Multicellular vs 511 B horns etc. Questions ...

[trollgurka]

Hi guys!
I built a nice pair of A7s this summer from heavy birch plywood. After listening to them for awhile I'm pretty satisfied with the sound. They have a richness and a musicality I really love! The bass and midrange is truely among the best I have heard.
There are two things though, that could be improved.



1. The treble has a tendency to feel a bit uncomfortable, a bit "ringy" and on the lower side of the treble spectre. Is there any way, except using eq, of hightening the really high frequencies? (Or dampening the "low highs")
I now use 511-B horns with 908-88 drivers.

I have another problem. Its almost sexual. Every time I see a multicellular horn on e-bay I fall in love because of the looks. I have realized from my own life that beauty isn't everything. (I tell my mirror so). I know nothing of the multicellulars. What do you think of them? What are the differences when it comes to sound. Would they improve my A7s?

[Tom Brennan]

Problem #1 can be helped with a compensation network that pushes down the mids, therefore extending the highs and improving tonal balance. Your 908s use symbiotic diaphragms and don't have quite the extension of aluminum diaphragms as used in 902s, aluminums may help.

There is much discussion of such networks on this forum and on the Altec users forum, they are very simple.

As for multicells; I like them but they're large format horns and I used them (1005s) with 288 drivers. Together they sound better through the midrange than 902s on 511s. However I used the 288s and 1005s as a team, I don't know that a 902 on a 1005 would sound better than a 902 on a 511.

[trollgurka]

Thank you! I tried to get the hang of that discussion but I must admit I didn't get the hang of it. I'm neither very technical nor experienced with this forum. Excuse my ignorance, but is a compensation system available to buy somewhere or do I build it myself? What does it consist of?

[Steve Schell]

It is true what Tom says; a proper network can bring up the highs and make even a large format driver like 288 sound great without adding a tweeter.

Most any compression driver will have a power response (total power out) that falls by 6dB/octave above 3kHz. or so. On a horn that disperses highs widely like a multicellular, the top can sound dull. Fortunately, the efficiency of compression drivers is so high that they have to be padded down several dB to match the bass section. If that padding is bypassed in the highs, it effectively brings the top end up and restores flat response to a significant extent.

A sophisticated way of doing this is to use a "tank circuit" that depresses the midband response but lets the highs run free. Jean Hiraga detailed a crossover that accomplishes this in the Altec A5 system, available here:

http://web.archive.org/web/20050206015233/www.soundpractices.com/images/HiragaA5.pdf

A less sophisticated way of doing it is to use a simple bypass cap that routes around the midband attenuation in a passive crossover. A 1, 2 or maybe up to 5mf cap (depending on driver impedance and taste) bypassing the midband attenuation will pull the high end up where it should be.

The 500Hz. Altec multicellulars and large format 288 drivers are a significant upgrade from the small format drivers and 511 or (egad!) 811 horns. Best are the early tar filled horns like 805, 1005 and 1505, but even the later cheaper construction B style horns will easily outperform the small format stuff.

The cast aluminum small format horns can be greatly improved with damping, as they do ring like crazy. Several ziplock bags full of sand placed on top provide a cheap and reversible improvement; several layers of latex paint loaded with sand may work even better, though it is not reversible. Another tweak you might like is to remove the rear covers from the compression drivers, though the diaphragms are then exposed to the cruel world and its hazards.

[trollgurka]

Thank you very much both of you!
I very much appreciate the possibility for a novice like me to get this kind of help.
I will have the Hiraga crossover made for me.

[RKLee]

I love the looks of the multi-cells also, I think they look so cool. But at the time I built my speakers I couldn't afford the 288 driver and horn. The 288 and multi-cell horns back in 1975 cost the same as the JBL 375/2461 driver and horn, way out of my price range.


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The 500Hz. Altec multicellulars and large format 288 drivers are a significant upgrade from the small format drivers and 511 or (egad!) 811 horns. Best are the early tar filled horns like 805, 1005 and 1505, but even the later cheaper construction B style horns will easily outperform the small format stuff.
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.When I built my Altecs, I used the 811 horns because they were less expensive than the 511s, and didn't take up as much room. My home built speakers were way better sounding than most factory assembled non-Altec or JBL speakers.