Projection engineering (Sept 1929-Nov 1930)

Projection Engineering, May, 1930 Page 21 that the carbon transmitter may be regarded as a source of constant e.m.f. acting in series with the impedance of the transmitter. The operation is as follows: With the transmitter acted upon by a sustained source of sound and the decade resistance box (R) disconnected, the voltage is then measured across the terminals B and C. This evidently is one-half of the voltage developed by the carbon transmitter. Then with the voltmeter connected across A and C sufficient resistance is introduced by the decade box R to reduce the voltage, as indicated on the voltmeter, to its preceding value. The value of R will then equal the a-c. resistance of the transmitter on the previous assumption that the transmitter is a source of constant e.m.f. acting through its a-c. resistance. The system is only accurate if the impedance of the retard coils and voltagedivider resistance are several times that of the transmitter resistance. Correct Load Impedance The input impedance of the transformer, under load, should at least equal the a-c. resistance of the microphone at the lowest frequency desired to pick up. Since reproduction is desired at as low a value as 16 cycles, it can be seen that a transformer for good frequency response must be of considerable size. As an example, one of the best transformers for use with a standard carbon microphone of approximately 100 ohms to the button, possesses an inductance of .95 henry with a resultant reactance, non-loaded, at 25 cycles, of 149 ohms. For best reproduction, an inductance of even higher value than this is desirable. As it can be seen, the reactance of an inductance of this value at 16 cycles would only be 95 ohms unloaded. The loaded transformer primary impedance would be considerably below this tigure. It also must be noted in the desieii of transformers that since a carbon microphone is essentially pure resistance, it does not vary its impedance with frequency. Condenser Microphones Theory The basic theory of condenser microphones is very simple, and we will explain their operation with the assistance of the diagram shown in Fig. 31. The condenser transmitter type 394 can be considered a very excellent condenser of small capacity. A resistance TRANS C i o 394 RANSMITTER Fig. 31. Condenser transmitter. One stage of amplification. "*: 6 •e v. R4 FOR 200 OUTPUT. STRAP B TO C . FOR 10"OUTPUT. STRAP A TO B AND C TO D. 6 + 200 V. SHELL OF AMPLIFIER (Ri), such as 10 megohms, is connected in series with it. The diaphragm of the condenser transmitter assumes a slightly convex surface upon application of the charging voltage. Sound waves, therefore, impinging on this surface cause the diaphragm to change position and vibrate in unison with the impinging sound waves. This varies the capacity of the condenser, thus changing the charging current fed to the condenser through the resistance Ri. This change in current through Ri causes a varying voltage drop across Ri. The grid of Vi is then connected pedance circuit of the characteristics of this condenser transmitter and voltage drop resistor, a long coupling lead to its amplifier cannot be tolerated. This is due to the additional capacity introduced, with resultant decrease in the high-frequency response. Construction and Theory of the Condenser Transmitter Unit A detailed diagram illustrating the internal construction of a condenser transmitter is furnished by the Bell Telephone Laboratories and is dis CDNDENSER ■ TRRNBMITTER TYPE-C Fig. 32. Internal construction of transmitter, in detail. so as to receive actuation from this voltage by means of the coupling condenser d and the grid resistance R*. The condenser transmitter in output is approximately 24 decibels below a carbon transmitter, and the necessity for an additional stage of amplification is therefore quite evident. It is also quite obvious that in a high im JL r-1 DECADE BOX Fig. 30. Hookup for determining the a-c. resistance of a microphone circuit. played in Fig. 32. This particular type is illustrative of one of the best types of condenser transmitter and we shall use this illustration for the discussion of the construction of a condenser transmitter. Theory The design of a condenser transmitter is a very much more complex undertaking than simply having two condenser plates separated from each other, one variable and the other fixed. Possibly the best idea of the attention which must be observed for detail is afforded by examination of the back electrode displayed in the illustration, Fig. 32. If a simple electrode were constructed with no perforations in the back plate, a most disappointing performance would result. In order to better understand the requirements for condenser transmitter