Hi Stewart,
thanks, that was an interesting read.
I gave the first page a try, maybe someone can add more (or turn the denglisch writing into real english
. ).
[quote author="Eugen Schachta"]
Condenser Microphone
by Eugen Schachta
Building microphones in the style of Reisz has been described repeatedly; and with clean work one will have gotten decent results, but every carbon microphone has the disadvantage of high self noise, which is irritating especially with high gain. This was the main reason 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 decent 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.
Abb. 1 shows the working principle and the schematic of the microphone. C is the microphone capacitor, 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
(i guess he means tube stages with plate resistors as opposed to interstage transformers that were not uncommon at that time). It is appropriate to supply the preamp from batteries only, this also prevents noise.
Abb. 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) 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 conected 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.
[/quote]
A wiki might be nice for things like this...