CX120-8 easy build

[BOBinGA]

Since this is a full build thread, I am putting it in the forum rather than the gallery for now.

https://www.parts-express.com/dayton-audio-cx120-8-4-coaxial-driver-with-3-4-silk-dome-tweeter-8-ohm--295-384

I have been asked a couple of times for a PEDS center channel. I made an MTM version of one over a year ago, but it was too complicated. Plus, I never thought it was what I wanted. The MTM layout created a lot of frequency response variations in different seating positions. After PE released the CX120-8 coax driver, I thought this might work out better as far as off axis listening and indeed it does. It also has the added benefit of a simple four element crossover. So here is a center channel that has the same bass roll off as the PEDS and a smooth response no matter where you sit.



I have not finished the boxes yet and my crossover is still external and attaches to the cable out the back. Now that I am satisfied with it, I will move the XO into the box and paint it. But that a project for Christmas vacation.

The box is mostly 1/2 inch MDF and is 5.25" H X 9" W X 5" D. The front baffle is 3/4 inches thick.

Now on to the design.

[BOBinGA]

This was a bit difficult to get working well. I first measured the T/S parameters of the one I received. It was a little off of the published specs, but it still modeled just like the published specs.
Published Fs = 87.5 hz Qts = 0.48 Vas = 3.0 liters
Measured sample Fs = 88.8 Qts = 0.53 Vas = 1.71

Its not unusual for drivers to have higher than spec Qts and lower Vas. Indeed, this was the case, but for this closed box it worked out to the same volume of 0.066 cu ft (114 cu in).



The Box size was pretty much determined by the height needed to allow the driver to fit the front. Using a regular rectangular box, we get a Qtc of 0.69 with full stuffing. Indeed, measuring the final box with my DATv2, I get a Qtc = 0.71 and an Fb = 118 hz. That's pretty close to predicted.

The WinISD graph above also includes a ported box 225 cu in with a 1.5"PVC port (1.59" actual diameter) X 7.25" length. You can see this will give you output down to 70 hz, but the size is double.

Here is my notebook page with the box details:



I didn't build the ported version, so that's up to you to try. I will be using the close box with a sub.

[BOBinGA]

You can see the different crossover versions I tried. The increased bass version was a little too bassy and lacked highs. The flat version is what I would use if it will be out from the wall a couple of feet on stands used as a stereo pair. The reduced BSC version works best when used directly below a large TV screen or when placed on a shelf close to a wall.

To illustrate the difficulties getting to make these work, here is the measured response in my large room. First the woofer:



You can see that the in box, in room uncorrected response has a large hump at 1700 hz followed by a big dip down to 4k hz followed by the cone breakup going back up to 8k hz. Luckily, this was massaged into a pretty close to 2nd order butterworth low pass slope at 2300 hz with just the 1.2 mH inductor and 4.7 mf capacitor.

Here is the tweeter:



The unmodified response is about 5db higher than the final woofer response. Again, I was fortunate to get the down to match the woofer with only a 3.3 mf capacitor and a 3.0 ohm resistor. This resulted in a 4th order butterworth high pass at 2800 hz.

Now finally, here is the predicted system response using the flat crossover:



You will notice that there is a small dip in the response at 2500 hz. This is because the woofer and tweeter crossover points are different - 2300 and 2800 hz. There was simply no way to get them closer together since these are the limits for the individual drivers. Fortunately, this crossover only creates a 3db dip, which is not even noticeable in use.

[BOBinGA]

Finally, here are the actual measured response of the finished speaker using the flat and reduced BSC XO circuits. (Disregard the stuff below 70 hz. I left my subwoofer on set to a 50 hz crossover for my regular speakers.)

First, with 1/3 octave smoothing to show you the response warts ad all.



You can see that there are lots of room modes below 600 hz. I could try gating them out in the measurements, but I find it somewhat easier to just mentally average out the peaks and dips to get a response that is closer to what I hear. The peak/dip/peak above 10k hz is a product of the coaxial tweeter and is not really audible.

To get an idea of any response problems that I can work out in the crossover, I also look at the one octave smoothed response. Its obviously not as accurate, but it gives you a better sense of the octave to octave balance and is pretty indicative of what you actually hear. Here it is:



How about them apples! Pretty close to dead flat with only four crossover parts. I don't think you can do any better.

[BOBinGA]

Finally for tonight, here are some shots of the crossovers I made using the PE small perforated boards. Even on the small 3" X 5" boards, I mounted both the flat and low BSC crossovers at each end of one board. I didn't have exactly the parts I needed, so I had to create them by paralleling some parts. But the actual values are all standard values that you should be able to source at PE.



I will try to add an actual wiring diagram later, but that's all I have time for tonight. Let me know if you have questions.

[wogg]

This is fantastic Bob! I have a plan in my head for one of these CX120's with a pair of 4" woofers flanking it for my someday HT build. I'll have to bookmark this for a starting point of the mid to tweeter crossover.

[brianbunge]

I too am looking at using this driver for a center channel (and maybe mated to a larger woofer for L/R speakers) except I am in a huge room with 20’ ceilings. So I’d like to build a center shaped more like a sound bar with a quartet of 5.25”-6” woofers using the Dayton Designer series paper cones. Let me know if you have any ideas concerning this.

[BOBinGA]

I have been using this as my living room center channel with the PEDS as left and right, PS95s as rears and an 8” sub I have not documented since the driver is long discontinued. It works great for this purpose since my wife and I don’t listen too loud. But the CX120 center does exactly what I need - it makes dialog very easy to understand.

I have considered using the CX120 in a LCR sound bar. I think that would be good high value build with a subwoofer and an inexpensive 5.1 AVR. But I don’t need this now, so I will leave that to others.

[davidroberts]

Did you ever post the complete xover schematic?

[BOBinGA]

Here is the final XO:

It has a very slight 1 to 2db upward bias in the 800 to 2000hz range to help with voice intelligibility. Otherwise, its flat.
C1= 3.3uF
R1= 3.0 ohms
L1= 1.2mH
C2= 4.7uF

[wogg]

Wow, that's way simpler than I expected! Well done Bob.

[djg]

This project wasn't in my mind until BOBingGA posted the XO. Always wanted to do a coax, so here's the last build this season. I won't do a build thread, just post a few pics here. Prebuilt the innards, the actual driver is back ordered for a month. Some fairly nice 1/2" ply, veneer with a 1/4" MDF baffle on all or part of the front. I plan to get them all built and waiting for the drivers in the next week. This is a pair of ported speakers for a 2.1 setup, the original being a single center channel.





PSA mahogany veneer left from last year Zingers build. 1/2" MDF baffle, glued to 1/2" ply front panel, 1" baffle, overkill.



CX120s should be here today.

[BOBinGA]

I hope they come back in stock. The similar CX150 seems to be discontinued, but Meniscus still has a few. I was planning to use the CX150s for atmos height speakers, but maybe they will be stocked again. If not, I guess I will use SB’s. But that’s a project for next year.

[djg]

Well, the product page says 11/06. Orphan boxes, hope not.

[djg]

BOB, I just ordered 2 from Meniscus, thanks.

[djg]

Baffles drying, maybe tomorrow. Excited to hear a coax speaker.

[davidroberts]

Yes it would be really nice to be able to purchase the S2000 woofers individually and use the coax in WCW arrangement for the center and CWW arrangement for the L/R in a soundbar.

I haven't found anything else that uses that same frame or basket.