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-   -   SDX15 x2 ...design suggestions please (http://www.diyaudio.com/forums/subwoofers/109584-sdx15-x2-design-suggestions-please.html)

Oscillate 6th October 2007 06:45 AM

SDX15 x2 ...design suggestions please
 
I will be building a subwoofer(s) mostly for music listening. The goal is hard-hitting, tight, accurate bass. Ability to play <20 Hz tones is desirable. High SPLs are not necessary. The drivers will be the CSS SDX15 x 2 (a mentoring friend gave me one and I will be ordering a second soon). These will have a dedicated power amplifier (Crown Xti2000 or XTi4000) and an outboard cross-over (if needed).

Parameters as per CSS website

Recommended sealed enclosure:
Home audio ...Low Q (Q.577) 6.5 cubic feet fully stuffed

Re: 3.6 (1.8 ohms per coil)
Le:1.8
Fs: 19.2
Qms: 3.65
Qes: 0.44
Qts: 0.39
Vas: 218.44
Cms: 0.25
SD: 790
Xmax 30mm
Xmech 40mm
BL: 16.5
Mms: 274.8
SPL: 87.3
1000 RMS watt power rated

For music, the TVC/active tube buffer preamp has 3, 'always active' outputs per channel (RCAx2, XLR). For HT, the pre/pro has RCA sub-out x 2.

I believe that building the Low Q (Q.577) sealed 6.5 cubic ft. enclosure to be the best choice(?). The two designs that I am considering are...

1) 13.0 cubic ft enclosure with the drivers in a push-pull configuration (front to back, wired out of phase). A mono-sub, connected to to the left stereo channel (music) or a sub-out (HT).

2) 6.5 cubic ft enclosure x 2. Two subs, seperately connected to left and right channel (music) or to sub1 out and sub2 out (HT).

Do you folks feel there is any added musical benefit derived from having two seperate subs, each one connected to a seperate stereo channel? Do you feel that a push-pull mono-sub would hit harder and tighter?

All thoughts and replies welcome. Thanks for your time.

BHTX 6th October 2007 07:14 AM

Re: SDX15 x2 ...design suggestions please
 
Quote:

Originally posted by Oscillate
High SPLs are not necessary.
So I'm assuming you're aware that you would lose quite a bit of efficiency with an isobaric push-pull? Since you were willing to increase the enclosure size to a significant amount for a low q, I'm also assuming that size isn't much of a concern, or at least a big priority? In that case, I wouldn't mess with an isobaric, as it'd probably be more trouble than it's worth, and you'd lose A LOT of output..

Honestly, if it were me, I'd do two separate sealed 3.6 cubic foot enclosures, run a Marchand BASSIS and/or Behringer DEQ2496 and/or DCX2496 for Linkwitz Transform and EQ (or build your own LT), and run them in stereo, as close as possible to your L&R channels. With these drivers and a Linkwitz Transform, you should be able to get in-room response down into the very low teens with a significant amount of output. As a matter of fact.. 114 dB before room gain at 20 Hz before 30mm xmax is reached (xmech is 40mm if i remember correctly), although with the 3.6 cubic foot .707 qtc boxes, thermal limitations might become an issue before that point, in which case you'd build the enclosures if more power handling was desired, although I seriously doubt that'd be necessary. ;) But this just gives you an idea.

I had considered this exact setup recently, and if I were still going for subwoofers, this is more than likely what I'd do. Aside from the LT and/or EQing capabilities, I'd also make sure that I could make necessary phase adjustments to match your front channels as close as possible, and as previously mentioned, run them in stereo close to your L&R channels. I definitely wouldn't just stick them in the front corners, as these subs shouldn't require that. I'd probably experiment and actually try to keep them away from the walls as much as possible while still keeping them separated and close to your left and right mains.

Oh and, yes, I most definitely do "feel there is any added musical benefit derived from having two seperate subs, each one connected to a seperate stereo channel"...depending of course on how they're crossed, as well as the positioning of the subwoofers, front channels, room, etc etc.

Amit_112dB 6th October 2007 11:28 AM

since all thoughts and replies are welcomed
:smash:
i'll jusy say 20hz is not easy to reproduce with a driver but its a peice of cake for a room to kill it.
Have you thought have standing waves etc....?

Oscillate 7th October 2007 08:20 AM

Regarding an 'isobaric push-pull': I guess my description was vague. Please allow me to clarify. From what I've read so far, images 1 and 2 refer to an isobaric push-pull sub? I was thinking along the lines of image 3. I am aware of the decreased efficiency of this configuration. But understand that it provides some vibration and distortion cancellation, along with a really clean, hard hitting sound?
http://www.nevada.edu/~baldob/pp_1.gifhttp://www.nevada.edu/~baldob/pp_2.gif

~~~~~~~~~~~~~~

However, since you've confirmed my suspicions that two seperate enclosures would be better than one for music, lets go with that. From what I've read about the Linkwitz Transform, it's a 'simulation' allowing a smaller enclosure to sound like bigger enclosure that has a lower Qtc. I would much prefer to build two larger enclosures (Q.577 = 6.5 cubic ft) than two smaller enclosures (Q.707 = 3.4 cubic ft) that are electronically compensated. The larger boxes would probably be less prone to thermal problems also? I certainly think your suggestion of using a Behringer DCX2496, in this case for the high and low pass filters ("depending of course on how they're crossed") has great merit and will do that.

"Have you thought have standing waves etc..." -Uhmmm, if your refering to the room itself? Then I can't adress that issue yet as I will buying a house soon and I am not sure of room size or it's acoustics. I thought to build two Sonotube style subwoofers to adress standing waves within the enclosure itself. Utilizing 24" or larger diameter Sonotube. These would be fully stuffed with "8 oz of polyfil per cu ft", as recommended per CSS. Is there any sonic benefit or detriment to having the drivers top mounted in the Sonotube (top-firing) versus bottom mounted (bottom-firing)?

Next design choice: I believe a sealed enclosure responds faster to transitions. Should I build two sealed Sonotube subs or perhaps build two, low tuned, ported Sonotube subs that have a very flat response curve? Which one do you feel would sound more musical?

[If the Mods will please allow for cross-posting]
This question stems from reading about a LLT design which (reportedly) lets the low frequency tuning itself resolve some of the ported design problems?

In fact, this is my first speaker build ever. Although I have the skills required to build whatever, I do not have the knowledge or experience using WinISD or UbiBox. If one of the forum members could please model the two following possible designs that would really help...

1) A sealed, 24" diameter, 6.5 cubic ft/184.06L (Qtc =.577) Sonotube

2) A ported, 24" diameter Sonotube, tuned to Qtc = .577 or perhaps to 14Hz?
(I am not sure how to express this, although I did see a 14Hz/300L model using a different diameter Sonotube on another forum)

...many thanks for your time and replies :)

95Honda 7th October 2007 09:13 AM

You don't loose efficiency using push-pull like your picture #3, only the Isobarik #1 & #2 would have lower efficiency over a conventionaly operating pair of drivers. You would be just fine with your push-pull set-up.

A pair of subs always is better than one in my opinion. I would be wiling to bet you would be very happy with that driver in the large sealed boxes, you would have alot of low end output. Personally, I would go with a large low tune vented alignment over sealed, but that is just preference. A sealed box doesn't neccesarily have faster transient response than a vented one, maybe better than a poorly designed vented box. Also some people make sealed boxes so large that they sound lifeless in my opinion...


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