Neumann M50

[riggler]

Martin,

I'll go look for Dale's contact information. This sounds like the way to go.

Earlier I thought I found some measurement capsules that would work from Cirrus. The MK:224 looked to be a good contender. 12.6mm. But they are pre-polarized electret condenser though. So, no go there. The B&K 4165 is discontinued and will become hard to find. The Gefell MK221 would also work but I can't find any for sale! And I'd imagine they're expensive.

Gus, I have a pair of the MXL omni capsules. On the 603 bodies (with upgraded caps), I think they sound okay. They're $45 for the pair! I used them on choir with spheres and sounded surprisingly good through a DW Fearn VT-2.

Been studying tube stuff, will see how it goes...

 

[riggler]

Emailed Dale. These are going to be too tight for him. Tolerances, spacing, etc.

I called Bill Hayes. He was an engineer at Altec, responsible for the 201B mics and associated capsules. He worked on my pair. He suggests looking into the AKG CK2. Peluso has their version of these for less than $200 per stereo pair. Trying to find where to buy the real CK2 and get some specs...

 

[Martin B. Kantola]

[quote author="riggler"]Earlier I thought I found some measurement capsules that would work from Cirrus. The MK:224 looked to be a good contender. 12.6mm. But they are pre-polarized electret condenser though. So, no go there. [/quote]

Why won't a pre-polarized work for you? Leaving out the three resistors used for the polarization voltage in the original M50 schematic will not affect the sound as I see it.

Good work finding all those options! Measurement capsules are generally expensive, unless you find them used.

Interesting with the AKG capsules. Wonder how a CK92 or Rode NT45-0 would work? Not expensive.

Martin

 

[dale116dot7]

I doubt that I can maintain tolerances to those required for an M50. It is possible that a really thin Mylar diaphragm could react the same way as Titanium, Aluminum, or Nickel (depending on which version you pick), but I doubt the longevity of Mylar stretched that tight. I also don't have a M50 capsule to take measurements on.

I've toyed with the idea of using a B+K capsule with the grille taken off and inserted into a sphere. But I have enough mics. I rather like my Altec M11 'Coke Bottle'.

I've found 5840's and most other submini tubes to require a long burn-in and selection process; even 6AU6 and 6AH6 tubes need special selection to use for low-output capsules. Maybe one out of ten or twenty 6AU6 tubes are useful. I might try a 6BA6 to see if I can use rejects from my compressors...

-Dale

 

[riggler]

[quote author="Martin B. Kantola"]

Why won't a pre-polarized work for you? Leaving out the three resistors used for the polarization voltage in the original M50 schematic will not affect the sound as I see it.
[/quote]

This is what I am unsure of. Can anyone else comment on any audible or functional differences between an externally polarized capsule versus a pre-polarized capsule?

:?:

 

[Marik]

[quote author="Martin B. Kantola"][quote author="riggler"]Earlier I thought I found some measurement capsules that would work from Cirrus. The MK:224 looked to be a good contender. 12.6mm. But they are pre-polarized electret condenser though. So, no go there. [/quote]

Why won't a pre-polarized work for you? [/quote]

Good and legitimate question. I have pairs of B&K4133, and Gefells MK202, and MK221 and wish had their prepolarized counterparts instead, not to mess with 200V bias.

Interesting with the AKG capsules. Wonder how a CK92 or Rode NT45-0 would work? Not expensive.

It is my understanding the Rode is the same as Behr B5 and SP C4. For the price of the capsule you could have entire B5, which will include cardioid, as well. It is a smaller capsule though, and IIRC, has lower output.

Cannot comment on CK92, but I have a pair of CK62-ULS, which are very good and sonically close to (discontinued) CK2.

Best, M

 

[Martin B. Kantola]

[quote author="Marik"]It is my understanding the Rode is the same as Behr B5 and SP C4.[/quote]

Oh, forget it then. Know that Rode makes many of their own capsules otherwise with good tolerances.

Martin

 

[riggler]

All,

I'm working with a local manufacturer that may be able to make capsules. They have not done mic capsules before, but can manufacture to extremely tight tolerances.5-axis CNCs, mills, lathes, etc. Does anyone have any drawings / technical information of any of the KK series capsules that were used on the M50 or M150? Here's what I have so far:

Diameter: 12mm
Diaphragm: Titanium, Aluminum, or Nickel over Mylar
Spacing from Backplate: Between 8 to 10 um (micrometers)

Does anyone know of a real similar capsule that is indeed close enough. I've looked quite a few of the suggestions here; it seems that the measurement type capsules are the only ones small enough with close enough backplate spacing. Today I'm going to look in-depth into whats available. If anyone else can think of something readily available, please chime in.

