A midrange box

[stevie]

Here are a few photos of my midrange cabinet project. It uses the cabinet from a non-working Celestion KRi2 that I picked up on eBay for not very much money. The first task was to remove everything from the Celestion so that I had an empty shell to work with.

[IMG]http://i59.tinypic.com/25asqp0.jpg[/IMG]

Despite all the internal bracing, the box itself rings like a bell and I’d want to apply lots of bitumen damping to the walls before I even considered using it as a midrange enclosure, never mind bass.

[IMG]http://i60.tinypic.com/153uslv.jpg[/IMG]

[stevie]

In this case, however, I’m going to fit a sealed midrange driver, an Eminence Alpha 8MRA. Although it is quite an old design, it performs surprisingly well, particularly for the money. The sealed back means that it comes with its own enclosure, so to speak. If you install it in an existing cabinet all you have to do is cut a hole the right size. The downside of this arrangement is that it pushes the driver’s resonant frequency up to around 500Hz. 800Hz is therefore the lowest crossover point that’s realistically possible with this driver, and only if you use steep slopes. As the BMS 15” I’m partnering it with performs beautifully well beyond this point, that’s something I can live with.



The midrange driver. A bit dusty but it works OK.

Edited June 6, 2014 by stevie

[stevie]

Just had a look at the price of the Eminence - they're not so cheap after all. They do appear on eBay for only a few pounds, but I think I'd probably use the Celestion equivalent (TF0818MR) if I were buying new (I got mine from eBay for a few pounds ). It looks like a more advanced driver all round with a slightly lower resonance frequency, a less pronounced peak at fundamental resonance, a smoother response and more controlled breakup at the top end. And it's almost half the price of the Eminence too! I have some TF0615MRs in my workshop but they are not loud enough for this system (which uses a 4 ohm bass driver).

The next version of this system (should I ever do one) will cross lower, probably to a 6.5" midrange chassis. But that's another story. Here, I’m going to fit the passive crossover inside the mid cabinet with two Speakon sockets on the back – one will be the main input from my amp and the second will be the filtered output to the bass cab. This means that I won’t need to carry out any modifications to the bass cab whatsoever.


Oh well. Now to drill a couple of holes in the back of the box for the Speakons.



Wire them up and screw them in. Time for a beer after all that hard work.



Edit for a missing indefinite article.

Edited June 6, 2014 by stevie

[stevie]

The only bit of woodworking needed is to cut out a spacer for the Eminence - the cutout in the cabinet is just a tad large. There's no need for precision here and my jigsaw was adequate.



I apply layer of foam strip before screwing the spacer in and trying the driver for size.



It fits, and is starting to look like a proper cab.



We're still missing the most important part, the crossover, but as that's where all the real work is, it will have to wait for another day.

Edited June 6, 2014 by stevie

[stevie]

I'm surprised nobody has asked any questions, like 'why bother'? I hope to answer that particular one when I post the measurements in a few days' time.

[VTypeV4]

I think this is great, adding sonic precision to your system to achieve the required result using a bit of home brew and ingenuity. I raise a glass to you.

[IBWT]

Why bother?

[Guest bassman7755]

[quote name='stevie' timestamp='1402085191' post='2470009']
I'm surprised nobody has asked any questions, like 'why bother'? I hope to answer that particular one when I post the measurements in a few days' time.
[/quote]

I guess the presence of the ashdown cab underneath it tells us all we need to know ..

[VTypeV4]

[quote name='bassman7755' timestamp='1402156428' post='2470552']
I guess the presence of the ashdown cab underneath it tells us all we need to know ..
[/quote]

I wonder what the implication there might be.. Hmmmm, now let me think

[xgsjx]

I get the theory behind your idea. Are you bi-amping or are the crossovers doing all the work there?

I don't doubt your skills for the audio side of things, so I think the big job for you might be trying to make the cab "look" nice.

It should take a fabric covering easy enough, such as tolex or carpet. If you have no pets, may I suggest for the ultimate coolness.... fur fabric?

[stevie]

[quote name='IBWT' timestamp='1402138173' post='2470329']
Why bother?
[/quote]

Good question!

The Ashdown cab spotted by the eagle-eyed bassman bears as much resemblance to the original Ashdown cabinet as this one does to the original Celestion cabinet. The only things that have not been swapped out or altered are the grille and the metal corners.

As far as appearance is concerned, I’ll let you judge for yourselves when I take some photos of the complete rig. It’s not a one-brand look, that’s for sure, but it looks OK to me. I’m more interested in how it sounds than how it looks. I suppose it would be possible to construct a wooden cab to match the Ashdown, but that would have been a lot more work.

Here’s a picture of the prototype crossover, which answers xgsjx’s question. For the cost of those components, you could easily buy yourself an electronic crossover, which is the option I'd recommend for most people. The passive crossover has the benefit of convenience - once it's installed it doesn't need any tweaking and you don't need a second amp.

[IMG]http://i58.tinypic.com/2rm8fnc.jpg[/IMG]

[Phil Starr]

Interesting, that looks like a fairly significant crossover. Did you consider using something simpler like the 'bright box' approach with just a simple 6db/octave high pass filter to the midrange and no filtering on the bass driver?

Obviously that would lead to some comb filtering effects and it would be almost impossible to get a flat response but it would be a lot cheaper and simpler and you would get the extra frequencies and dispersion from your midrange driver.

How did you decide on the crossover frequency? I assume there are a few octaves of overlap.

[stevie]

Hi Phil, yes, it is indeed a heavy duty crossover - the system is 500 watts. To give you an idea of scale, those resistors are 30W jobbies - not the usual 9/11 watt wirewounds.

The crossover frequency was dictated by the midrange driver, which has a large peak and a nasty delayed resonance at around 500Hz that needs to be suppressed. This is therefore a 24dB Linkwitz Reilly filter at 800Hz - roughly where the 15" starts to beam, which works out OK. So there is very little overlap and no comb filtering. With a better behaved midrange unit I could have used a 12dB per octave x-over.

The bright box idea is OK, but I'm trying to minimise compromises here, as this system is for my own personal use.

[stevie]

It's now finished and this is what it looks like. I took a lot of measurements when I was designing the crossover but didn't save any that I can post. So, I'll do another set and then try to explain what this setup does that most others can't (IMO).

[IMG]http://i57.tinypic.com/2meaa1g.jpg[/IMG]