Diy Push-Pull symetrical undamped F8 capsule

[kingkorg]

When you design such a capsule there are two main challenges to consider. Both of them are linear and predictable, so it should be quite straightforward to control them:

1. Obviously such capsule is way underdamped for linear response, so you will need to correct its response. The peak will be around the frequency of the diaphragm tuning resonance. Since the system is resistance controlled below that frequency you will get stiffness controlled response and above--mass controlled. The parameters are linear, so for the de-emphasis even a simple mid cut series RCL should do the job adequately. Of course, with DSP you could fine tune it very nicely.

In comparison to condenser cardioid capsules one of the main advantages here is a lack of chamber resonances with their peaks and other anomalies. Also, phase behavior should be quite uniform. Very good indications will be this capsule taking EQ very nicely.

2. The top frequency response is additionally limited by the front-to-back distance traveled by the sound wave--effect very well known from ribbon microphones. Many years ago on one of the forums I wrote a little primer on its details, so will just re-post it here. For some reason the Italics are not working, so I am putting it into quotes:

"In order to understand how the ribbon microphone works, how its flat response is formed, and how this response is different from capacitor mic, it is important to know three concepts: 1) Resistance Controlled system, 2) Mass Controlled system, and 3) Forces on the diaphragm.

1) The best example of the resistance Controlled system is a condenser microphone capsule, where the diaphragm is tuned in the middle of the band (usually somewhere between 900Hz to 1500Hz, depending on a mic). That exhibits a huge (sometimes up to 60db) peak. Naturally, for the flat response we need to damp this peak, which is done by means of air cushion trapped between the diaphragm and back-plate. The amount of viscosity of this cushion is regulated by the certain size holes (or sometimes grooves) in the back-plate.

2) On the contrary, the ribbon microphones are tuned into the lowest frequency of the band, so actually their natural response FALLS with 6db8 rate.

So, where is the flat response in ribbons coming from?

In the "native" fig8 pattern the sound wave strikes the front of the diaphragm and creates acoustic pressure p1. Since the back of the ribbon is exposed, the same sound wave flows around the ribbon and pole pieces/magnet structure some distance (called acoustic path d) and creates some acoustical pressure p2 at the back of the ribbon. This results in pressure difference p1-p2, which in fact, is a driving force to move the ribbon.

For example, why there is a null at the 90 degrees polar response? Because the sound wave reaches both, front and back at the same time, so there is no pressure difference.

The interesting feature of this driving force is that it doubles with every octave, so the acoustical response of the ribbon actually RAISES with 6db8 rate. Now, remember that the Mass Controlled system naturally has a falling response? When we combine those two that gives us an overall flat response.

Now let's see what's going on on the extremes of the bandwidth.

1) Low end:

Obviously, the tuning frequency of the ribbon would determine the lowest response. However, in the real system there always will be a slit between the ribbon itself and magnet/pole, so because of the viscosity of the air in that slit below some certain point the system turns into the STIFFNESS Controlled one (the one, which defines true omni operation) and the response below that point rapidly falls--that's why it is not practical to tune the ribbon much lower that point.

2) Top end:

As we talked earlier, there is a distance d, which represents the path between front and back and obviously this distance can be translated into the wavelength.
As we talked, the driving force p1-p2 increases with each octave, but only to the point where the d represents 1/2 wavelength, because when the d reaches the full wavelength our driving force p1-p2 becomes ZERO. That is, in the condition when the wavelength of the signal becomes equal to the distance of the acoustic path d obviously the p2 will become equal to p1 (because it is 360 degree shift) and the ribbon won't be moving.


That's why if we know the acoustical path d and ribbon dimensions, it is very easy to calculate the top frequency response.

It is important to notice, the top response will also be somewhat affected by diffractions caused by magnet system cavity, but there are special graphs, which help to correct the calculations to the very high degree of accuracy."

Hope this helps,

Best, M

This is priceless

 

[molke]

The responses are very, very sad face shaped.

By the way, did you ever try with the stock hole size, or go straight for re-drilling?

My own attempt at a rather flat symmetric figure 8 capsule (24 mm diameter) now has very few and small holes, and I still didn’t get the response to be as flat as intended.

 

[kingkorg]

By the way, did you ever try with the stock hole size, or go straight for re-drilling?

