Old ribbon mic threads, anyone have them archived?

[Bjorn Zetterlund]

If it's of any interest, there's two STC 4038 ribbons at work that I can take pictures of (as in the insides). Need hosting, though. And PM me please, just in case I forget!

Bjorn

 

[smilinfu]

The conversation has gone over my head so I?m of to Google and and the library to see if I can learn enough to catch up.

Marik,
I have not posted any pictures of my last attempt yet, they did not turn out very well, and probably would not illustrated much as I snapped the ribbon again (it was on a very flimsy backing) maybe I?ll try again tonight

I had the magnets set up so the large flat surfaces were facing each other, the neodymiums I could find had their north and south poles on those surfaces, so I figured the field would be strongest with those facing each other, like yours but with the magnets on their sides. The downside was that I had the ribbon sitting 1/4? down in a valley between the magnets, probably not letting much vibration into the ribbon. I want try 2 new things, one with my current magnet orientation but the ribbon sitting closer to the front and the other with the magnets oriented as you have them. So I may just wait until I get those done to post again.

The question:
Were you able to find magnets that had their axes running through the side? If you are using the same ones I am (with north and south on the large flat surfaces, from indigo or wonder magnets) do you have them arranged so both poles face the same direction or are they reversed (one with north facing front, the other facing the back)?

p.s.
I posted here as the other thread seams good for looking at existing designs (thanks again zebra50) and didn?t want my uninformed attempts to pollute it.

 

[PRR]

> I made a few experiments with Neodymiums, using 1/4"x1/4" (across) ironm as pole pieces. Closing the path did not have almost any affect on the sensitivity.

I was wrong, you are right. These new-tech magnets are so strong that closing the magnetic path makes very little difference. Here's the word from a friend who used to design hard-drive motors (the seek motor is somewhat like a ribbon mike magnetic structure):

-------------------------------------------------------------------

John-

What do you know/recall about modern super-magnets?

A "ribbon microphone" has a magnetic gap about 1" tall, at least 0.1" deep
(0.1 square inch pole area) and about 0.25" long from N to S. As always, we
want a strong field.

Classic designs had a large U-magnet or a hunk of Alnico, focused down
through pole-pieces.

The new super-magnets suggest using the magnet itself as the pole-piece.
You can get them about 1"x0.2". Mount two of these 0.25" apart and you have
a magnetic field the right size.

A friend built one. He does not have a closed magnetic path: the magnets
are stuck to 0.1"x0.5"x2" iron straps on aluminum spacers. So in addition
to the 1"x0.2", 0.25"L gap, there is an inch-long return path through air,
off the larger-area edges of the iron straps.

I said try closing the iron path. He says it makes nearly-no difference on
test (it should).

Is is possible the magnet faces are saturated? I know Alnico saturates
easier than iron, and we use tapered poles to focus low flux density at the
magnet face to high flux density in the gap. But I thought the
super-magnets didn't suffer so much from this?

What flux density would be typical in a hard drive seek motor, the kind
that has magnets without pole-pieces to focus on the coil?

-=- PRR -=-
------------------------------------------------------------------

Fm: John

>>>> The new super-magnets suggest using the magnet itself as the pole-piece.

You can get them about 1"x0.2". Mount two of these 0.25" apart and you have
a magnetic field the right size. <<

Ok. Yes, using the magnet as the pole piece works very well, and
surprisingly, the return path makes relatively little difference to the
flux in the working gap.


>>>> Is is possible the magnet faces are saturated? <<


<g> Of course they're saturated. They're the magnets!

With Neodymium-Iron-Boron magnets (I presume that's the kind you're talking
about), you tend to get the most bang for the buck (the flux density in the
gap relative to magnet volume) when the thickness of each magnet is about
the same as the gap "length".


In other words

    NS    NS 
 |s|m |  |m |s|
 |t|a |g |a |t|
 |e|g |a |g |e|
 |e|n |p |n |e|
 |l|e |  |e |l|
 | |t |  |t | |

With an arrangement like that, you get, typically, about 9000 Gauss. The
main effect of the steel is to reduce stray flux.

Of course in hard drives, we were often looking for 1% or 2% {advantage},
so steel and return path and so forth were carefully designed.

In the picture above, a hard drive would have had about a 3/16" (4.5 mm)
gap. The magnets would be about 3/32" (2 or 2.5 mm) thick each, and the
steel about 1/16" (1.5 mm) thick. That would get about 8000 Gauss. Double
the magnet and steel thickness and get a little over 9000 Gauss, keeping
the same gap.

As each magnet in a hard drive is really two magnets (half polarized one
way and half the other) the steel around the edge, separating the two
magnets, doesn't really carry much flux. As much as anything, it's used to
tweak the circuit to try to get the working flux density as even as
possible.

Magnetics is very non-intuitive. Nothing is linear and everything happens
in 3 dimensions.

-- John -- Santa Clara, CA -- OZWin 2.33

 

[NewYorkDave]

Nothing is linear and everything happens
in 3 dimensions.

Very much like life itself! :wink:

 

[Marik]

PRR,

Thanks a lot for contacting your friend--now we know for sure what is happening there.

Were you able to find magnets that had their axes running through the side? If you are using the same ones I am (with north and south on the large flat surfaces, from indigo or wonder magnets) do you have them arranged so both poles face the same direction or are they reversed (one with north facing front, the other facing the back)?

I used similar to ones you are using now (10 mm thick) and they were polarazed the same way. I would not use them again, unless wanted to make an omni ribbon. The problem--they are way too big for nice and short acoustical path between front and rear of the ribbon. The way you arrange it, creates a tunnel in front of the ribbon, and it creates all kinds of diffractions and other nasty things. If you move the ribbon close to the front, the tunnel will still be on the other side, which will create very non linear forces on the ribbon--I don't really see how it would be possible to make a ribbon with decent HF response with magnets that big. Also, I suspected that bigger magnets have very little affect on the output, and it seems that last PRRs message confirmes it.

 

[Marik]

Thank you Larry and Andrew,

The complete file of BBS monograth is here:


http://home.comcast.net/~markfuksman/BBC_ribbon_info.pdf

 

[smilinfu]

Thanks for the input. I will put moving the ribbon up to the front at the bottom of my things to try list. I still want to try one with your orientation, and might play with using some tapered iron pieces to focus the field, allowing me to keep the same orientation but open up the space around the ribbon. I?m looking forward to trying some that are a little shorter (I think that?s what you were recommending), much easier to only need 1 inch of good ribbon.

Thanks for everything!
:guinness: :guinness: :guinness:

 

[Marik]

Try using T rod as pole pieces and drill holes in it for shorter acoustical path. Face the magnets as you do and glue them on "top" of T, while "I"s of T will be drilled and the ribbon placed between them. Find some means of frame which spaces and holds the pole pieces (with magnets), and you can also mount the ribbon on it.
It should work.

 

[Whackamole]

Would any of the magnetics gurus care to comment on materials for making pole pieces? Everybody talks about "iron" but we all know there's gotta be more to it than that!

 

[andre tchmil]

Please don't forget the B&O BM types,
I have a few of these babies. :green: :thumb: