Guys,
thank you all, i got it now and... feel a bit like i had a brain fart! :
Ln76d:
Cap in M49 is to get connection between both diaphragms without loading grid vith high voltage as also to make flipped phase between front an back.
About the m49 if the back membranne isn't connected to the front one
through c1 (1nf) there is no multipattern possible as there is no mix of front and back signal so only 'pure' cardioid (front side of capsule) pattern availlable.
I think i've been misguided in studying the G7, as i was never bothered to see that same topology (the m49 use) in the u48 schematic. ???
Ln76d:
The best cardioid is pure "cardioid" (like U47 and M49b/c - with use of the switch S2) - completely without use of back diaphragm.
There's also C414E type of topology (for now the best sounding for multipattern) but it needs negative voltage - most of capsule can't work here with high voltages above 100V.
There's also ELA M251 topology which is pretty good but need separated backplates.
Cathode follower (currently working on it) can be used without any capacitor for multipattern.
G7 topology is odd, because it utilize two different types of connection, but it can use only one.
Each topology have it own limitations. Worse response or/and lower level comparing m-p cardioid with pure cardioid.
The higher values of high ohm resistors and higher values of capacitors = worse response.
Usual 470pF - 1nF is enough (especially in M49 topology) - some capsules and circuits allows to use lower values.
Not without reason C414E use 4.7M resistors
All depends on the circuit, tube or FET and capsule.
Ok. I've had to read it multiple times but this is enlightning for me about other things i've had wandering.
I now do understand why the k67 backplate is used in so many clone of ck12: the need for separate isolated backplate, using an 'easy' to machine/reproduce design.
I'm working for some time on a CF headamp too and came to same conclusion as you. ;D
Now about the value of this diaphragm bridging capacitor: it seems counter intuitive to me that the lower the value the better the response. Could you expend on this a bit please?
Ln76d
I'm writing about overall input topology concepts.
And i thank you for doing this!
C414E type of topology is almost perfect, but utilse negative voltage to get phase difference. Notice that both diaphragms are polarised and the signal is taken from backplate.
Ok, i ve read many times this kind of comments from you about the c414 topology. I don't know the circuit but i'm interested in understanding it. When you talk about negative voltage are you talking about
relative negative voltage to some biasing or real
negative rail?
Are you talking about this schematic:
http://gyraf.dk/schematics/AKG_C414_old.GIF
If yes could you explain further as i don't see where there is negative voltage in there?
Timjag:
to get a figure of 8 the beck capule needs to be polarised at twice the voltage as the front.
Please excuse me but i don't follow you there... In m49 in fig of 8 you have backplate at 58 or 60v, back membranne at 116v or 120v and front at +2.4v. The polarisation voltage is not twice the front membrane, it is twice the backplate.
Timjag:
The M49s is a weird one because you can switch it internally to be a series connected omni. I guess this was so the studio could decide - if it was planned only orchestral work then the pattern selector was unecessary and both capsule are linked via the switch like a u47.
If i remember correctly the m49 is the first mic with remote pattern control and i think the switch made a comeback once the Neumann engineer whitnessed that in 'pure cardioid mode'(or u47 mode) there was a difference of 4db of sn. The thing which i find interesting is that in the latter 'c' version the switch is omitted in the schematics i have.
Timjag:
The M49 really was a redesigned 47
As is the u67... the 87, the...
I'm kidding about the 87 but most of the tube circuits are evolutions upon the 47, or are presented like this in the Neumann history i've read.
Timjag:
The reason its not there in the C12 is because the c12 polarises the diaphragm not the back plate and the signal is taken from the backplate not the diaphragm so the two never need to be connected - it could be argued that omni on the C12 isn't true omni because of the two separate voltages on the two diaphragms even if they're identical. theoretically they can never be 100% as one, but the plus side is it isn't lossy like the omni U47 so the output isn't decreased
I agree.
Timjag:
The M49 really was a redesigned 47 Neumann got a lot of complaints at the time regarding the u47 - the body was too big, you could either have figure 8 or omni, not both, the tube was pretty much obsolete by 1955, at the same time AKG had come out with the C12 which is just about as perfect as you could get and they'd modified it to be the ELA-M for Telefunken, and Neumann didn't want to loose the market.
So here was the M49 aU47 but better, or so they thought.
The complaint came from the film and broadcast (tv) about the size... which is ironic if you ever seen an Elam! The c12 was indeed smaller in form factor, but to my knowledge in EU most used the Elam rather than the c12 in film or broadcast (mainly because of the 'standard' ac701 tube and psu compatibility).
Anyway i really thank you! I learned a lot and you made things clearer for me.
Timjag:
You're barking mad.
I agree too!
But Ln76d is mainly right even if definitely biased upon some subject.