604 back mounting questions

[rogerh113]

Hello,

I just got a pair of 604-8g, and am starting to think about cabinets and build details. One thing that I think I would prefer to do (or I am pretty sure my wife would prefer me to do) is to rear mount the 604s. I have seen little info on this, and hopefully someone can provide some perspective and insight from actual experience.

I was thinking of using 8 of the steel threaded inserts on the back of the front baffle to secure the driver. Probably add a bit of glue to them when installing to assure they are secure.

Is there any particular issue with back mounting the driver and is there a limit to the mounting board thickness? I think I saw somewhere, perhaps on the '1974 Altec Enclosure' manual, that the baffle should be 1/2" max. Not sure if I interpreted that properly or if it meant something else.

Any insight would be appreciated. There are probably some issues I need to take into account that I have not thought of.......

regards -- Roger

[Altec Best]

Hi Roger, Welcome to the Board ! The baffle should be very stiff 1/2" is too thin and would flex.... Use 3/4" or thicker for best performance.

[rogerh113]

Thanks. I was planning to use 3/4 inch, and probably braced unless that causes a problem. On page 12 of the Altec 1974 Enclosure Manual, it says on footnote 1 - "Use mounting baffle no greater than 1/2". I could not locate what footnote 1 referred to, and thought maybe it was just a general statement to prevent the builder from creating sort of a horn by having the baffle too thick. I also cannot imagine mounting this beast on a 1/2 inch thick board.

regards -- Roger

[VolvoHeretic]

I can't really answer your questions, but you could, depending on how much room the screw-in inserts leave you, round-over the speaker cut-out with a 3/4" or 1/2" router bit, or front mount with the speaker recessed flush and have a rim piece of 1/8" copper or aluminum water jet cut to cover the gasket and screws painted or clear coated to preference.

This is what I would like to build.

[rogerh113]

Thanks for the idea on the round over, and I will add that to the design if the threaded inserts allow. At a minimum I should be able to do a generous easing of the cut out edge, which should be sufficient.

Impressive layout. I think you may want to add another chair or two !!

regards -- Roger

[mah]

Roger, that is what I did, including the glue. I used flat-head(Allen) machine screws to avoid screwdriver slip. Used 4-bolt mounting - I consider that adequate.

I also rounded the cut-out front edge with a 3/8" radius round-over router bit. Plenty of clearance to the screw(bolt) heads.

My cut-outs are 13 3/4 inch diameter.

Cheers.

[GM]

Hello,

I just got a pair of 604-8g, and am starting to think about cabinets and build details.

Greets!


Something like this or .......? http://www.diyaudio.com/forums/multi-way/259659-gpa-604-8h-iii-build.html


Using a double baffle, you have the option to 'blindly' use the 12-24 T-nuts/black machine screws like Altec use to provide with each driver as well as provide a better 'roundover', even better than what he did, which is mostly cosmetic.


The 1/2" baffle thickness limit refers to the small front mounted 405 driver, which would need a >45 deg chamfer on the rear if any thicker one to keep reflections from it comb filtering/modulating the diaphragm.


That said, a 1/4" thick BB ply baffle would be sufficient for the 604s if made not much bigger and ideally we want a motor support/mass loading cab brace, further reducing front support requirements. Where cosmetics isn't an issue I advocate using small baffle 'inserts' with rear mounts for ease of periodically rotating these old tech gooped surround drivers, etc., to ensure the cab can be made very rigid, massive.


GM

[rogerh113]

I have not determined the cabinet yet - something like the one you show, the Stonehenge III, or Billfort's version. Probably more tall and narrow than short and squat.

First I am planning to build up a 'test mule' for open baffle to see what that sounds like. I got a 'scrap' piece of 3/4 mdf from a local building site (happen to know the contractor), and with a little effort will put together one open baffle. That should give me a good sense of the sound, and also the trade off with regards to diminished bass. I imagine that it is quite system, room, and design dependent. Based on what it sounds like to me, I will decide on whether I want to proceed with a real open baffle implementation, or move on to deciding on a specific cabinet build. I will also try to find out about TL cabinets as well, just to cover the bases. I've only looked at that a bit, and have been unable to find any plans for TL.

regards -- Roger

[VolvoHeretic]

So, I made a quick diagram showing the 604-8G mounting requirements, sorry I didn't have time to draw the speaker to scale: 604-8G

A knife cut threaded insert for either a 10-24 or a 1/4-20 bolt are the same size; 3/8" dia. x 1/2" deep:
1/2" Brass Slotted Drive Knife Thread Insert with 1/4-20 Internal Thread Size

[GM]

You don't think the 4 bolts leads to more vibration of the speaker frame against the baffle?

Original Altec frames are really rigid, but not so sure about the later cast ones [seen too many damaged ones in prosound apps] and use good quality composite gaskets, so [4] bolts is sufficient for the former and enough folks are fine using [4] with the latter, so your call, though in theory more is better and continuous clamping best overall.


That said, the truly best is none, like was done in the early days of 'subs' where they were floor or stand mounted and the baffles just close with an inner-tube like leather seal spanning the gap. Regardless, the motor brace will go a long ways towards quelling any vibration concerns and supporting the majority of its weight.


GM

- - - Updated - - -

Probably more tall and narrow than short and squat.

I will also try to find out about TL cabinets.......

Everything you need to know about the various TL alignments: http://www.quarter-wave.com/index.html


There's no TL designs for Altec drivers AFAIK, only vented TLs [MLTL] due to driver specs and since tall is acceptable this would be the preferred alignment if vented is desired.


GM