low rpms or even a hand drill. Or would a router be preferable?

Anybody tried this?

Eric

 

[capslock]

Thought this was a prime DIY subject, but it hasn't elicited any posts yet.

I should receive some used Eton speakes shortly and, unless somebody posts a report of failure, will try option c, i.e. drill holes in the basket to vent the space between diapragm/spider and magnet. I will measure TSP before and after modification.

Any ideas of how to keep little pieces of aluminum from getting into the gap? I had thought of a vacuum cleaner and very low drill rmps.

Eric

 

[planet10]

a) should be doable. One would have to cut a big circular hole in the dustcap and then stick some sort of metal column through the hole and glue it to the magnet

Dustcapectomy. usually beneficial. Phase plugs can also be made of wood, woolfelt, lipstick tops

dave

 

[Variac]

Model rockets such as Estes use balsa and plastic nose cones in lots o sizes which could be used as phase plugs. You can order just the cones from Estes and others

 

[planet10]

You can order just the cones from Estes and others

Got some URLS?

dave

 

[capslock]

Dustcapectomy

Got experience there?

Do you remove the whole cap or cut a circle and leave a dustcap rim behind?

What should the gap between the plug and the hole be? Too large and you have an acoustical short, too small, and you don't improve mechanical loss.

Wood would have no cooling effect but then I doubt that that is much needed.

 

[planet10]

Re: Dustcapectomy

Got experience there?

Do you remove the whole cap or cut a circle and leave a dustcap rim behind?

What should the gap between the plug and the hole be? Too large and you have an acoustical short, too small, and you don't improve mechanical loss.

Wood would have no cooling effect but then I doubt that that is much needed.

not as much as i'd like.

seems to work best if the entire dust cap goes (my experience ranges from 2" to 8")

A gain in cooling comes from just letting the VC breathe.

dave

 

[capslock]

optimum gap between phase plug and cone

The question about the optimum gap still remains to be answered. What is the gap in typical Seas drivers?

Had a look at the hard paper Vifa drivers in my T+A. The minimum cone diameter is definitely smaller than the dust cap diameter. It wouln't hurt (and might improve stability) to let some of the dust cap remain.
Wonder if it could be done with a hot aluminum or stell cylinder. Would work on plastics, but I doubt it would work on the coated fabric dust cap in the Vifas.

Eric

 

[capslock]

boring the magnet

Had a look at two different Vifa drivers. The magnet seems to consist of one or two ferrite rings, chromium-coated steel pole plates and an aluminum or steel seal in the center of the pole plates. Should be possible to drill through the seal without hurting the ferrite.

How does one keep metal particles from entering the gap, though? Is there any way to deglue the surround from the basket or the basket from the magnet?

Eric

 

[planet10]

Re: optimum gap between phase plug and cone

The question about the optimum gap still remains to be answered.

I meant to answer, but was doing too much multi-tasking. I try to get the PPlug the same diameter as the pole piece.

As to how you drill without metal filings getting into the works, i have no real idea. I have seen some drivers after-the-fact that have had serious surgery like this. I could ask how it was done (although knowing him he probably demagnetized the ferrite, did surgery and then remagnatized the magnet).

dave

 

[navin]

i copied the plug from a Audax HM170Z0 driver. it is solid. so i figured one can make them out of epoxy resin moulded in shape too right?

 

[planet10]

i copied the plug from a Audax HM170Z0 driver. it is solid. so i figured one can make them out of epoxy resin moulded in shape too right?

Sure, why not? Or Ceramic (get the potter's wheel out boys), or leggs "boxes" filled with X, a 35mm film canister filled with "X", lipstick caps, wood, metal, foam, felt, plastic, cement ...

Each material will have a set of resoncance/absorption/reflection/transmission characteristics which may or may not be beneficial to the performance of a particular driver. And then there is the shape ... With such a large space to explore we might as well dive in with what we have and establish some data points.

dave

 

[navin]

tip toes?

 

[Variac]

Not to forget those Estes and other nosecones on the "speaker company" thread
Also: how come Pinoccio's nose isn't animated?

 

[capslock]

surgery

Aquired two vintage Eton (German variety) Hexacone (Kevlar honeycomb membrane) 8-480/32 drivers through ebay. One of them was a free extra to the other because it scratched. Clearly, it was manufactured at a different time (differnt color of membrane, different basket, diffent magnet, though all the same size).

