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Old 23rd June 2014, 12:43 AM   #1
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Default The "Force 8" collaborative design

The “Force 8” speaker design project

I’m starting this thread as a design exercise – a virtual design, aka a thought experiment. I’ve been thinking about it for some time, and I hope that it reaches fruition in a physical DIY design. Should it make it that far, then it will be my wish that the plans and specs will be shared amongst the audio DIY community.

What is the “Force 8” project? Well, the Beaufort wind scale describes a force 8 as a “fresh gale”. My aim is to create a modern 3-way design, paying visual and performance homage to the Gale GS401a. (Hifi enthusiasts of a certain age may now begin to reawaken longings, mentally drool, etc.)
Yes folks, the plan is to design a speaker that visually evokes the silver end-capped “amp-killer” that was the stuff of dreams for many of us.

In any design process there have to be some rules, and so here they are:

THE RULES

1. This is NOT a cloning exercise. The speaker will use new, currently-available drivers, not vintage units or NOS (new old stock).

2. The speaker will maintain the L:W aspect ratio of the original GS401a, and will be sized somewhere from original actual size to 115% of original. (I figure that extra internal volume will permit some extra scope for driver selection, without overly compromising the object of “visual homage”.

3. The design will be a sealed cabinet.

4. The complement of drivers will be the same: two 8” bass drivers, a 4”-ish single mid-range and a single tweeter. (I’ve given thought to using a modern full range paired with a super-tweeter, but that will be a topic for further debate.)

5. For maximum flexibility in driver selection, the speaker will be tri-amped. Similarly, it is my suggestion that a miniDSP be used in preference to a XO. (In the fullness of time it may be that a suitable XO can be developed, but for the moment, we’ll use a miniDSP (or similar). That will facilitate the sharing of different approaches to crossing between driver units.) Drivers of any impedance may be suggested. Bass & midrange should be cones and the treble should be “visually similar” to the original treble unit, & all are to be flush-mounted on the baffle.

6. The speakers will play as low as the original GS401a’s and as high as the original GS401a’s. Watts are now cheap, so the overall sensitivity is perhaps less critical, but let’s agree that the speakers should be aiming to recreate (at the least) the frequency range & SPL that the originals could muster.

7. The arrangement of the drivers on the baffle will maintain (as neas as possible) the original spatial relationship (to maintain the “visual homage”).

8. There will be end caps. I’ve had some thoughts about this feature, and I have a “cunning plan” – but more of that later. The end caps will be a cosmetic feature and may be chromed metal, painted (two-packed?), or wood-grain. My concept is that they should be removable, perhaps upgradeable? (More about this topic later. It is not essential to the driver and cabinet design.)

9. The GS401a’s’ cabinets featured an 18mm proprietary 3-layer material. For simplicity’s sake, I’ll specify void-free 18mm ply. (Please DO NOT chime in to ignite a flame war about MDF, etc.!)

10. The GS401a’s cabinet had an internal brace. The force 8 design will feature two “holey braces” (nod to Planet10), one behind each bass driver.

11. The GS401a’s were designed as stand-mounters. The Force 8’s performance should mirror the placement options of the GS401a.

12. To borrow a term from engine-building I propose that contributors submit ideas and comments in 3 categories:

Phase I – True DIY “frugal-phile”, making the best of drivers and materials of limited cost, to achieve a nevertheless pleasing performance;

Phase II – Where I anticipate most effort will be made, using drivers and components readily available and reasonably priced. Nothing exotic.

Phase III – Cost-no-object. Go hard – exotic drivers, yak belly fluff stuffing (cryogenically-treated, of course), silver wiring, etc. - but don’t expect to have lots of company.

13. The final rule is that I want to make it clear that I don’t want contributions telling me I need to use 6” or 10” woofers, one woofer instead of two, a vented cabinet, or any other fundamental change to the approach. Find your own sandpit. (Any such suggestions will be referred back to this rule.)

I have done a considerable amount of research on the GS401a and I will be posting links to web resources, the Gale Yahoo Group, etc.

