DIY Large Diaphragm True Condenser Microphone Capsule - From Scratch!

There comes a point in time in every DIY microphone builder's life where making a microphone by soldering together PCB circuits and off-the-shelf capsules is not enough. They want to get full control over the sound of the microphone, so they embark upon trying to build their own condenser microphone capsule.

Historically, home built capsules are often mediocre in sound, and this is mostly due to the tight tolerances needed to get a diaphragm membrane hovering within 100µm above a smooth metal backplate, and also difficulty in getting hold of materials such as gold sputtered Mylar.

Here I'll show you how to do it and get good results, without needing a micrometer tolerance lathe or a clean room or a gold sputtering vapouriser...

Microphone Capsule Types (skip this bit if you know your condensers from your ribbons):

There are several types of microphone capsule employing different physics to generate a signal:

Studio quality (high fidelity) stuff:

• "Condenser" capsules:
• Electret capsules = permanently charged (static) backplate, no need for bias voltage
• True condenser capsules = conductive backplate, needs a bias voltage
• Dynamic capsules = a diaphragm attached to a coil moves around a magnet
• Ribbon capsules = a strip of thin metal moves between two bar magnets

Low fidelity designs:

• (Piezoelectric capsules = a quartz crystal gets squished by a diaphragm and generates a voltage, lacks fidelity)
• (Carbon microphones = a box of carbon granules gets squished by a diaphragm and generates a voltage)
• (Optical / laser microphones = laser is reflected off a diaphragm onto a light-dependent resistor or photocell and the variance in diaphragm position causes a variance in voltage. I suspect we'll see studio quality versions emerge in the next decade.)

We will be talking about condenser capsules from now on, and will be making our own true condenser capsule.

Off-the-shelf Condenser Capsule Designs:

90% of a condenser microphone's sound is down to its capsule, yet there are only a few designs available on the market as stand-alone components:

• Electret capsules = permanently charged backplate, no need for bias voltage:
• Small diaphragm electret - the kind every circuit component shop sells, found in laptops, lapel microphones, cheap computer mics. Limited bass response.
• With inbuilt FET transistor = have a little transistor built within the microphone container which provides amplification (more common)
• Without inbuilt FET transistor = require an external FET transistor
• MEMS microphone (surface mount electret) - found in modern smartphones, tablets, smart home devices like Amazon Echo. Limited bass response. All have inbuilt FET transistors.
• Large diaphragm electret - available in 16mm and 25mm versions (TSB2555 being particularly high quality), found in lower cost studio microphones such as AT2020.
• With inbuilt FET transistor = have a little transistor built within the microphone container which provides amplification
• Without inbuilt FET transistor = require an external FET transistor (most common for large diaphragm electrets
• True condenser capsules* = conductive backplate, needs a bias voltage
• (Small diaphragm true condenser - less than 16mm diameter, hard to obtain)
• Large diaphragm true condenser - greater than 16mm diameter and typically 25mm active area diameter (with total capsule diameter typically being 34mm)
• K47 style = single backplate, centre terminated diaphragm (two wire)
• K67 style = double thickness single polarity backplate, dual diaphragms, centre terminated (three wire)
• K87 style = double backplate each with own polarity, dual diaphragms, centre terminated (four wire)
• C12 single style = single backplate, edge terminated diaphragm (two wire)
• C12 dual style = single polarity backplate, dual edge terminated diaphragms (three wire)

* Neumann has a great chapter on large vs small condenser capsules.

The Limitations of Circles:

There is one stonking limitation with all these capsules: they are all circular.

Ehrlund microphones of Sweden have a video which explains succinctly why circular shapes are cursed by a resonant peak. Simply put: circles "ring" like a gong does. This is not what you want in a microphone because it would make the frequency response very uneven and gives poor "temporal resolution".

If you've ever looked at a standard condenser capsule backplate you'll see lots of holes drilled. On closer inspection, some are blind-ended and some are straight through. The straight through holes make the capsule directional, they allow sound through from the back but delayed. The blind ended holes are all about killing off the membrane's resonance. In other words, we're having to drill an elaborate network of holes where many of them are just to compensate for the undesirable property of a circle.

Surprisingly few professional microphone manufacturers make non-circular capsules. The ones I've found:

• Ehrlund microphones (triangular true condenser membranes - see patent)
• Audio-technica - AT5040 & AT5047 (quad rectangular electret membranes)
• Pearl / Milab (rectangular true condenser)
• Bock Audio 5zero7 (elliptical membrane)

The rest are all circular membranes. This is largely because we've settled on making capsules using a lathe.

