Recommendation for DIY Altec A7 cabs

philajoe73

Member
2015-03-03 4:43 pm
Built a 2/3 scale reproduction of A7 cabinets designed for 10" drivers. Horn is a Heil AMT transformer with an extended waveguide. This project has been assembled from items I have accumulated, so not a big budget production. Nonetheless, I did pay careful attention to the plans I found online and am proud of the outcome. Problem is that the only 10" drivers I have are lower quality and designed for sealed cabinet. I have a 2 LR crossovers built for 1200Hz and 1500Hz that I have tried, but the mid-bass seems muddy and I suspect it is due to the driver not liking the vented design.

Any recommendations for a mid-bass that won't break the bank, yet provide better clarity? Amplifiers are in the 25W range.

TIA
 

Eldam

Banned
2012-09-13 12:25 am
France
Let's try...

The genuine design was made with a the 16" Altec 416-8B if I'm correct : it has a low resonance frequency but the cabinet didn't allowed to go lower than F3 80-60 Hz, maybe a little less in some rooms. So you must choose something close in the T/S parameters. Could a 2/3 sized cabinet will match is an other story, the load should match the driver you choose !

The Heil 1 is better upon 1k6, but maybe your horn will allow to us it lower : measurement will tell you !

The game with the FLH of the cabinet and the one designed for the AMT should be a smooth transition at the XO...

As for the wood : a thicker or another wood will not give you surely a better result !

Trained designers or experienced guys with the genuine A7 will have more accurate inputs !
 
Altec reproduction

Thanks, Eldam. I am beginning to see the lower end limitations you mentioned. I really wasn't looking for the earth to shake with this enclosure, but I find that I am having to heavily equalize for the bottom end to compete with the Heil. I know that the efficiency mismatch is causing a lot of this and my intent was to bi-amp this setup to overcome any mid-bass limitations. My hope is that there is a 10" driver better suited for this setup.

BTW, I couldn't be happier with the add on horn. Thanks to Jeff Poth for his inspiration. The AMT has always impressed me in whatever design I hear it in. They really are easy to work with and this modification seems to make it even more versatile.
 

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Eldam

Banned
2012-09-13 12:25 am
France
Some thoughts : you should use an active design to save money because it is hard to find the correct XO and slope : a 2 way MiniDSP may save your wallet ! And also allow to apply some EQ as making bi-amp easier !

You have also the possibility to tune the "port" with a moving piece of wood to close more or less this apperture in relation to your room !

Cabinets looks very nice :)

There are nice 10" Beymas or B&C drivers, Maybe JBLs are easier to find where you live ?
 
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Eldam

Banned
2012-09-13 12:25 am
France
I wisch I was able to make such cabinet !

You shoud list yours 10" and give your rear cabinet volume to allow experienced people here to help you (I'm not). I assume you have to find a 10" driver with a low resonance frequency and which climb a little to match the Heil horn !

http://www.beyma.com/products/lowmidfrequency/110WR308 ? For instance ?

Or a 10" Full range to make easier the XOs ?

You may find a 10" with the lowest FS, not low efficienty to get closer from the Heil AMT efficienty with a not too high Qts (<=0.5)...

The MiniDSP comes also with a possibility to make measurement with an USB mic for setuping & EQ; many threads on the net and MINIDSP site. There are some cheap Dayton calibrated mics with USB plug made for the MiniDSP system.

You need to measure first the Heil/horn combo t find the lowest crossable frequency ! Off and On axis...

That's the basics ! From there, experienced members may help you; but you give not enough datas yet ! The task may be rude as one choose firstly a driver before to make the cabinet, so I assume you followed maybe an existing DIY around a littlier Voice of the Theater ?

It's an original thread and design, I hope you will have help of members here...
 
The genuine design was made with a the 16" Altec 416-8B if I'm correct : it has a low resonance frequency but the cabinet didn't allowed to go lower than F3 80-60 Hz, maybe a little less in some rooms. So you must choose something close in the T/S parameters. Could a 2/3 sized cabinet will match is an other story, the load should match the driver you choose !

No, just was one of many drivers with similar specs used with the assumption that each app would be EQ'd as desired. The 803 was the original driver, which had a 50-60 Hz Fs, ~25 ft^3 Vas, ~0.2 Qts, which when coupled to a period correct amp with variable damping tone controls had an effective Qts up to ~0.38.

Regardless, a 2/3 scaled cab will have a frequency response, horn BW, cab Fb shifted up in BW, so guessing your Beyma might work well enough frequency response wise, though its Vas spec is probably too low to properly load the cab.

GM
 
GM, please excuse my parochial understanding of TS parameters. Could I affect the Vas spec by adjusting the vent size. Everything I read on this parameter discusses air compression. The only variable I have, as Eldam has mentioned, is the opening.
 
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Eldam

Banned
2012-09-13 12:25 am
France
Thanks GM for the inputs ! Hey your avatar shows a double drivers in FLH : everyone should have one like that :) ! Must rock in the mid-bass with fauelous transcient !

