Loading caps

[bfish]

The loading caps Altec used on small-format MI drivers (i.e. 807, 808, 909) have been described as a protective means of limiting of diaphragm excursion. Does this mean that the diaphragm actually strikes the inner surface of the loading cap upon maximum excursion, or is it more like the name implies, in that the diaphragm is acoustically loaded by the small amount of air trapped between it and the loading cap?

[Old Guy]

Hi Bfish-

My understanding is it's the trapped air. I would think the diapraghm striking would be audible, and also damaging to the diaphragm.

Actually the effect of the loading caps can be quite audible. To prove it put a loading cap on the driver of one channel by leaving the other off. The rolled off high end because of the loading cap is readily apparent.

Anyway my 2 cents.

[GM]

Greets!

The latter, to the point where on the 511 it rolls off below 1200 Hz, hence Altec's eventually switching to 800 Hz XOs when using this combo to get a 3rd order acoustic XO on it, otherwise there's a big suckout in the response if a 500 Hz XO is used.

They knew this, yet still initially offered the 511/807 or 808 with the 500 Hz XO. The marketing dept. must have had access to medical grade 'products' back then.

GM

[Russellc]

I can think of no reason to use the loading caps in a home hi fi application. I remove them and install the felts, always sounds better to me. Some where or the other I have a box full of loading caps I have removed.

Russellc

[Russellc]

I can think of no reason to use the loading caps in a home hi fi application. I remove them and install the felts, always sounds better to me. Some where or the other I have a box full of loading caps I have removed.

Russellc

[bfish]

Old Guy,
That's what I'd always thought, but I puzzled over the word 'limit' as possibly being literal. By the way, Happy Belated Birthday! Will you be changing your monicker to "Really Old Guy" anytime soon?

GM,
You answered my next question before I could ask it. Maybe marketing and publications shared more than employers....

I'm assuming for the cap to be effective, a gasket of the correct thickness, which provided an effective seal would be required. Just curious if a thicker gasket which allowed an increased trapped volume would lower the rolloff point while maintaining some form of protection?

This is all related to my curiousity regarding the performance of SF drivers used without any horn at all except for the short integral one, and how a loading cap might effect it. In other words, would a loading cap allow a lower XO point for a given slope, or is that strictly a function of the horn?

BTW, I've gathered all the tubes your buddy needed, but can't find his addy to ship them to. Could you please send me that info?

Russellc,
That's what I've always done too, but had always wondered how the caps really worked (or didn't).

[Old Guy]

Hi Bfish- thanks for the birthday wishes.

I notice there's another poster named older Guy. Maybe I'll catch up to him yet!

[bfish]

It's depressing to know some of my greatest fun is had when I'm gigging those more-over-40 than myself. I haven't started reading the obituaries daily.......yet.

Seriously though, I can't imagine how stupid(er) I'd be today, were it not for the time that several 'Old Guys' have taken to share their experience and wisdom with me. A Toast to "Old Guys" everywhere!

(RIP, Jack Brown 1908-1982).

[GM]

Greets!

Yes, it uses a very dense rubber gasket, and yes, you can tune it by increasing the gasket thickness. Me, I just removed the gasket and spaced the loading cap out to the practical limit of its screws to make a two stage loading scheme, so no need to swap screws or add the felt lining.

WRT using the driver for tweeter apps, ideally the gap needs to be much closer to the diaphragm to reactance annul it, but never having done any experimenting, don't have a clue how close. Anyway, with a short horn, the XO will set its roll off/protection and as Old Guy noted, it's best to remove the cap for best HF extension without EQ. The W.E. driver it's derived from initially used a large, well stuffed rear chamber to further 'soften' its roll off to extend its HF, so stretching some fine nylon material over the diaphragm and stuffing the rear cap with wool might improve the HF even more.

I don't recommend using the driver without a rolled over flange though since its sharp edge transition causes refections back to the throat, something that W.E., Altec either didn't know or didn't consider it a big deal, and it's not for PA/cinema apps, but for HIFI it is.

This brings up another problem with Altec drivers that W.E. addressed and for whatever reason, JBL (the man) didn't copy, is that the mounting joints he chose cause these same reflections in spades if not perfectly aligned and smooth, which with the thick cardboard gaskets and the so-so horn manufacturing QA, really makes a mess of its HF response. I couldn't believe how much better a pair of max tweaked 511s sounded after I did away with the gasket and align honed them with the drivers attached.

I emailed the info back on 3-29, but will resend.

GM

[bfish]

For the transition smoothing, are you talking a fine-wheeled dremel/good eye, or machine shop work? If the 800s' transition was bad, the even-smaller opening of the 900s when coupled to a std. SF horn would be even worse. Would a DIY, short "spacer", with its' opening having the same expansion rate as the driver, and thick enough to match the horn ID be advisable, or better to build/buy a matching horn?

Go ahead and call me an Old Guy too, the addy was there all along and I didn't see it. That situation should improve, as I ordered new glasses last week 'cause my buddy was finding more mushrooms that I was!