The original German document is here

We are translating it into English. Once done this will be published and shared at The Lab.
Translation by EZ81, additional notes by PRR, Stewart Tavener
Condenser Microphone
by Eugen Schachta
Originally published in "Radio monthly magazines", Vol 8, August 1932, Weidmann, Berlin

Building microphones in the style of Reisz has been described repeatedly; and with clean work one will achieve decent results, but carbon microphones have the disadvantage of high self noise, which is irritating especially with high gain. This was the key motivation for me to start constructing a condenser microphone. The result of my experiments are described here. A note in advance: it takes extreme accuracy to get a good result, but afterwards one will have fun with the device. The microphone is designed in a way to allow the DIYer to experiment with improvements.


Fig. 1 shows the working principle and the schematic of the microphone. C is the microphone capacitor (capsule), cap plate 1 is a perforated disc which lets sound pass to cap plate 2, a membrane (very thin metal foil). The sound hitting plate 2 changes the capacity of C, and these tiny changes of charge are amplified by the first amplifier stage which is set up the usual way. This connection is naturally the most susceptible one in the whole circuit. Let me add here that this small amount of amplification is usually not enough, it is recommended to add two more resistance stages [See NOTE 1 below]. It is appropriate to supply the preamp from batteries only, this also prevents noise.

Fig. 2 shows a cross section through the DIY microphone. The capacitor is resting inside a brass enclosure a. Inside the latter is a isolating pertinax (= FR-2 [Note 2 ]) ring b, which surrounds a contact ring c, which is connected to the terminal screw m by a wire. Next to this contact ring c is the membrane d and the distance ring f, which sets the distance between the membrane and the fixed cap plate e (a perforated disk). This disk is connected to the enclosure a by the second contact ring g. The end ring i fixes the construction on the front side, the disk k closes the back side.

After this description of the microphone the parts are to be described in detail. As most DIYers do not own a lathe, it is recommended to have parts a, c, e and g made by a precision mechanic. The latter will also happily provide proper screws for the terminals and for fixing the end rings. It is advised to wait for the metal parts to be ready before building the other parts to make sure everything can be fitted together.

As can be seen in Fig. 2 and 3., ring a has a recess, into which the isolating ring b is fitted. The latter is made from 3mm pertinax according to Fig. 6. The notches on the outside contain small pieces of pertinax, whose dimensions are also given in Fig 6. These distance pieces prevent the membrane and the contact ring from touching the enclosure. A piece of high frequency litz [Note 3] is soldered to the inside of the first contact ring c, and c is fitted into b. Both c and b must fit thightly, if necessary,gaps must be filled with thin pertinax (as used for making speaker membranes) or mica.

Now the membrane is made. Cut a circle with diameter 52mm from the thinnest possible sheet metal, so called 'template sheet metal', which can be bought cheaply from any engraver [Note 4]. The thinnest i could get was 0.07mm. I achieved very good results with this. It is left to the DIYer to experiment with thinner material if available. There may be some improvement this way. The frequency response was already very good with the thickness mentioned above. As the membrane sits in a very small distance from the other cap plate, it needs to be absolutely flat. It is also cleaned thoroughly on both sides with cleaning sand (Emery / sandpaper?) and a damp sheet. When fitting it into the isolating ring, it needs to fit exactly and the edges must not get bent, as this would lead to a short-circuit. The spacer ring following now will have to be made from paper, as other materials with proper thickness are probably not available. Cut a ring according to Fig. 7 from the yellow cover paper of the "Funk" magazine, which is just thick enough to prevent the cap plates from touching. This ring is soaked in liquid paraffin wax and put on a string to dry. One can try making this ring from mica (which is available in oven stores), which is split to the right thickness, but the paper ring worked good enough for me. This spacer ring is followed by the perforated disk e and the second contact ring g. The top of g will probably not be even with the top of a, so cut some rings from the template sheet copper and put them between e and g until g sticks out 0.5mm. After the capsule has been assembled this far, the pertinax end rings which press al parts firmly together can be mounted. The front-side end ring i is made according to Fig. 8. Silk can be glued to its inside to protect the microphone system; afterwards a ring from "template copper sheet metal" is cut to the same dimensions as i, its purpose is making electrical contact between enclosure and microphone system. The back side end disk is drilled according to Fig. 9. A screw is fixed in the middle hole and connected to the litz wire that was soldered to the contact ring c earlier.

Afterwards this disk is fixed to the enclosure; one of the fixing screws can -- with an additional nut -- be used as a contact to apply the bias voltage. With everything finished so far the microphone can be checked for short circuits. The membrane is connected to the negative pole of an anode battery and 40 volts are applied to the enclosure with a voltmeter in series. If the meter moves, the microphone has to be opened again to locate the cause of the short circuit. If the meter does not move, the voltage can be raised in 10 volt steps. Usually the voltage van be raised to 220 volts without the membrane being pulled against the back plate. If this happens anyhow the bias voltage in operation needs to keptbelow the critical voltage. After the microphone passed this test, it can be connected to the amplifier. The resistor values I found to work well are given in Fig. 1. It cannot be ruled out that other values lead to good results, and I want to encourage experiments especially in this regard. An A411 tube excelled as the first amplifier stage, but here experiments are in order as well. Those wanting to achieve very good results will place the first amplifier stage directly under the microphone -- which does not necessarily have to be shock-mounted -- with a designated shield. This way the grid connections can be kept short. The connections to the battery and to the next amplifier stage are shielded as well, otherwise there would be interference from the mains which cannot be prevented by other means. A suggestion for mechanical construction can be seen in the pictures on page 351. Maybe another DIYer will find a better design. I would be particularly happy to receive reports from the readers in this regard.


Notes
1 Assumed to mean tube stages with plate resistors as opposed to interstage transformers that were not uncommon at that time.
2. Pertinax is actually the trade name common in .de for the dark brown phenolic PCB material with the nasty smell.
3. Litz wire. see http://en.wikipedia.org/wiki/Litz_wire
4. Assumed to be copper, although any thin conductor could be tried.

Fig. 3: The dimensions of ring a.















Fig. 4: the contact rings a and g












Fig. 5: the perforated disk











Fig. 6: the isolating ring with the distance pieces












Fig. 7: The spacer ring f













Fig. 8: the front ring f












Fig 9: the end disk k