Wouldn't it be an advantage to be able to easily create capsules of any shape from home?

The Limitations of Home Equipment:

Condenser capsules are essentially capacitors with ~50pF-250pF capacitance where one plate (the diaphragm) is able to move, and the other plate (backplate) is fixed. To achieve a decent capacitance, the diaphragm membrane must be close to the backplate. The other way to achieve a decent capacitance is to make the area of the conductors larger. However there is an upper limit on the dimensions of the diaphragm because the larger it is, the less well it responds to higher frequencies.

The ideal size of an active membrane seems to be around 15mm-40mm diameter to be able to span 20Hz - 20kHz audio frequency range, thus to get a decent capacitance at this small area, the diaphragm needs to be very close to the backplate, within 100µm. This is our first limitation: getting a backplate so smooth that it doesn't have ridges and burrs which would rise up and hit a membrane is hard even with a mid-range lathe. The second limitation is getting some sort of fine spacer to produce a gap of less than 100µm.

The third limitation is in the diaphragm membrane material. It needs to be conductive on one side so it acts as a capacitor, but insulated from the backplate so our "capacitor" isn't shorted out. The membrane needs to be low mass so that relatively small movements of air from sound waves cause the membrane to move. Metalised PET plastic film is what you'll find in commercial true condenser microphones, however this film is normally produced by vapourising gold ("gold sputtering") so that it condenses on the plastic film on one side... not something easy to do at home.

However if we keep to the same physics principles of a condenser capsule but radically change the design and build process, it is possible to get around these limitations.

The Workarounds:

Backplate smoothness:

There is a cheap, mass-produced, easy to cut-and-shape material, with a very flat surface (<10µm variance): copper clad PCB!

You can pick this up for ~£4 per 100mm x 150mm. Go for FR4. Most is 1.6mm thickness total with the top 35µm being smooth copper. If you can get thicker total thickness (2mm or even 3.2mm), then go for it because the less flexible the backplate, the better the high frequency response.

Backplate-membrane separation:

Once again, there is a cheap, mass-produced, easy to cut, adhesive insulating material with a really consistent thickness of 62µm: Scotch tape / sellotape! Alternatively, double-sided tape is adhesive on both sides and is just 32µm thick.

Either tape can be stuck on the backplate around the edges of the intended active area, upon which the diaphragm membrane can be affixed with superglue or directly.

Diaphragm material:

The ideal material for a condenser capsule membrane is conductive on one side only, and very good under tension. 6µm Mylar (PET) sputtered with gold is the most commonly used membrane material commercially.

Would you be surprised if I told you there is a cheap, mass-produced, easy to cut, conductive on one side high tensile thin plastic film available too? Yep: insulating thermal "space" blankets.

You can pick these up for less than £3 for a 1000mm x 1500mm sheet on Amazon! Instead of gold, they are coated on one side covered with aluminum. They are folded, but the panels are at least 50mm x 100mm, perfectly big enough for any capsule we'll be making. Make sure you get ones made from PET (Mylar). The cheapest ones tend to be made with the thinnest Mylar at 6µm, presumably cost saving. Something this thin makes for a fairly poor thermal blanket, but absolutely excellent for our nefarious purposes as a diaphragm membrane!

Consumables:

• Backplate: 35µm copper-clad PCB board, FR4 with total thickness 1.6mm-3.2mm (the thicker the better)
• Membrane: Thermal insulating "space" blanket: 6µm aluminium-coated PET/Mylar film
• Capsule-membrane spacing:
• Scotch tape + superglue, OR
• Double-sided tape
• Frontplate: more of the same copper-clad PCB board
• 6 x 2.5mm bolts + nuts (use Nylon bolts if you wish to use the same bolts to mount the capsule to a metal mic body frame, otherwise metal is ok)
• Solder
• 15cm insulated thin equipment wire (for capsule leads)
• PVC "electricians" tape - for tensioning the membrane and insulating capsule edges if you wish.
• Cleaning capsule surface: more Scotch tape (stick and lift dust and dirt)

Equipment:

