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Thread: 755E mating with tweeter

  1. #1
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    755E mating with tweeter

    Hello. I have a pair of nice 755Es. I have had a pleasure listening them in open baffle for a while and I realised I really miss the top half of the octave. I do not know how high my ears will take me, but I can still hear the high pitch of CRT tubes just fine (and it's annoying too).
    755's are nice and all, but I need a tweeter, or all my jazz will sound oily...

    What to use?
    Horn (closer pattern match)?
    Textile dome (velvety sound)?

    How high to cross? Do I leave the 755 rollof naturally?

    How about the bottom, do I attenuate the lowest freqs in "open baffle"? Can I damage the drivers if I don't? Should I put them in a box? Sealed? Vented? ported? TM? TQWT?

    I just have to make justice to these babies!

    Thanx for any suggestions...

  2. #2
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    What exactly is a 755E? Do you have pics/specs?

    John

  3. #3
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    755E is the last in the famous series of 755 fullrange 8" speaker drivers. Western Electric started with it's fullrange driver, which later became 755A with Altec Lansing and was produced in the early 50'. It was later on replaced with 755B *(very briefly) and continued with 755C (alnico) and 755E (ferrite) speaker drivers called "the pancake". The C and E versions extended their bass an octave lower, but somehow "lost" the magic in the mids and highs... For whatever it's worth.. the C and E versions posess a magic, austere quality of sound, sweet and laid back, tammed but full of expression.

    755's are considered some of the best 8inch drivers ever produced on this planet.

    They all share a wide frequency response, have a high Qt and a (C and E specially) have a midbass hump of about 3 db in the range from 90-120 Hz due to high Qt.

    755E will play hapily from 60-12000 Hz and has specs from 54 (fr) to 15khz, I found that there is some enegry radiated at 16khz, but it seems like a resonance of some kind and is not usefull (too much).

    One of my drivers is slightly brighter than the other in terms of timber, but then again, I also see different shades of colors with each of my eyes, so this might be an overshot, especially because both drivers measure 6.8 ohm exactly and are nicely mached.

    While I am at this, I might as well ask... one of the dustcaps has a slight dent, which does nothing sonically, but looks ugly... Apart from putting a pinhole in the dustcap, is there an other way about it?

    Anyway, have a look at this:
    http://members.myactv.net/~je245/755survey.htm

  4. #4
    Dang. Amateur speakerdave's Avatar
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    Well, I'm not the expert that you should get talking to you, but since no one else is jumping in here I'll try to get things started. My single connection with your dilemma is that back in the day I owned several of the 755a's and recognized their quality, but really did not have the electronics to appreciate it fully.

    The pattern you need to match depends on the crossover point, so the two need to be considered together. But since I would assume you are not trying to turn the 755 into a woofer, but rather want to use as much as possible of its very natural midrange and treble, pattern matching may need to take a back seat. Can I assume you are only thinking of cutting the tweeter in at 10k Hz?

    I've read reviews of a successful speaker that uses a ribbon to supplement the Hartley 220 msg driven full range. That might be one thing to consider. I am presently working on a similar project with the 220 msg and a quad of Janszen panels on each side, but have no results to report yet. The Janszen panels are a little hard to get. Somewhere in the future I'll be doing what you're working on with the Stephens 80FR, so if you get some good results with your experiment I hope you will report them here.

    David

  5. #5
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    Hi OutThere,

    - I'd start out trying the 755 in a 2 cu' sealed enclosure . This was W.E.s original recommendation and I don't see any reason to question it .

    - Based on the following response studies for the older W.E. 755a , I would add an inductor to the 755 to start to flatten out the highs from 5K upwards .
    - The idea to first flatten the response of the 755 may not apply to your "E" version if it is more linear than these "A" graphs . If no"bump" above 5K, you'll need to use a smaller inductor to move the crossver point upwards to meet the tweeter point.

    - You can see through the magic of image manipulation that this addition of an inductor to the woofer limits it's useful HF response to about 9K .

    - So I would bring in a tweeter around that point . Metal/Mylar/Silk/ or paper ? I don't really know what would match up best with the 755.

