How high one can x-over the A-7 ?

[aditya]

Hi everybody,

I am a new member, and I am really happy to be amongst so many Altec-fans.

I just love the Altec-A7s for their looks, and open-sound. I had many oppurtunities to own them, but never went ahead due to its size & space requirements. Recently I have built a house for myself, and being an audio-fan, the A7 fire has kindled again as I now have a 39'x20' space for my personal use. But the irony is that the A7s are no more easy-to-get types on the 'used' side (and I will not go for the new ones). So I have decided to build them myself. I am into DIYing as a hobby. So this will be okay for me.

I have collected most of the datas of the A7 thru my research in the past month or so, and have the confidence that the outcome will be a good pair of boxes. But I find two questions un-answered.

1) In the several A7 box-plans that I have gathered so far, I find that......in some designs, the cavities behind the horn are exposed & are a part of the space behind the woofer, whereas in others they are completely sealed off.

2) What is the highest x-over f value for an A7 ? I know the best to be around 500Hz, and anything beyond 900/1000Hz will bring in modulation distortion to the midrange. But my predicament is that I don't have the horns with me, and I am also not sure when will I get them. But I have a pair of horns (British make) which though very good sounding, will only x-over at 2000/2500Hz or above. What is the f-response pattern of the A7 with the woofer only ?

Many thanks in advance

Aditya

[gearfreak]

Welcome!

First off, the A7/A5 will be an 828 or 825 cabinet for the section you are referring to. Some will refer to A7/A5, some more specifically to 828 or 825. Consider including the terms in your searching lexicon.

Locating a copy of the 'small v.o.t.t.' paper (Jim D.) will be a good starting point for you.

Regarding the cavities, you'll likely find some information by searching regarding the 'horn flares'. 'Foam' or (expanding) 'Foam Filled' will likely bring back some results as well, as a typical conversation is with respect to whether or not to a) seal them off b) fill them with foam/sand/cement etc.

In general, earlier editions did not seal off the horn flares, more recent editions do. You may find the opinion posted that creating a sealed version is a preferred platform, better anchoring the woofer/providing some benefit. Most of the discussion will likely turn towards the woofer model that you are using and it's 't/s' specs, and how sealing off the flares will affect the box volume, and thus tuning. You will no doubt be interested in searching for threads related to 'optimum' (in the eye of the beholder) port size as well. Look for the thread titled something to the effect of 'A7 port size that old saw' for a start. Member GM seemingly tirelessly provides input on this sort of topic, so posts by him with 'port' in the text may be a good place to start searching as well.

Do consider the bolt circle size of your selected woofer when looking at plans. There are small and larger frame versions of many altec woofers. Larger generally being more recent. The 515 (subtype omitted) may be a specific and particular exception.

[gearfreak]

An afterthought...

I believe that the woofers in many later editions (515-8G) are spec'd at topping out at 4khz. I know that the N-1285-A (or B?) xover in mine is selectable 500/800/1200. I (I) assume that they'd intended it to be used @ 1200 as such. You could easilly argue against that assumption though.

You might find mention of dispersion or polar response charts in the same doc that spec the 515-8G (and other later variants, like 'HP' (?) ) where e.g. they began listing efficiency WRT the woofer/cab combo.

[Old Guy]

There is a lot more to crossover than the highest frequency a woofer will do though.

As a speaker goes up in the frequency it is reproducing, the pattern narrows.

This puts more energy on axis, and accounts for how a large speaker like a 515 can go that high.

The problem is, that response is restricted to a very narrow "sweet spot" right in front....off to the side it will be way down...

Unless you have a very deep and narrow room you will have a problem.

I would not go higher than the 1200 Altec used. Honestly, with a 15 I think you may even be compromising at that point. The 1200 was for reliability not improved sound...HF units will live longer at 1200.

[aditya]

Many thanks for the responses.