Turns out the scratching is evident if I push down one side of the membrane by only 1 mm (the other one will exhibit no scratching even if I move the membrane by as much as 6 mm).If I distrubute the force more evenly, i.e. push down on the center of the dustcap or move up on four points on the rear, I can get +/- 3-4 mm without scratching noises.

- Is this dirt in the gap (the magnet hole is not covered by a sieve) or rather too tight tolerances?
- If dirt, what is the most promising way of opening the thing:

a: unglue the magne from the basket? how?
b: unglue surround (rubber) and spider from the basket? how?
c: demagnetize the magnet? how? why at all?

Eric

 

[mrfeedback]

Slide a strip of cellotape down the gap and run it around the polepiece - this will usually grab any bits of debris in the gap.

Eric.

 

[mrfeedback]

Also by selecting the right thickness paper or cardboard, you can slide a sleeve down into the gap to centralise the voicecoil.
Heat the cone/voicecoil glue joint carefully with a hot air blower until it eases a bit, let it cool fully and remove the sleeve and you might fix it.
Do not heat the spider because this will make it go loose and alter the characteristics.

Eric.

 

[capslock]

Very good tips, thank you very much. Before I can try, I'd have to know how to gain access to the gap.
Remove dustcap? Will only give me access to the inner side which is already something.

Remove magnet? how? If I heat the magnet-basket joint, I will probably demagnetize the ferrite and ruin the spider.

Also, I have the impression that scratching occurs no matter which side of the membrane I press down. So either they got the tolerances (uniformly) wrong or there is dirt in all directions.

Eric from the other side

 

[mrfeedback]

Very good tips, thank you very much. Before I can try, I'd have to know how to gain access to the gap.
Remove dustcap? Will only give me access to the inner side which is already something.

Remove magnet? how? If I heat the magnet-basket joint, I will probably demagnetize the ferrite and ruin the spider.

Also, I have the impression that scratching occurs no matter which side of the membrane I press down. So either they got the tolerances (uniformly) wrong or there is dirt in all directions.

Eric from the other side

It's a pleasure.
Yes, remove the dustcap with a hot air source to weaken the glue, and with an exacto (scalpel blade or stitch cutter blade) knife both lift the cap and carefully slice/seperate the glue web without damaging the cone or dome.
Typical glues go soft with heat and become much easier to work.
Now you have access to the pole-piece.
If the magnet is fixed by screws, by all means do so carefully, if not do not attempt it.
Some drivers run very fine tolerances and will scrape if not pushed or pulled perfectly evenly.
Other reasons for diminished gap is white powder corrosion of zinc plating of the pole-pieces, or the end of the voicecoil former having mushroomed by mechanical bottoming, or the voice-coil having 'sprung/shifted' due to thermal overload.
A thin stiff blade (a broken retracting tape measure is good) can be slid down the gap and used to scrape any corroded zinc, or ream the former back to shape.
After this, use a fine strong jet of compressed air and sticky tape to remove any smeg.
Then if needed, fit an appropriate thickness sleeve down the gap and cook the VC/Cone glue joint and allow it to self align and let it cool for a long period. (hours).
Practice on a junked old driver FIRST before you try any of this on a valuable treasure.
A very carefully used paint-stripper hot air gun is an appropriate tool, or a hairdresser dryer that gets HOT.

If you need more help, just ask.

Eric Von Downunder.

A low frequency high excursion drive free air test will tell you of any dynamic operating scraping.

 

[capslock]

partial success

Hi Eric,

tried your tips. Wrapped the speaker in aluminum foil except for dustcap and used hot air gun in low setting. Could remove dustcap easily with knife.

VC former was surprisingly modern, i.e. coil wrapped on paper that had an inner liner of this stiff brown, semi-transparent plastic.
It had also 4 mm holes to let the air trapped by the spider escape into the 10 mm hole in the center of the magnet. The inner slit was very narrow, could not even slide in a piece of very thin cardboard (file card).

Found thick paper that worked. Afterwards, scratching was gone but slight ticking sound remained when using 20 Hz for test.

Inserted a whole sheet of paper slightly less long than the inner circumference of the VC former. Then added three additional strips of the same paper 120° apart.

Rewrapped the speaker, heated and cooled. Clicking sound was gone but cone had sagged by about 5 mm. Redid the paper trick with a little extra paper and fixed cone in nominal position. Heated only rubber surround and spider. Assume spider is the problem as it still looks slightly deformed. Clicking back, but only at large amplitude.

Eric