NEW mid-top for 3 way!

[Dr.EM]

Hi all! I am creating this thread for documenting progress on the mid-top section of my 3 way design.

The design will use Hi-Vi M8a woofers, Dayton RS52 mids and Hi-Vi RT2E-A tweeters all configured in a tri-amplified active crossover setup. The plan is to have seperate top units and floorstanding woofer enclosures; this is my progress on the top section.

Progress will be slow as I spend a lot of time away from home at uni and don't have my building equiptment there.

Anyhow, here is my starting point. I'll first post a rough sketch of the front of the mid-top section to give an idea as to what is being aimed toward:



A real wood front would be nice and so would brass fixtures (as the tweeter has a gold/brass element) but we will see about these. I am undecided on whether the drivers will be recessed in as normal (very difficult with this shape!) or recessed in but with felt around the drivers, a more triangular overall cutout can be used then.

Construction will be laminated, with basically numerous baffles glued together to make up depth. No actual enclosure is required per-se as both drivers are self enclosed, however a baffle on it's own wouldn't look right imo. Quick question, would having a solid wooden front baffle then MDF "baffles" behind it cause any issues with expansion rates being different, after being glued together?

Onto the first progress. I have been using my router to cut the aluminium faceplate of the Dayton RS52, party for aesthetics and partly to reduce the CTC spacing (at around 4.7khz crossover, this is important to minimise). I used a TCT 6mm bit and a home made circle guide to cut the arc form. This is my router:

http://www.machinemart.co.uk/shop/pr...scuit-jointers

Here is the cut shown:



And here it is butted up against the tweeter. A fairly seamless joint!




There of course was a problem. Not in routing, that went well, but the grille fell off of my other unit (I have a pair naturally) and when I tried to re-attach it with superglue a small amount leaked onto the faceplate. I made the error of trying to immediately wipe with whatever was to hand (tissues) when really leaving the blob to set would have been better. Needless to say, after failed attempts with acetone and hot soapy water the glue hasn't come off and the finish is spoiled further . I still have not decided if I will be replacing the unit or trying matte paint to hide the damaged area. I shall continue anyhow.

I hope to update after about 2 weeks. In the meantime I can try to answer any questions!

[sreten]

Hi,

That is a strange tweeter to use with a dome midrange. I'd expect
the DM to be harder to c/o than a normal small mid or bass mid.

/sreten.

[Dr.EM]

Hi,

I've had the tweeters a while and originally had a different mid. To me these domes seemed like a good choice to go with them. This way both the tweeters and mid are low mass drivers with similar transient characteristics. The dome mid also allows for a suitably high crossover frequency (tweeter needs at least 3khz, ideally higher to lower distortion) without breakup issues or large driver spacing issues. So far listening tests have found them to be a very detailed combination, which I like. Here is a design using very similar drivers to mine, although differently implemented:

http://www.lonesaguaro.com/speakers/cyclops/CYC.htm

For the crossover I will be using 24db LR, with a breakup notch on the aluminium dome. The tweeter will have it's protection capacitor forming the 4th filter pole with 18db actually active

The dispersion is different for the drivers however. Both have a wide horizontal dispersion, but the tweeters have a very narrow vertical dispersion. I hope this wouldn't cause too much issue

[Dr.EM]

Sorry for lack of updates on this! A few reasons, firstly I don't have my camera back home, it's here at uni! Also, there is less to show than I had hoped

I went to get some more layers done this weekend but annoyingly the router guided trimmer bit fell apart (bearing broke open) nearly as soon as I started . It did damage the workpiece a little when it came apart but not too seriously. Woodfiller should fix it but only once it is all finished otherwise.

So, I have just 3 layers on each so far of about 20 required . 2 new TCT bits are on order and I will have a break for easter when I can hopefully get some done

[Dr.EM]

Hello again!

This is still in progress! It's Easter break so I am getting a bit done. I've been working on a prototype crossover to go with a test speaker.

The test speaker is a baffle of correct shape (though poorly cut) holding the mid and tweeter, perched atop my mini computer subwoofer. It is the correct woofer (M8a) but is ported instead of sealed and does not have enough stuffing inside for midrange duties really.



Crossover is nearly as I intended. It features 24db slopes for all drivers (tweeter has 18db active, 6db protection cap), a notch filter for the woofer breakup and another for a mild peak in the response caused by the baffle. A notch for the mid breakup was experimented with but abandoned, it added a lot of noise for unknown reasons but it also caused phase shifts at the crossover point since the reduction required is quite dramatic. It's at 13khz and the crossover is 24db at 4.65khz so it'll be significantly attenuated by then regardless; it's right near the top end of my hearing anyhow so isn't that offensive!

The 24db electrical slopes should work well as all drivers overlap nicely. For instance, the woofer rolls off about an octave above Xover (once the peak is nulled) and the mid rolls off an octave below. A similar situation with the mid to tweeter but with more overlap.




What I want to experiment with is phase correction between the mid and tweeter, although they are very close to each other depth wise, mabye it isn't required?

It does play music and the result so far is promising. Obviously hard to judge in mono with one channel missing though! Some recordings from the 40s I have that are in mono sound great


On the actual speaker build the laminated layers are still going together, it's a slow process but I have my new router bits and they are much better!

[harry09]

Very Very cool!

[Per]

A bit ot, but how did your use your router on aluminum? Often thought about doing the same thing...did you cool the bit or turn the rpm down?

/Per

Edit: instead of using bits with bearing, use a copyring and a regular 10 mm (or diff. dimension) bit. The bearings will get worn by the dust...

[Dr.EM]

Thanks Harry!

Per, to cut I just lowered the RPM to minimum (about 7000RPM on mine) and sprayed WD40 into the grooves I was cutting in the material before each pass. I used about 4 passes, so 1mm a time roughly. To cut I used a 6mm twin flute TCT bit. Most difficult was keeping a constant feed rate, it tended to move suddenly as I had to follow a guide to cut it (the radius was too small do do directly with the router compass).

Thanks for the tip, I will see how my new bits hold up then decide what to do. I think finding a bushing and bit of matching size might be tricky as the router bit won't fit through the bushing? Perhaps one with a smaller shank and larger cutter could be fitted with the router plunged...


My problem now is flush mounting those mid and tweeter units! The tweeter shape is a nightmare . I just tried and failed making a cardboard guide by scanning in the shape and adjusting the size up before printing, what I forgot was that preserving aspect ratio would elongate the shape, I just needed a 1cm border all around! I may try this again or buy a big enough bit of board to swing all the arcs on, but then those corners are a problem . Any ideas appreciacted here!

[kyrie48]

I'm jealous... super quality cut!!
Nothing that looks better than two drivers blended together like that!!

[gainphile]

Even your bread board work is tidier than mine !

http://s188.photobucket.com/albums/z...t=DSC_8803.jpg

I think at least the tweeter should be flush-mounted? I found the diffraction is audible even with 2mm deep flange.