Phase Plug design for MW65xx

Gardenstater

Tony I wish you could hear it now! I wonder how many of these Polk would've sold if they sounded like this in the showroom. I am now playing the Dave Grusin stuff you recommended and the qualities are really highlighted now with the interplay of cymbals and vibraphones and triangles. Really spectacular.

plastic_avatar

I love seeing the experiments, the tweaks, the builds, the mods.

I always cringe when I see:

'Why do you think you know more than the designers when they built the speakers?'

or

"Don't you think they would've done that if it was the right thing to do?'

And I feel like cheerleading when people respond with:

'Hold my beer.'

[much glorious beer holding here]

Tony M

Gardenstater wrote: »

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

Poster picture.

Gardenstater

Maybe I'll have that framed lol^^

I think with these phase plugs I'm getting cleaner bass at high volumes. Blasting Supertramp, Crime Of The Century right now, which I have an ingrained audio memory of, and the bass is the tightest and most impactful I've ever heard from my system.

It would make sense that the chunk of aluminum in intimate contact with the pole piece would be conducting heat away that originated in the voice coil. Cooler voice coil means higher efficiency and less compression losses. Also, with the phase plug, any air exiting which would have been trapped under the dust cap to a large extent, has to flow directly between the aluminum voice coil former and the phase plug, helping to cool it better.

xschop

Was pretty good beforehand, but now the string bass instrument is stunningly accurate and articulate with the plugs in the 8TLs. Reverberations of those low notes are now more revealing than ever...

Gardenstater

I am noticing it at high volume for sure. Probably a cooler voice coil which conducts better.

xschop

You actually removed more moving mass deleting that heavier dust cap.

Gardenstater

xschop wrote: »

You actually removed more moving mass deleting that heavier dust cap.

I suppose so.......all 0.025 oz of it according to my digital scale. That doesn't include what more I shaved off of the glue line afterwards though.

xschop

A few years ago, I passed up a set of Black 15T's. That won't be happening again now that I've built these small MTMs.
Decato-XS 15TLs...

If you can, would you do a freq. sweep on a felt vs. latex painted cone center....

Jazzhead

Nice work guys... xschop you are a machine shop wizard and Gardenstater needs no encouragement to dive in. So, should I mod my old RTA-12's? Part of me doesn't want to leave any stone unturned if there is performance to be gained. However, I'm a bit afraid come at them with an exacto knife as it is hard to replace those old 6500's. Also - they will end up looking like Madonna??

Gardenstater

Jazzhead wrote: »

Nice work guys... xschop you are a machine shop wizard and Gardenstater needs no encouragement to dive in. So, should I mod my old RTA-12's? Part of me doesn't want to leave any stone unturned if there is performance to be gained. However, I'm a bit afraid come at them with an exacto knife as it is hard to replace those old 6500's. Also - they will end up looking like Madonna??

I was a bit nervous on the first one, and the 2nd one was a breeze. As long as you don't put the Xacto knife in more than, say 3/16" or so, you won't even touch the voice coil former and as long as you leave a little dust cap over the wires rather than trying to follow the glue line right there, you'll be fine. It is easy to come in after and shave a little more away to clean it up after the dust cap is off. If I did it again I'd use my shims and my angled Xacto Knife. Rob has done plenty of them successfully without those tools. I used a magnifier for assistance lol.

My drivers are the MW6500 as well. Crossed over at 3kHz. I suppose one of your drivers has a lower XO point because you upgraded to the RTA 12B crossovers? I don't see that as a problem and the benefits are several, to these.

PS: Madonna sends her blessings, in an unbuttoned white lab coat to boot

xschop

I always cut the cap off close as possible to cap glue perimeter BUT not close to the glue near the wire leads. Then I come back and trim closer to the leads after cap is fully removed.

I take it you swapped crossovers to fix lobing issue? If so, then plugging them will pin-point the imaging and clean up the last bit of distortion since you've already done Gstater's foam basket mod.

And...i've discovered the 6500s are just like Madonna. If you don't give them a facelift, they'll remain saggy.

