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Thread: Magnetic Return Structure, Help Me Understand

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    Senior Hostboard Member bowtie427ss's Avatar
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    Magnetic Return Structure, Help Me Understand

    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?
    Not all vegetables make good leaders.

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    Senior Hostboard Member Audio_by_Goodwill's Avatar
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    Re: Magnetic Return Structure, Help Me Understand

    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.
    Audio_by_Goodwill
    Michigan, USA

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    Senior Hostboard Member bowtie427ss's Avatar
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    Re: Magnetic Return Structure, Help Me Understand

    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.
    Not all vegetables make good leaders.

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    Senior Hostboard Member BillWojo's Avatar
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    Re: Magnetic Return Structure, Help Me Understand

    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

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    Senior Hostboard Member GM's Avatar
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    Re: Magnetic Return Structure, Help Me Understand

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

    GM
    Loud is Beautiful if it's Clean! As always though, the usual disclaimers apply to this post's contents.

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    Senior Hostboard Member Earl K's Avatar
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    Re: Magnetic Return Structure, Help Me Understand

    Quote Originally Posted by BillWojo View Post
    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

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    Senior Hostboard Member Earl K's Avatar
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    Re: Magnetic Return Structure, Help Me Understand

    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 ).


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    Senior Hostboard Member bowtie427ss's Avatar
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    Re: Magnetic Return Structure, Help Me Understand

    Quote Originally Posted by Earl K View Post
    Hi Bowtie,



    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 ).

    Thank you, Earl K!

    I am reminded of something a retired engineer once told me "to design and build are rudimentary, to explain how it works is the real talent"
    Not all vegetables make good leaders.

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    Senior Hostboard Member GM's Avatar
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    Re: Magnetic Return Structure, Help Me Understand

    Wish I had time to post all that we learned back in the mid '60s by advancing SOTA slot car motors to performance levels that the then electrical, physics, Ga. Tech Professors claimed was impossible till we gave them a few to play with and not much good at dumbing it down since quite frankly I don't understand the physics of the situation, just witnessed the cause and effect of probably hundreds of DIYer's experiments, but hopefully we have some folks who do and with easy access to the various book, articles, etc., so FYI, FWIW: Google Scholar

    GM
    Loud is Beautiful if it's Clean! As always though, the usual disclaimers apply to this post's contents.

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    Senior Hostboard Member Steve Schell's Avatar
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    Re: Magnetic Return Structure, Help Me Understand

    GM I loved slot cars in the mid '60s! There were two local places I frequented in Pico Rivera and Whittier, CA with my mediocre cars and Cox controller. I had an inline 1/32 scale and a sidewinder GT-40 looking 1/24 scale. My mom said that it was all a fad and would be gone within a year. I argued that oh no, slot cars were here to stay, but both racing tracks were closed within a year or so. Mom was cool enough not to say "I told you so."

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