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Old 19th November 2009, 04:43 AM   #1
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Default Has anyone tried SDX7 in a folded TL?

Hi everyone,

I've been kicking around a few ideas for a set of lounge room speakers to replace a pair of truly terrible old Sony's.

I'm starting to gravitate towards an active 2 way FAST-ish design with an SDX7 and a Mark Audio CHR-70 per side. Alternatively, I'm also considering a 3 way with an SDX7 actively crossed to Zaph's ZA5.2.

Anyway, I'm wondering if I can get away with building these as floorstanders with the SDX7 in a transmission line. The straight MLTL's CSS show on their website are a little too tall, since they'd put the midwoofer well above the seated listening axis by the time I'd put it on top.

Which brings me to my question: has anyone tried designing a folded TL for an SDX7? If it could be brought down to about a metre in height, I think that'd be about right as a floorstanding base for a fullrange or 2 way.

Cheers,
- Chris
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Old 19th November 2009, 05:57 AM   #2
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Yes. I built a push-push TL for a pair of SDX7. I ended up stuffing until aperiodic. I'm going to replace them with a quad of Eastechs and build a sealed box, same size and dimensions for the SDX7.

I have this design and at least one other kicking around.

dave
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Old 19th November 2009, 06:04 AM   #3
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Hi Dave,

I take it the TL was too small with 2 drivers, hence the high level of stuffing?

If so, do you think a design with a single driver might fare better?

These don't need to go particularly loud, so I was mostly looking at the TL for extension - the other option is a linkwitz transform, a quick spot of modelling suggests that I could get flat to 35hz and still be good for peaks up to 92dB with a single driver in 14 litres or so.
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Old 19th November 2009, 06:09 AM   #4
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No, the problem was, as seems the case with some XBL drivers is that they run out of steam very abruptly and make some awful noises)... you need something that controls the drivers excursion as it goes low.

A half cu ft, and a bit of boost at 25-30 Hz does a pretty amazing job. That is what i'm using right now with a set of uFonken.

dave
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Old 19th November 2009, 06:24 AM   #5
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OK - so what I'm thinking, from an electrical point of view:

A TL has the effect of flattening the impedance curve, so the driver wastes less power near resonance, allowing it to play lower because excursion will increase for a given input power.

A linkwitz transform is a tuned equaliser which forces the amplifier to deliver more power in order to counter the rising impedance curve.

Thus, assuming power is sufficient and you're limited by excursion, maximum output levels would be the same for a TL and an LT'd sealed box.

(sorry about the phrasing, I'm studying for an Electronics and Circuits exam at the moment)

In brief, what I'm inferring is that TL's aren't actually magic, so, assuming I have a decent amount of power available, a linkwitz transform is just as useful, and easier to build.
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Old 19th November 2009, 06:52 AM   #6
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A linkwitz transform is usually used to put a driver in too small a box and then change its roll-off to that of a larger box. The SDX7 already fits into a teeny weenie box, an LT is just not necessary.

dave
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Old 19th November 2009, 07:03 AM   #7
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A linkwitz transform acts to change both Qts and F3, so it can be used to change both the apparent alignment and the extension.

ie. I can take a system with Qts 1.1 (ie. too small a box) and F3 80hz, and use a Linkwitz transform to make it behave as though it has Qts 0.7 and F3 40hz. In this case, it will apply both a dip above 80hz to remove the response peak, and a 12db/oct boost between 80hz and 40hz.

(And yes, I do know what I'm talking about with the Linkwitz transform. I've never built my own sub, but I've designed several LT's for other people doing ported to sealed conversions)
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Old 19th November 2009, 07:22 AM   #8
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I am well familiar with LT. In the case of the SDX7, it is just use of electronics for electronics' sake and IMHO a waste with the unnecessary extra circuitry degrading the sonics.

To get the SDX7 Q up to 1.1 you need to stuff it into a <3.5 liter box (so small you have trouble fitting the driver into the box. Unless you are trying extreme miniturization (which if you want something a metre high is not the case), an LT is overkill. A 21 litre box (in practise 14-17 litre and some stuffing works just fine) has critical Q and an F10 of just over 30 Hz, most of the time any boost at all is too much.

KISS.

dave

(the curves in the chart coorespond to 3.5, 7, 14, & 21 litre net apparent volume)
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Old 19th November 2009, 07:55 AM   #9
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Indeed, saying Q1.1 was just an exaggeration for the sake of illustration. Since I want to use the sub section as a stand, it could be 30L as easily as it could be 7L.



I'm more interested in your statement that it's more reasonable to look at F10 than F3. A quick look at WinISD suggests that in 21L, F3 is about 75hz, while F10 is about 35hz.

75hz seems very high to me....am I just overestimating the importance of flat extension through the 40-80hz octave?
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Old 19th November 2009, 08:00 AM   #10
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People look at F3 all the time, but it is a waste of time. There is no correlation between F3 and perception of bass... it is also a number that does not consider that the woofer is in a room. F10 is much more meaningful. Put the woofer into a room, in a corner or at least the junction of 2 walls and you usually have effective bottom down at least that low. The slower roll-off also meashes better with the room gain.

Design a woofer with an F3 of 20 Hz and i guarantee you will have a BIG hump and bass will not work out well.

You do not want to exceed a net apparent volume of 21 litre with this driver.

dave
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