CBT36 clones .. sort of

[rhubarb9999]

So .. I built these. Lots of holes. I put them together as a prototype to see if I liked the concept.

Basically I tried to build a super low budget CBT36 clone. The drivers are the $1 Aura Neo specials and the $.50 Apex Jr. tweeters

There are 19 mids and 45 tweeters per side. There are two sealed NHT1259 woofers per side. I built the woofer cabinets 20 years ago to use with a set of Carver 60" ribbons (with a custom active crossover).

The cabinets match the original CBTs .. 5 feet tall with a 36 degree curve. The baffle is 1/2" MDF bent over the arc and screwed down every 10 inches.

I am using a modified DCX2496 for the crossover. Power for the prototype is from a pair of garage sale receivers .. a 100x5 Kenwood with direct inputs (running the mids and tweets) and an old Marantz driving the woofers.

Right now there is no shading. The woofers are wired in series/parallel (groups of three and four). The tweets are in five groups of nine.

On an Ohm meter, the tweeter array is about 8 Ohms, the woofers are 10 Ohms. Actual resistance is a little higher. The old Kenwood gets warm but has not shut itself down yet.

Tuning by ear I found that I like the woofers crossing at 150Hz with a 24dB Butterworth slope.

The mids cross to the tweeters at 6KHz with a 12dB LR. I added a 3KHz notch of about 4dB with the DCX EQ.

So .. I did this has an experiment and I would say that it surpassed my expectations.

First off . .the little Aura is a really nice driver. Very smooth, vocals sound great. The dynamics of the system are awesome. To quote an often used word .. effortless. None of these drivers are working very hard so distortion is low. The Auras are also carrying the bulk of the load. So you get the coherence goodness of an (almost) full range driver. The tweeters are really just helpers to add a little sparkle at the top. Without them, you could probably EQ the Aura line and be pretty happy.

And then there is the imaging. Sitting on the floor, sitting on the couch, standing up .. the sound never changes. Walk to one side of the room, still a nice stereo image. These have a giant sweet spot .. both horizontally and vertically.

At this point, I don't think I am going to play with the shading. Most listening is from the couch so "anywhere imaging" is not a big deal right now.

Next up is some measurements and more tweaking.

So here are some pics ...

[rhubarb9999]

Re: CBT36 clones .. sort of

More pics



So the budget ...

Aurasound NS3-194-16A 3" Paper Cone 16 ohm ($1 each at 50pcs) plus shipping - $72 http://www.madisoundspeakerstore.com...r-cone-16-ohm/

Apex Jr. Tweeters - 100 @ $.50 each plus shipping - $62 http://www.apexjr.com/speakerstuff.html

3/4" MDF for cabinets, 1/2" for baffles - $45

Hole saws for woofer and tweeter - $25

So for around $200 you can make yourself a pair of curved arrays that sound pretty darn good. Because the Aura cannot go as low as the ND91, I think that actual woofers (rather than subs) in stereo is the way to go. When I cross over at 80-100Hz, you can tell that some of the low end grunt is missing. Maybe an array of buyout 8's or 10's ... hmmmmmm.

Do they sound as good as the real CBT36s? I have no idea, never heard them. Would the shading make a big difference? Don't know. I don't think that it would help with the dynamic impact, seeing as most of the drivers are attenuated to some degree. My understanding is that the shading has the greatest impact on the vertical imaging .. something that I am not really concerned with.

So for 1/10th the price and a little more build effort, I don't think these are too far off the mark.

Many thanks to PE and Don Keele for publishing so much information about the originals.

[kenny_k]

Re: CBT36 clones .. sort of

Love to hear those fellas. I agree with your assessment of the Auras.

[the kid]

Re: CBT36 clones .. sort of

...I'm not sure if you have enough speakers...:rolleyes: Pretty amazing!

[tomzarbo]

Re: CBT36 clones .. sort of

Wow, those are impressive. I would have been a nervous wreck when it came time to glue the baffle to the curved cabinets...

Please don't break, please don't break, please don't break!

I've done a few curved baffles like that and sweated the glue and clamping process a bit. Yours came out very nice.

