HP filter for Altec Lansing 299-16A?

[critofur]

I'm not at my PC where I have various speaker software... I will be home briefly later and pickup some parts.

I'm looking for a high pass filter to protect vintage Altec Lansing 299-16A compression drivers mounted on huge horns.

My first guess is a ~ 20uF cap. Just need something basic that will protect these from blowing without being too detrimental to the sound...

I'm helping out an art school graduate student who'll be using these for sound in an "installation"...

[Pano]

20uF should be just fine. That's a 1st order at about 500Hz. You don't want to hit it too hard with just the cap, but it will offer protection, for sure. If you are using some other crossover, remember to add in this filter. Should give you approx a 3rd order acoustic at 500Hz.

[critofur]

Well - he was running them with NO protection off his Sony receiver and said they were REALLY loud, then the voice coils blew.

The 20uF should give them WAY more protection than just feeding them full range, eh?

I'm just wondering what kind of bump I'll see at FS, might need a notch filter - or, maybe just adding an ~ 8 Ohm shunt resistor after the cap will suffice. Don't have time for any software modeling for the next week or two with finals coming up...

[critofur]

Oh - I should mention, I was not THERE when he blew them, of course :P

[GM]

Originally Posted by critofur

Well - he was running them with NO protection off his Sony receiver..........

..........maybe just adding an ~ 8 Ohm shunt resistor..........

Assuming an expo horn, then choose a cap based on 1/2 the horn's Fc, so for a 16 ohm driver on a 500 Hz horn choose a cap for a 250 Hz 1st order XO = 40 uF.

As a rule-of-thumb for a very low output impedance, using a matching damping resistor is desirable, or 16 ohms in parallel with the cap in this case.

GM

[Pano]

Yep, what GM said.

[GM]

Sadly, not the capacitor part. In my haste I responded from rote, i.e. assumed a distributed 70 V line sound system. For NO protection, i.e. 1st order XO, choose based on 4x Fc or 5 uF for a 500 Hz expo horn since we want the driver down -24 dB/125 Hz.

GM

[Pano]

That seems high.... But it will keep him out of trouble, for sure.
Also think the parallel resistor is a good idea.

[kevinkr]

Anyone looked at what replacement diaphragms cost for these drivers? I'm assuming that relatively inexpensive modern drivers could fit the bill, and these expensive vintage drivers could be put on eBay as is and easily pay for a modern replacement driver.

Unfortunately here ignorance is not bliss - an expensive mistake for sure.

Data sheet here: http://www.altecpro.com/pdfs/vintage...F%20Driver.pdf

Diaphragm Data Here: http://www.altecpro.com/pdfs/vintage...TER%20LIST.pdf

Great Plains Audio for replacement diaphragms: http://www.greatplainsaudio.com/parts.html

Note that the Pascalite diaphragms originally used in these drivers are not generally readily available.. (No longer made apparently by anyone.)

Here is what is currently available on eBay, probably cheapest option, and it might be OK: http://cgi.ebay.com/ws/eBayISAPI.dll...fvi%3D1&_rdc=1 I can't vouch for the quality but for $110 for a pair as compared to the genuine item at maybe 4X - 5X this cost, it's reasonable.

[Pano]

Yep, frams are not cheap. Even the knock-offs.

I was interested in what a single cap might do and also cap+resistor, so I ran a few graphs based on the impedance curve of an Altec 291-16K on a 300Hz horn.

You'll see various values and the nearest target slope. Remember, this is electrical, not acoustical. Some values make a nice shelf.