I heard a Kef Blade two this week-end, and I find it borring and with a poor mellow mid-bass... maybe the amp or the huge room in which it was ! Borring despite the good medium-treble !

I believe despite the difficulty, FLH or straight horn is the way to go if one has the chance to have cave, dedicated room for the upper bass to medium.... or at minima vertical array if direct coupling with the cone drivers !

Emminence should be a good path to follow as well ! Delta 10" ???
 
a coupe of years back(6 maybe) i designed a version with a 12 inch altec driver, i sealed the bottom of the wave-guide and used the bottom volume for a 12 inch sub
the altec driver was sealed and had a f3 of 80hz and the same a7 flare at 140hz
it was about 2/3 the size of the a7 and kicked *** as the sub was tuned to a f3 of 25 hz with a 300 watt bash amp, top was a 811 crossed at 800hz
presently its in a store in tokyo
i once saw a miniature a7 with a 5 inch fr driver and a 3/4, and one day i will build one for my desktop
 

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Ok.

This is really fairly simple.

Two key points to understand.
First, the A7 was designed as a PA speaker.
Second, the front loaded horn was designed to increase the SPL above the direct radiator's (the speaker's) normal output level so that it might nearly reach the LF limit of the horn above at about the same SPL.

Not hi-fi.
Never going to be hifi IF used per the original design.
Can't be.

So, skip the horn part for the moment.
You have a box of some volume. Figure out the volume.
Now you need to run box simulation programs and see what TS parameters will produce LF output down to some frequency you can live with.

My 2 cents worth says that is 40Hz or lower.

Next you need to figure out what relative sensitivity you need for that driver.
Of course you are fighting the parameters. Bigger drivers have higher sensitivity (all other things being the same), but have a larger VAS, which means LESS LF extension, in the same volume.

IF you are biamping - DEFINITELY a very good idea. Otherwise you will need to pad the HF section down considerably to meet the LF.

So now you have the balance points, VAS vs. Vb, and F3 vs Fb all vs. SPL.

Next you have to consider the effect of the acoustic response of potential driver candidates WRT to the effect of the front loaded horn. Usually this is a disaster for hifi.

You can compensate for the HF rise due to the front loaded horn in the x-over, of course defeating the whole point of front loading, but that's the price to pay for this box.

The advantage of the design originally is A) the rising HF response of the bass driver due to the front horn loading and B) the set back of the LF driver, permitting the acoustic center of the driver to be close to the acoustic center of the HF horn.

You don't have quite the same situation with the AMT as you would with an Altec 803 or 1003, or 805/1005 horn on top. Or even the 811 cast horn (ick, imo).

So you'll need to balance these factors to make this box usable.

I have done this box up, but used a JBL driver that was the modern equivalent of the one used in their classic 50's home horn. It had the benefit of a better VAS, lower F3 and so was loadable down to ~40Hz in the FULL size A7 box, without the open "vent" and with 4 tuned ports. Also I chose to make a hyper sharp cutoff low pass x-over which flattened the response of the horn rise. Then the client used a high power solid state amp (eventually...) for the bottom end. The result was fairly spectacular in a normal size room. We met the horn at ~300Hz. Higher up the problems become more difficult due to the polar response of the driver and the way the front loaded horn behaves.

short version on driver selection for the smaller box, you probably want a 12" driver to keep the VAS down to a manageable value, so that you can get some LF out.

Edit: You designed it for 10" drivers, so best start by looking there. Due to the design, assuming enough space you could
change the throat board and use a 12" IF you found one that simulated better. The choices amongst 10" drivers may be
somewhat limited, but there are still quite many to pick from.

Edit 2: forgot to mention Qt. I'd aim for higher than usual Qt for your enclosure. Maybe ~0.30 - 0.35 will work out well. Low Qt drivers will likely fight you with F3 and VAS - again the balance thing, plus watch the response above that ~100Hz point - not the idealized one that some mfrs show, the real one. :D
 
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Another thought. Would it make a difference if I fit a board into the lower (rear) section, angled a 45 degrees, which could reduce the box volume by 11L (6H x 6D x 19W) and maybe help with standing waves? Should get my Vb ~ 81L.

What do you think about a driver like this?
Speaker Detail | Eminence Speaker

this driver is NG. By dropping the volume of the cabinet, you lose the benefit of the volume available. Also the F3 is way way too high.

You want an F3 in a driver of this range of parameters an octave below the tuning freq of the box, so in this case for ~60Hz cutoff you'd want a 30Hz F3... (usually)

Drop the actual box volume - figure it out as closely as you can, deduct for the interior stuff, and then take off 10% anyhow - into a simulator where you pick the Fb (low freq limit) and dump in VAS/Qt/F3 values and see what you can find. Also watch that 100Hz-1kHz freq response CAREFULLY!!

_-_-