• Computer for creating PCB design (can use something simple like Inkscape)
• For etching and cutting the PCB:
• Desktop CNC milling machine with 1mm-2mm PCB mill, OR
• UV etching tank + design mask + bandsaw/coping saw, OR,
• Outsource the PCB etching and cutting to someone like PCBway
• Decent scissors
• Membrane tensioning:
• An 80mm ring (e.g. top 4cm of paint tin) for stretching the membrane across to tension it, OR
• Homemade tensioning jig from small length of 80mm-110mm pipe + 4 bolts + 4 wingnuts + 2 pieces of plywood
• Soldering iron
• Wire strippers (you could get away with scissors or your nails as it's thin equipment wire we're stripping)
• Dremel with sanding attachment: for rounding corners and taking off copper burrs
• Scalpel or craft knife
• Needle (sewing is fine, but 23+G hypodermic is even better): for puncturing the membrane at the mounting holes so you can put bolts through without detaching the membrane

Other optional useful items:

• Acetone (nail varnish remover), for removing traces of tape adhesive or superglue to have another go if you mess up sticking the membrane
• Superglue debonding agent: for removing superglue to have another go
• Q-tips (cotton bud tips) for applying glue, or cleaning off glue with acetone / debonding agent
• Multimeter / ohmmeter to confidently identify the conductive side of the space blanket

+ some sort of donor microphone body + circuit to test this out on. I recommend:

• Cheap BM-800 microphone body, hang onto the XLR connector that comes in it (or a small metal biscuit tin + some aluminium mesh + 3 way terminal blocks, if you're testing capsules out)
• DJ Jules' DC-DC Hex Inverter multiplier circuit (circuit schematic, PCB on PCBway)
• DJ Jules' OPA Alice circuit (circuit schematic, PCB on PCBway)
• Phantom Power source

First, you want to design your capsule shape. Your design will be used to etch (and if using a CNC milling machine, cut) the PCB.

Before you do anything else, measure the microphone body's headbasket you are going to use. You will need to fit the capsule into this, ideally with at least 1mm spare between capsule edges and headbasket mesh to provide some shock resistance. This will define the upper limit of the the of capsule you can make.

I use Inkscape for the design, though Adobe Illustrator will work as well. They both allow you to create vector graphics and make shapes out of Bezier curves.

Your design needs to have at least two areas of equal dimensions:

1. The backplate: etched to outline the active area and separate it from the non-active mounting area.
2. The front plate: with an aperture cut in it

If you are using standard 1.6mm thick PCB, you should also have a third area marked:

3. The rear backplate: An extra PCB etched with a mirror image of the active area, used to thicken up the backplate

If using Inkscape, go to File > Document Properties and ensure the project is set up in cm or mm, and you have adjusted the page size to be twice the width of your capsule and height as once (if not making a rear backplate) or twice the length (if making a rear backplate).

There are important features in each of these areas:

Backplate:

• Active area - this can be any continuous 2D shape. Experiment with different shapes!
• Etched valley separating the active area from the rest of the backplate
• A copper trail connecting the active area to one corner of the backplate - this is where you'll solder the backplate wire
• Mounting holes: At least four 2.5mm diameter holes around the edges of the backplate area, so you can bolt it to the front plate. These should all fall into the non-active area of the backplate
• Ventilation holes: Several full thickness 2mm diameter holes through the backplate inside the active area. This will make your capsule directional. You can vary the number, layout and size of the holes to see what effect it has on sound and directionality.
• Make sure one of these holes is in the dead centre of the active area. This will allow you to add a nylon tensioning bolt if your capsule ends up too bass-heavy
• A corner cut off at a different corner to the copper trail soldering area. This is to provide space for the frontplate lead to reach the frontplate
• (optional) If using a rear backplate: Partial thickness holes which will be cut through the backplate, but not through the rear backplate.

Front plate:

• Aperture - this should be the same shape as your backplate active area and just be slightly larger than it. This area will be cut out, providing a window for sound to reach the membrane
• A corner cut off - should be the mirror corner to where the copper trail on the backplate leads. This will be cut off to provide space for the backplate lead to reach the backplate.
• Etched valley in the mirror corner to the backplate copper trail and at least three times as wide as it. This is to ensure the backplate's copper trail does not short to the frontplate
• Mounting holes: 2.5mm holes in the mirror position to the mounting holes in the backplate

The front plate will have its copper side facing the copper side of the backplate.

Rear backplate:

(if using 1.6mm PCB, or if you want a thicker backplate)

This allows you to thicken up the effective backplate by adding a second layer. A thicker more rigid backplate gives better high frequency response. It will be mounted back-to-back with the backplate, so that its copper face faces outwards.

