[quote author="ToobieSnack"]marik do you add pole peices only?
OR
do you add a magnetic return circuit and pole peices?
if you add pole peices the distance between the actual magnets increases by the thickness of the pole peices ...
with air as a MRC magnets in close proximity may behave differently.. if...pole peices are added the "air" mrc will become less efficient ... correct?
so maybe if you add pole peice AND MRC flux will increase..
this would be very interesting to know.[/quote]
I tried pole pieces only. I don't think the air works as a MRC at all in this context. There are many other factors, such as field cancellation because of close proximity of S and N of the magnet, which are much more predominant here.
Anyway, considering that with steel on back of the magnet increases the magnet energy by only about 20%, and closing the MRC path by another couple %%, the "air" effect will be negligible, if any.
ALSO: remember the idea i had to use razor blade for pole peices?
just about 1-1.5mm and somehow attach razor to flat plate (another razor perhaps)
i can't figure out how to attach the 2 together ( 1mm x 50mm x 0.5mm pole peice and the 0.5mm x 5mm x 50mm plate)
my theory is that : decreased ditance with tiny pole peice AND MRC will be very efficient.
Sorry, I don't recall it. I think such small pieces of metal might saturate very easily, reducing efficiency even further.
[quote author="rodabod"]
On a side note, has anyone had a bash at making (or know the theory behind the operation of) a cardioid ribbon mic?
I would imagine you would just need some suitable venting to the back, but I'm not sure.[/quote]
There are two ways of getting cardioid pattern in ribbon mics:
1) Electrically, combining pressure and velocity elements. This route went many cardioid RCA models, and hybrids like multipattern Altec/Western Electric 639, American DR330.
2) Acoustically, with the venting to the back, you are describing. This way built Shure, Beyer, Fostex printed ribbon cardioids, and use pretty heavy acoustical resistance.
The RCA 77D and DX are also went this way, but completely differently, having an elaborate labyrinth, as a back chamber acoustical loading.