Phase Plug design for MW65xx

xschop

Have to share this knowledge. After doing quite a few of these plugs, Ive found that not all dust caps are the same diameters. So I cut the known ID out of the felts for the voice coil with the 1.1" die and will let the end user trim the OD for their individual caps as they are removed.

I'm now looking forward to a factory set of CRS+TL's to be auditioned with them..

F1nut

Are the different size dust caps found on the same MW model or is the size difference related to the different models?

xschop

Honestly, I believe it has to do with non-uniformity of the dust cap installation on any given MW. But the 6516's in general had a smaller OD. The 6502s I had, and 6510s you sent were a little larger OD, but same size. I still want to do a set of 6511's.

I also did discover one large pinhole on a 6502 that looked like it passed QC from the RM3000 (think that was model) subwoofer I had yanked them from.
There's no way it would have passed the PR test as it was @ 3/16" hole under the dust cap between VC and dust cap glue perimeter.

I did the repair by using a small span of the old dust cap, applying Eileens front and back then glued over hole. It passes PR test with flying colors.

Gardenstater

Wow. Crummy QC to say the least!

My NOS circa 1990 MW6500 purchased on eBay has a larger dust cap that hid the VC wires. Also sounds different from the earlier ones and therefore I ended up removing from my system until I might happen to stumble on another NOS one of similar vintage. It was probably around the end of the time that they stopped making them for replacements/old model support.

xschop

Here's a textbook example of quality issue. These were the last MWs I plugged. When I cut the cap off, I could clearly see the incongruent offset of the VC to cap OD concentricity. I decided to apply the black window crafters paint that rubberizes when dried. Ive been using it to recoat the leads on the front of the cones on all the other MWs. I had tested it out on black craft paper previously and it sticks to it as strong as the factory doping compound that I believe is a water/latex emulsion.

It dries to the exact cone sheen, and by ear I can't sense any difference in sound/playback between it and the felt one I swapped out.
Maybe equipment might differentiate the two, but I can't.

So the plug swap process and black paint application took about 10 minutes start-to-finish.

F1nut

Gardenstater wrote: »

My NOS circa 1990 MW6500 purchased on eBay has a larger dust cap that hid the VC wires. Also sounds different from the earlier ones and therefore I ended up removing from my system until I might happen to stumble on another NOS one of similar vintage. It was probably around the end of the time that they stopped making them for replacements/old model support.

Replacement drivers of that circa were made in Mexico. The dust caps are larger and the doping is different.

Gardenstater

Yeah I forgot to mention that it had the Made In Mexico sticker. Didn't know that about the doping. Interesting.

Gardenstater

xschop wrote: »

Have to share this knowledge. After doing quite a few of these plugs, Ive found that not all dust caps are the same diameters. So I cut the known ID out of the felts for the voice coil with the 1.1" die and will let the end user trim the OD for their individual caps as they are removed.

I'm now looking forward to a factory set of CRS+TL's to be auditioned with them..

I'm sure you've already though of this but since you have the I.D. punch already made I can't help but wonder about punching the hole in the dust cap that has just been removed and using a thin line of the Aleene's Tacky Glue to re-install it. Would it be too hard to get the hole in the exact right place?

I think your window paint and the felt are both great solutions, just thinking might look nice this way as well.

I think some people may even want to anodize the plugs in black or gold. I'm leaving the original silver bling when I do mine soon.

RandyCroissant

This is an interesting mod, do you use shims to center the phase plug?

xschop

The dust cap is too thin a material as compared to the felt, but you could try it. The felt has the best aesthetic IMO, but @5-6 feet away, it is difficult to distinguish the between felt vs. dare I say...black paint.

xschop

RandyCroissant wrote: »

This is an interesting mod, do you use shims to center the phase plug?

I programmed 0.020" off the diameter for these glue-on plugs for two reasons.
They can be set by an overhead light and eyesight, and if you ever blow the voice coil, you can machine the pole-piece to fit a common voice coil replacement. I have successfully done so and it wasnt too difficult.

RandyCroissant

Thank You Rob and Happy New Year!!

Gardenstater

Installing the phase plug in my R speaker now. Went pretty smoothly with my shims that I made which are a combination of my 0.0065" plant label stock (wish I had more) and some Frost King PE tape which is 0.0055". Seems to be a foolproof method, if I am not speaking too soon. Will see if everything is OK when it fully cures. One of the beauty aspects of these shims is that I was able to give the phase plug a good turning in both directions, to distribute out the Loctite Red, without losing alignment at all.

Gardenstater

These are the shims I used for the MW6500 with the tight clearances they have. Would probably need a thicker base shim material than 0.0065" I used for the pole piece to VC ID gap, for the later models but the 0.0055" Frost King PE tape should work for the 2nd layer which centers the phase plug on the pole piece. I actually cut the length of these down to 2-7/16". It is crucial to cut the ends to a curved shape to get the shim started:



I used this to cut the dust caps off. Xacto blade Super Glue Gel'd to a wooden dowel that I belt sanded an angle on first.





After doing this one I don't think people need to be intimidated and it will be fun to get some other people's reactions to how it sounds.