• Mirror image of the backplate etched valley separating the active area from the rest of the backplate. This is to ensure that the backplate does not get accidentally connected to the frontplate via the rear-backplate.
• A copper trail connecting the active area to one corner of the backplate - this is where you'll solder the backplate wire
• A corner cut off - should be the mirror corner to where the backplate's cut off corner is. This is to give space so the front plate lead can be soldered to the front plate
• Mounting holes: 2.5mm holes in the mirror position to the mounting holes in the backplate
• Ventilation holes: Several full thickness 2mm diameter holes through inside the active area, in a mirror image to the backplate ventilation holes.
• DO NOT copy the partial thickness holes from the backplate. Only the full thickness ventilation holes!

The example I've provided "Bowtie", is a square with its edges bowed in. Yellow areas (ignore the text) are where the copper is fully etched away. Black lines are where cuts are made. Blue circles are where mounting holes are drilled. Green circles are the ventilation holes. Light green circles are the partial thickness holes.

You can experiment with your own hole layout and active area of the backplate. Try various shapes. Circles are bad because they ring at a resonant frequency, because they have the same distance across the circle's centre to the edge of the circle. Other shapes have a variety of distances from centre to edge, meaning that different wavelengths of membrane vibration will be induced, thus less unwanted resonance. If Ehrlund's hypothesis is correct, the further you deviate from a circle, the better damped the unwanted resonance will be.

The Bowtie.svg file (below) can be opened in Inkscape. We will later convert it to a file for etching and cutting the PCB.

Variables you can change to alter the sound:

1. Overall shape of active area (and faceplate aperture)
2. Number, diameter and location of ventilation holes
3. Number, diameter and location of partial thickness holes
4. Tension of the membrane
5. With or without a central tensioning Nylon bolt
6. Point heights and ridges in the active area to "tent" the diaphragm (deposit some solder on the active area copper to achieve this)

This step is different depending on whether you will be etching and cutting your PCB with a milling machine, using an etching tank, or sending it off to a PCB manufacturer such as PCBway

CNC Milling Machines:

If you wish to make the "Bowtie" capsule, I've included the file below

1. Save your design as a "plain SVG"
2. Use an SVG to GCode converter, such as the awesome online jscut.org:
3. Set 96 as px per inch if importing from Inkscape
4. Set up the dimensions in mm
5. Put in the correct mill bit diameter (whatever PCB milling bit you are using)
6. Set the appropriate speeds. I suggest:
7. Step over = 0.3
8. Rapid = 800mm/min
9. Plunge = 20mm/min
10. Cut = 120mm/min
11. Open SVG > upload the Inkscape SVG that you created in Step 1
12. Select all the yellow etch areas
13. click "Create Operation"
14. select "engrave"
15. set depth to 0.045 (45µm, ensure a 0 is between the decimal point and the 4!)
16. click "generate"
17. Select the mounting holes, ventilation holes:
18. click "Create Operation"
19. select "pocket"
20. set depth to the thickness of your PCB plus 10% (e.g. 1.8mm if using 1.6mm board)
21. click "generate"
22. Select the partial thickness holes (if wanting to drill these):
23. click "Create Operation"
24. select "pocket"
25. set depth to the thickness of your PCB plus 10% (e.g. 1.8mm if using 1.6mm board) if using both a backplate and rear-backplate, else set it to LESS than the PCB thickness (e.g. 1mm for a 2mm thick PCB)
26. click "generate"
27. Select the frontplate aperture
28. click "Create Operation"
29. select "outline"
30. set depth to the thickness of your PCB plus 10% (e.g. 1.8mm if using 1.6mm board)
31. click "generate"
32. Select the diagonal corner cuts
33. click "Create Operation"
34. select "engrave"
35. set depth to the thickness of your PCB plus 10% (e.g. 1.8mm if using 1.6mm board)
36. click "generate"
37. Select the final plate separating cuts (diagonal and horizontal)
38. click "Create Operation"
39. select "engrave"
40. set depth to the thickness of your PCB plus 10% (e.g. 1.8mm if using 1.6mm board)
41. click "generate"
42. Save GCODE > save to local disk, give it a sensible name (e.g. capsule-octagon.gcode)
43. You may need to manually open your GCODE file in a text editor to specify the drill spin speed by adding the following lines just after the "move to clearance level" line:
44. S80 (change the number to whatever % of max RPM will give you about 5000 rpm)
45. M3 (to make the bit start spinning at that point)

PCB etch tanks:

1. Move all the objects onto another layer in Inkscape apart from the yellow etch areas
2. Turn the yellow etch areas black
3. Obtain an acetate sheet and borrow the use of a laser printer
4. Load the laser printer with the acetate sheet
5. Print your black design onto the acetate sheet. This will be your etching tank overlay
6. Then unhide the holes and cuts in Inkscape, but and hide the etching areas.
7. Print this onto another acetate sheet or tracing paper sheet. This will be your hole and cut guide.

