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Two things first. I am not the Marshall who drew these plans and I do not know if they are accurate. I am posting the link because it will interest anyone like myself who is about to construct cabs for an A7 pair.
The site is also of general interest for Altec fans -
http://mfk-projects.com/vintage_altec.htm
Happy building,
Marshall.
A follow-up to my above post. The drawings do not show the full width speaker mounting baffle board that is used in the 828 cab. I am not familiar with the 828B model so cannot say whether the drawing is correct or not.
Marshall
Awhile back there was some discussion/debate on the different iterations of the 825/828 cab and someone posted that the full width baffle sealing off the horn cavities debuted in the 1981 828G.
Notes for anyone contemplating DIYing this cab:
1) For HIFI/HT apps it ideally needs to be made with ~3/4" (19 mm) Baltic Birch, Appleply, or void free marine grade plywood to minimize its bracing requirements.
2) It needs more bracing everywhere even when using the recommended plywood, but AFAIK there's no drawing available. Even though this cab is so outdated it needs to be relegated to the museum IMO, it's still popular enough that a bracing drawing is on my long DIY 'to do' list if no one beats me to it.
3) IMO only cabs loaded with either 803 series, 416A and early 416B drivers or re-cones known to have similar specs should use the narrow baffle board; all others should either use the full width baffle or fine tune the cab's net volume (Vb) as required if not driven with a high output impedance amp.
4) If the horn cavities are blocked off, then fill them with expanding foam (Careful! You can deform/blow out the flares), sterile sand, kitty litter (my preference) or similar to keep them from drumming.
5) For both bracing and sealing issues, I recommend using the small rear access panel.
6) I recommend experimenting with tuning the vent to a much lower Fb. FWIW, some folks prefer a ~75"^2 (483.87 cm^2) opening, but this is so room/position dependent that I believe it should be used only as a starting point. The main thing is that the cab must be airtight or all your efforts will be a waste of time and final tuning will be higher than you think if you're not actually measuring its in-room response.
This is all that comes to mind at the moment, but feel free to add/whatever to it, then let's turn it into a 'sticky' we can refer to.
GM
<font size="2" face="Verdana, Helvetica, sans-serif">Your correct the drawing doesn't show the wide baffle board. However when it came time to build it, the pictures show he went the full width. It's not that big a deal to add panels to an existing narrow baffle..... Many have done it. It's one of the things I'll do to my cabinets if I ever get around to rebuilding them.Quote:
Originally posted by mah:
A follow-up to my above post. The drawings do not show the full width speaker mounting baffle board that is used in the 828 cab. I am not familiar with the 828B model so cannot say whether the drawing is correct or not.