metal and magnetics

[ToobieSnack]

hey all,
I just started another ribbon mic while I wait for new neodymium magnets

I decided to replicate my RCA 74b as closely as possible with easily available materials.

I will start a new post for that later, I have some good pics of my progress.

The RCA uses a brass material for the main mount. (similar to a yoke inside the mic body)

MY question is:

is there some reason RCA used brass rather than steel?
will this metal I have around my magnets reduce my flux? or interfere with it's focus through the poles?

I hope not... I really don't want to build that part again...even though it's relatively simple. I am 90% complete at this point...i have ribbon mounts and wiring left....oh yeah and the body...ugh
my brother in law is a master at copper works and he is going to build me a base enclosure...I'm still waiting on that detailed info from El Fito on his excellent grill work...fito where are you???haha


OH yeah I guess i should mentions that the matereial i used is readily available at any hardware store ...where... the material for iron railing construction is located. I guess this is termed "wrought iron"

so anyway...brass or steel/iron??????????? that is the question

thanks
ts

 

[CJ]

Brass hardware is used in high end x-formers also, because it is non magnetic.

 

[ToobieSnack]

thanks for your input CJ.. i had a feeling this may be a problem... i hope not too much....maybe it's just to keep metal particles and jewelry from being sucked to the mic but it may affect the power too. i hope not.
thanks
ts

 

[adrianh]

Brass does not effect the magnetic circuit as it is non ferrious.

 

[ToobieSnack]

here's another thing that may be a factor.
RCA uses this brass yoke as the magnetic return curcuit.
could this be why it is brass instead of a ferrous metal?
will noise get into the signal from the magnetics?
I plan to use insulated wire for my mag return may be this will be ok?

marik PRR fum (and others) what do you think?
thanks
ts

 

[PRR]

> RCA uses this brass yoke as the magnetic return curcuit.

No. Impossible. You must be misunderstanding something.

In electric circuits, we have conductors (copper) and insulators (plastic). Actually, we have a broad range: copper is very good, iron less good, but any metal is many thousands of times more conductive than plastic, dry wood, etc.

In magnetic circuits there are only two materials:

1) Iron

2) anything else

Empty space, air, wood, brass, animals minerals and vegetables are all "the same". We define the magnetic conductivity of empty space as "1.000000", and these other materials run 1.0000 to 1.001 or so.

The main exception is iron. Iron has magnetic conductivity around 1,000, a LOT more than air or brass. Wave a magnet around the world, and there is no doubt when you hit some iron.

Small exception: Nickel and some other metals and alloys are magnetic, just not as high as iron. Stainless Steel can run from not much more than 1 to way over 100. (And generally, the "better" it is for stainless-ness, the less it sticks to a magnet: really-stainless steel has so much Chrome in it that it breaks-up the iron magnetic domains and seals the raw iron away from air and water.)

When reading a magnetic circuit, ignore all the non-iron to get the basic shape. THEN go back and remember that all the air/brass/etc also conducts magnetism a little. 1/1000 as much as iron, but there is air everywhere: a small leakage over a large surface is a significant shunt on the intended magnetic-gap.

The "mount" that holds the works inside the case would normally be brass (or plastic). The juice from the magnet should go up the iron poles, not out into the case (which was iron on some of the older RCAs). In this position you want an insulator, just like the rubber biscuits mounting the engine in your car, or the plastic block in a home fuse-box.

 

[adrianh]

Brass no workie in an magnetic circuit.

 

[ToobieSnack]

thanks prr
i gotcha ... I don't want any opposing force acting against my gap.

I have since replace the internal mount ...from iron (magnetic) to aluminuim (non-magnetic)

I only have a few hole to drill ( and they'd already be done if I had my good drill...this one is way crappin out on me....:sad

and i guess I was a bit confused about a magnetic return circuit...and I still am sort of....anyway

RCA does use the brass mount to "return" current from the top of the ribbon on both sides...which kinda made 2 loops (humbucking) and this is where my confusion lies.(2 wires run from the ribbon mount to the brass mount on both sides)

I will not use my aluminum mounts like RCA but I will use insulated copper wire on either side of my magnets (2 loops and twisted at the bottom to the trafo) I might experiment with it but will plan on using the wires unless ther is some wild difference.

 

[BYacey]

A little off topic, but I thought stainless steel was a mixture of nickel and iron / steel alloy?

In steady state magnetic fields non ferrous metals have no effect as PRR pointed out, but if you have a varying magnetic field such as an electromagnet powered with AC or a physically moving field, eddy currents can be induced into the non ferrous metal thus creating it's own little magnetic field. Mechanical car speedometers work on this principal, and I recall seeing a project years ago in a Popular Electronics (1960's) on how to build an eddy current super magnet that would pick up brass, aluminum, copper, gold, etc.

 

[PRR]

> I thought stainless steel was a mixture of nickel and iron / steel alloy?

Not according to "Tool and Manufacturing Engineers Handbook" and AISI:

STAINLESS AND HEAT-RESISTING STEELS

This family of steels is categorically described by composition as containing 4.00 percent or more chromium and generally more than 50.00 percent iron, and it may contain such alloying additions as nickel, molybdenum, columbium, titanium, manganese, sulfur, and selenium that confer specific characteristics to enhance their corrosion resistance, scaling resistance, and mechanical properties at subzero, room, and elevated temperatures, as well as their fabricability.

Steels with chromium contents between 4.00 and 10.00 percent, such as types 501 and 502, are not considered stainless steels but find applications almost exclusively in high-temperature service, such as in the petroleum-refining industry where corrosion resistance and high-temperature strength are required up to approximately 1,100F.

Steels containing 11.50 to 27.00 percent chromium are termed stainless steel by commercial usage, stemming from the early development of 12.00 percent chromium steels for cutlery purposes. Chromium confers remarkable corrosion- and heat-resistant characteristics, generally extending them with its increasing content. The extent to which they may be described as truly stainless depends upon the environment to which they are exposed.

There are three broad metallurgical (microstructure) classifications that have a marked bearing on their utility; namely, the martensitic, ferritic, and austenitic stainless steels. More recently the precipitation-hardening stainless steels have been added to this group. Each of these classes contains a minimum of 12 percent chromium, that amount being necessary to obtain the unique passivity or stainless character possessed by these alloys.

> if you have a varying magnetic field

Good point. AC eddy current effects "should" be negligible in field-magnet structures, though speaker coil inductance shows eddy current loading and makers sometimes increase this on purpose.

> build an eddy current super magnet that would pick up brass, aluminum, copper, gold, etc.

The US-standard electric-company meter is an Aluminum disk moved by magnetic fields. You can hold a magnet against it all day and never budge it, but shake that magnet 60 times a second and it induces a current which induces another magnetic field. A little trickery and it rotates. Interestingly the "brake" that governs its rotation is a stationary magnet, which induces yet another current in the rotating disk. If friction and counter-load is small, the number of turns is the number of volt*amp*hours, and a good number for billing because it (almost) represents the shovels of coal thrown under the generator boiler and the wear-n-tear on distribution equipment.

 

[BYacey]

I have seen aluminum rings inset into the center pole piece on speaker magnets around the inside edge of the gap. I would think this helps carry heat away from the coil as well as creating a conductive ring that would provide some dampening to the coil movement; but being stationary in the gap I am not sure how effective this is. I have seen in other speakers thin (3/16 high X .075 thick) aluminum rings on the voice coil former that I would assume dampen movement due to the increased mass and the shunt effect whenever the coil moves.

 

[ToobieSnack]

Spectacular reference on magnetics

http://www.ibiblio.org/obp/electricCircuits/DC/DC_14.html

later
ts