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Where is the Rest of the Speaker Driver?

72.192.28.43
Posted on July 16, 2014 at 13:44:51
Gerry E.
Audiophile

Posts: 2192
Location: Saratoga Springs, NY
Joined: February 19, 2000
 Hi:

A Western Electric 750A speaker recently sold on eBay for $6,700 (see photos below). I have seen photos showing the front of the 750A before but this is the first time I have seen the back.

There's not much to it. Just a donut shaped magnet and a couple of metal pieces holding it to the back rim of speaker. There's no frame per se. How does this thing work? Thanks.

Gerry










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RE: Where is the Rest of the Speaker Driver?, posted on July 16, 2014 at 19:54:37
gilmorneau
Audiophile

Posts: 288
Location: Colorado
Joined: August 10, 2004
 Just guessing (never seen one in person), but there probably isn't any more to it. There wouldn't need to be. The cone is aluminum with annular compliances, so it can only move in and out. The rigidity of the aluminum would prevent any lateral motion. The voice coil (on a former)is surely fixed to it.

With the magnet (and thus the coil gap) fixed into position by the brackets (connected to the perimeter frame), and the coil fixed to the cone (also connected to the perimeter frame), the only possible motion is for the cone to go in and out. Exactly what's needed. Elegant, really.

 

w magnet, posted on July 17, 2014 at 07:53:40
RCA-fan
Manufacturer

Posts: 801
Location: Ontario
Joined: July 26, 2003
 This has a "W" magnet, or it is shaped like a torus. There is no back plate. I believe it is the most efficient magnet structure available, as there is no break in the magnetic circuit.
University used this type of magnet..

bill

 

RE: w magnet, posted on July 17, 2014 at 11:49:31
freddyi
Audiophile

Posts: 3852
Joined: December 6, 2001
 yeah and even in "low end" products - I'd like to see a 312 fetch $6000.

hey Bill - look at my post below concerning your 70Hz horn and a funky simulation


Karlson Evangelist

 

70 hz horn, posted on July 17, 2014 at 18:18:51
RCA-fan
Manufacturer

Posts: 801
Location: Ontario
Joined: July 26, 2003
 Hi Freddyi,

If you offset the driver from the throat, your simulation is exactly what happens. You can boost the lower bass, but then you get a notch in the upper frequencies.
In a way, it is like adding a front cavity, which has similar results.

If the driver is very far forward, the the horn becomes a sort of "box"... an odd shaped box, but it ceases to act as a normal horn.
If the woofer has a rear chamber, then you have an bandpass-ish system.
The Karlson has sort of a weird shaped bandpass front chamber in much the same way.
It is important to note that the shape of an enclosure from a pipe, to a horn, to a box, all affect the output with considerable effect.

In the case of the 70Hz horn, the woofer while appearing to be offset, is really a lumped circuit combing a small front chamber and throat in one "mess".
The 70Hz horn wasmade to find a small horn to go from 90Hz to 1kHz.
It does this despite being folded due to the lower reflector, and careful choice of the bend point. While very interesting, no further work was done.

I have the numerical expansion rate if you would like to see it.
I was surprised how high this horn went, although it is very directional at 1Khz.

One other note about the W.E. driver... it has no spider.

I hope this helps. bill

 

RE: 70 hz horn, posted on July 18, 2014 at 01:00:10
freddyi
Audiophile

Posts: 3852
Joined: December 6, 2001
 thanks for the explanation - what would you suggest for the lower reflector? can it benefit from an upper reflector? would a couple inches more width be useful? I've got a Community branded 12 which might work well.

on the Karlson front, an offset T-TQWT variant designed by Greg B. has proved good in several builds ranging in size from ~4 inch driver to 12" as the plan below.


BEST,
Freddy

response of a Visaton BG20 outdoors in the Karlsonator 12 vs
a smaller Karlsonator 8 - the builder said there was no damping in the large cabinet so that may account for the dips (?) - he only had the pointy stub lined on the 8" driver version. I have a rough prototype of a 6" version which sounds pretty nice with the cool L. Cao F6 fullrange

Karlsonator12 Plan

Karlsonator8 Plan

Karlson Evangelist

 

reflector 70 hz horn, posted on July 18, 2014 at 12:01:06
RCA-fan
Manufacturer

Posts: 801
Location: Ontario
Joined: July 26, 2003



Reflector is 10" x width of horn. ( at the bottom facing mouth.
If you e-mail me, I can send you more data.

The Karlsonator is a nice idea. Is the SPL accurate?

bill

 

RE: w magnet, posted on July 21, 2014 at 13:28:24
freddyi
Audiophile

Posts: 3852
Joined: December 6, 2001
 thank you so much - I could not get an answer yet on the Karlsonator graphs as the builder is a farmer and working 12 hour days harvesting wheat
Karlson Evangelist

 

RE: Where is the Rest of the Speaker Driver?, posted on July 29, 2014 at 12:03:09
Steve Schell
Manufacturer

Posts: 1440
Location: So. California
Joined: December 16, 2001
 Gerry the 750A dates to about 1940, so it is an ancestor to W.E.'s postwar line. A similar outer suspension is described in L.G. Bostwick's U.S. Patent #1,967,223. Despite the rudimentary construction, it uses a 4" edge wound aluminum voice coil in a fairly tight gap. I had one years ago and it sounded very good. Later examples used a stamped steel basket instead of that inadequate three bracket arrangement.

 

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