My own attempt at a rather flat symmetric figure 8 capsule (24 mm diameter) now has very few and small holes, and I still didn’t get the response to be as flat as intended.

If I went with stock hole size I would get damping, that's what they are for in stock size. You can't get it to be flat if the capsule is supposed to be undamped. However if you want flat, damped symmetrical capsule, that would be a nightmare to achieve.

 

[molke]

If I went with stock hole size I would get damping, that's what they are for in stock size.

Sure, I was just wondering whether the stock hole size would remove one “very” above. But if you’re happy with the result, then there’s no need.

 

[Marik]

The parameters are linear, so for the de-emphasis even a simple mid cut series RCL should do the job adequately.

Sorry, made a mistake here. The mass controlled response above the tuning frequency should be flat until half of the front to back wavelength, where it starts falling due to cancellation. The mid cut we would need in the undamped cardioid capsule because in that one the force on the diaphragm will not double with each octave going up, so the response will fall with 6db8.

IOW, the de-emphasis in the fig8 should be not mid-cut, but high shelf cut. I edited original post.

Best, M

 

[global-records]

@kingkorg Have you already used this capsule for the RF circuit? Which resistance has the metallized mylar that you used for the diaphragm? In my case, it was 10 ohms/square and I ran into difficulties and I'm still convinced that this is an important parameter for AM RF applications. more precisely, not the resistance of the film itself, but the total impedance of the capsule on the RF. So I'm still looking for a suitable film.
Specifically for the @rogs project, I see an ideal capsule like this, only with reduced gaps, increased hole area and a well-conducting diaphragm.

 

[thecr4ne]

Kinkorg,

I remember seeing somewhere a cutout of Schoeps ingenious CM 66 3 pattern capsule with all mechanical switching. There was a different diagram for each pattern, IIRC. Try to find it—it will give you a very good idea how they achieve a good diaphragm damping with relatively open area to avoid resonances.

Best, M

I've been hunting for this for a while, as I have a Schoeps CMT-56 which is now limited to fig-8 only. Anyway, After a lot of poking around, I stumbled onto a reference to where the schoeps MK-6 capsule diagram can be found: Fig 5-17 in John Eargle's "The Microphone Book" which I accessed by "borrowing" it here: "The Microphone Book" (archive.org membership required)

Anyway I chopped up and re-organized the relevant imagery here:

 

[kingkorg]

I've been hunting for this for a while, as I have a Schoeps CMT-56 which is now limited to fig-8 only. Anyway, After a lot of poking around, I stumbled onto a reference to where the schoeps MK-6 capsule diagram can be found: Fig 5-17 in John Eargle's "The Microphone Book" which I accessed by "borrowing" it here: "The Microphone Book" (archive.org membership required)

Anyway I chopped up and re-organized the relevant imagery here:
View attachment 115643View attachment 115644View attachment 115645

This is insane, thanks!

 

[GeorgeToledo]

Grosskopf patented this version of pattern switching as well, not just M49/50, and subsequently assigned it to Schoeps.

 

[Murdock]

I've been hunting for this for a while, as I have a Schoeps CMT-56 which is now limited to fig-8 only. Anyway, After a lot of poking around, I stumbled onto a reference to where the schoeps MK-6 capsule diagram can be found: Fig 5-17 in John Eargle's "The Microphone Book" which I accessed by "borrowing" it here: "The Microphone Book" (archive.org membership required)

Anyway I chopped up and re-organized the relevant imagery here:
View attachment 115643View attachment 115644View attachment 115645

Have you pulled the capsule out of the headbasket?
I would love to see a picture of the actual capsule.
Where is the „on-axis“? Looking at the headbasket it seems to be a side adress mic. I know the drawings are simplified but there it would seem to be top addressed.

 

[bremusound]

I've been hunting for this for a while, as I have a Schoeps CMT-56 which is now limited to fig-8 only.

Daniel Andreev of Reso Labs posted this in 2018, maybe he can help. A great guy. Unfortunately he‘s located in Russia what doesn‘t make it easy to send mics over for repair.
The CMTS contains two MK6 capsules.