I've been doing a lot of reading off of the Neumann site and other places, and am getting a better handle on this. It seems the place to put the money and get it right in this mic is the capsule. I got to hear some samples of the M150 on orchestra in Decca Tree, and it has a little different character than the RCA Bartok recording I have that used M50's, but that could well have been due to tape machines vs. digital, post production, etc. What I did hear is the omni-ish bass presence, and high end articulation.

Is the M150 a better candidate for DIY? For one thing, the tube is readily available and cheap. It's a JAN6111. It's transformerless, which may account for some lack of color in regards to the M50, but would bring our cost down and mean one less headache in sourcing a part, although we can get similar transformers from Lundahl, even Jensen.

So, anyone have an M150 schematic? Methinks the capsule has not changed from the M50, but maybe I am wrong.

Gus, thanks for prompting me to go out there and read. I've learned quite a bit. On the M50 schematic I can now identify a voltage divider, a DC blocking cap that blocks the bias coming off of the plate. This "Calibration Input" --- is there a trimmer in the PSU? It looks like it would adjust the cathode/heater voltage.

Sorry for asking so many questions!

 

[riggler]

OK, this Cirrus is interesting, keep coming back to it:

Cirrus MK:224 -- Prepolarized electret condenser.

Sensitivity: 50mV/Pa, -26dB ±1.5 dB re 1V/Pa
Noise floor: <15dBA
18.5 pF @ 250Hz

Frequency response looks flat until a slight rise at 10k.
Free-field corrected.

Manufacturer notes these are compatible with B&K 4155 & 4189.

They make 2 other capsules just like this that are less expensive. The MK:226 has a more uneven frequency response having a rise at 8kHz, and sensitivity is 37mV/Pa.

Could one of these be the candidate?

 

[riggler]

MK224: $950 list
MK226 and other equivalents from other manufacturers: ~$600

 

[burdij]

Here is a first attempt at a DIY omni capsule. It is sized roughly the same as the M50 capsule. The outer diameter is .75 in. (19 mm). The active electrode diameter is .5 in. (12.7 mm). The diaphragm is 6 micron polyester with an aluminum coating. The spacer is 12 micron polyester. I am getting about 70 pF of capacitance. The original capsule has 76 pF capacitance. The coin in the photo is a dime. Wrinkling on the lower edge of the diaphragm was caused by slippage of the spacer during diaphragm install. I need to figure out a method of holding the spacer in position during the install process.

 

[riggler]

:grin: Wow!!!

Did you buy the spacer material at that thickness? Are you making the components on a CNC? ...And, would you be able to make a batch for us if you're successful, or give some instruction to those of us that don't have the skills yet to design this.

Is this diaphragm edge terminated somewhere?

70pF is where we want to be, great job, and many thanks! :guinness: :thumb:

 

[burdij]

The polyester material I am using was prepared for mylar capacitor manufacturing, I think. They are both about a foot wide and several yards long. This type of material is usable for this capsule since the diaphragm is edge terminated and supported on the insulated body of the capsule which is delrin. The conductive side of the aluminized polyester is out so it makes contact with the brass ring.

I tried to keep the machining to a minimum. The capsule body and backplate were done on the CNC mill. The only part made on the lathe was the brass ring. Getting this far is the easy part. Now comes the "tuning" phase, getting the hole size, depth and pattern right. Dale can probably attest to the relative difficulty of this process. I took some inspiration from Dale's small diameter capsule shown on his web site here:

http://www.10000cows.com/SomeOtherDIYCapsules.htm

 

[riggler]

Is the backplate flat and smooth, or does it have holes / pits in it?
I assume the holes that I saw on Dale's are done to smooth the respone. Is that right?

Do you have any pictures during construction showing the pieces?

 

[Martin B. Kantola]

The backplate needs to be perforated, otherwise very little air is trapped behind the diaphragm and it can't move much. But at the same time the backplate needs to be mostly solid to create a capacitance with the diaphragm. So we have to balance these two. Gets much more complicated for a cardioid... Is this a way of explaining it? (already edited once...)

Martin

 

[Marik]

Since the omni capsules are stiffness controlled, there are two ways of building. The first is to tune the diaphragm to the top of the bandwidth and not to worry much about damping. Another is to use lots of damping and tune the diaphragm in the middle of the bandwidth.
Both have their pros and cons. In the first case you will need to have a lot of bias on the capsule to make up for severe sensitivity loss, in the second case you will need to do A LOT of experimentation to get the damping right. Of course, the spacing is absolutely crucial, you will also need to get the back chamber volume absolutely right. I spent almost two years designing this thing. It is something in between B&K measurement capsules, and CK62 style. It has quite elaborate threaded system, so I can adjust any spacing and fine tune it. I am close...

