Neumann M50

[Marik]

[quote author="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..
[/quote]

You can do it whether with Dale's method, or build a tension jig. I think Tim Campbell once posted pics of one.

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

NO. Capacitance depends on the spacer thickness (distance between diaphragm and backplate) and backplate open area (amount of holes and their diameter).

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

This one seems kinda messed up. We need to vent the backplate for a certain amount of damping. The atmospheric pressure balance achieved by means of a little cappilar tunnel, connecting the backchamber to the outside.

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

We don't equalize the response, we damp the resonance.

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.

It is confusing what wents are you talking about? If it uses a moderate tension, then it gotta have lotsa damping for a flat response.

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

Considering there is 2 micron spacer difference I am not that sure. As a side note, frankly, I don't see a good reason of taking M7 backplate as an example and starting point. It is COMPLETELY different animal and the only resemplance is both M50 and M7 are microphone capsules.
The M7 hole pattern is optimized not only for a damping for CARDIOID mode, but also for a right time delay and is much more complicated. We do not need it in an omni capsule.

Best, M

 

[riggler]

Marik,

I now realize the foolishness in some of my questions. Things just "clicked".

It's the air volume and electrical characteristics of the diaphragm and backplate setting the capacitance. Hence, tension won't affect it, but holes in the backplate will.

I now understand the role of the vent as well.

What I meant to say about the M50 tension should have been written:
The M50 capsule has (according to Neumann patents in general) a medium amount of tension of the diapghragm, requiring damping. We can only speculate what the backplate of an M50 capsule looks like.

I see your point on the differences of the M7 and M50 capsule. Can we achieve the damping we need with only a few holes? I'll search out some information on the backplate in the KK series capsules and see what I can find.

OK, scouring the net and Neumann's site gives no hint on the backplate construction. Will have to experiment, unless someone has some information to post.

Waiting for my copy of the AIP Mic Handbook to get some formula to do some math...

 

[gyraf]

scouring the net and Neumann's site gives no hint on the backplate construction

Google some capsule images?

 

[riggler]

So we are looking *through* the diaphragm here? And the web-looking material with gold color behind is the backplate?

 

[ioaudio]

http://www.twin-x.com/groupdiy/thumbnails.php?album=294

 

[Marik]

[quote author="riggler"]

Can we achieve the damping we need with only a few holes? [/quote]

Yes, we can, but since it has less damping we'll need to use high tension on the diaphragm and high bias voltage. Not a big problem in a tube mic.

Best, M

 

[dale116dot7]

There are a few ways to get rid of the ripples. First is to cut the spacer slightly smaller than the mounting ring so some slippage can be tolerated. Another is to have a couple of small locating holes with pins so the spacer is located by the tooling and not the screws. Once tightened, the pins can be removed. Another option, I like it best, is to machine the spacer right into the backplate. This is easy in a capsule like the M7. The tighter the gap the more precise the lathe needs to be. For the 50 micron or so gap of the M7, my CNC Sherline works well. For a much smaller gap, you'd need a precision lathe.

 

[just.sounds]

Later M50's where build with the km83 capsule. This explaines the differences between the two pictures of capsules i think.
KM83 capsules are not that expensive i think.

Maybe there is something in the microtech gefell range.

They build the only metal diafragm capsules i know of these days.

http://www.microtechgefell.de/eng/home.htm

 

[burdij]

I am constructing a fixture that will provide two clamping rings. The inner ring will hold the sheet of spacer material with respect to the capsule body. I will make a stencil of thin brass to cut the hole in the spacer. The outer ring will clamp the electrode material. This will be attached to the base of the fixture. A salvaged micrometer drive will be mounted on the bottom of the fixture and simultaneously drive the capsule body and the inner clamping ring upward into the sheet of electrode material and provide the tension control. A sharp stylus will be used to punch the holes for the screws of the clamping ring through both the electrode and the spacer.

 

[burdij]

Here are some capsule assembly photos:

Installing the diaphragm spacer.

Installing the diaphragm.

Capsule with diaphragm installed.

Making the capsule recess in the acrylic sphere.

The capsule installed in the acrylic sphere.

