The Plum Dingers - PS95-8 and ND20FB-4 in a Desktop MTM

[KEtheredge87]

Hello Everyone!

Speaker building is a hobby not always appreciated by our spouses or significant others... so when the opportunity to fix one of their problems with a set of speakers arises, you jump on it! My wife is a teacher and tutors children with dyslexia. This service used to be done face to face at school or at home, but the pandemic has forced all that work to be virtual over Zoom meetings. As you can likely imagine, trying to hear the nuances in human speech over laptop speakers is not a fun task. Enter the handy husband with a speaker for every need .

Etheredge Audio Engineering is proud to present "The Plum Dingers." I designed this small desktop MTM using the Dayton PS95-8 full range driver and ND20FB-4 rear mount tweeter to help deliver clear speech (well... as clear as a VOIP connection / zoom meeting will allow!). The speakers were made with a matching set of stands to put the tweeter right at ear level to be in the sweet spot of the MTM while seated at the desk. The only requirement my wife had was that the speakers needed to be purple. I'm more of a veneer man myself... but the lady asked for purple, so purple she gets! I went deep into unknown territory and used powder based milk paint applied via HVLP spray gun to get as smooth a finish as possible. That said... honest milk paint that comes in a powder like this does finish rather chalky, and still feels that way after some sanding. The color itself is called "Plum" and it has been finished with 2 coats of hemp oil, both products sold by the Real Milk Paint company. The hemp oil deepened the color from a light, almost lilac purple to the deep purple that's pictured. The camera doesn't quite pick up how deep the color is, but the wife loves it, so score! To top it all off, I bought some purple techflex sheathing for the speaker wire, because I really wanted to lean into the purple on this one!

Specifications
Dimensions (WxHxD): 6” x 11” x 8”
Materials: 3/4" MDF baffle and internal brace, 1/2" MDF walls, 3/4" BB strips for back panel frame
Enclosure Type: Two-way Bass Reflex
Estimated Frequency Response: 75 to 20,000 Hz
Fb and F3: 93 and 75 Hz
Port: 2” PVC (1.6125" ID) x2.625" long, outside flare
Nominal Impedance: 4 Ohms
Crossover Frequency: 4930 Hz
Tweeter: Dayton ND20FB-4, flush with baffle though rear mounted
Woofers: 2x Dayton PS95-8, recessed into baffle
Cabinet: MDF Baltic Birch rear access panel frame
Cabinet Finish: Real Milk Paint "Plum" color with Hemp Oil finish
Binding Posts: Dayton Audio 5-Way HD Binding Posts – Gold Finish

Enjoy the next few posts of pictures! I didn't intend to make a build log for these since time is short with an infant crawling around the house. Frequency Response, Impedance measurements, and more in the next posts

Thanks for reading!
Keith

[KEtheredge87]

XO Details and Measurements - The PS95's are marketed as full range drivers, and they have been used that way by others. I however am a sucker for phase cones... especially fancy copper colored phase cones! I decided to use these full rangers with the ND20FB-4 to handle the high frequencies. The high end of the PS95's frequency response chart looked a bit off to me, and I'm handy enough with a soldering iron to create a crossover. Besides, the rear mount tweeter allowed me to achieve a nice close CTC distance and avoid the press fit style used by the ND16FA-6 (a nice little tweeter in it's own right). I don't normally aim for flat XO response, but this one sounded really nice to me as a flat response, though I expect it will be best that way for speech intelligence as its "day job"

I used parallel LCR filters on both the woofers and the tweeter to handle resonances. These may or may not have been really needed, but I liked what I heard, so they stayed in.

[KEtheredge87]

Physical XO layout and install - I cut HDF board to fit between my rear access panel frame and the internal brace, then arranged XO components to fit within the space. Inductors were spaced as appropriate, including between the two sides of the cabinet. XO boards were held in place by high strength velcro, same as I've done on other speaker designs where I didn't see the need or space to deal with PCB standoffs.

[KEtheredge87]

Cabinet construction pics - It's a fairly basic cabinet with 3/4" MDF front baffle, and 1/2" MDF walls, top, and bottom. I used 3/4" thick strips of baltic birch plywood inside the cabinet as a mounting frame for my rear access panel (didn't want to chance repeated installs / removes with screws in MDF). Also, there was no way I was getting XO boards into the cabinet through the small clearance holes for the PS95's. This was arguably the biggest pain on the project, since I rushed a bit making a nearly exact template for the rear panel cutout. I screwed that up more times than I care to admit. Ultimately I spaced it ~1/32" on all sides to allow clearance, without a big gap. I also took into account gasket tape thickness on my rear wall access panel, and actually planed the MDF down a bit to compensate.

