I've been doing the thing that you're not supposed to do: building my own condenser capsules. I thought I'd share my technique.
I wanted to experiment with the shape of the capsule, membrane tension, and pattern of backplate holes to get a feel for the difference it makes. However my metalwork skills suck, and I only have soldering equipment and a desktop CNC milling machine to hand. I also wanted each capsule to be cheap to make because the intention is to make a variety of shapes / hole patterns to see what effect this has.
So I've been building them from unetched sheets of copper-clad PCB (exploded view):
Cheap space blankets seem to make viable membranes for experimenting, being conductive on one side and about 20?m thick. Scotch tape is a great insulation spacer being only 60?m thick. The generic PCB you can get seems to have a very uniform surface though I've not measured it.
I'm using my desktop CNC milling machine to drill the backplate and faceplate holes and to cut the faceplate aperture. I'm then using a Dremel to polish the backplate and take off any little shards on the cut edges.
To tension the membrane, I've simply been stretching it over a 80mm large metal ring conducting side down, using PVC tape to tack it down so it's visibly flat. Then once I've applied the Scotch tape perimeter to the backplate with a very thin layer of glue, I place the backplate face-down on the membrane and position a weight on top. After about 15 minutes I can cut the membrane free from the ring. Then I position the faceplate, prick through the mounting holes with a needle and push through the nylon bolts and fasten with nylon nuts. I then solder the backplate and faceplate leads. The final step is to fold the Mylar edges over the faceplate and tape them down to make electrical contact.? I found that putting the faceplate copper-side down always caused shorts to the backplate, likely around the mouting holes and it would happen no matter how careful I was.
The capacitance measures 250-800pF depending on capsule size. Some of that is from the clamped section of the membrane.
So far the capsules are quite resonant in the mid frequencies, but disappointingly lacking in the highs, rolling off above 10kHz. The low end response seems to depend strongly on tension, the slacker leading to more prominent lows. The output levels are pretty low, needing about 3 o-clock on the gain to get a reasonable signal. I'm not sure if this is to do with the mass of the membrane or due to the membrane resistance (~3 ohms per 10mm).?
Does anyone know of source of low resistance 6?m aluminiumised Mylar? I'm not keen to go ripping apart electrolytic caps if I can get my hands on factory samples!
