Magnetic Return Structure, Help Me Understand

[bowtie427ss]

I have long been curious about Altec's move in the early 70's from welded magnetic structures to non-welded structures in loudspeakers using alnico magnets.

Recently, i discovered this footage of Elvis from 1970 at the Culver City studio(three A4's for playback) . He's singing into the Satan of microphones, the EV 666. Watch carefully early in the video, he gets a hellish shock on the nose from it, even makes a comment to the boys in the booth.



This led me to do some research on the Electrovoice 666 family of mics. In the sales slicks they tout as a selling point a "non-welded magnetic return structure".

It now occurs to me that the "new and improved cast steel return pots" that Altec advertised for the newly introduced 416B/802G drivers didn't contain any new design features or characteristics that made them particularly special. It seems more likely that the sole improvement was the elimination of welds in the magnetic circuit.

My guess is that the welding process creates irregularities in the grain structure of the alloy that cause a discontinuity in the magnetic flow through the structure.

Can anyone expand on this or offer a more accurate explanation?

[Audio_by_Goodwill]

Good video! I enjoyed that, and I've never been an Elvis fan.

Had other products/companies switched to cast at this time? Grain structure certainly seems reasonable, but I wonder if it was more of a marketing thing... a marketing spin to sell the new cheaper way to build the product as a benefit to the customer.

[bowtie427ss]

Good video! I enjoyed that, and I've never been an Elvis fan.

I agree completely. That's a special performance to be sure, and gave me a new appreciation for The King. I feel lucky to have discovered it and am happy to share.

[BillWojo]

Boy is this web software screwed up!
Anyway, I just wanted to comment on the cast structures properties. I had both a welded pot and a cast pot tested with a handheld XRF elemental analyzer that gave me the chemical makeup of both metals.
Working in metal warehouses as the traveling saw repairman has it's perks. Could never afford one of those 30 to 40 thousand dollar units.
Anyway, the cast pot is made of cast steel and has just a tiny percent less FE in the make up. I was rather surprised as casting steel is not a cheap way to manufacture a product. Cast iron is much cheaper.
Oh, the older welded steel pots are cadmium plated under the paint and the cast steel pots are zinc plated, at least my samples were.

BillWojo

[GM]

I'm going with the usual reason for 'improving' a well proven product, cost reduction.

GM

[Earl K]

Boy is this web software screwed up!
Anyway, I just wanted to comment on the cast structures properties. I had both a welded pot and a cast pot tested with a handheld XRF elemental analyzer that gave me the chemical makeup of both metals.
Working in metal warehouses as the traveling saw repairman has it's perks. Could never afford one of those 30 to 40 thousand dollar units.
Anyway, the cast pot is made of cast steel and has just a tiny percent less FE in the make up. I was rather surprised as casting steel is not a cheap way to manufacture a product. Cast iron is much cheaper.
Oh, the older welded steel pots are cadmium plated under the paint and the cast steel pots are zinc plated, at least my samples were.

BillWojo

Bill,

Interesting.

I'd like to know which Altec models these ( tested ) pots belonged to .

Thanks <> Earl

[Earl K]

Hi Bowtie,

My guess is that the welding process creates irregularities in the grain structure of the alloy that cause a discontinuity in the magnetic flow through the structure.

Can anyone expand on this or offer a more accurate explanation?

My understanding was the cast return pot had a better Magnetic Reluctance figure ( ie; lower in magnetic resistance ) and was therefore more permeable to the flux lines of the magnet.

Unfortunately, I can't find any corroborating stats ( via Google ) that would show it's any better than properly prepared/machined ( & properly welded ) cold rolled, grain oriented, silicon steel casings.

Maybe long term, casting ends up being a better QC approach ( compared to counting on those temperamental, highly paid welders who would rather make the big bucks on some pipe-line project ).