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Multi-Way Conventional loudspeakers with crossovers

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Old 6th November 2015, 04:54 PM   #1
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Default New member, new OB project, questions...

Been lurking here for quite awhile, but this winter I'll have time to build something, and I thought I'd start a build/questions thread on what I'm doing and going to do. I've got some electronics experience with doing repair work. I've rebuilt an ST-70 and my current Citation II and built a preamp, but have never done speakers. I'm quite good with wood, though - and plan something kind of special with the baffles. I've been reading numerous forums and have developed the idea of what I'd like to implement, but there are issues (many of which I'm sure I have not even thought of yet). My idea is a 2 way passive OB (to start with!) using the Eminence Alpha 15 driver for the bottom end and an Altec 811 horn with an 806a (8 ohm version) driver for the mid and upper end. I've got a pair of Eminence PXB2:800 crossovers (800Hz 18 dB/octave HP, 12 dB/octave) as well.

I put everything together in a mock up cardboard baffle, and the Eminence drivers are very weak/quiet. I'm sure this is due to the large efficiency difference between the drivers - the Altec 806a (I have the 8ohm version built for Heathkit; the specs I found are from the 16 ohm Altec version) is listed in old Altec literature as having 109.5db efficiency, while the alpha 15 is only 97db.

So... I need to tame the output of the 806a a bit. What are recommended ways to do this? I've used the online L pad calculator - thinking about a 10bd drop on the 806a would bring it down more in line with the output of the alpha - so I did the calculation with a 10db drop at 50w per channel and got the values for my resistors for an L Pad of:

Click the image to open in full size. Parts List
Resistors
R1 = 5.47 Ohms 34.19 Watts
R2 = 3.7 Ohms 10.81 Watts

Q1: Do the knowledgeable folks here recommend this route, or is there something else that I should be considering?

I know there are lots of other variables to consider - baffle size, room size, etc., but the baffles are still under sketchup development right now. Many thanks in advance for any input received!
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Old 6th November 2015, 07:48 PM   #2
badman is offline badman  United States
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An L-pad is the typical way to damp the level, but given that you're new to this, I'd suggest getting a minidsp and doing this active- it's a lot easier than the nuance of passive XOs.
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Old 6th November 2015, 07:57 PM   #3
DanHat is offline DanHat  United States
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You could make this work. I think 97db is a bit optimistic for the alpha. I would think it is more in the neighborhood of 94 to 95db. Generally you would want about 6db more out of the ob woofer than the mid/tweet. It could be closer to 20db attenuation for the horn. You might try a variable lpad from a place like parts-ecpress. Once the levels sound right you could measure the lpad and replace with 2 resistors.
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Old 6th November 2015, 08:35 PM   #4
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I didn't realize partsexpress had variable L pads - should have known. Unfortunately, they don't give you any info on the resistance applied to the circuit - or am I missing something? How do I know, for example, that this P.E. L pad would give me even 10db of attenuation? Speaker L-Pad Attenuator 50W Stereo 1" Shaft 8 Ohm

Thanks for the replies - much appreciated!
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Old 6th November 2015, 11:26 PM   #5
GM is offline GM  United States
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Quote:
Originally Posted by DanHat View Post
I think 97db is a bit optimistic for the alpha. I would think it is more in the neighborhood of 94 to 95db.
Even lower just as its published response plot shows, baseline efficiency is ~92.5 dB using published specs, so the one to reference from.

For a passive XO, adapting the M19's adjustable one [AKA N-1201-8A] is the 'hot ticket' for the 811 as it also accounts for horn woofer offset, XO delays, off axis blending their polars.

For a simple XO, Altec's N-800-F and -K were popular for decades and if in the budget, finding/DIYing early potted N-800Ds with its tapped auto-former is arguably the best overall sounding, especially if high output impedance tube driven.

Anyway search/post on the Altec User's Forum to get schematics, parts lists, known tweaks, etc., and of course with a much lower efficiency woofer there will be some components needing different values.

Other forums:

High Efficiency Speaker Asylum
Lansing Heritage Forums
Audiokarma Home Audio Stereo Discussion Forums

FWIW, all things considered, I'd rather spend my time/$$ adding a second woofer/channel to get efficiency up there with the Altec 803/416, 811 combo these XOs were designed for rather than doing significant XO tweaking beyond adjusting for a ~ 4 ohm woofer section.

GM
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Old 7th November 2015, 12:50 AM   #6
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Going outside of a 2 way is not an option.
Active crossover is not an option.

I'm open to suggestions as long as they fit within the framework of my original post.

I've specifically avoided looking for Altec 15" drivers (like the 416) because the low qts is said to not work well with smaller OB designs - and my baffles are going to be on the small side. I've got my eyes open for a pair of 415's or 420's but I've got the Alpha 15's right now.
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Old 7th November 2015, 06:26 AM   #7
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Hi, total absence of any good advice - sorry, partecipants !

Totally wrong speakers, no-good crossover ( what's so difficult in the first two pages of the"manzanita Aka fast, inexpensive tec, OB " ? ), no description of where they are to be "put" .

The only schematic I see is 2 resistors and a speaker

And as it goes driven by a tube amplifier, it might be a good match

I've always read about the different behavior of tube amps that make the rising impedance peak > to acoustical bass boomines, due to the low damping factor

...so maybe with the current generic crossover ( probably not as if contouring is applied also to...) the equalization would be determined by the amp/driver response
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Old 7th November 2015, 07:02 AM   #8
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Look - I did not intend this to be an esoteric philosophic discussion about what is better than which. This is an experiment with parts that I have. I understand that they might not be the parts that everyone would prefer for this application. But - they are what I am working with. If you want to help answer some questions for me, then help - by all means.

I just ordered a pair of the variable L-pads from PE. That should get me the attenuation in the 806 drivers that I need to see where to go next. Many thanks to those constructively contributing!
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Old 7th November 2015, 07:07 AM   #9
GM is offline GM  United States
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Quote:
Originally Posted by jcoll View Post
Going outside of a 2 way is not an option.

I've specifically avoided looking for Altec 15" drivers .........
OK, but no one has suggested more nor switching to Altec drivers, only me suggesting using proven Altec XO designs to help with designing your own XOs since you're using Altec horn/drivers and doubling up the Alphas/channel for the reasons I mentioned.

GM
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Old 7th November 2015, 07:15 AM   #10
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Quote:
Originally Posted by GM View Post
FWIW, all things considered, I'd rather spend my time/$$ adding a second woofer/channel to get efficiency up there with the Altec 803/416, 811 combo these XOs were designed for rather than doing significant XO tweaking beyond adjusting for a ~ 4 ohm woofer section.
GM
Sorry - thought that's what you were saying here.

I'm sure I'll have to do some crossover tweaking. It's a long winter here, though! And maybe I'll find a pair of 420's in good shape in the meantime...
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