    - Personally, I would look for a tweeter with a 92 to 93 db rating with nice response from 8K and up . Audax makes a whole range of nice sounding tweeters. Audax publishes specs for their products which is a necessity in this exercise. They usually include AC impedance curves which are also necessary, to compute the correct value of capacitor .
    - Another idea is to find a nice set of Altec H3000 , mylar-diaphragmed horn tweeters. If you want to preserve the vintage concept of what you've got / this would be a good approach. I've included a pic of a set of ultra-rare H3000 horn tweeters, of the style using a radial horn-bell . The "much" easier to find H3000 are the type that look like mini multi-cell horns. I haven't heard either to offer any sonic advice .
    - BTW ; 92 db efficiency for a tweeter is definately pushing the limits and is more the exception than the rule .. *** ie ; = Premium Pricing ( assuming smooth reponse ) ***

    - Given those 6.8 ohm DCRs' you've just mentioned and the fact that most woofers AC impedance rises with frequency , you should run AC impedance studies on your 755Es. I imagine that by the 5 to 10Khz, the AC impedance is most likely, well over 10 ohms .

    - A 6000hz F3 point for 8 ohms = @ .21 mH
    - A 6000hz F3 point for 10 ohms = @ .27 mH
    - A 6000hz F3 point for 12 ohms = @ .32 mH
    - A 6000hz F3 point for 16 ohms = @ .42 mH

    - You can see why , it's important to get some "real" AC impedance curves .
    - Assuming the unavailability of these impedance curves you are stuck finding the best fit "empirically" ( trial & error ) . Buy a bunch of coils of various sizes and try them out.

    - I'm aware of some fellows who have used Altec/W.E. 756a 10 inchers with Fostex T90 tweeters . ( here's one pic ) . I imagine this approach was taken because the 10" is a couple of db more efficient than a 755. Those extra 2 db put the 756 woofer out of the realm of ever matching up to a dome type tweeter .


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  6. #6
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    Earl K-

    You rock.

  7. #7
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    Thank you very much for this info Erik and speakerdrive.

    I would love to get my hands on H3000, but alas, chances of that driver in EU are slim. I got my 755Es from Denmark and they are very clean (if it weren't for a little glued tear in one of the cones and the sodding dent in the other dustcap). Getting the H3000 is impossible (and perhaps too expensive if I consider the bargain price I got 755Es for...)

    I live in Slovenia EU and Audax is a no-show here. I could order it through madisound, but the price of shipping, tax and customs... not worth it.

    Audax TM025F1 textile dome or PR125T1 horn with 5db attenuation would be a good choice. I was thinking in the lines of Visaton DHT9 (http://www.visaton.com/english/artikel/art_156_1_7.html) with 6db attenuation and a 6db/octave gradual blend with the 755E driver... Or perhaps Visaton DSM 25 FFL (http://www.visaton.com/english/artikel/art_166_1_8.html) without the attenuation...
    Te reason is that Visaton I can get locally and Audax I cannot. Maybe Fostex FT-17H would be a good choice too... but again... I have to order it from Germany...

    Because the information on 755E is scarce, I would not like to fiddle with a crossover. I'd even let the midbass hump there, or perhaps try to use it to aid in baffle step correction... naah... perhaps the hump is too low... have rethink that one...

    For now I am leaning towards Fostex or Visaton horns, a simple attenuation and 6db roll-off controll.

    Do you think I am oversimplifying?

  8. #8
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    Quote Originally Posted by OutThere
    Do you think I am oversimplifying?
    - I'm not sure what you mean by your question / but I would keep the crossover network simplistic . Ie ; a coil on the 755E and a cap ( maybe an Lpad ) on the tweeter of your choice .

    - I suppose when experimenting like this in the matching of old & new components , it's a good idea to keep costs down . That means buying local .

    - FWIW ; here is an older Audax tweeter that's worth a scruntiny. Having the tweeter have a lower AC impedance ( than the woofer ) means you can get a couple more db out of it ( from the extra current delivered ). This can help in balancing two different efficiencies. This also means you can also look at the specs for a bunch of different automotive tweeters . Car tweeters are typically 4 ohms and are usually built quite robustly .

    - Also , here are the FR plots for the two Visatons that you've mentioned .
    - The middle pic is the dome tweeter, while the last pic is the horn-tweeter.

    - The world is awash with Tweeter-Horns that are driven by plastic/mylar diaphragms. If they mostly have reponse curves like the horn-loaded Visaton / then you have more choices to consider .

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  9. #9
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    Thanx again.

    But consider, that the nasty thingamabub at the end of visaton's horn response is between 20 and 40 khz... Something most tweeters don't even think about...

    Still pondering.

  10. #10
    pelly3s
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    if you dont mind spending a couple bucks the EV T-350 is an impressive tweeter. 105dB efficient and very smooth sounding ive seen a couple of different rigs with 755's and t-350's

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