PCB manufacturing service (e.g. PCBway):

This method requires the most technical knowledge and familiarity with PCB design, but probably produces the cleanest finest tolerance capsules. You need to be familiar with Gerber files. You can start in Inkscape, but also can build using a Gerber editor (e.g. gplEDA for Linux, EasyEDA web-based (cross-platform), Osmond PCB for Mac, FreePCB for Windows)

1. Consider amending your design so you can produce multiple capsules of different shapes on one PCB.
2. If the manufacturing service offers thick (2mm+) double sided boards, go for that:
3. Mirror the rear-backplate area so that it is the same way round as the backplate design. Use the rear-backplate design as the bottom copper layer for the backplate section, ensuring all other parts of the bottom solder copper layer are clear (i.e. no copper remains)
4. Use the main backplate design and the faceplate design as the top copper layer
5. Otherwise, if only 1.6mm board is available stick to single sided and make the entire etching happen on the top copper layer.
6. (optional) Add a Top Solder Mask layer to the backplate to cover every area EXCEPT the active area and the active area's solder tab on the corner. This will add 5-10µm to the separation between membrane and active area which is probably helpful.
7. If starting from Inkscape, convert your SVG file into a Gerber file package using a tool like Gerbolyze
8. You will need to invert / subtract the etch areas from the unetched areas to give a the areas where copper should remain. This will be for the top copper layer (and bottom copper layer if using double sided board)
9. You want the black diagonals, horizontal cuts, vertical cuts and aperture to be registered as PCB board outline layer (keep out layer)
10. Holes should be registered as drill guides
11. Ensure the top solder mask layer is registered to insulate all but the active area

Now you will need to actually produce your PCB:

CNC Milling Machines:

CNC Milling Machines all differ, but almost all will happily accept GCODE commands. I have a cheap Chinese 3018 3-axis desktop milling machine. I've had to do a few minor mods to get it reliable (upgraded the power supply, replaced the motor grub bolts with fatter M4 bolts, added limit switches).

I am using Tungsten Carbide PCB End Mills, which will happily etch, cut and drill PCB. A 1.5mm bit can do the entire job without needing to change tool. You will not need any coolant for PCB.

1. Set up milling machine and connect it to your computer
2. Ensure your computer has its USB port suspend mode switched off so the job won't be interrupted
3. Place your copper clad PCB on a thin, totally level piece of wood or stiff cardboard approx the same size to protect the milling machine platform
4. Ensure your PCB edges are parallel to the X and Y axes (your PCB should be "square" in alignment). Clamp the PCB to the wood and platform. You will want to ensure the top left corner is not covered by a clamp. Also keep the clamps away from any drill / etch areas.
5. Use a CNC milling machine program like Universal G-Code Sender (UGS platform). Open your Gcode file.
6. Position the platform and milling bit so it is at the zero point (top left of PCB board, bit just touching board)
7. Reset the zero point on your Gcode platform so that the milling bit's position correlates with where the computer thinks it is.
8. Run the Gcode job.
9. Ensure you keep the piece of PCB that was the aperture of the frontplate. This will be useful as a guide to cutting the Scotch / Double Sided tape.

Tidy up the PCB:

1. Using a Dremel with a coarse sanding bit, gently round the sharp corners off the edge of the PCBs without taking so much off as to lose the copper solder tab of the active area.
2. Using a Dremel with a fine grinding bit, gently but quickly run it over the surface of the active area without taking off all the copper. This will remove any burrs caused by drilling.

PCB etch tanks:

1. Use the acetate overlay as a mask for the PCB etching tank
2. Etch the PCB in the tank (ensure acid present, turn on the UV light, leave for appropriate amount of time)
3. Remove the PCB from the tank, wash
4. Overlay the holes and cuts acetate/tracing paper sheet and stick down at edges with tape. This will be your guide for cutting and drilling
5. Cut the holes (2mm drill bit + 2.5mm drill bit required)
6. Cut the aperture and diagonals and PCB edges (you can use a Dremel with a fine cutting bit if very careful and the PCB is held in a vice)

Tidy up the PCB:

1. Using a Dremel with a coarse sanding bit, gently round the sharp corners off the edge of the PCBs without taking so much off as to lose the copper solder tab of the active area.
2. Using a Dremel with a fine grinding bit, gently but quickly run it over the surface of the active area without taking off all the copper. This will remove any burrs caused by drilling.