 

[Marik]

I've been hunting for this for a while, as I have a Schoeps CMT-56 which is now limited to fig-8 only. Anyway, After a lot of poking around, I stumbled onto a reference to where the schoeps MK-6 capsule diagram can be found: Fig 5-17 in John Eargle's "The Microphone Book" which I accessed by "borrowing" it here: "The Microphone Book" (archive.org membership required)

Anyway I chopped up and re-organized the relevant imagery here:
View attachment 115643View attachment 115644View attachment 115645

Yes, that’s the one I was thinking of… just did not remember exactly in which book.

Needless to mention, with all mechanical engineering marvel and ingenuity of this exercise none of the patterns are exactly optimized and Schoeps pattern dedicated capsules are more desirable.

In general, making undamped PP Fig8 has a few contradictory parameters. On one hand, we want to make the tuning resonance as low as possible. Say, we are able to cut tuning frequency in twice then the de-emphasis will be 6dB less, meaning we do not lose as much S/N. This means we would want to make this capsule larger (and need to watch stability of the undamped diaphragm). But then, on the other hand, because of the cancellation (I posted earlier) we (severely) lose top end response. To take care of this part we might want to consider some well tuned resonator system. Perhaps rather middle sized diaphragm, or more narrow rectangular will be a better compromise...

Just think--in a ribbon mic all those contradictions are taken care naturally--all we need is a very low tuning resonance to get a mass controlled system for perfectly linear response...

Thanks, M

 

[thecr4ne]

Daniel Andreev of Reso Labs posted this in 2018, maybe he can help. A great guy. Unfortunately he‘s located in Russia what doesn‘t make it easy to send mics over for repair.
The CMTS contains two MK6 capsules.

I reached out and was told they are no longer producing these, nor are they providing schoeps service. sadface.

 

[Marik]

I happened to have a Schoeps MK6 mechanical three pattern capsule for repair on my bench. Here are pics.

 

[Marik]

I've been hunting for this for a while, as I have a Schoeps CMT-56 which is now limited to fig-8 only.

Yep, same problem here. So far, was able to make it into a solid cardioid and fig8. It seems there is no good seal of the back in omni pattern. My main suspect is a thin piece of rubber ring, which probably deteriorated and lost its elasticity. Will try to soak it in some kind of rubber rejuvinator, or find a way of making a new silicone gasket...

Best, M

 

[thecr4ne]

Yep, same problem here. So far, was able to make it into a solid cardioid and fig8. It seems there is no good seal of the back in omni pattern. My main suspect is a thin piece of rubber ring, which probably deteriorated and lost its elasticity. Will try to soak it in some kind of rubber rejuvinator, or find a way of making a new silicone gasket...

Best, M

First, Thanks for sharing the pics, I have yet to disassemble mine, and this will prove very helpful when I do.

As for the rubber rings, I asked a few follow up questions of Daniel Andreev of Reso Labs as mentioned here, and he was very helpful...

Daniel Andreev of Reso Labs posted this in 2018, maybe he can help. A great guy. Unfortunately he‘s located in Russia what doesn‘t make it easy to send mics over for repair.
The CMTS contains two MK6 capsules.

...with Daniel's permission, I'm happy to share that he provided this 3d model of a mold to replicate the rings with silicone. The mold should be produced with a resin 3D printer to achieve the necessary detail. As for the gasket material, he just mentions silicone, and that off the shelf construction silicone can work, also care should be taken to avoid bubbles in the silicone.

I'm currently awaiting a resin print of the mold from a colleague who has access to a resin printer. Once in hand I'll experiment with creating new gasket rings and share my results.

I've also created a separate thread to chronicle maintenance of my pair of CMT-56's.

 

[adamasnan]

Hi all, this thread has encouraged me to take a look inside my MK6's stuck in fig8 also. I have the full working mechanism, and can see the arms shifting the assemblies back and forth, but I cannot quite understand why I am unable to derive a cardioid pattern, I suspect that the insulator (white rubber seal/gasket?) is too dried out to work effectively.

I do hear some slight difference to the rear between the patterns, it seems that when I bought them, the mechanism arms were removed (and provided in a bag) and it was left not quite open enough for full fig8; just ever so slightly duller (more so than what might be detected from the front/rear differences in the MK8). Weird decision to remove the arms, if the fixed position was not optimal without them.

So, how might I achieve a good cardioid pattern out of this?

Thanks!