DIY capsule

[Tim Campbell]

I was asked quite a long time ago about some dimesions for microphone backplates. Sorry that it's taken so long to get back to this. Anyone wanting to make a capsule should start with a 4.07 thick backplate. Using a 1.6mm drll bit, drill to a depth of 3.75mm. Through holes should be drilled using a .6mm bit.

Many chinese capsules use a 1.3mm bit drilled to a depth of 3.25 into a backplate that is only 3.72mm thick. On most chinese capsules all holes are then drilled through using a .6mm bit.

There's a lot of room for experimentation. I believe these differences in dimension contribute very much to the differences in sound we hear from one capsule to the next.

 

[Gus]

Thank you Tim!

 

[dale116dot7]

I assume those dimensions are for a large-diaphragm double backplate design (K87-like). A single backplate (like the Debenham capsule, or the M7 or KK47) would have to start at around 6 mm or so. Does anyone know the thickness of a KK47 or M7? It looks like maybe 6.5 mm from the photos I've seen.

Also, one of my Chinese capsules use about 0.9mm holes in place of the 0.6mm. It was out of an Apex 430.

 

[Marik]

[quote author="dale116dot7"]A single backplate (like the Debenham capsule, or the M7 or KK47) would have to start at around 6 mm or so. Does anyone know the thickness of a KK47 or M7?
[/quote]

M7 diameter (without middle rim)--29.85mm (1.175")
Diameter of gold plating--25.41mm (1")
Thikness--5.73mm (0.225")

 

[dale116dot7]

Thanks! That's great information! That's actually quite a large diaphragm - almost 30mm!

 

[saxtim]

Using a 1.6mm drll bit, drill to a depth of 3.75mm

Tim thanks, for posting this info! Just curious, in the K67/87 design - the depth of the 3.75mm does that apply to both:
a. the holes that don't go all the way through
and
b. the depth of the first step of the tapered holes that do go all the way through?

I'm still trying to find a source for Metallised film. I'll give a capacitor a go - I remember ages ago it was said you need a low voltage one that is at least the diameter of the capsule. Just curious though, how much actually film is inside a capacitor - I'm imagining there's probably not much.

tim

 

[PRR]

> I believe these differences in dimension contribute very much to the differences in sound we hear from one capsule to the next.

Of course.

There's no difference between these mikes and a kettle-drum.

Except you want the kettledrum to ring a lot, the mike should ring just enough so it won't be "dull/dead".

And of course, the mike has to be tuned to the top of the audio band.

So first step is reduce all dimensions. The 30 inch skin becomes 1 inch or so. The kettle gets much shallower.

But actually, we want a solid electrode very-very close to the diaphragm. So the air space can't be a "kettle". In a simple design, we use the fact that the electrode works best in the center 2/3rds of the diaphragm, and leave a recess all around it that is is the desired air space. That tends to leave a strong resonance. There is very little resistance damping of diaphragm motion. The air in the center is trapped, and the air at the edge of the diaphragm just slides right out into the air space. We want a lot of little air spaces and resistances all over the diaphragm.

A big wide air space has little resistance and acts as a spring. A deep narrow air space has a lot of resistance and acts as a damper. The idea is to get the right balance of deep and narrow.

The AIP microphone book has some calculations for omni mikes. I have never seen detailed calculations for the classic bipolar mike. I suspect that such calculations are HARD work, and require a lot of insight to arrive at an answer before your fingers fall off. For one-off DIY, I think "research" (i.e. copying) is the wisest plan to arrive at starting dimensions. Unless you can copy precisely, your mike won't come out exactly like the one(s) you "researched", so then you change dimensions to find a better balance for your particular construction.

Seems to me there must be some rule-of-thumb, similar to the way an experienced speaker builder can hack-out a bass-reflex box for a new speaker from a few thumps and insight into box and port proportions.

 

[PRR]

> much actually film is inside a capacitor - I'm imagining there's probably not much.

A half-inch diameter cap should have around 100 inches of foil.

They are not kidding: it is plenty for lots of experiments. If a $5 cap yields 50 mikes, you are paying $0.50 per diaphragm. It does not pay to buy a 1,000-foot roll of cap-film unless you are going into production.

The bigger the better. The outside of a 1" diameter cap will give smooth foil, the inside layers of a small cap will be all curled up and harder to flatten.

The old wax and paper caps were easy to cut up. I pulled a couple feet off a wax-cap in a few minutes. Dunno how you do it with modern caps.

