Improving Vifa P17

[paulspencer]

I'm currently working on tweaking my Vifa P17 drivers, following instructions posted by Mark of McKenzie Acoustical Design

http://www.madspeaker.com

He wrote about the tweaks on this thread:

http://www.diyaudio.com/forums/showt...719#post687719

Unfortunately the linked instructions are no longer on his site, but it involves removing the dustcap and adding a phase plug as well as glue rings to reduce vibration mode problems. He showed measurements of improved time domain performance and flatter response.

So far I have removed the dustcap and made a prototype phase plug. The real one will be solid copper. With dustcaps removed, it's hard to pick much difference, if it is there at all. I think I can detect some change in upper frequencies, but it's subtle.

Dustcap removed:

[paulspencer]

Prototype Phase plug added

The tricky part will be attaching it to the pole piece. I think I will have to use glue, but care will be required to get it right. I don't want to glue the VC former to the pole piece!

Any suggestions anyone?

[paulspencer]

I will be able to do an AB test by running the speaker as TM and switching from top to bottom driver.

Will it sound better? I hope so, but if it doesn't, I'll make sure this phase plug looks good!

Phase plug vs stock P17:

[sreten]

Hi,

If the phase plug is turned, a steel screw in its centre
(from the back, countersunk head) should work a treat.

/sreten.

[paulspencer]

My concern is centring it correctly. In the magnet there is a 15mm deep hole in the centre drilled into the back of the magnet. Also I have concerns about changing the motor by drilling into the pole piece.

I've also thought about contact adhesive, but that could go wrong if not placed correctly as it tends to set quick!

[sreten]

Hi,

I meant a steel screw in the phase plug not pole piece.
(Assuming a solid pole piece - not as you say hollow)

An accurately placed 15mm steel washer on the phase
plug will be centred and hold itself onto the pole piece.

/sreten.

[ashok]

Droping or placing a steel washer can be very dangerous . Alignining it while it's stuck to the pole piece might be really tough. In addition if it goes out of center while putting it in place you could ruing the top edge of the voice coil former.
Be careful. Use a slow ( or slower ) setting glue. Use some hand wound ( paper ) spacer to keep it centered with respect to the voice coil. You can pull it out after it sets.

Paulspencer: What solvent did you use to remove the dust cap ?
Thanks.

[paulspencer]

Do you know of a glue that will work for this application? Metal to metal. The actual phase plug will be copper and I'm not sure what metal is used for the pole piece.

Solvent wasn't required to remove the dust cap. The cone is basically plastic, the adhesive is clear like silicone. It's a simple matter of getting a nail under the dust cap edge and gently working at it until it comes off. The dustcap comes off without damage and there are no marks on the cone.

Unfortunately the VC former is very thin and does not form a perfect circle. I'd like to trim it down if I could, but I think it could readily be damaged. I could fill the space around it with something to make a neater finish, but I suspect I might just have to not worry too much about it, as it's only visible very close up.

[ashok]

Hi Paul,
Thanks for the info about removing the dust cap.

If I were you I'd avoid any magnetic metal work on the pole piece or work with any magnetic pieces near the coil. If you have ANY magnetic metal scrapings it will become embedded in the air gap for ever ! There goes your speaker ! No magnetic screwdrivers either !!
Non magnetic is safest. Copper is great . You could use a VERY thin film of quick set Araldite epoxy glue. It hardens in an hour or so and then gets max strength in several hours. It is called Araldite Rapid over here.

You need just a thin film . It must not run when squeezed between the ploe top and the plug. You 'could' implant a SMALL steel screw ( 3mm ?) in the middle of the base of the copper plug. This should have enough magnetic pull to keep the plug in place while setting and low enough to move it around if required. The screw need not touch the magnetic pole piece .
I would like to use a glue with less strength so that the plug could be removed easily later on if required. Like plastic glue sticks ! If you heat the copper plug it should come off easily . If you are very sure of not removing the plug later on , the Araldite Rapid should be enough.

Alternatively, A 'right sized' steel screw in the base of the plug might be enough to keep it in place. A very "small" blob of epoxy at the joint will ensure that it really stays in place.

About the voice coil former. You will see that many other commercial drivers have coil formers that do not look centered exactly with respect to the center plug.
On my Mission 780 it is VERY noticeably off center. But not noticeable unless one looks at it up close. The edge of the former is too close to the plug to see if it is non uniform or not.
Cheers.

[paulspencer]

Ashok, thanks for the suggestions, very helpful.

Another challenge is putting rings of glue (PVA) on the rear of the cone. I need to put a ring which is close to the centre of the cone, and this is difficult as I need a long pointy nozzle which I don't have on this glue. I'm looking at making up some kind of nozzle for it ...

The ring needs to be 36mm in from where the surround rubber ends (on the back of the cone). I could do it on the front, but at the rear it will be hidden.