The other thing to consider was flush mounting the tweeter. Normally this isn't a big deal, but when flush mounting a driver from the rear... some extra attention to your router cut depth and overall depth calculation needs to be taken into consideration. I used #4 x 3/8" pan head screws to install the tweeter from behind. These were maybe 1/4" long, so as not to poke through the front baffle. I also had to account for a layer of gasket tape here too. No leaks in these cabinets except for the ports!

Speaking of... the ports are 2" PVC pipe that was recessed into the rear of the access panel, epoxied into place, then routed with a roundover bit to give an external flare. These did not paint as well as I would have hoped (was looking for a smooth continuous coat of purple all the way down the tube... but whatever).

[KEtheredge87]

Last post unless questions come up - Painting. This was my first go with the HVLP spray gun. I bought a DeVilbiss FinishLine 4 set, and I enjoyed using it... not so much the cleaning part. That said I did clean it after every spray session! First I sprayed a couple coats of some primer on the bare MDF. Zinsser 123 grey primer was recommended by the Real Milk Paint people, so that's what I used. From there I switched to the milk paint and began my learning curve. I tried mixing the milk paint in an old blender because I heard it was tough to get all the solids in the powder to dissolve. This was a big mistake. It mixed... but it also made a crapload of plum colored milk paint cappuccino foam. Trying to strain that through a 190 micron HVLP filter took forever, and I'm sure I wasted a lot of the paint product. Next time I will just mix it in the can with the little glass marble they provide and stop trying to overcomplicate things.

Beyond that, the paint itself comes out a light purple, almost magenta color. Using the recommended hemp oil finish (which has a nice earthy smell) deepened the color dramatically and sealed up the porous milk paint. That made it feel a good bit smoother too.

The HomeRight spray booth is a glorified camping tent with cheaper materials. It contained *some* overspray, but when I had my settings wrong I was just fogging up the garage anyway. Better than nothing, but still not a real paint booth.

[a4eaudio]

Awesome! Nice write up too, much appreciated!

[KEtheredge87]

You're welcome! Glad you like them.

[djg]

Nice.

[tomzarbo]

Those are really nice Keith. Your wife has exactly what she needs to do her job now. Just set my wife up with my old Micro-B 2.1 setup for her work at home teaching setup as well.

Good audio is important for this teaching stuff, especially with younger kids because they don't always pronounce or annunciate speech well yet. They need to hear, and even see our mouth to know how to form the various sounds they should be making. I'm always pulling down my mask to show my students what position my mouth is in when im speaking more challenging to pronounce words.

Those speakers came out really nice! And dang, you have your own logo now!

TomZ

[KEtheredge87]

Those are really nice Keith. Your wife has exactly what she needs to do her job now. Just set my wife up with my old Micro-B 2.1 setup for her work at home teaching setup as well.

Good audio is important for this teaching stuff, especially with younger kids because they don't always pronounce or annunciate speech well yet. They need to hear, and even see our mouth to know how to form the various sounds they should be making. I'm always pulling down my mask to show my students what position my mouth is in when im speaking more challenging to pronounce words.

Those speakers came out really nice! And dang, you have your own logo now!

TomZ

Thanks Tom! Joy loves the speakers. She came rushing into my office space to tell me how much better her tutoring sessions went with the clear audio. Mission accomplished! Also the logo stuff is fun, but can be a bit expensive for some bling. There's a shop in town that runs a Trotec laser among other things, and they charge me $15 per plate that I have engraved. I wish I could do that kind of work myself to save some money... but that Trotec is a $15K laser, so I'm not going to recoup that investment... EVER! Ultimately I just like to be more and more polished with every build. little touches like dataplates are part of that evolution.

[Wolf]

Badges rock!
Nicely done, Keith!
Wolf

[ugly woofer]

I'm just happy that they aren't snake skin print, but seriously well done.

[KEtheredge87]

Haha Nick... a purple snakeskin print does exist, but I had enough tolex work for quite a while. Thanks for the compliment!

[Derekj]

They look great Keith!

[thekorvers]

Great job and very professional looking!

How do you like the green Frogtape? I have never used it. Any advantages over the regular blue painter's tape?

[KEtheredge87]

Thanks bud! I highly recommend the frogtape for many applications. The adhesive is much better quality than blue painters tape, and certainly better than standard tan masking tape. It sticks, but doesn't leave a messy residue behind on surfaces if you leave it for a couple days. I also feel like the frog tape seals better at the edge than regular blue painters tape. I think 3M has caught up with their "edge lock" blue tape in that regard, but I'm already sold on Frog Tape. It even comes with a container to keep the edges from getting all gummed up with dust.