 

[SonsOfThunder]

[quote author="PRR"]If a $5 cap yields 50 mikes, you are paying $0.50 per diaphragm.[/quote] Want to try that math again? :razz:

Peace!
Charlie

 

[gyraf]

[quote author="PRR"]The bigger the better. The outside of a 1" diameter cap will give smooth foil, the inside layers of a small cap will be all curled up and harder to flatten.[/quote]

And lower voltage rating caps has thinner foil base. Don't use a 400V capacitor if you can find a 63V type..

Jakob E.

 

[dale116dot7]

saxtim: The 'step' in the drilled-through holes occurs at the same depth as the stop on the blind holes on the 797 capsule (K87-like) I have in front of me; I think that was how your question was worded.

 

[saxtim]

Yes, all 1.6mm holes are drilled to a depth of 3.75mm. First drill all holes with the 1.6mm bit and after through drill whatever holes you want to act as a phase/resistive network with a .6mm bit. These hole sizes only apply to a 25mm solid backplate design.

Cool, thanks.
Hopefully soon I'll have some pictures that I can post of the progress I've made on the Debenham capsule. I'm going to start on giving a k67 style a go to.

A half-inch diameter cap should have around 100 inches of foil.

They are not kidding: it is plenty for lots of experiments. If a $5 cap yields 50 mikes, you are paying $0.50 per diaphragm. It does not pay to buy a 1,000-foot roll of cap-film unless you are going into production.

Much more than I imagined. Thanks, I'll have a fish through my bits and pieces and try and find some caps to take apart.

tim[/quote]

 

[dale116dot7]

I guess I'm the rebel in the group that wants to deposit his own gold... but if this is sort of a capsule update thing, I've started on an M7-like capsule (glued-in mylar diaphragm), doing cutting and facing tomorrow, then drilling 153 holes per capsule over the weekend... ugh. I won't get the diaphragms done 'till probably March or so, though.

 

[Tim Campbell]

No Dale,
You're not alone in wanting to deposit your own gold on membranes. I've made friends with Denmark's best expert in this field. His recommendation was to not try to piece together a setup as you're doing. He told me it's a long, expensive, and frustrating process. He recommended finding an old evaporation machine from a university. Many working setups can be found for cheap or free simply because they're not good enough for modern work. So I'm pursuing this also.
Good luck.

 

[dale116dot7]

I've looked around but up here it's such a large distance between an old machine and here that the shipping is way, way, too much. Even a vacuum pump costs $400 to ship, let alone a whole evap machine. Plus up here there's not the funding for universities to replace machines - they keep the old ones going 'till the bitter end. I might look at seeing if I could use a machine for a nominal fee - that might be a lot easier.

 

[dale116dot7]

I was drilling some holes (152 of them) over the weekend:

http://www3.telus.net/~dulan/MyM7.JPG

It needs to go back in the lathe for the centre hole and insulator, and also to finish the surface machining.

 

[saxtim]

I was drilling some holes (152 of them) over the weekend:

Looks great! Is this a direct copy more or less of an M7 backplate? I've never seen one before (disassembled that is)

Got any dimensions of this capsule? (diameter, depth, hole sizes/depths?)

I'm keen to try building as many different designs as I can.

I'm going to go and by some caps in the next few days to pull apart to see if I can get some useable metallised mylar film

tim

 

[dale116dot7]

I think most of the dimensions were mentioned earlier in this thread, but I guessed at the through-holes as 1/32" (0.031"), and the blind holes are around a #58 or 57 bit. I was using 0.155" as a stop depth - that is one dimension I don't have, so I guessed that it would be most of the way through the backplate. I arrived at the hole locations by carefully looking at any photos I could get of M7 or K47 capsules, and using my PC board CAD program to generate X and Y locations for the mill. You could adapt this to be a clamped diaphragm, but I was planning on building it as a glued-in diaphragm.

 

[Marik]

Hi Dale,

Great work! Are you going to lap it or sandblast?
What kind of glue are you planning to use?

 

[dale116dot7]

Probably I'll lap it on the lathe with 2500 grit paper to polish it up, then maybe a quick pass with slightly coarser paper to prevent sticking. I was thinking of using either epoxy (I use epoxy to attach the diaphragm onto the tensioning ring and it's solid), or maybe Araldite 502 or something like that. The finish is rough now because I put in some roughness intentionally to keep the drill bit from 'walking', so after I machine the diaphragm 'spacer' (it's machined right into the brass) and spot for the glue, it'll be much smoother than you see here.