tagger Posted March 6, 2016 Share Posted March 6, 2016 I'd like to hear some opinions from those running Altec 288's in either two-way or three-way setups. I built a pair of Cornscala/Alcorns with Altec 288 drivers and MR594 horns on top (crossed at 700 hz). I really like what I hear, but I feel it's missing the upper frequencies I hear from my Forte II's and Heresies with Crites tweeters. Not much but enough for it to be noticeable to me. I haven't taken any measurements but I would expect to see the 288's drop off between 15000 and 16000 Hz. I assumed my hearing wasn't worth squat beyond that, but after living with these for several months now I guess I was wrong. I'm thinking of adding tweets and crossing over around 9000 Hz. Would I be nuts for not letting the 288/MR594's run all the way up? Quote Link to comment Share on other sites More sharing options...
Chris A Posted March 6, 2016 Share Posted March 6, 2016 Have you considered Beyma CP25s, EQed flat and crossed about 3-5 kHz? The polars of these tweeters are really good and the price is also good if you look around for bottom dollar. Almost all 2" compression drivers using aluminum (the present case) or titanium diaphragms will experience diaphragm breakup modes above about 12 kHz. Most people less than 70 can hear that frequency, although harmonics at that frequency are not audible. What you can hear is rough response or the absence of smooth frequency response at those frequencies. Chris Quote Link to comment Share on other sites More sharing options...
tagger Posted March 6, 2016 Author Share Posted March 6, 2016 The only problem with the Beyma is the size of the horn. The outside dimensions are 6.3" (L) x 5.3" (H) and needing a 4.72" (L) x 5.12" (H) cutout. I built the top sections of my Alcorns with enough room for a 5" (H) OD lens and no more than a 4.25" (H) cutout. My thinking was that I would use a B&C DE120 / Eliptrac HF horn combo up top. As such, I didn't leave enough room to experiment with a larger variety of tweeters, unless I rest them on top of the cabinet. I attached a photo -- the cabinets are still unfinished at the moment. Quote Link to comment Share on other sites More sharing options...
Chris A Posted March 6, 2016 Share Posted March 6, 2016 Placing something like CP25s on top so that you can align them with the Altec drivers toward the rear of the cabinets would actually work fairly well. I'd recommend crossing at 2.7-3.5 kHz, looking at your midrange horns. I think that would kill any higher frequency hash that you might run into. Chris Quote Link to comment Share on other sites More sharing options...
tagger Posted March 6, 2016 Author Share Posted March 6, 2016 I plan to use ALK extreme slope tweeter networks so I'm not overly concerned with time aligning them. Also, if I were to have the tweeters recessed on top of the cabinets, wouldn't early reflections off the top of the cabinet be an issue? As it is the 288 drivers are almost sticking out the rear -- the top section of the cabinets are open in the back -- so to physically time align them the tweets would need to be all the way in the back. As for the crossover point, my thinking is to allow the 288's to play to their strengths, which in my mind is from 1200 - 9000 Hz. The MR594's are constant directivity horns and are very well behaved in that range. Al K has 5800 Hz, 7500 Hz and 8500 Hz versions of his extreme slope networks, and I intended to go with one of the higher ones. However, I may be easily swayed otherwise. Quote Link to comment Share on other sites More sharing options...
Chris A Posted March 7, 2016 Share Posted March 7, 2016 (edited) The issue is the 2" wide slots in the midrange horns, which will turn on higher order modes above ~3900 Hz. Time alignment of the tweeter to the midrange is a big deal--and time misalignments between the midrange and tweeter have even more effect the steeper the slope of the crossover filters happen to be. You'll hear the whole loudspeaker come into focus when you find the right location of the tweeters on top--the soundstage will simply appear and envelop your listening position-sort of like Polk SDA monitors do when you plug in the opposite channel patch cable. You don't have to worry about HF reflections above ~2500 Hz, they basically don't bounce--they instead get absorbed, whereas midrange frequencies will bounce off side and front walls and nearby furniture. It's good to keep the equipment low and away from the midrange horn mouths for that reason. Chris Edited March 7, 2016 by Chris A 1 Quote Link to comment Share on other sites More sharing options...
tagger Posted March 7, 2016 Author Share Posted March 7, 2016 Time alignment of the tweeter to the midrange is a big deal--and time misalignments between the midrange and tweeter have even more effect the steeper the slope of the crossover filters happen to be. This is very interesting. Why is it a bigger problem with a steeper slope? I figured with the extreme slope network (~120 dB / octave) the frequency overlap would be sufficiently small enough to not be noticeable. I do have a Behringer DCX2496 active crossover I could play with to time align everything. I'm not sure it would be a good permanent solution since it's a pro audio unit. I suspect my Emotiva USP preamp doesn't have a high enough output voltage but perhaps I'm wrong. Quote Link to comment Share on other sites More sharing options...
Chris A Posted March 7, 2016 Share Posted March 7, 2016 With the gentle slope networks (like those used by PWK in the original Heritage crossovers), the time misalignments are disguised even more because of the blending of the two drivers over more than an octave. However, it makes that octave or two sound fuzzy and its timbre somewhat shifted. Once you hear the time-aligned drivers, you'll wonder why you ever got used to misaligned drivers. You'll be able to hear the music much more clearly and transparently. Chris 1 Quote Link to comment Share on other sites More sharing options...