PCB manufacturing service (e.g. PCBway):

1. Submit your Gerber files to your choice of PCB manufacturing service, await the boards in the post!

1. Give the boards a rinse under water, blot dry with kitchen towel. Alternatively clean with isopropyl alcohol.
2. Use Scotch tape to remove dust from the active area by sticking it to the backplate then immediately lifting it to pull dirt away
3. Carefully cut Scotch tape or double-sided tape to the same shape as the space around the active area of the backplate.
4. If you used the CNC milling technique the bit of PCB you cut out as the frontplate aperture is a useful template! (see picture)
5. Apply the Scotch tape or double-sided tape to the backplate in the areas away from the active area.
6. If using a rear-backplate to double up the backplate thickness:
7. You may find it easier to get the backplate and rear-backplate aligned by pushing a couple of straightened paperclips (or solid core equipment wire) through the backplate holes from copper side to non-copper side, taping a bit to the copper side and then sliding the rear-backplate over these bits of wire.
8. Spread a thin layer of superglue to the back (non-copper side) of the backplate, avoiding the holes
9. Line up the backplate over the rear-backplate with the rear-backplate's non-copper side facing the backplate's non-copper side ("back to back"), so that the cut corner, ventilation holes and mounting holes line up too.
10. Press together and place a weight on them. Leave for 20 mins to bond
11. Using the needle clear any excess glue from the holes once dried.
12. Apply a dab of solder to the backplate solder tab corner and the frontplate solder tab corner

If you have used a rear backplate to add mass to the capsule so as to improve the high frequency responses, then you can go a step further and add more mass by adding a thick layer of solder to the rear backplate.

Your rear backplate should be facing with its copper side outwards.

1. Make absolutely certain you are looking at the rear-backplate. You do NOT want to add a layer of solder to the active area of the main backplate as this will eliminate the gap between membrane and backplate!
2. Using a hot soldering iron, deposit a ~1mm thick layer of solder over the rear-backplate's central region. Avoid covering up any holes
3. Allow to cool

You will find that this will improve both high frequency response and by making the holes effectively longer, will improve the low frequency response to. So this is a great intervention to do if you find your capsule sounds too "mid" heavy.

This involves stretching the membrane across some sort of gap so that you can lower the backplate onto it. The tension will either need to be set using a tension jig, or by placing some weights on the backplate.

1. Ensure you have the non-conductive side of the membrane facing you (upwards) if using a paint tin to stretch over. If using a jig, the orientation will depend on the jig.
2. If you are going to stick the membrane to the backplate, you want it so that you are sticking the backplate's copper side face down onto the non-conductive side of the membrane
3. Alternatively, if you are wanting to stick the membrane to the inside of the faceplate, you want it so the conductive side is facing up to you so you are sticking the faceplate onto the conductive side.
4. Cut 3-9 panels worth of membrane from the space blanket.
5. When handling the Mylar material, only touch the edges. You do not want fingerprints on the middle part of the membrane!
6. You want it so that there is a decent area in the middle of the jig / paint tin where there are no folds, so need to cut enough such that one panel can be centred over the jig / paint tin.
7. Stretch the membrane across the jig / paint tin.
8. Tape down firmly on the edges using PVC tape.
9. Adjust the PVC tape so that you get equal looking tension (a mirror-like appearance). Ensure your PVC tape is stuck firmly to the jig / paint tin.
10. If using a jig, tighten the jig up so that the membrane has a tight mirror-like appearance.

Several people have reported that they have had more success with sticking the membrane to the inside of the faceplate, because this ensures you can prevent excess glue leaching onto the active area.

If sticking the membrane to the backplate:

1. If using superglue (double-sided tape users can skip this)
2. Apply a thin layer of superglue to all areas of the copper side of the backplate apart from the active area and active area solder tab
3. Spread the glue using a q-tip (cotton bud). You need to be quick and have about 15 seconds total, and should not have any large blobs but rather have a smooth thin layer.
4. Place the backplate copper side down (face-down) onto the stretched membrane (which should be having its non-conductive side facing up)
5. Position a 100g-250g weight on top of the non-copper side of the backplate which should be facing up (I use some offcut of brass to do this)
6. Leave it for at least 30 mins, ideally longer.
7. Once bonded, take the weight off.
8. Carefully cut around the backplate to cut it free of the jig/paint tin without accidentally ruckling the membrane or peeling it off the backplate
9. Lay the backplate down on the surface with its membrane side facing UP
10. Using scissors or scalpel, very carefully trim off any excess membrane. Ensure the copper solder tab corner area is uncovered and the edge of the membrane is not touching it
11. You may find it easier to remove excess membrane by using a soldering iron to burn a wide border around the active area into the membrane, and then peel off the membrane that lies outside of this. This has the additional benefit of reducing parasitic capacitance.

