[quote author="xvlk"]
Yes, I have seen this somewhere in the net ...
This MUST be omni, because no additional resistance to the ribbon.
In bidirectional this way is not long - life.
Nice mic, but with one problem. How can be done hum-bucking?
There is not any frame, or connection, nothing...
Is inside the pole pieces (pole pieces are drilled and trough it
two wires symetricaly? Or no hum bucking ? It is interesting for me, if
I want to remember this mic in my thesis.[/quote]
I am not sure how they did it. In fact I'd love to have this mic in my hands for awhile. From a picture it seems that there are clamps on the top and bottom.
Pay attention, the clearance between ribbon and pole pieces is pretty large. Also, I find interesting the circled shape of the pole pieces, which should acoustically create a raise around probably 5KHz--in a manner of Schoeps, or Neumann M50 omnie balls.
There is only one "better" solution. Beyer s setup, done with
permendur wire. It can be flux efficient and shielded in the some time.
But very costly and ribbon setup complicated. (And with parasitic resonances.
)
Yep, "parasitic resonances". So, how come is it a better solution? Besides, short ribbons (esp that thick, as in Beyer) are much more prone to resonances, and should be very effectively damped.
Here is Beyer s setup (copied from Neumann brochure):
(you mus cut and paste link to the other browser)
http://mujweb.cz/www/xvlkxvlk/m130.GIF
There is one inconsistency in the Beyer mics ? Two ribbons.
Two ribbon mics was historically developed by Goerike (founder of
AKG) in early 50 - th s . It was stated as analogy to von Braunmuehl
principle of two- membrane gradient mic.
But without success. Mainly because ribbon is gradient as is, but
condenser must be made gradient by resistance. (resistance controlled)
I see double ribbon mics as anachronism. PRR, please, please, comment it for me.
xvlk
In fact, Beyers M130 and M160 are usually more desirable, becasue of more extended top and bottom of double ribbons.
Let's speculate for a litlle...
The top frequency response in the gradient (bi-directional) ribbon is mostly limited by length of signal path between front and rear. When signal length approaches length of this path, there is a 360* phase shift, and cancellation is occured, thus limiting bandwidth on the top. In reality, because of reflections and diffractions, it is not as pronaunced, but for now lets accept it.
And for now, lets see what is happening in omni one. To make it work right (i. e. flat response) the back-chamber should be infinite (that's why Olson puts a tube, which with filling simulates this infinity). If you simply close the back, you will get excellent, wistling highs, with no bass, becasue of 6db slope with each octave down.
I see the second ribbon in a sense, as a combination of a gradient principle, but with introduced resistance. IMO, in this case the path length is not as critical in this case, as a back ribbon introduces time delay, acoustical resistance, and mass of air trapped between ribbons, so the the "360* cancellation" occures at higher frequencies. Of course, it introduces all kind of resonances, but the second ribbon and compliance of trapped air work as a natural damping.
Since it involves all these parameters, distance between ribbons is very critical. It is probably really very hard to put all these into math. When I worked on my double ribbon, experimentally I found the ?optimal? one, with good balance between top range (which is more extended, compare to single ribbon with the same baffle size) and bottom (which goes away with even slight (about 0.2 mm) distance increase between ribbons). Proximity effect is noticeably lower than in a single ribbon.
But once again, here there is more troubles than it is worth. Besides, they don't sound as "natural", so I've abondoned this idea.