Chris A Posted March 7, 2016 Share Posted March 7, 2016 I figured with the extreme slope network (~120 dB / octave) the frequency overlap would be sufficiently small enough to not be noticeable. Even if the crossover filters were infinite slope, the time misalignment would still be audible, albeit with less timbre shift than using gentle slope filters. I do have a Behringer DCX2496 active crossover I could play with to time align everything. I'm not sure it would be a good permanent solution since it's a pro audio unit. If you simply move the tweeter toward the back of the cabinet to approximately align with the midrange driver, you've solved the problem without having to use the Behringer. It's also very cheap and easy to do. If you don't like it, you can easily undo. You're right about the "permanent solution" comment: the DCX2496 is good to hear for temporary use, but I think that it isn't hi-fi enough in terms of its analog circuitry to be a permanent replacement. Chris Quote Link to comment Share on other sites More sharing options...
Pete H Posted March 7, 2016 Share Posted March 7, 2016 Placing something like CP25s on top so that you can align them with the Altec drivers toward the rear of the cabinets would actually work fairly well. I'd recommend crossing at 2.7-3.5 kHz, looking at your midrange horns. I think that would kill any higher frequency hash that you might run into. Chris Nice, clean housing for the Beyma! Quote Link to comment Share on other sites More sharing options...
tagger Posted March 7, 2016 Author Share Posted March 7, 2016 First, thank you for the advise thus far! Now I'm thinking of performing some cabinet surgery to remove the top 5 inches above the mid horn to keep the tweeters closer to ear level. Do you have a suggestion on a 3.5 kHz tweeter network to add to my ALK 700Hz network? The ones Al builds start at 5800 Hz, but I'm not adverse to building a 3.5 kHz network myself if I have the plans. A different commercial solution would be fine too. Quote Link to comment Share on other sites More sharing options...
Chris A Posted March 7, 2016 Share Posted March 7, 2016 I'd use xsim (freeware) from Bill Waslo to simulate the change in values in your present ALK crossover, but I'd have to have a pretty complete schematic of your present network to use it to any advantage. Al K. (ALK Engineering) himself might also suggest some new values for the chokes and capacitors in his network. It's certainly worth a try to ask him. All he can do is bite your head off or ask for an unreasonable amount of money Chris 1 Quote Link to comment Share on other sites More sharing options...
tagger Posted March 7, 2016 Author Share Posted March 7, 2016 My current ALK network is the AP12 700 Hz network. It is a two-way network that includes an output to an optional tweeter network. There are no changes to my existing network required to add one of his tweeter networks. Would that not also be the case if adding a different tweeter network design? I have the schematics for the AP12 network, but since Al no longer publishes it on his site I will not post it here. Al K. (ALK Engineering) himself might also suggest some new values for the chokes and capacitors in his network. It's certainly worth a try to ask him. All he can do is bite your head off or ask for an unreasonable amount of money No need to risk that! I revisited his site and realized I missed that he also offers a 4000 Hz version. That's probably close enough. Quote Link to comment Share on other sites More sharing options...
Chris A Posted March 7, 2016 Share Posted March 7, 2016 Yes, 4kHz is the limit. The formula for the cut-on frequency of a horn producing higher order modes is: f(co) = 1.84c/πD = 7956/(throat diameter or diffraction slot width) units are in inches. For your midrange horn, if the slot diameter is 2", then the cut-on frequency is 3978 Hz, which is effectively 4 kHz. I'd go for that 4kHz HF crossover from ALK. I know that the Beyma CP25 is good down to about 2.5 kHz crossover frequency, so you're in good shape as far as sound quality, power handling, and coverage angles on it, especially since you're using steep slope filters that limit the power output below your crossover frequency relative to a gentle slope crossover filter. Chris Quote Link to comment Share on other sites More sharing options...
tagger Posted March 7, 2016 Author Share Posted March 7, 2016 (edited) The 288 is a 1.4" driver, and the MR594 was designed for Altec's 1.4" drivers. I haven't measured the horn slot diameter, but I doubt it would be 2" at the throat. What is "c" in that equation? EDIT: The Altec Lansing spec sheet shows a 1.406" throat diameter. Edited March 7, 2016 by tagger Quote Link to comment Share on other sites More sharing options...
Chris A Posted March 7, 2016 Share Posted March 7, 2016 C = speed of sound in air, room temperature = 1132 feet/second or 13584 inches/second The "c" comes from the word "celerity". If the midrange horn diffraction slot is only 1.4 inches wide, then the cut-on frequency is about 5.7 kHz. Chris Quote Link to comment Share on other sites More sharing options...
Marvel Posted March 8, 2016 Share Posted March 8, 2016 All he can do is bite your head off or ask for an unreasonable amount of money Almost choked at that... Quote Link to comment Share on other sites More sharing options...
djk Posted March 9, 2016 Share Posted March 9, 2016 The MR594 is a CD type, and will require about 6dB/oct boost above about 4Khz or so. This can be done in a passive network for a 100dB (or less) woofer. Quote Link to comment Share on other sites More sharing options...
tagger Posted March 9, 2016 Author Share Posted March 9, 2016 (edited) The MR594 is a CD type, and will require about 6dB/oct boost above about 4Khz or so. This can be done in a passive network for a 100dB (or less) woofer. I assume if I crossed at 4 kHz this would not be necessary? How would it be accomplished in the ALK networks if I instead crossed at 5800 Hz? How is it accomplished in general in a passive network? Edited March 9, 2016 by tagger Quote Link to comment Share on other sites More sharing options...
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