Best, M

P.S. Burdij, good work! I'd recommend to find different means of diaphragm clamping, rather than bolting it down. That might be fine with LD capsules, but takes too much space (the ring is too wide), so you lose lots of effective area, and get much worse pattern on HF. Also, watch for the cavity created by the ring. Look at the B&K, which have very thin rim for those reasons.

 

[riggler]

Marik,

How do you determine "the bandwidth" range?
Tuning the diaphragm means tensioning it, so it resonates at a particular frequency, correct?
Damping the diaphragm: is this done by venting the backplate more, or is it done by physically damping the diaphragm edge with a soft material, like a drumhead? Or is that done electronically?

Can anyone point me to a good link on condenser capsule design? I looked via Google but can't find anything relevant. I did find some information on the Neumann page, but it wasn't really design related from the ground up.

I'm trying to understand how the back chamber volume is determined, how the diaphragm tension is determined, etc.

Thanks for all of the information everyone!

 

[Marik]

[quote author="riggler"]Marik,

How do you determine "the bandwidth" range?
Tuning the diaphragm means tensioning it, so it resonates at a particular frequency, correct?
[/quote]

Yes.

Damping the diaphragm: is this done by venting the backplate more, or is it done by physically damping the diaphragm edge with a soft material, like a drumhead? Or is that done electronically?

Damping is achieved with perforating (or grooving) backplate and backchamber volume.

Can anyone point me to a good link on condenser capsule design? I looked via Google but can't find anything relevant. I did find some information on the Neumann page, but it wasn't really design related from the ground up.

I'm trying to understand how the back chamber volume is determined, how the diaphragm tension is determined, etc.

The first few chapters in API handbook dedicated specifically to true omni measurement capsules construction and should have all the answer. VERY heavy on high math and theory, though.

Otherwise, the best and the only place on planet earth is Meta on this very forum :wink: .

Best, M

 

[burdij]

The backplate has 20 .043 in. holes, .173 in. deep currently (dimensions are the same as a closed hole in a M47 backplate, no reason for picking them other than as a good starting point). I believe Dale's backplate has a similar number. The amount of area covered by holes must be close to the M50 as the capacitance is fairly close. My spacer is ~2 microns thicker than the "real" M50.

The clamping system I am using is the same as that used on an actual M50. You can see one in the photo at the bottom of the page at this URL:

http://www.neumann.com/download.php?download=docu0064.PDF

I derived the rough dimensions of the capsule from this photo because we know that the sphere is 40mm in diameter. It looks like there is a little ramp shaped ring around the outside of the capsule to provide a transition, I suppose.

I ordered some acrylic spheres but the common ones are 38mm in diameter (1.5 in.) so there will be a slight difference in the front to back pattern performance.

I will get some photos when I put a new diaphragm on to replace the wrinkled one.

 

[riggler]

Burdij, Marik,

How are you determining the proper capsule tension, and how are you providing that tension to the diaphragm when clamping? Dale's method of a washer makes sense.

To determine the tension, are you measuring capacitance trying different weights?

To sum some of this up:

1. We need to vent the backplate so that atmospheric pressure won't affect the response -- in essence a change if capacitance.

2. We then drill pits into the backplate to equalize the response of the capsule, countering the resonant frequency.

3. The M50 design does use a vent, and also has a moderate tension on the diaphragm, using the backplate pits and vent to balance the response.

4. Burdij must be very close with the backplate drilling and spacing, since his capacitance is close.

Burdij, you have any photos of your components before you assembelled the capsule? Maybe you can make the spacer wider so that you can drill alignment holes in it. Then the screws will hold it in place as you screw it down. Also, I was thinking if you tightened the screws only a quarter turn at a time, jumping around the capsule this might apply more even tension more slowly. But, maybe you're already doing that.

Thanks for all the good links guys, I'm reading them, have learned alot on Tim Campbell's site, which has the Neumann patents.

Looking again at the M50, this large brass looking ring around the capsule...
We see a lead running into the bottom of it. It looks like the diaphragm lead, right? This ring must be heavy. I wonder if the reason for that was to kill off some resonances in the Lucite. Remember, cutting the capsule in, which goes pretty deep, leaves a perfect ring of plastic around the capsule.

So, is that ring tied to the diaphragm electrically? I wouldn't think.