TomZ

[rhubarb9999]

Re: CBT36 clones .. sort of

The baffles are not glued on .. they are screwed. There are screws every 10 inches on each side that bite into the 2x2 cross supports. I did not glue them so I could remove the baffles and do the wiring for the shading ... someday.

When I paint the rest of the cabinets, I will probably fill the screw holes. You will never see them.

[Thumper Tom]

Re: CBT36 clones .. sort of

Very nice and on a poor man's budget is even better.

[duanebro]

Re: CBT36 clones .. sort of

I love it! Glad to hear that you like them so well. Any problems getting the drivers or baffles to seal? What did you use for gasket material? I'm going to build the same thing at some point this summer. Only I think I will try to shade them and use a passive xo... Still with eq though. How much of a PITA was it to cut all the driver holes? I have thought of getting mine done at my college FAB Lab CNC router. Not sure of the cost - they never seem to charge me.

They look great!

[rhubarb9999]

Re: CBT36 clones .. sort of

The drivers are all rear mounted. I used hole saws to cut the circles (I'll find the sizes). The woofers are a nice tight fit. They have a little lip around them that fits perfectly in the hole and seals them up. The tweeters were a little trickier. What I did was glue a bottlecap to the front of one tweeter that was the exact size of the hole. Then I put this template (more of a jig actually) into each hole and drilled the holes for the screws. Then replace the jig with a real tweeter and put in the screws. Everything lined up nicely. I just used regular gasket tape for the baffles. I marked up the baffles and drilled pilot holes while the baffles were flat. Then I temporarily screwed them to the cabinets to cut the driver holes. A CNC would be MUCH nicer but doing it by hand was not bad. I took me about an hour per baffle to cut the driver holes.

I would be interested to see what you come up with for shading and a passive crossover. I would love to get the tweets and mids to be xover-ed passively and then actively cross to the woofers. Then I could use a Mini-DSP for the system and free up my DCX for other projects.

[basketcase]

Re: CBT36 clones .. sort of

Could you explain how you wired the auras? I have a case and have been trying to figure out how to get to them close to an 8 ohm load in an array. Thank you.

[rhubarb9999]

Re: CBT36 clones .. sort of

I did groups of 3 or 4 drivers (19 total) in parallel and then tied the 5 groups together in series. On an Ohm meter I get about 12 Ohms .. so in reality its probably as 16 Ohm load on the amp.

[raiderone]

Re: CBT36 clones .. sort of

I followed the link to this thread from another about the CBT36.... I was wondering if you listened to just the mids array (tweeters off) and if you did what did it sound like? I ask because there are other threads about using only 3" drivers and sounding good, but none of them were curved, well actually one or two were curved but "concave-ly."

[Wolf]

Re: CBT36 clones .. sort of

I followed the link to this thread from another about the CBT36.... I was wondering if you listened to just the mids array (tweeters off) and if you did what did it sound like? I ask because there are other threads about using only 3" drivers and sounding good, but none of them were curved, well actually one or two were curved but "concave-ly."

Comb filtering would not allow any kind of decent treble response from the mids only. Now- if the drivers had an inherently rising response up to and past 20Khz, then it could possibly comb-filter the response back to flat destructively. I have seen this happen in one instance, and it was not bad.

Later,
Wolf

[Sydney]

Re: CBT36 clones .. sort of

I followed the link to this thread from another about the CBT36.... I was wondering if you listened to just the mids array (tweeters off) and if you did what did it sound like? I ask because there are other threads about using only 3" drivers and sounding good, but none of them were curved, well actually one or two were curved but "concave-ly."

Visualize that the distance from the drivers ( reproducing the same signal ) to your ears will NOT be the same, and it become apparent that the sound produced by the more distant driver will arrive later.
In classic engineering texts* this is often represented as an integration of multiple trigonometric relations.
The result is summation that produces lobes.

* One example is Sound System Engineering
By Don Davis, Eugene Patronis

[rhubarb9999]

Re: CBT36 clones .. sort of

I did not notice much comb filtering with just the mids playing. The curve helps reduce this. With a good bit of EQ, the mids can sound OK .. but the tweeters really help.