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Thread: A7 Port advice

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    Junior Hostboard Member Tax's Avatar
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    A7 Port advice

    Hi A7 Owners/Tweakers,

    Greetings from Sydney, Australia!

    I have recently bought a pair of Altec A7's consisting of 825 cabinets with 515B woofers and 802D CD's in 511B horns. The xo's are original N800-D units.

    The previous owner had the 511B mounted inside the A7 cabinet ala "Magnificents" with the bass flare on the ground. I have played around with the speakers in my room and prefer the speakers with the 511B mounted on top of the box and the bass horn sitting directly underneath it (what I call theatre or original style).

    I would appreciate your kind advice in relation to the dimensions of a baseline port I could start off with to fill the void of where the 511B used to be inside the box?

    My preference initially is for a rectangular vent. I read somewhere on the hostboard that GPA had recommended a 26" x 3.25" vent.

    Could I have the slot closer to the bottom of the cabinet (as in closer to the floor) or stick to the conventional middle of board placement?

    Any advice you can offer is appreciated.

    Thanks!

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    Senior Hostboard Member Elitopus1's Avatar
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    Re: A7 Port advice

    Mine is 10"x7.5". I read somewhere that 75" is a good number for the port. My cabs also had the horns stuffed in the bottom when I got them. I had to cut a piece of plywood for the port area because that piece was missing.
    The current port sounds much better than A7's I have heard with factory ports. Mine are tuned much lower. I would try other sizes and experiment but I like this setup too much

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    Senior Hostboard Member LowOhms's Avatar
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    Re: A7 Port advice

    Quote Originally Posted by Tax View Post
    Hi A7 Owners/Tweakers,

    Greetings from Sydney, Australia!

    I have recently bought a pair of Altec A7's consisting of 825 cabinets with 515B woofers and 802D CD's in 511B horns. The xo's are original N800-D units.

    The previous owner had the 511B mounted inside the A7 cabinet ala "Magnificents" with the bass flare on the ground. I have played around with the speakers in my room and prefer the speakers with the 511B mounted on top of the box and the bass horn sitting directly underneath it (what I call theatre or original style).

    I would appreciate your kind advice in relation to the dimensions of a baseline port I could start off with to fill the void of where the 511B used to be inside the box?

    My preference initially is for a rectangular vent. I read somewhere on the hostboard that GPA had recommended a 26" x 3.25" vent.

    Could I have the slot closer to the bottom of the cabinet (as in closer to the floor) or stick to the conventional middle of board placement?

    Any advice you can offer is appreciated.

    Thanks!
    Hello,

    I run 825s, 515Bs, and 802Ds like you. Got my 825s for free, last December. The drivers I got for free, two years ago, from audiophiles in Kansas City I had audio and triode amp- mentored in previous years.

    Find a port suggestion, many here will give you good input, use it, and proceed in this order :

    The first thing I suggest is to stabilize the stock 5/8ths thick 825 box,

    I suggest you do as I did, and put NO BRACING inside. The cheapest, easiest way to stabilize the 825 box, AND KEEP IT STOCK, is to add mass loading.

    We have a large hardware chain in the USA named Lowes, and in their Garden Center, they have " concrete patio tiles ", which are about 11 1/2 inch square and roughly 1 3/4s inch thick. Each weighs about 17 pounds, cost about $1.48 a tile. I added about 300 pounds of these tiles to each enclosure, on top, mostly at the rear, and the AMOUNT of weight was selected by MY ear, till I was satisfied. Total cost, maybe $75, and it is REVERSIBLE. Do this first. Tiles come red and gray, I chose reddish ones.

    The second thing I would do is optimally wire the amp to crossover, the crossover itself, and the crossover to the drivers with FAR better ( heavier in most cases ) wiring, to improve your system's transfer efficiency. See details of what I did, wiring wise, in my post in this thread :

    URL :

    See my post #8, regarding decent wiring, in this URL : ..Proposed Parts List for Model 19 Crossover

    The 800HZ 12 dB crossover is what I prefer ( A7-800 ) over the 500 HZ frequency. But the third thing "I" would like to suggest is you re-do the crossover with FAR better caps, better wiring, and maybe better inductors. One cap that NEEDED to be in " my " crossover for the VOTTs to display magic pleasing to me, was a DynamiCap 4.0 uF E series, in all four positions, as part of a "bypass" to an other special high-current, uber low ESR cap I used to get the 8.62 uF Butterworth / 16 Ohm crossover cap value. I will be happy to share the cap specifics privately.