I live in Brisbane, Australia (no, not Austria!) and it would be great if some local DIY enthusiasts joined the endeavour – but hey … everyone is welcome. Bring your friends.

cheers

Doug
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Old 23rd June 2014, 01:25 AM   #2
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subscribed. looking forward to see where this heads
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Old 23rd June 2014, 01:34 AM   #3
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Default Basic background info about the Gale GS401a

As promised:

GS401 Speaker | galeaudio.com

The overall dimensions are:
605mm L
330mm W
270mm D

The optional stands placed them 18" (450mm) above the floor.

cheers

Doug
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Old 23rd June 2014, 02:24 AM   #4
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Internal volume calculation:

((605-36) x (330-36) x (270-36) /1000000 = 39.14L

This is inclusive of the space occupied by the internal brace(s) and the mid-range enclosure.

So let's say that the two woofers "see" 37L. (If we allow an extra 15% as discussed in post #1: 42.5L)

cheers

Doug
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Old 23rd June 2014, 12:31 PM   #5
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Quote:
Originally Posted by Brisso57 View Post
I’m starting this thread as a design exercise – a virtual design, aka a thought experiment.
The GS401 is a W-MT-W 1978 speaker. Awards

The Sony SS AR1 TM-WW is a 2011 speaker. Awards

A great exercise would be to compare the two designs using the same drivers. How much sound quality do you give up for style?

"1978 style layout" versus "2012 performance layout"
Attached Images
File Type: jpg GS401 SS_AR1.jpg (71.0 KB, 566 views)
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Old 23rd June 2014, 10:40 PM   #6
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Hi Linesource

Inherent in my project is the acceptance that the "GS401a arrangement" will have ultimate physical limitations. I accept those. The challenge is to revisit the limitations of the physical format, utilising modern knowledge and drivers, etc. to extract the best performance whilst keeping the style (/WAF).

cheers

Doug
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Old 24th June 2014, 04:35 AM   #7
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One last pitch for 2014 tech in the 1978 GS401 cabinet .... two 10" woofers, plus the 90H x 40V polar pattern SEOS12 waveguide with a 1" compression driver centered above. Xover at 1000Hz with LR4. This will provide decent controlled directivity, plus decent bass with help from MiniDSP equalization. As a bonus, you can get ~94db/watt efficiency from two 10" woofers wired in parallel.

There are several well reviewed high-efficiency Studio Monitors which use dual 12" woofers plus a waveguide/horn centered above them. GREAT DYNAMICS! After reading a couple reviews, you might consider building a slightly larger cabinet with the same aspect ratios as the GS401. MiniDSP equalization + WATTs = deep_bass in a small box.
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Old 24th June 2014, 08:20 AM   #8
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Quote:
Originally Posted by Brisso57 View Post
Hi Linesource

Inherent in my project is the acceptance that the "GS401a arrangement" will have ultimate physical limitations. I accept those. The challenge is to revisit the limitations of the physical format, utilising modern knowledge and drivers, etc. to extract the best performance whilst keeping the style (/WAF).
From your link the -3dB point at the low end is in the 50s which is reasonable but the small volume will mean less efficient drivers. Is a 5" midrange within the rules or must it be a 4"? The larger midrange is likely to help widen the choice of suitable woofers.

The Gale woofer to mid crossover is at 475Hz which is quite high but probably appropriate for a small midrange. Do the rules require it to be kept?

The crossover from mid to the tweeter is at 5kHz! If this is to be kept then we can forget a 5" midrange but we can use pretty much any tweeter. This crossover point is going to influence the sound.

Are metal drivers and/or phase plugs within the rules?
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Old 24th June 2014, 08:38 PM   #9
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Just off the top of my head:

SB Acoustics SB23NRX45-8 x 2

BMS 5S117

Monacor DT300 + waveguide.

I'd move the xover points to about 250-300Hz and 2.5k.
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Old 24th June 2014, 08:51 PM   #10
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Originally Posted by Charles Darwin View Post
Just off the top of my head:

SB Acoustics SB23NRX45-8 x 2

BMS 5S117

Monacor DT300 + waveguide.

I'd move the xover points to about 250-300Hz and 2.5k.
The SB 8" woofers want twice the volume available. It looks too much of a squeeze given the Gales had tight bass and more bass extension.

I also would lean towards a 5" midrange but not if the original crossover frequencies are to be kept. Is the 475 Hz - 5 kHz for the midrange an important part of the character of the Gale? It will certainly influence the sound.

Is a waveguide in the spirit of the original speaker?
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