Jazzhead

The lobing/comb filtering issue was addressed by hyperdamping the outer driver of each speaker. Westmassguy came up with this novel idea and it worked way better than expected for a couple of reasons. The stock version of this speaker had the lobing issue but also produced too much mojo at certain midrange frequencies (a shouty, "in your face", character). The center brace between the two drivers was extended down to the lower reaches of the drivers creating a four-sided box around the outer driver of each speaker that was completely lined with Sonic Barrier. This mod solved the lobing issue (one can now casually hear the difference between inner/outer drivers), and luckily, perfectly, solved the shouty nature as well. After doing this along with a tweeter/top hat diffraction mod they image quite well (as a right/left pair). I also believe that having the damped brace between the inner and outer drivers has stopped standing waves between the drivers that contributed to muddy midrange. They still have polyfiber behind all drivers, and a 4x4 piece of Sonic Barrier behind the inner drivers. Originally, my speakers were loaded top to bottom with polyfill. I couldn't physically fit it all up behind the drivers so some of it went. In all my mods I believe the internal volume changed and after reading some info posted by F1nut and Gardenstater I experimented with PR weights and found that one steel fender washer per PR worked some bottom end magic. Kick drums, low piano notes, bass guitars, bari sax etc. have all gained significant realism. Maybe now they need the phase plug facelift.

xschop

Sounds like you've remedied all the backwave issues. The plugs remedy the front wave issues by preventing their cancellation and destruction/distortion at the cone center.
I myself listen to alot of jazz and real instruments is where you notice the improvements right off the bat.
I'll spin out a set for you if you're up for the challenge.

gp4jesus

@Jazzhead: sorry that your post pushed a few buttons this morning.

Your “shouty*” reference reminded me my RTi A7s seem to “shout” or “bark” when turned up LOUD! I’m confident the drivers are not reaching Xmax.

That said, which type or version Sonic Barrier (SB) did you use to tame them? Did the SB do the trick alone or post-Foam was the final step to cure the “shouting?” What did 4 X 4 mean? Inches or feet? Did you post pics?

Gonna add SB to a PE order today unless you know of better pricing elsewhere. I believe I’lll need a lot less as my tower’s mids’ space is less than a cu ft. My Center… maybe double that but still less than half of one of yours.

@Rob/George: the cone’s side benefit of add’l VC cooling is great news for those of us that like to “crank up” - another great reason to do the surgery.

@Rob PM pending w/address & PP info a little later. You’re gonna do some for Jazzhead. Add 4 to the job for me but do his first.

Gotta get busy w/breakfast for the family, the honey doo’s, add an Exacto knife to my Lowe’s list, and tidy up my hobby bench

Thanks for all your inspiration, help, and patience.

Tony

Gardenstater

@gp4jesus It would be an interesting experiment to see if the RTiA7 drivers would also benefit from phase plugs. Would probably require a re-design of the plug dimensions.

What midwoofer driver is in those and what is the VC ID and pole piece OD? What is the diameter of the cone and depth and included angle and dust cap dia? What freq. are they crossed over at?

Jazzhead

@gp4jesus

I used the 3-layer Sonic Barrier product (Black Hole 5 was unavailable). It seems effective. The 4x4 piece I mentioned is inches. Such damping material behind each driver is effective at absorbing some reflected backwave, but you can overdo it. Many on the forum rightfully warn that too much can suck the life out of your speakers (search the forum). I had to experiment and listen repeatedly. Gardenstater's "foam on the inner basket" mod also helps tame my speakers ability to lose composure at high volume. To my ears both of these mods together are optimal but with a reduction in the amount of damping material behind the driver if you do both. I don't often listen loud, but if I crank some power my speakers no longer lose composure before my ears cry uncle.

Jazzhead

@xschop and @Gardenstater

Did you two ever decide which was better around the plugs - felt ring or Windowcrafters paint?

Yes, xschop I would be interested in a set of plugs. I will PM you soon.

Gardenstater

Jazzhead wrote: »

@xschop and @Gardenstater

Did you two ever decide which was better around the plugs - felt ring or Windowcrafters paint?

Yes, xschop I would be interested in a set of plugs. I will PM you soon.

Rob has done both. I'm still deciding what to do here. I am *almost* hesitant to change anything it sounds so good

BUT, I have done some measurements/calcs and, for my MW6500s, the dust cap material is heavier if it were layed out flat, compared to the crafter's felt Rob sent me. 1.78x heavier.

