Seas CA15RLY X2 cabinet volume suggestions

ShaneF

Member
2015-12-02 4:42 pm
I've just purchased 2 pair of Seas CA15rly bass drivers and I'd like to use them in Dappolito configuration. What cabinet volume would you suggest? I would like to keep them as slim as possible because they are for a desktop system.

I have seen suggestions for 1 bass driver at 7L. Would 10L be okay or would you go more... 15?
 

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H1216-08 CA15RLY

Entirely a matter of taste, I'd think. 7L per driver upwards for sure. :)

SEAS 5INCH

The conversion to D'Appolito is straight forward if we ignore the ideal 90 degree phase BW3 filter aspects that improve dispersion. Simplest way is to wire in basses in series, double bass coils and resistors, and halve the bass shunt capacitor. Then double the volume, and cut the bass reflex to half length. You can also simplify the filter.
Vifa PL14WJ-
 
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It's up to you. You can parallel the bass drivers, by all means, but the filter conversion is less straightforward. :D

I have certain theoretical reservations about parallel wired basses, but it's YOUR hobby.

Sounds a like a fun speaker you are building.

The Vifa PL14WJ link should give you some ideas. I don't suppose it's a wildly different 5" driver.
 
I'm getting the feeling that the port is about 50mm (2 inches) diameter X 60mm (2.5" length) length for about 24 L in either case. About 50Hz tuning. I would think that is about near enough in an inexact science. Because damping affects things too.

TBH, Are you gonna hear it? I often think the perfect is the enemy of the good. I'm no great shakes on reflex calculations, other people might do this stuff better than me, but do I notice that at a higher driver Fs, the port gets smaller.
 

ShaneF

Member
2015-12-02 4:42 pm
I'm looking forward to building these things over the weekend. Normally, I would spend stupid money on stuff that costs way more but my intuition -while looking at the response curve- sends up a green flag because they are relatively well-behaved, moreso than their aluminium-coned counterparts. I think I will match these up with the 27tffc tweeter. There are a couple of reasons: I find that the aluminium dome tweeter that I have in another set replicates metalophone percussion and brass instruments very well... but strings and woodwinds "suck". Second, piano could be better. Knowing one cannot have everything, I think I will opt for this fabric dome for a better balance, musically speaking.

Would you have any suggestions for a crossover and driver placement for these guys?

And lastly, there's a part of me that might even want to go one step further. I have 4 of these drivers and the last 2 bass drivers are still available. I am considering having one operate as a mid range and the other two handling the bottom end. I know that having 2 5 inch drivers does not equal 1 10 inch driver but width on my desktop is a major concern.
 

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Lojzek

Member
2012-02-10 12:12 pm
Croatia
Hi Shane,

are you absolutely sure that you really need an MTM configuration?
If there is anything wrong with how the metal dome tweeter performs,
assuming that it is a quality piece of a driver, then the XO filter is wrong
and you might be a little too subjective in your assessment. IMO. :)
 

ShaneF

Member
2015-12-02 4:42 pm
Hey boss! :)

I was considering MTM because: given the volume and height of the cabinet, I wanted to put the tweet at ear-level with a couple of degrees of back angle.

I assume your question leads me to understand that MTM (acoustically) is more difficult owing to lobbing problems and differences between driver, et cetera?

W 7.5" H 20.5" D 11" is what I am working with.
 
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ShaneF

Member
2015-12-02 4:42 pm
I am afraid I don't have any measurement gear... other than a tape measure :)

I am just building the boxes now and I think I will put off mounting the battens before I know whether I will MTM or not.

Thanks again for your advice and help!

PS: I am considering using the the tweet and X-over you helped me out with before, for now. "Save money and all that" :)

Of course, with the bass drivers impedance half (X2 drivers) I will change out the bass X-over to a 1mh and 20uf. I have an excellent 1.5mh goertz coil that I might pop in there and lower the shunt cap instead.
 

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I do share your misgivings about metal tweeters with classical music. They always make grand piano sound like a metallic harpsichord to me. :D

IDK if fitting a tweeter zobel of a about 7.5R and 0.68uF helps. A bit, IIRC. But I don't like spitty soft domes either. TBH, a ring radiator would be fun to try. Vifa XT25, SS R2604, SB Acoustics? Something to do with pinning the centre of the flapping soft dome that works.