If sticking the membrane to the inside of the faceplate (I've had better results this way):

1. If using superglue (double-sided tape users can skip this)
2. Apply a thin layer of superglue to all areas of the copper side of the faceplate.
3. Spread the glue using a q-tip (cotton bud). You need to be quick and have about 15 seconds total, and should not have any large blobs but rather have a smooth thin layer. Ensure you cover right up to the edges of the aperture.
4. Place the faceplate copper side down (face-down) onto the stretched membrane (which should be having its conductive side facing up)
5. Position a 100g-250g weight on top of the non-copper side of the faceplate which should be facing up (I use some offcut of brass to do this)
6. Leave it for at least 30 mins, ideally longer.
7. Once bonded, take the weight off.
8. Carefully cut around the faceplate to cut it free of the jig/paint tin without accidentally ruckling the membrane or peeling it off the faceplate
9. Lay the faceplate down on the surface with its membrane side facing UP
10. Using scissors or scalpel, very carefully trim off any excess membrane.
11. You may find it easier to remove excess membrane by using a soldering iron to burn a wide border around the aperture into the membrane, and then peel off the membrane that lies outside of this. This has the additional benefit of reducing parasitic capacitance.

1. Orientate the face plate so that it is copper side down, with its solder tab corner on the same corner as the cut corner of the backplate. You want to have the backplate's solder tab visible from the front on one corner, and the frontplate solder tab visible from the back at a different corner. That's why we put the diagonal cuts on different corners.
2. Carefully position the faceplate down onto the backplate so that the mounting holes line up. You should be seeing the non-copper side of the faceplate with the silvery membrane visible through the big faceplate aperture.
3. Pinching the faceplate and backplate together firmly, take the needle and push it through one of the mounting hole from the backplate side to the frontplate side so that you puncture the Scotch tape and then the membrane. Wiggle the needle around a bit to ensure the full 2.5mm hole is clear of membrane or tape
4. Pass a mounting bolt through from the backplate side to the frontplate side (this ensures any spurs of membrane get pushed onto the frontplate and do not short to the backplate), and secure with a nut.
5. Do the same with the other mounting holes, (prick with needle, wiggle to clear tape and membrane pass bolt through, secure with a nut), one at a time.

1. Electrically test your capsule:
2. With an ohmmeter (multimeter on resistance mode) set to the highest range, place one probe on the solder tab of the backplate and one on the solder tab of the frontplate. They should be isolated (resistance unreadably high). If the resistance is low then there's a short between the frontplate and backplate. You will have to disassemble the capsule and find it.
3. If you have a capacitance meter, keep the leads in the same position and measure the capacitance. It should be 50-250pF, though I've had capsules working with 850pF before!
4. Cut two x 5-7cm of equipment wire and strip 3mm from each of the ends
5. Solder one end of the first lead to the backplate soldering tab corner.
6. Solder one end of the second lead to the frontplate soldering tab corner. You will probably want to cover over the aperture with a bit of tape or paper (but without touching the membrane) to stop bubbles of rosin jumping onto your precious membrane!
7. Place this capsule into the head basket of your microphone body
8. Solder or connect the capsule backplate lead to the bias part of the microphone circuit
9. Solder or connect the capsule frontplate lead to the signal part of the microphone circuit
10. Replace the head basket and body case

1. Connect the assembled microphone to a Phantom Powered microphone XLR input
2. You may need to turn the gain up on your mixer
3. Listen to it!

Troubleshooting:

Generally the tendency is for people to design capsules that are too large, thus sound bassy and muddy. Most commercial high end condenser capsules are 34mm in outer diameter, with the active part of the membrane usually less than 25mm. Bigger isn't necessarily better (unless you are going for a bass-heavy mic).

I suggest shapes where there is a variety of distances from edge to edge (across the centre) of 15mm - 30mm.

Generally speaking:

• Larger distances mean better bass response. Smaller distances mean better treble response
• Larger distances mean worse temporal resolution (close together sounds like hi-hats might mush together). Smaller distances mean better temporal resolution.
• Slacker membrane (lower tension) means better bass response and a wider bandwidth (wider frequency response) overall. Tighter membrane (higher tension) means better treble response and a narrower bandwidth (narrower frequency response)
• Slacker membrane (lower tension) means mean better temporal resolution. Tighter membrane (higher tension) means worse temporal resolution (close together sounds like hi-hats might mush together).