Gardenstater

^^Actually these shims may work fine for other MW drivers as well, because (correct me if I am mistaken) but I think what Polk did is they made the I.D. of the framepiece larger but they kept the voice coil ID and OD and pole piece OD the same. So they increased the air gap by about 0.0085" but that is all on the OD side of the Voice Coil. Doesn't come into play with the phase plugs install.

xschop

As long as your pontifications get it centered, you're all good lol.

You should try the Sharpie curved end on the VC outer lip that protrudes from the cone, smooth it perfectly circular, and set one by eyesight.

You should also try one with felt and the other without.

Gardenstater

A sharpie may work but I took advantage of having a micro Lathe and made this PVC plug gauge with 0.001" radial clearance. Works very well for making VC round, although mine were also forced round by the shims. :

xschop

Thats another tool on my list.... nice

Gardenstater

Well I have the system back with only the R speaker phase plugged. I'm happy with the centering I got. This picture is slightly off angle but you can see how good the centering is I think. First listening impressions are that my R speaker has disappeared a BUNCH more! Also, I was hearing details I hadn't heard before right off the bat and, most surprisingly, cymbals sounded different with more information coming through I guess, around the crossover point of 3khz. Just guessing here but everything with cymbals now sounds more live than ever. Easy Blues by Jimi Hendrix from People, Hell & Angels sounds incredible, with even more air and decay times in the cymbals than I ever heard before.

Vocals in Mercy Street by Peter Gabriel are even more separable than ever. I'm just hearing more detail than ever and much more smoothness especially.

I can't wait to get the L speaker done and get this thing balanced out.

At this point I can already stick my neck out and say it would be nuts not to do this mod

xschop

Nice job. First plugged 6500s then.

Gardenstater

I'm first?? Cool lol These puppies *may* be the highest performance MW's with the smallest airgap and then they later chickened out or decided discretion is the better part of valor, shall we say, and opened the gap up by 0.006" or so....

Who's next? Step right up.........

xschop

Thanks to Larry, we all know that to install a pair, it takes a pair...

Gardenstater

Well I say enjoy them to the max. Life is short. The Smithsonian doesn't want our vintage Polk drivers anyway, so don't worry about that lol.

Gardenstater

As I impatiently await the complete by the book 24 hour cure time of the phase plug in my L speaker (only necessary for the OCD'd), I found this Phase Plug patent for fellow NERDS if I am not alone:

https://worldwide.espacenet.com/patent/search/family/038873608/publication/US7949146B2?q=pn=US7949146B2

Gardenstater

I'm listening to the first ever pair of xschop Phase Plug modded Polk 7Bs right now and to say I am pleased is an understatement!

Center image of vocalists etc., is now so far removed from the speakers as a source that it is almost spooky. Hard to believe it isn't live. Very deep image too, even better than I already had.

Percussion instruments, whether wood or metal are so detailed and lifelike that it is just stunning.

Vocals smoother than ever.

Gardenstater

Don't have recent "befores" (my bad), but here is what it is looking like with admittedly poor measurement equipment (Android mic and Advanced Spectrum Analyzer Pro app). I believe the gold line is a better gauge, being the peak reading. Other line was bouncing all over the place. This is with whitenoise and the reading position is 12" from baffle, between tweeter and MW. There is a dip on both but that is above the crossover point of 3kHz.

L speaker:



R speaker:

xschop

I do like how older recordings project a better image than before the install. Listening to familiar Mingus and Oscar Peterson old-school, and they do have more of a live presence than before.

Gardenstater

I found no need to rebalance when I had only done one speaker. Whatever is lost in effective cone area (Sd), which is admittedly small, is apparently made up for somehow. I don't know if it is eliminating some cone breakup from the dust cap, or the negative effect of the "oilcan resonance" caused by the previous air cavity between vc former, pole piece and dust cap, or something else like the interferences being blocked from waveforms originating on opposite sides of the cone that might be out of phase with each other. Whatever it is it clearly WORKS in THIS driver.

It was funny that the magic came in mostly from only doing one driver although it was the icing on the cake doing both for sure. The foam inside the basket was different, where the magic only came in from doing both, and there WAS a significant relative drop in output of around 0.4dB or so, when I did only one.

Gardenstater

I also should add, for anyone out there champing at the bit for more info ( ), that I can't detect any change in time for the leakdown test when pushing in and holding the passive radiator and I have the MW6500 dust caps that you can hardly even see any light through, when you shine a flashlight under them. Obviously the long annular restriction between the phase plug O.D. and the voice coil former (0.012" clearance) is enough to make up for the lack of the dust cap.

gp4jesus

George: that competed driver pic* belongs in a museum! I can scarcely imagine the SQ improvement as I recall awesome “pre-plug imaging and soundscape!
*full frame of the driver uncropped

Rob: do you take PayPal? (Wait patiently for a PM) I admit I’m scared to perform the ”intrusive” surgery mids. I have “bigger non-audio fish to fry” around here so if I did, I won’t get to them*, or much else audio related for a while - that includes foaming the inside of the mid* frames.
* LR & Center channel mids only - 4 total

Tremendous thanks to you pioneers!

Tony