Anyway, I had a look at the MTM PL14WJ design. http://www.troelsgravesen.dk/VifaPL14WJ-R2604.htm. There's scarcely a cigarette paper between the Vifa PL14WJ and the SEAS CA15RLY so you could use most of the ideas: SEAS 5INCH.

20L is about right. I think these might be great little speakers. MTM's gain 6dB level without extra distortion, so go much louder and sound authoritatative. IMO, combing just sounds like off axis rolloff. I had a good listen to something like this recently and liked them a lot.

532765d1456057805-classic-monitor-designs-mtm-scanspeak-raal-ribbon-selah-audio.jpg


I don't think you can go far wrong here.
 
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ShaneF

Member
2015-12-02 4:42 pm
I have a 1mh tape coil but it's only 16ga. I wish is was 14.

I have a 1.5mh 12ga tape coil that's just sitting around. I wonder if I could use that and pull back slightly on the shunt cap? I have some 5.25mfd Siderials kicking around.

Thanks for finding that Vifa Dappolito cabinet drawing! I'll be ripping that off... "thanks very much" :D

I think I will port on the front. I have an extra 1cm on the height of my cabinet. There's room.


That SS2604 looks quite interesting! I wish it was 6 ohms so I could re-use some of the Xover parts I have.
 
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I've just purchased 2 pair of Seas CA15rly bass drivers and I'd like to use them in Dappolito configuration. What cabinet volume would you suggest? I would like to keep them as slim as possible because they are for a desktop system.

You want it for a DESKTOP - right? The problem is, for a real D'Appolito configuration, you have to use a very small tweeter and a very low crossover frequency. That means, you only get a homogene radiation pattern with a low xo-f, the formula is

f < 2c / 3d

(f = xo frequency, c = speed of sound, d= distance in m from the middle of the woofer to middle of the 2nd woofer). Which equals to an xo of ~1050Hz if your woofers are as close as mechanical possible to a tweeter of 6,5cm diameter (or less). It's very hard to find a that small tweeter which can be xo'd at such a low frequency. Ofcourse, you don't have to follow D'Appolito but that means to introduce a lot of vertical peaks and dips, moving out of the main radiation axis woule mean you'll encounter a lot of side-beams and holes. If that's really for a deskop and short distance listening, I would strongly recomment ditching the D'Appolito configuration. Besides that, you'll have to do a lot of work to equalize the short reflections of the desk anyway.
 

ShaneF

Member
2015-12-02 4:42 pm
Thanks for your thoughtful response ICG.

Of course you are right about many things, here. I'm attempting to kill many birds with one stone and yes, we can't have everything. As a stand-alone unit, I'd like to build them so that I can use them for applications "including" my desktop but my situation may change and I'd like to have the option to use them as a room speaker too. I know that's a lot to wish for --and quite frankly, impossible.

And yes, I know of the situation regarding the lengths of waveforms. My academic background is classical music composition. This acoustical problem comes up constantly in orchestra concert performances. The low brass, strings and percussion will occasionally be asked to come in slightly ahead of the conductor's baton at the bottom of the ictus. They do this so that the band will sound precisely together at a big "thump" in the music.

https://www.youtube.com/watch?v=6GmrlY0pA_Q
 
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Shane, I'm looking forward to your build here. :D

I don't think you should get too bogged down in detail with an essentially simple project. I think it'll be a good speaker.

532765d1456057805-classic-monitor-designs-mtm-scanspeak-raal-ribbon-selah-audio.jpg


This one, which we can't afford or design, is actually rather elegant. It's D'Appolito improved IMO. There are all sorts of complexities with diffraction, combing and lobing and dispersion which Joe D'Appolito largely addressed with his BW3 filter. But most of these things are built into nature, and everything we do is a compromise. The issues never completely disappear.

But phase aligned LR2 filters like Mr. Troels does with the MTM Vifa sound OK too. I have a centre speaker like this. I wish I had a pair for stereo. :)
 

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