Question about capsule and capacitor

[ln76d]

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 in m49 as used in the u48 schematic.  ??? 

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?

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?

If yes could you explain further as i don't see where there is negative voltage in there?

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.
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.

Yes, but the polarisation voltage of back diaphragm in M49 needs to be "off" to works properly in "pure cardioid"

G7 is rather a mix of M49 and C12 topologies - changing only to M49 type you will get better response, also lowering cap to max 1nF and reducing 1G to 100M (btw.).

Original CK12 and teflon versions have same pattern as original K67, difference is that the CK12 is edge terminated and have chambered design with extra resonator disks In K67 much easier to make two identical backplates and rotate them properly than screw up and make some offsets in pattern placing

Capacitor value? Hmmm... Don't have a head to think even about that Probably just a capacitance impact as the whole input is just one big RC. Each half of the capsule is like separate capacitor, values are different some 55pF, some 75pF and more (per side) Maybe some theorist will do that job?
Take two 470pF and connect them in parallel but one from it thru the switch on one side and you should hear the difference
Or take 470pF and 10nF (like in G7).
The brighter capsule - the effect more audible.

C414E DC converter - it's what it's - it outputs positive and negative voltage.
Here's the same schematic with the values:
https://groupdiy.com/index.php?topic=44737.20
There isn't anything superior in negative voltage. Making the same type of input topology in classic tube m-p circuit will not work with voltage above ca. 70V on the back diaphragm.  Only thing why it's superior over other topologies, is that you will get similar sensitivity and response to pure cardioid
In tube mike, back diaphragm  (sometimes both) will collapse  to the backplate. When 60V is permanent polarisation voltage, then in 9 pattern mic only 6 will work - from omni to the first supercardioid. I tried chinese k12, k47, and three different k67 (even one which usual can be polarised much higher). No - i will not use Neumann capsules or brass ck12 for tests
Effect will be the same
From the input topology also depends how much voltage you can apply to the capsule. Usual diaphragm polarisation can be used with higher voltage than backplate polarisation but not in this case
I'm thinking about adaptation of negative voltage inside tube mike PSU
Anyway - U87Ai DC converter also use negative voltage, and schoeps can be converted to use it (some chinese MP FETs are exactly done this way).
Back to C414E converter - it works only with DC voltage (there's no AC conversion), front diaphragm is permanently polarised with 60V, back:
- cardioid 0V
- omni  60V
- fig. 8  -60V
- supercardioid -30V
This microphone was designed to use with 9-52V input voltage.

R3 is typical current limiter btw

Look for U1 transformer on schematic much more like for couple inductors connected to each other

Could you paste schematic for M49C without switch? - that is something new for me?!?

Don't forget that there were microphones before U47 - CMV3 and CMV5 looks much more like a predecessors of later plate followers Then Neumann wanted to save some cash, so they decided to use VF14 with common voltage for plate and heaters

 

[KrIVIUM2323]

Yes it is much clearer.  8)

Could you paste schematic for M49C without switch? - that is something new for me?!?

Sure, here it is:

http://repforums.prosoundweb.com/index.php/topic,37067.15.html?PHPSESSID=9qas9njlq5bh2sodqmroj5d4s0

It's interesting because this thread give a clear view of how the circuit evolved with the different versions.

One thing that i find interesting is the difference between the 'b' and 'b3': the feedback loop is a bit different between the two, taking the output cap outside the loop. Don't know what to be gained that way, maybe less phase shift?

It gave me better understanding of Oliver Archut m49 conversion too. It is clearly a cross bewteen the early one and the 'c'.
He commented that using 'c' self-bias in 49 circuit with one of his AMI transfo gave a nice result for him:

From Tab funkenwerk/AMI website:
I also did a variation that Neumann never did; the early NWDR transformer with AC701K and C circuit, that can focus the M49 sound in ways the original designers never could dream about.

 

[KrIVIUM2323]

Usual diaphragm polarisation can be used with higher voltage than backplate polarisation but not in this case

This is interesting. In first mic circuits i builts years ago it appeared to me but i knew lot less than now and as i destructed some capsule either way of biasing i thought it was my brain deducting things which had no sense...

Do you have an idea why diaphragm polarisation can take higher voltage?

I'm thinking about adaptation of negative voltage inside tube mike PSU

I remember a schematic Gus traced and gave long time ago of the VM1.
He gave most of the circuit but prefered to keep psu private as it used some Brauner proprietary trick for mp bias.
Maybe this is related. I will probably never know because all users of vm1 i known never let me open the psu to check what was going on there...    ;D

 

[ln76d]

Yes it is much clearer.  8)

Sure, here it is:

http://repforums.prosoundweb.com/index.php/topic,37067.15.html?PHPSESSID=9qas9njlq5bh2sodqmroj5d4s0

It's interesting because this thread give a clear view of how the circuit evolved with the different versions.

One thing that i find interesting is the difference between the 'b' and 'b3': the feedback loop is a bit different between the two, taking the output cap outside the loop. Don't know what to be gained that way, maybe less phase shift?

It gave me better understanding of Oliver Archut m49 conversion too. It is clearly a cross bewteen the early one and the 'c'.
He commented that using 'c' self-bias in 49 circuit with one of his AMI transfo gave a nice result for him:

Thanks! Two new schematics in my collection

Difference between b and b3? Different phase shift (rather worse in "b"), no DC thru the feedback. I didn't tried so i can speculate only, that b3 version could mess a little with second feedback via C4.

Oliver mod is exactly something between. This is just typical, simplest option of plate follower. It acts like  a little castration of any m49 sound characteristic. I did many similar circuits, it works nice, but there is no m49 "magic". In the very first versions it was specific due to hiller tube and type of biasing. Later models had a lot phase shifts and eq of frequency characteristics.
Only one thing which i don't like in Oliver schematic are 1Gohm resistors

This is interesting. In first mic circuits i builts years ago it appeared to me but i knew lot less than now and as i destructed some capsule either way of biasing i thought it was my brain deducting things which had no sense...

Do you have an idea why diaphragm polarisation can take higher voltage?

I remember a schematic Gus traced and gave long time ago of the VM1.
He gave most of the circuit but prefered to keep psu private as it used some Brauner proprietary trick for mp bias.
Maybe this is related. I will probably never know because all users of vm1 i known never let me open the psu to check what was going on there...    ;D

There's hard to collapse backplate
Topic isn't simple - it depends on really many factors. Lets focus on pure cardioid connection for simplification. Standard polarised backplate and diaphragm to grid vs. polarised diaphragm and backplate to grid. 
Still if you will apply to much voltage on diaphragm it will be stressed, but in reversed scenario it will collapse totally.
Not grounded capsules are less sensitive for higher voltages as also any hum issues.
Capsule type matters also - edge terminated, center terminated, diaphragm tension (for example teflon ck12 - huge low end, less high frequencies, can work with voltage around 100V for cardioid ) and many, many other things

Here's a nice topic about modification of VM1, i think you could contact MS Vienna, he's a really good guy and probably will give you some answers about voltages
https://groupdiy.com/index.php?topic=63305.0