Sinds I have a SEM-35 I have noticed that the mic really sucks (excuse me for the langue). But these carbon microphones have a tendency to become clothed. It come with age and moisture.
So after searching the web i found a side that has nice description how to convert it to a electret microphone. The site is in dutch, so i’ll try to translate some passages.
The site and source of this info in dutch can be found here: http://www.pi4raz.nl/index.php?option=com_content&task=view&id=2413&Itemid=46
The main thing is that they wanted to keer the “old sound” to it. To achieve this, the microphone signal is first passed through a high-pass filter; which here consists of the network C1 / R2. The low-pass filtering at 4.2 kHz will take place with the aid of C3 and R4; the latter components are included in the feedback network between T2 and T1. The capacitors C2 and C6 have the task to suppress RF interference signals, which may be picked up by the wiring or the electret microphone. The network R6 / C4 is added to improve the stability of the amplifier. The DC-behavior of the amplifier is such that it resembles that of a carbon microphone, namely a non-linear resistor. The diodes D1 … D4 at the output of the amplifier form a full-wave rectifier, which is a power supply voltage which is reasonably independent of the line current (namely, which can vary between 15 and 150 mA, depending on the device used, etc.) yields. The rectifier also ensures that the polarity of the supply voltage is always accurate. For the audio signal of the rectifier bridge is absolutely no impediment, since guiding the diodes as a result of the current flowing through the line. The fact that have been used for Zener diodes D1 and D2, has everything to do with the protection of the circuit. As long as the line voltage stays below the zener voltage, borne D1 and D2 like normal diodes. If the voltage is higher, they keep the supply voltage of the circuit combined with R12 within safe limits.