So the ring shaped piece of felt with: O.D. = 1.5, I.D. = 1.0, slant height = 0.3, would weigh 0.017 oz. vs 0.025 oz. for the dust cap, so as long as you use less than 0.008 oz of Aleene's it is less weight in total than the dust cap.

As far as the paint goes, I haven't been able to locate the stuff Rob used but I found some Black (shiny black) Fabric Paint that I am going to paint on a piece of cardboard and weigh it before and after and evaluate the appearance of.

xschop

Jazzhead wrote: »

@xschop and @Gardenstater

Did you two ever decide which was better around the plugs - felt ring or Windowcrafters paint?

Yes, xschop I would be interested in a set of plugs. I will PM you soon.

I have excellent hearing and I cannot tell the difference between the two. Maybe audio equip could distinguish the difference, but I cannot.

gp4jesus

@Gardenstater & xschop: I’d find that very interesting and it could broaden Rob’s market for the plugs.

All RTi A series 6.5s look similar if not the same.

‘A7 mid: RD6546-1-E 9139 (took notes ages ago) though they all have slightly different model#. eg:
‘A6 mids*: RD6545-1-E, 7355 2 drivers, 2* plugs please
‘A3 mid: RD6548-1-E
*Center channel

Dust cap diameter: ~1.5”.
Cone diameter including surround, ~5 3/4” Minus surround? I’ll get back to ya
Depth? Is the cone “bottom” where it meets the dust cap?
Cone angle? I’ll get back ya

VC ID and pole piece OD: please school me on how to obtain this data. I don’t where to start looking or measuring.

Factory XO frequency: 2.7K. I EXO’d them @ 2K.

Thanks for your help and unspoken caution: I assumed mine would use the same plugs OOPS!

Gardenstater

@gp4jesus I think you or someone else will have to donate/lend one for dissection to make it happen. I saw some for sale in the flea market I think, for $25.

gp4jesus

😬 thanks for the Flea Mkt tip

I shall return

xschop

From your tweeter felt thread pics, those RTi Mids look like 1' VC dust cap sizes. Knowing Polk's Mfg techniques pretty much is almost a given. I'd also be interested in the RTi MW outer ring dimensions. Maybe they could be modded to fit the vintage MW outer edges?

gp4jesus

That’s a 1” tweeter above the mid…
https://us.v-cdn.net/5021930/uploads/editor/bc/1eif2fpr54lr.jpeg

Big dust cap compared to the tweet. Pic copied from my RTi A7 build threads

I have a spare ‘A3 mid in a box. Think it would make a good guinea pig?

Wish I kept the A3 mid a malfunctioning amp wasted right after I mounted them.

xschop

That's the pic I was referencing. Im sure it's a 1" VC MW. What is the ID and OD of that MW ring?

Cut the cap off the A3 mid and take a look. It may even have the same pole-piece to cone center depth as the early MWs. If so the phase plugs would be same dimensions as well.

Gardenstater

Result of a test I did with some Black Fabric Paint. 2 coats seemed to provide good coverage on a 4" x 4" piece of cardboard. Some distortion of the cardboard but goes away pretty much after the paint dries. Will check back tomorrow to see if the cardboard is totally flat again. Color seems not bad. Looks like 2 coats on the truncated cone region left after removal of dust cap would only be about 0.0047 oz., which for my MW6500s, is about 19% of what the dust cap weighed (0.025 oz.).

I think if I do this, given how tight the clearances are on my MW6500s, I would install the shims while the paint dries fully. Will also serve to mask off the phase plug.

Gardenstater

I'm going to see if they have a Brown and try a mix of 3 parts Black to 1 part Brown. The cardboard remained a bit dished (concave on the paint side) but it isn't really a fair test because cardboard absorbs/soaks up the paint whereas the cones seem to be a woven textile, probably synthetic non-absorbent like fiberglass. I may have not given enough time between coats and may have applied it too thick. The first coat actually dished it the other way. I'll try one of my decrepit original MW6500s first and will be able to see if there is any distortion of the cone that changes the voice coil alignment in the pole piece.

xschop

Looks like the same compound. I only tested on about 1"×1" area initially of Black paper and it remained flat. On a large area it would inevitably warp some for any compound.

Gardenstater

This stuff is very thick, but it can be brushed out easily. I wouldn't call it self leveling. Water based confirmed by brush cleaning easily in water.