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Old 14th July 2004, 03:11 PM   #1
Jennice is offline Jennice  Denmark
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Default Newbee question: Can this be done (well)?

Hi folks,

First I thought I would build a pair of 2way speakers with:
One DX25TG05-04 or XT25TG30-04 tweeter (which to choose, and why?)
and one Peerless SDS or HDS164 woofer.
(I found this as a kit somewhere - would give an easy start with hopefully good sound).

Then I thought I'd run out of bass at higher levels with this modest woofer size. I'm used to the oldest B&K 630 series speakers which are 2.5way (not the passive radiator like 620 has) with 2 x 6.5" woofers.

Anyway...Then I thought about putting 2 woofers in parallel, maybe the w-T-w (M-T-M) way (I did read the thread on distances between the woofers), but started wondering about the resulting sensitivity compared to the tweeter sensitivity. Will this be a problem?

Also, I considered to make it 2.5 way and put one woofer in closed enclosure, and the other in bass-reflex. My idea was to have one with tight bass, and one that went lower. Would that work?

anyway... I know it's a lot to ask, but...hey, everyone starts somewhere

Any help appreciated
Jennice
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Old 14th July 2004, 06:00 PM   #2
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Using dual woofers will give a 6dB increase in voltage sensitivity but that is largely negated by the baffle step anyway, so don't worry about it. Chances are you'll still end up padding the tweeter down a bit.
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Old 14th July 2004, 06:45 PM   #3
Lusso5 is offline Lusso5  United States
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Default Re: Newbee question: Can this be done (well)?

Quote:
Originally posted by Jennice
DX25TG05-04 or XT25TG30-04 tweeter (which to choose, and why?)
The tweeters behave very similarly up to 20K.

The DX is the one of the best bang-for-the-buck tweeters out there IMO, and its 1/2 the price of the XT.

DX and XT distortion measurements.
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Old 14th July 2004, 08:35 PM   #4
Jennice is offline Jennice  Denmark
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Quote:
Originally posted by BillFitzmaurice
Using dual woofers will give a 6dB increase in voltage sensitivity but that is largely negated by the baffle step anyway, so don't worry about it. Chances are you'll still end up padding the tweeter down a bit.
Intersting... (Well, both replies are interesting, but this one makes me wonder).

Can you tell me why it's 6dB and not 3dB (is that because we're talking power and not voltages?), and what's the "baffle step" ???


The DX is rated 93.5dB, while the HDS164 (which behaves nicer at 2-3kHz than the SDS164) is rated 87.6dB. Gaining 6dB gives a paired sensitivity of 93.6 dB which is close to the tweeter. Now that mysterious baffle step!?

Based on the experiences and talk about spacing considerations with MTM designs, would I be better off with TMM (tweeter on top) design, or do the same spacing rules apply to the mid/woofers? (would be easier to implement, anyhow, if I wouldn't have to "squeeze" in the tweeter.

If I went for the DX, what order of filter would I need for crossing at 2kHz (is that too low for the DX? (No recommended frequency range stated in data sheet).

Any knowledge out there on the use of 1 woofer in BR and the other in sealed box design, sharing the same overall enclosure for each speaker = 2 woofers each side?


Finally (for now)... any knowledge of how 2.5 way speakers are filtered? If the 2nd woofer kicks in at the lowest frequencies, and the tweeter is matched in sensitivity to the 1 woofer-mid, then a sensitivity bump will occur at the lowest frequencies. -- or am I mistaken?

Thanks -
Jennice
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Old 14th July 2004, 09:22 PM   #5
cjd is offline cjd  United States
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3db gain for halved impedance (assumes you parallel the drivers)

3db gain for doubled surface area.

Wiring in series give you a net gain of 0db but doubles power handling.

Baffle step is the point at which the speakers no longer project into half-space (forward only) and project to full-space. Check this link for a good writeup.

If you go with a 2.5 way, you'll want the mids mounted together (MMT). If you just go 2-way, MTM is a possibility.

2khz cross I would probably go with a 4th order LW slope. What you're watching out for is excursion troubles, and a steeper roll-off keeps you further away from danger as you lower the crossover point.

C
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Old 14th July 2004, 09:28 PM   #6
Lusso5 is offline Lusso5  United States
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http://www.tlinespeakers.org/tech/ba...intro-bds.html

Benefits of MTM (D'Appolito) vs. TMM depend on your crossover phase relationships.

2k should be fine with the DX at 2nd order or greater, but you shouldn't pick a crossover point without doing the proper measurements and simulations.

There are many supurb documented DIY designs out there with build plans/schematics. You should build a tried and true design.

Edit - sorry CJD, looks like I posted over you.
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Old 14th July 2004, 09:48 PM   #7
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Can you tell me why it's 6dB and not 3dB (is that because we're talking power and not voltages?), and what's the "baffle step"


Because despite the differing impedance loads the voltages seen by the woofs and tweeter will be the same.
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Old 14th July 2004, 10:10 PM   #8
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http://woodartistry.com/linkwitzlab/faq.htm#Q21
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Old 14th July 2004, 11:19 PM   #9
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Jennice, I'm in a process of building a speaker like the kind you are describing. I will use the DX tweeter (XT is too expensive for me) and two 7" CSX woofers. For 4ohm impedance, I would wire the woofers in parallel and have a parallel crossover. Or, for 8 ohms, I would instead have a series crossover. For a 2kHz XO, I would use a third-order, just to be safe.
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Old 15th July 2004, 12:48 AM   #10
Ap is offline Ap
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Assuming drivers are actually 8ohm nominal
(most are but Ive seen drivers stated as 8ohm with Re, resistance, varying from 5.2 to 7.5 ohm)

For series driver you would have 16ohm.
This is not the same thing as a series xover.
Most (typical) xovers are paralllel.

With two bass drivers you could have
series drivers / parallel xover,
or parallel drivers / series xover,
or series drivers / series xover,
or parallel drivers / parallel xover.
See how complex it gets adding anpother driver! Not to mention MTM lobing issues.

Andyg is a fan of series xovers & can help with more info if interested.
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