PAGE No. 24

An Improved Microphone Inset
July 1936

Continued effort directed towards the improvement of the microphone embodied in the Bakelite hand micro-telephone, results in minor changes which are embodied after lengthy tests have been carried out to prove their utility and constancy.

These slight changes are usually made without necessitating any alteration to the code of the inset, occasionally, however, a modification takes place which is of a major character and as a result a new code must be allotted.

The N7750 transmitter (pictured right) is an instance of such a change although its contour is exactly the same as the older type N7745 with which it is fully interchangeable. The assembly of the electrodes and the type of electrode has been modified (pictured below) and this together with improvements in the production of the carbon granules has effected an important change of characteristics.

One consideration leading to the design of this new inset was the desirability of having a microphone that would give satisfactory efficiency and service on both C.B. and L.B. magneto instruments, so that in the case of the convertible instrument, for instance, the change to the different conditions of working could be effected without the necessity of changing the microphone.

The characteristic resistance of the N7745 inset was approximately 75 ohms. This was satisfactory for C.B. working but too high to give the necessary volume efficiency with the local battery instrument. The new N7750 inset, however, has a characteristic resistance of approximately 50 ohms, which has been obtained without detracting in any way from the high efficiency of the old N7745 when used for C.B. working, and furthermore, full efficiency with local battery working has been secured.

The low characteristic resistance of the improved inset will also result in better service under those difficult tropical conditions where an increase in the characteristic resistance occurs for reasons not yet properly appreciated. The new inset will give longer life under these conditions with greater freedom from ‘frying” and background noise than has yet been obtained.

It retains all the special features of the N 7745, that is, the completely closed granule chamber to prevent the ingress of moisture, the special lacquer used to coat the diaphragm to effectually minimize corrosion, the complete absence of varnished silk or linen which rapidly deteriorates under tropical conditions or where conditions are moist and humid, and, as in the case of the N7745, the N7750 inset is repairable and can be taken down and reconditioned, using a few simple tools.

Regarding actual efficiency, the transmission reference equivalent is on the average 8 db worse than the C.C.I. datum under conditions of test defined in the C.C.I. year book 1931 pp. 53 to 61, this briefly may be said to be at least equal in efficiency to the Post Office standard telephone No. 162.

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Last revised: October 14, 2019