My points were....
.....by shunning off the cavities behind the woofer thru the use of a full baffle, one ended up removing nearly 3000 cu-inches, which is approx 1/10th of the total box volume. Then how this was adjusted by the Altec designs as no other changes were mentioned ? The only changes I find are the LF drive units themselves, and probabely that is the answer, and for my use I shall have to do the tunning once more and adjust the port accordingly.

.....yes it is true that for a clean & un-muddied midrange sound, there is no alternative to a x-over point around/below 1K. Actually if possible one should always allot the 350-3500Hz bandwidth to one single driver only. And when the LF driver is a 15" type, all these become more important. But my point was.....what is the f-respose of the A7 with only the LF driver fitted ? I mean how much it differs from that of a 15" mounted in a normal ported enclosure without a horn ? Does the horn design of the A7 attenuete the HF response beyond a certain point, say 1200 Hz. ? Can somebody throw in some experience here ? For a tight listening area, one can xover a 15" even at 2500/3000 for that matter.

Thanks
Aditya

[whitebroncoii]

I know this will stir up the hard-core guys out there, but, here goes . . .

The A7 at 500Hz was designed for cinema applications and not for near-field monitoring. The cinema need is to put the speaking parts on the end of your nose 40 - 50 feet from the screen. Near-field the A7 at 500Hz is very mid-range heavy and many younger listeners will immediately think they are lacking bass and extended highs. "Music" versions of the A7 used 800Hz for a crossover. I experimented with a N1285-8A (selectable for 500 - 800 - 1200) and finally selected 1200Hz for my listening space. The decision is based on my room, the playback material, and the high levels.

IMHO your crossover frequency is a choice you make for yourself.

[Old Guy]

I know this will stir up the hard-core guys out there, but, here goes . . .

The A7 at 500Hz was designed for cinema applications and not for near-field monitoring. The cinema need is to put the speaking parts on the end of your nose 40 - 50 feet from the screen. Near-field the A7 at 500Hz is very mid-range heavy and many younger listeners will immediately think they are lacking bass and extended highs. "Music" versions of the A7 used 800Hz for a crossover. I experimented with a N1285-8A (selectable for 500 - 800 - 1200) and finally selected 1200Hz for my listening space. The decision is based on my room, the playback material, and the high levels.

IMHO your crossover frequency is a choice you make for yourself.

I have a strong tendency to agree with much of that...the A7 is too middy near field. And 1200 does tend help that.

I'm just so use to running active and being able to play with crossovers....that a hard choice isn't necessary...

I've noticed the opposite to be true for me with ported boxes though...I prefer the lower xover generally.

YMMV= your mileage may vary.

[aditya]

Hi,

Thanks to you all......
I think I have got the answers now.....

As per "gearfreak's" (many thanks) suggestions, I searched with the key-words, and I was amply rewarded with lots of postings & threads.

It seems that the 825/828 boxes are quite flexible enough to withstand minor volume changes, and that at the end they can always be re-tunned accordingly. Closing the cavities behind the horns will not only increase rigidity, it will also augment the reflex action for the port, better.

The woofer-box combo has a hump in the f-curve at around 180-220 Hz zone. But it also kind of extends well up to the 2K-3K zone and beyond, and the energy level also is not bad. So I think I need not anymore deffer the whole project of building the boxes till I get the HF units. I can always temporarily use the existing ones. May be, the sound of high-mid and above will be a bit axial, but then my room also is no theatre. At least I will hear thru these great looking boxes !

Hope I am right.

Many thanks again
Aditya

[Mustang Marvin]

Since you are a DIY guy and so are others here, I have a question. Why did Altec use 5/8 inch plywood in their A7's and not 3/4 inch? So many times I hear of reenforcing the boxes on the original 825's and 828's. The most generally accepted version installs 2x4's inside the boxes. The 828's have reenforcement on the backside, so do I need to reenforce my 825's. I don't want to fool around with the original design if it's not necessary. I could use a little more bass though. I still love those A7's...
Thanks

[Audio_by_Goodwill]

...... Why did Altec use 5/8 inch plywood in their A7's and not 3/4 inch?

Some were 3/4"...... the three that I had were.