CX120-8 easy build

[cjbruce]

Hi everyone. I apologize for my long absence. I am still alive, but I moved on to another hobby that has taken all my time and energy lately. I built a small DIY solar power system that now runs my computers and home theater completely independently from my house grid. Next, it's on to DIY a whole house grid tied system, but that will be posted elsewhere if and when its more complete,
I am glad to see continued interest in some of my designs. Over the next few weeks, I will try to review all the related posts and see what I might need to clarify. Thanks again for your interest and I am looking forward to a getting back to some speaker work even if it will be my secondary hobby for a while.

-Bob

What a great project! Way to go! We are considering doing solar/battery backup on our house as well. Looking forward to hearing how it progresses!

[BOBinGA]

Hi everyone. I apologize for my long absence. I am still alive, but I moved on to another hobby that has taken all my time and energy lately. I built a small DIY solar power system that now runs my computers and home theater completely independently from my house grid. Next, it's on to DIY a whole house grid tied system, but that will be posted elsewhere if and when its more complete,
I am glad to see continued interest in some of my designs. Over the next few weeks, I will try to review all the related posts and see what I might need to clarify. Thanks again for your interest and I am looking forward to a getting back to some speaker work even if it will be my secondary hobby for a while.

-Bob

[cjbruce]

The 3D printed version made it to the homepage of www.instructables.com!

https://www.instructables.com/3D-Pri...-Home-Theater/

I put the CX120 build together with the TEBM46 surround speaker build. Thank you BOBinGA for posting your crossover and for the inspiration. This is my first crossover build and I was afraid to just go for it and trust my ears. My version of the crossover isn't perfectly flat, but it was the smoothest I could make it with the equipment I have.

EDIT: The project just earned a prize in the Instructables "Home and Garden" contest!

[cjbruce]

Thank you! With the marble PLA there is no finishing required. The speckles in the material hide most of the 3D printing flaws.

[cjbruce]

Thank you! The cloth grill was necessary to make the room look cohesive. I'm happy with it too.

Its funny that you mentioned putting the cloth over the port. With my surround speaker design the air velocity in the port was so great that it was causing buzzing. I redesigned the grill cutout on all my speakers, including this one, to prevent this. I didn't occur to me that a simpler solution would be to not cover the port.

[LOUT]

I didn't expect the grills to look as nice as they do. I'm glad you went with the cloth, it seems to fit the look really well.
I also didn't expect to see the port covered by the grill for some reason, so that's kinda inspiring (I don't have a good excuse..just that thought never occured to me, lol).

[Chris Roemer]

Looks nice.

[cjbruce]

Finished! This is version 3 of the enclosure. Here is the link to build instructions on Thingiverse:

https://www.thingiverse.com/thing:5371462

Sound is good! The speakers compare well to a pair of Edirol monitors I have been using for the home theater, but these blend better visually with the gray fabric grills.

[cjbruce]

Testing is complete, and I've settled on first order filters for both low and high pass. The filters don't sum to perfectly flat on my iPhone microphone, and the speaker placement near the floor is causing some challenges that seem to disappear when I place the speaker a few feet higher.

Here is my final parts list:

Dayton Audio DNR-1.2 1.2 Ohm 10W Precision Audio Grade Resistor
4.0uF 100V Electrolytic Non-Polarized Crossover Capacitor
Jantzen Audio 1.0mH 15 AWG P-Core Inductor Crossover Coil

Total cost at the time I am writing this is about $20 for the crossover.

Here's the finalized design:

[cjbruce]

The speaker was tested close to the floor. This speaker will be used as a center channel below a pull-down screen below a 77" soffit. There is very little space so the speaker must be as slim as possible.

[cjbruce]

I am having trouble with some peaks and dips around the crossover region. The speaker didn't sound flat initially, compared to some old Edirol monitors and a TEBM 65 BMR I put into a small box. I don't have access to a calibrated microphone, but I do have a Frederickson 2502 Function Generator in our lab and my iPhone 12 running the free "Decibel X" app. My uncalibrated phone mic is showing a big drop around 3 kHz compared to the TEBM65.


I didn't have the exact crossover parts specified in the "Reduced BSC" list, so I ended up going with the following:

C1 = 5.6 uF NPE
L1 = 1.0 mH steel core inductor
C2 = 4.0 uF NPE
R2 = 1.2 Ohm 10 watt

Not satisfied with what I was hearing I did a bunch of sweeps at 2.83 volts on the function generator, listening and watching for peaks and dips. Here's what I tried:

1. Inverting tweeter polarity - This replaced the big dip with a very unpleasant peak around 2-3 kHz.
2. Increasing the value of R2 - This did little to eliminate the dip, but padded down the tweeter so the highs started to disappear. With inverted tweeter polarity it helped reduce the 2-3 kHz peak a little, but caused a dip starting at 4 kHz.



I eventually gave up on the tweeter side and started looking at the low pass. I didn't have another inductor, so I started playing with C1 instead:

3. Adding capacitors in series to reduce C1. I was thinking by changing things around I could get rid of the some of the phase cancellation that was occurring between the woofer and the tweeter, but no matter what I tried I could get the big dip (normal tweeter polarity) or bump (inverted tweeter polarity) to go away.

Finally, out of things to try unless I buy more components:

4. Remove C1 entirely. With C1 removed suddenly the crossover region sounded better. The crossover is now a simpler (and cheaper) 1st order on the low pass with a 1.0 mH inductor. There is still a narrow dip at 3.9 kHz, but the speaker is measuring and sounding much flatter:



Now that I have a better baseline I would like to go back and redesign the grill. It is currently a 3D printed design with 3mm holes covered by speaker cloth. I suspect there is some diffraction happening due to the hole pattern that can be eliminated by opening up the entire thing and using cloth fabric only.

[cjbruce]

Absolutely. I feel like I am really pushing the limits of my 3D printer with an enclosure of this size. The sides of the enclosure are already eating up a roll of filament, and when I tried to do 10 mm thick top and bottom pieces I was getting air leaks and rattles trying to press fit the pieces together. The outside of the slotted port is not fitting well and is in dire need of some sort of rigid glue and sealant to stop buzzing.

I'm almost at the point where I want to give up and just make an MDF box.

[djg]

Very nice. I think you would want thicker top/bottom pieces.

[cjbruce]

Just finished the 3D print and initial assembly. I added a little polyfill which seemed to help a bit in our initial listening. I'm happy with the sound on the test bench so far, but I will need to put it in the room with the subwoofer to see how it integrates.

TOP - 3D printed 2 mm marble PLA - Looks surprisingly good but doesn't fit well.

BOTTOM - Laser cut 3 mm baltic birch plywood - Fits well, but I would like to go with something a little nicer.

TO DO:
1. I am currently using a 1.2 Ohm resistor to attenuate the tweeter. I will need to listen to the speaker in the room to see if this tiny bit of tweeter attenuation is enough.
2. Neither the current 3D printed top nor the 3 mm plywood bottom will be in the final version. Need to figure out what looks and fits the best.
3. Need to design the magnetic grill and choose a grill fabric. I tried 3D printing a grill but it looked a little like a 70's Marshall amplifier grill -- not the look I am going for.

[cjbruce]

I did an initial for check for the 3D printed driver mount today. The fit is snug but not too tight. I’m going ahead with the full 52 hour print now. Fingers crossed for the next two days!