The whole point of this capsule design is it is cheap to knock up at home. This means you can experiment with all sorts of shapes and tensions without breaking the bank!

Problem: No sound

Causes: No phantom power / short between the membrane and backplate

Resolutions:

• Check the resistance from one capsule lead to another, it should be very high. If not, you have a short. (do this powered off)
• Check your capsule leads are properly soldered
• Check the mic circuits are working (test with another capsule)
• Check Phantom Power is actually switched on
• Check mic is connected to the line in properly
• Check your mixer gain is up enough

Problem: Output signal too low

Cause: Likely to be too much parasitic capacitance between the active area and non-moving parts of membrane.

Resolution:

• Ensure your etched lines which separate active from non-active areas are wide (at least 2mm)
• If using a rear-backplate, ensure you have the same pattern etched so that you don't get accidental capacitative coupling from active area backplate <-> rear backplate <-> non-active area backplate
• Trim off as much membrane from non-active areas as possible without ruining the adhesion of the plate
• Check the wires feeding your capsule. Avoid twisting them together and increase the distance between the membrane lead and any ground shielding to reduce the capacitance.

Alternate resolutions:

• Add more gain on your mixer!
• Increase the bias voltage (in the DJ Jules OPA circuit, bump the Zener diode up to a 15V one)

Problem: Part of the membrane is glued to the active area

Cause: Excessive amounts of glue applied.

Resolutions:

• Try the capsule out. You might find it sounds good like this!
• Remove the the membrane, remove glue residue with acetone / debonding and have another go with less glue this time
• Remove the the membrane, remove glue residue with acetone / debonding and have another go but with double-sided tape

Problem: The capsule is lacking in the high end (treble) / sounds too bass-heavy

Cause 1: Active area shape has large distances in each dimension no small distances

Resolutions for 1:

• Design a new capsule with a different shape where there are smaller dimensions from one side to another
• Shrink the active area to <30mm max dimension

Cause 2: Membrane tension too low

Resolutions for 2:

• Add a tensioning Nylon bolt to the middle (prick the membrane over the middle hole and push a Nylon bolt through, securing with a Nylon nut
• Redo the membrane, this time with more tension / bigger weight when sticking it on

Cause 3: Backplate not thick enough

Resolutions for 3:

• Add a rear-backplate (see guide above). this can be done as a later step provided you remove the mounting bolts and faceplate first before gluing on a rear-backplate
• Dismantle the capsule, then add a thick layer of solder to the rear-backplate (see above)

Cause 4: Parasitic capacitance between the active area and non-moving parts of membrane. (Affect higher frequencies more than lower frequencies)

Resolution:

• Ensure your etched lines which separate active from non-active areas are wide (at least 2mm)
• If using a rear-backplate, ensure you have the same pattern etched so that you don't get accidental capacitative coupling from active area backplate <-> rear backplate <-> non-active area backplate
• Trim off as much membrane from non-active areas as possible without ruining the adhesion of the plate

Problem: The capsule is too resonant in the mid frequencies

Cause 1: Backplate not thick enough

Resolutions for 1:

• Add a rear-backplate (see guide above). this can be done as a later step provided you remove the mounting bolts and faceplate first before gluing on a rear-backplate
• Dismantle the capsule, then add a thick layer of solder to the rear-backplate (see above)

Cause 2: Membrane tension too high + dimensions too large

Resolutions for 2:

• Redo the membrane, this time with less tension / smaller weight when sticking it on
• Redo the capsule with a smaller design (try <30mm)

Cause 3: Too many ventilation holes

Resolutions for 3:

• Tape over some of the holes at the rear of the capsule
• Dismantle the capsule, add a rear-backplate if you've not used one, then add a thick layer of solder to the rear-backplate (see above). You can solder over some of the holes

Problem: The capsule is too tinny / bright / high frequency

Cause 1: Copper burrs tenting the membrane / not enough insulating space between membrane and backplate

Resolutions for 1:

• Remove the membrane, lightly touch with a fine grinding bit on a Dremel to remove burrs
• Add a second layer of tape to the insulating Scotch / double sided tape

Cause 2: Membrane tension too high

Resolutions for 2:

• Redo the membrane, this time with less tension / smaller weight when sticking it on

Cause 3: Active area shape small distances only and no larger distances from edge to edge

Resolutions for 3:

• Design a new capsule with a different shape where there are larger dimensions from one side to another
• Increase the active area size to >30mm max dimension