MWAF contestants, B-Stock RST28F-4 info...

[Wolf]

Okay, progress!!!

I'm using number 2 as the guinea pig, but it should work on both. The key is to attack from the rear. I heated up a small screw-driver to slowly pry off the back cup. In most cases, this will likely chew up and ruin the cup. I found some Ball baby-food jar caps at Walmart to facilitate the replacement parts. They are a touch loose, but will work. 2" PVC flat caps are a touch too small, but would also likely work.

Initial:




No back cup on 2:



Removed the foam plug in the pole-piece:



Pulled out string of black adhesive (that was across the front of the pole) from the rear using needle-nose pliers, and then inserted a 1" ball of cotton to flush with backplate. There is still no back cup installed:



More in the next post...
Wolf

[Wolf]

And finally added the Ball-back-cup with a thin layer of wool batting over a material similar to Ultratouch, slightly overstuffing the cup, and then wrapping with masking tape to hold and seal the assembly for testing:



I did absolutely nothing in front of the felt pad that covers the vented pole piece, and this looks really good!

Then I went one step further. Not cool enough to do much else today. I had to shut off the ceiling fan and the box fan for these, and the family was nice and kept totally quiet for the 10 minutes it took.

I attempted what I call the 'Jeff Bagby driver measurement setup sans cabinet'. I set the tweeters on a sheet of egg-crate on the carpeted floor, then laid a circle of open-cell 1/16" sheet (with center cut out for the driver) atop of it. Then I surrounded the foamed face with 3-4 pieces of Sonic-Barrier. Jeff uses felt, but I had this handy. The mic was about 2 feet from the driver facing down. A 70uF cap was used in series for driver protection. These plots are not to be held as if in an IEC baffle or baffled box, nor are they likely for use for much of anything else except comparison to each other.

Bstock1, unaltered:



Bstock2, modded, same levels and setup. Nothing but the drivers have been changed:




I know this is not quite apples to apples as they are 2 different drivers. However, I feel it's very revealing of the change. I will be modding the second tweeter and posting those results for a more ideal comparo. The guinea pig wasn't measured beforehand as it could have just been a lost cause and not worth taking measurements at all. Since it was a dramatic difference, I feel it is worth it for the other unit. Obviously the setup will change for the next go around as it's been put away. I will do my best to get them similar in arrangement.

Later,
Wolf

[Paul K.]

Just so I understand, are you telling those that received RST28F-4 tweeters in their MWAF grab bags got B-Stock drivers from PE? (I'm not meaning to criticize, just want to know).
Paul

[Billet]

I wonder how these would sound with the back of the tweeter opening into a heavily stuffed 1 liter midrange type enclosure?

[Wolf]

Yes, I am, Paul. Donna told me when I picked up my grab-bag that they were all 'matched B-stock tweeters' that they were giving us. My guess is that these were the units that were discovered to be less than specified. I'm just showing what I did to make mine perform better than they were out of the box. Basically, the foam plug is the main offender in them. Swap with a cotton ball, and you'd likely get better results and likely spec measurements. I went further than that with mine. I still have to measure the BSTOCK1 tweeter after the mod to verify the results, but my gut says it will mirror the 2 unit.

Later,
Wolf

[ugly woofer]

Nice job Ben! Have you thought about heating the pvc cap and stretching it over the back of the tweeter? Just a thought.

[Wolf]

I had not thought of that, but it's too late now. Thanks! I had a little bit of experience from experimenting with the Airborne tweeters back in the 'Black Box' contest when I did my SchwarzXL project. I kinda figured the stuffing and cup damping was the cause of the problems.

I glued them up with E6000 last night with masking tape as the clamps. I pulled the tape off this afternoon, and reapplied Gorilla-tape for good measure. You never can trust that things stay as you want them.

So now, these are the final sealed up measurements of the pair:




That hump in the Mod1/Bstock1 driver at 200Hz in the impedance is audible in HD sweeps, and shows as higher 3rd order. You'll see that in the plot that follows. Why it's there or what is causing it is unknown to me, but it does not appear to have been in the initial measurement. The rest of the HD plot does not show anything different than the Mod2/Bstock2 measurement.




And, just for fun, I threw an old RS28A-4 in the same setup and got this:



Being that the FR plots aren't really that different, nor really applicable to design, I'll omit those this time around. The point was to get better performance from the B-Stock tweeters, and get them closer to each other hopefully, and I think I managed to do that.

Later,
Wolf

[Wolf]

Slight correction and clarification; After reviewing my work, it appears that Bstock1 driver did have a 3rd order hump in the HD back in post #2 of this thread around the same point of 200-250Hz. It appears that was a blemish inherent to the driver when I received it. Still, a xover above 2kHz would likely negate this issue entirely.

Later,
Wolf

[dynamo]

Nice work Ben, the differences you made in the measurements are not insignificant.

[Wolf]

I wonder how these would sound with the back of the tweeter opening into a heavily stuffed 1 liter midrange type enclosure?

I would not advise this. The chamber volume should be kept as close to original as possible to keep the Xmax and Pe specs happy under mechanical operation.
Later,
Wolf

[dynamo]

Since the back cap is a pain to remove, from what you’ve seen could one drill a large (1/2” ?) hole in the middle of the back cover to access the material that needs switching, and epoxy a penny or something over the hole when completed?

[Wolf]

There is a thick felt pad that is glued in the cup very securely, so that's not as easy as it sounds. It was more work to remove the felt than the cup itself.
Wolf

[Paul K.]

Ben, out of curiosity, how thick is the back cup? I looked at the back of my pair, which I hadn't done yet, and saw each was marked with an "X", apparently identifying them as not to spec.
Paul

[Paul K.]

I was going to edit my Post #13 but thought it would then be suspected of being Spam; so I assume the felt pad is attached to the inside of the cup itself? Approximately how thick is the felt pad and how big is its diameter?
Paul

Ben, out of curiosity, how thick is the back cup? I looked at the back of my pair, which I hadn't done yet, and saw each was marked with an "X", apparently identifying them as not to spec.
Paul

[Wolf]

The cup itself is not very thick. The felt pad fits inside the entire area of the cup, and is about 3-4 thin layers glued together to be about 3/16" to 1/4" thick, and it is glued to the inside of the cup. It is not easily removed.



Hope that helps,
Wolf