    After you do all three suggestions FIRST, you can play with port size. NO rush. The correct size YOU and YOU ALONE select by ear. Its a function of many factors, but YOU alone can decide that by listening.

    If you have any questions, or need help, email me as I check that often [email protected]

    What sotra nice amp you going to use on them.??

    Since April, I have been working on a SET DC coupled Type 45 amp, 1.5 Watts or so, and whatever I did in the amp-build, was heard, LOUD AND CLEAR - on the speakers. Suggest a two stage 2A3 amp, DC coupled, as a good design. I can help you, if you DIY, with that.

    Consider these three suggestions, then YOU need to choose the port size for your system, by your ear !!!

    Have fun, I am.

    Jeff Medwin Low Ohms
    Last edited by LowOhms; September 12th, 2016 at 01:13 AM. Reason: added URL

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    Senior Hostboard Member Elitopus1's Avatar
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    Re: A7 Port advice

    Jeff:
    I'm curious how you have your horns mounted. With all the pavers on top it seems like there would be very little room. You mentioned before you run some EH500 clones.

    I put some extra bracing in mine and have considered adding some more support to the flares. Mine are the newer MDF 828 cabs. I think if I added weight on top it would crush them

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    A7 Port advice


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    Re: A7 Port advice

    Jeff I don't see any reason to keep an 828 cabinet "STOCK". unless it's in near factory condition with original drivers.
    you can add / change the portboard to modify the port area without modifying the cabinet. and changing the port is by far the most effective way of getting more out of an 828 cab. the next would be putting effective bracing inside the cab. along with foaming or packing the horn flares with sand to keep the
    hornflare buzzing to a minimum. (the latter adding "mass loading")
    Last edited by Phil-G; September 12th, 2016 at 07:53 PM.
    Sonic Barbarian

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    Senior Hostboard Member mah's Avatar
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    Opinion is only as valid as its verifiable supporting evidence.

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    Junior Hostboard Member wweigle's Avatar
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    Re: A7 Port advice

    I modified my ports to 10" x 6". Can't remember exactly what my rational was for choosing this size. At the time I was running some JBL D130s. Currently using a set of 604Es as woofers. Sound pretty good to me.
    Attached Images Attached Images

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    A7 Port advice


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    Re: A7 Port advice

    nice mess of records.
    Sonic Barbarian

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    Re: A7 Port advice

    Opinion is only as valid as its verifiable supporting evidence.

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    Re: A7 Port advice

    Quote Originally Posted by Phil-G View Post
    the next would be putting effective bracing inside the cab. along with foaming or packing the horn flares with sand (the latter adding "mass loading") to keep the
    hornflare buzzing to a minimum.



    While I'm a big fan of mass loading when required, rigidity can be gained 'quicker' by bracing, then selectively add mass as required, so brace the panels to both raise their resonances and cross brace as required to keep it from 'breathing', then experiment with added mass and once it's determined how much is enough, add it in the form of a [massive] removable decorative top. Large potted plants were popular and the foliage made for great HF diffusers.


    Once the cab's Fs is pushed high enough to get it above its pass-band [~2*Fs/effective Qts], then in the case of these horn cabs they are already heavy enough if they were just boxes, so all that would be needed is a double thickness top plate and I imagine one reason why HF horn 'sleds' were used initially; but the horns have a higher pass-band, so easier overall to push these resonances down below it, so stiffen them with bracing to handle the acoustic loading, then add mass to quell this higher resonance pass-band.


    What folks can't seem to 'wrap their mind around' is that while rigidity increases with the cube of thickness, bracing [and better yet, starting with a very rigid material] shifts the cab's resonant pass-band up in frequency and since acoustic power rolls off exponentially [1/f], it's increasingly easier for the material's mass to damp it, ergo external damping requirements are reduced also.


    Having done several pairs of 825 cabs and one pair of plywood 828s decades ago, I know that when properly braced and horn cavities and HF horn are stiffened/damped with a high mass such as sterile sand, Portland cement or similar, no further mass loading is required. Of course tar filled horns are more than sufficiently rigid/damped, though the heavy drivers need some help.


    GM


    Loud is Beautiful if it's Clean! As always though, the usual disclaimers apply to this post's contents.

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