 

[rodabod]

Someone needs to say...

What you've done there looks awesome. Excellent work. Hope it sounds good in the end.

I feel I haven't contributed much recently, but I have something of an East-German/Russian re-build on at the moment which i hope to post soon. A circuit which I think nobody has tried yet, as far as I know.

Roddy

 

[Marik]

[quote author="rodabod"]Someone needs to say...

What you've done there looks awesome. Excellent work.

[/quote]

Indeed, very nice. The hole pattern looks like a very good start. Have you listened to it yet?

Best, M

 

[riggler]

Burdij,

WOW! :thumb:

I just got the Condenser Microphone handbook. I, too, am curious in how it sounds. Do you have any drawings for what you've done so far?

 

[burdij]

The capsule is installed in a test mike with the usual suspects (ef86, 2461 nico). It actually works, too. It even has a pretty good omni response, although there is a bit of high shading when the source is close. The farther the source is from the mike, the less the shading. The overall frequency response is somewhat lacking in highs which tells me that more hole area is needed to reduce the damping somewhat. Quite the opposite of the problem I usually have with LDC capsules. Here is the capsule mounted in the test mike:

And compared with a U87:

Just from some rough testing, the capsule has about 6db less sensitivity than the U87, which is an ai version. The suprising thing to me, anyway, is that it works so well, because I can clearly see a little piece of junk trapped between the diaphragm and the backplate. Need to do some housekeeping, I guess.

 

[riggler]

Burdij, that is great! Are you using a cathode-follower configuration?

From the AIP book:

"The theoretical ratio of backplate diameter to diaphragm diameter for maximum sensitivity is approximately .816"

Do you have measurements for your backplate/capsule?

How "damped" is the high end? Can you post an audio sample somewhere? From recordings that I have, none have anywhere near a hyped high-end. So you might be closer than you think.

 

[ioaudio]

burdji,

looks very nice!
maybe also try the backplate pattern from the akg ck1/2

 

[burdij]

Thanks for the photo, ioaudio. Clearly, a lot of area is open in this capsule.

In my capsule, the active electrode is .5 inches in diameter. The area inside the clamping ring is .540 inches in diameter. The electrode is surrounded by a gutter .020 inches wide and .040 inches deep. Here is a drawing of the capsule face and a side view of the sphere showing the capsule.

The outer diameter of the capsule is .750 inches.

The capsule sounds good on voice and crazy harmonica playing but the U87 sounds hyped in the highs by comparison. This usually means to me that the test capsule is actually lacking in highs. The M50 capsule has a 6db rise in the high range so it should sound brighter, I would think.

The circuit is the same as the circuit in a U47, transformer coupled to the plate, tube wired as a triode, grid bias developed from the cathode connected to the unbypassed filament circuit.

 

[olafmatt]

Hi burdij,

when I get back home next week I will see whether I can take the KK-53 capsules (aluminum) out of the grilles of my KM-253 mics. Really don't want to damage them, so I can't promise anything. But if I remember correctly there is a metal ring sitting in a groove that has to be taken out in order to get at the capsule. (picture of mic body with whole capsule taken off can be seen here: http://www.vacuumsound.de/diy/km253/DSCF8603.JPG)
I could then upload some pictures and also take measurements. Unfortunately the hole pattern won't be visible due to the aluminum diaphragm.

Olaf

 

[strangeandbouncy]

Hi,

justsounds says that he only knows of Microtech gefell making metal doaphragms nowadays. I'd like to add hebdensound(formerly calrec)who do nickel capsules for £150 (cardiod, omni, sub and hyper cardiod) Amazing mics at a rediculous price! v v v recommended!), B+K/DPA, ans Sonodor.

Kindest regards,



ANdyP

 

[olafmatt]

Here are the pics of the KK53 capsule that I promised. Seems it would need some cleaning after all those years.

The capsule diameter is about 16.3 mm, the depth of the back part is 11.7 mm. Seems I forgot to measure the ring on the front. :roll:

And here the the mic with a DIY perspex spehere (1.5 inch / 38 mm diameter because I bought it in London).

Olaf