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:
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!
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:
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!
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.
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!
Thanks for the replies - much appreciated!
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
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.
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.
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
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
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!
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!
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
Some? A lot!
I used to run the 811 horns above Selenium 15" woofers on open baffle. It took a loooong time to get the crossover right.
With the Alpha 15, the challenge will be just as big. You will lose a good bit of efficiency on OB to get a good tonal balance from the woofer. Were you cross the woofer and the size of the inductor needed will be determined by the baffle.
Then you will have to pad the horn way, way down to match - and figure out where to high pass them.
I used to run the 811 horns above Selenium 15" woofers on open baffle. It took a loooong time to get the crossover right.
With the Alpha 15, the challenge will be just as big. You will lose a good bit of efficiency on OB to get a good tonal balance from the woofer. Were you cross the woofer and the size of the inductor needed will be determined by the baffle.
Then you will have to pad the horn way, way down to match - and figure out where to high pass them.
I thought I had a csp file around here that modelled the Eminence crossover, but had to re-do it. Granted, I'd have to go look to see if the Eminence components use textbook values (what I used when modelling the pxb2:800 crossover with these drivers) - which I have not. But, the summed curve using 2nd order Butterworth for the lowpass and 3rd order for the highpass. The summed response doesn't look bad - but this was only the 3rd time I've tried to do this.
To me, it doesn't look like the Altec frequency response is high enough, but... the curve I traced for the Eminence A15 came from Eminences' website - and I don't know what kind of enclosure they had the driver in for their test - so the mid-tweeter part of the summed curve is probably anomalously low in the curve. BUT... with attenuation, I think it should be pretty good (if I did any of this correctly anyhow).
Funny thing. There is a pair of 420's on fleapay now. Hmm.
To me, it doesn't look like the Altec frequency response is high enough, but... the curve I traced for the Eminence A15 came from Eminences' website - and I don't know what kind of enclosure they had the driver in for their test - so the mid-tweeter part of the summed curve is probably anomalously low in the curve. BUT... with attenuation, I think it should be pretty good (if I did any of this correctly anyhow).
Funny thing. There is a pair of 420's on fleapay now. Hmm.
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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:
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!
You may get the Altec padded down, but without some kind of compensation to extend its range, it will fall off response wise to early. Even with said compensation, it still will not extend as far as an 802 or 902 version Altec will.
Yes the horn is more efficient than the bass driver. This is what allows the compensation filter to work. It shaves off efficiency to the level of the woofer near the crossover frequency, then less and less as frequency extends into the higher range, thus compensating for physics, which will always cause a compression driver to roll off as frequency rises.
I''ve been down the horn road before, but this open baffle stuff is also new to me. I have no idea how horns do on open baffle.
Open Baffle is not like a closed box. The woofer will have a pronounced rising response on OB. Or a falling response toward the bass, looking at it that way. The level difference between the usable bottom end and the point at which you want to cross can be 10dB or more. A conventional box crossover will NOT take this into account.
Now the Alpha is a small magnet, high Qts woofer with a low end bump. That's why it's often chosen for OB, the high Qts bump helps fight level the bass and midrange.
It took me a very long time to figure this out and abandon normal crossovers for OB. It's just a different animal and needs a different crossover. You have to (cough cough) think outside the box.
Now the Alpha is a small magnet, high Qts woofer with a low end bump. That's why it's often chosen for OB, the high Qts bump helps fight level the bass and midrange.
It took me a very long time to figure this out and abandon normal crossovers for OB. It's just a different animal and needs a different crossover. You have to (cough cough) think outside the box.
Yes, I've seen a lot of folks talk about the shortcomings of the 806a. I didn't realize that's how the compensation filter worked. Do you know of a schematic and/or some specific background info on a compensation filter for the 806a? Many thanks!
Yup, that's it! If you search out Zilches work in the gigantic Econwave thread, he tried many drivers including the 806, and got all there was to get. It is a giant thread, but the search function may get you there. Zilch was one of the best and I miss his input. He could dumb complicated stuff down to where even I could understand. Zilch, rest in peace.
Russellc
Russellc
Once you know what your high pass is for the horn, you'll bypass the series resistor to let thru more top end to the horn. You may be bringing the horn down 20dB to match the Alpha on OB, so that gives you room to boost the highs. (not really boost, just attenuate less).
Your horn crossover would look something like the plot below, which high passes at 1200Hz and gives the horn a 12dB boost at the top end. It's something you'll want to tune by ear, using different values for the cap. The cap labeled C1 is the shelf/boost cap.
Schematic is from XSim crossover software.
Your horn crossover would look something like the plot below, which high passes at 1200Hz and gives the horn a 12dB boost at the top end. It's something you'll want to tune by ear, using different values for the cap. The cap labeled C1 is the shelf/boost cap.
Schematic is from XSim crossover software.
Attachments
Thanks guys - I had read part of that thread, but man it's a lot to get through. I had seen Zilch's name everywhere, and had come across some of the ideas that he had, and had done some reading on filters. The placement of the cap and what it does make better sense to me now with that schematic than it had before - thanks!
Have to say though - I'm thinking more and more about the 420's. Thing is though - their rated efficiency is also 97db. Why is there such a disparity in the efficiency rating of the Eminence A15 - also rated at 97db? Differences in measurement techniques between the 1960's and today? Just curious.
Have to say though - I'm thinking more and more about the 420's. Thing is though - their rated efficiency is also 97db. Why is there such a disparity in the efficiency rating of the Eminence A15 - also rated at 97db? Differences in measurement techniques between the 1960's and today? Just curious.
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