A discussion of the 630A microphone, Western Electric’s answer to the STC 4021, can be found at https://coutant.org/we630a/index.html Reproduced from the Bell Laboratories Record of October, 1935, author R. N. Marshall gives a thorough report on the research and development that went into the design of the Western Electric 630A microphone, which is quite similar to the STC 4021.
The 4021 microphone is a pressure-operated moving coil (dynamic) microphone and is a carefully engineered and precision built instrument, the general design of which has been proved over many years of use in high quality studio work and acoustical measurements. Each microphone is tested in free-field conditions to stringent performance requirements. The polar response is omnidirectional; i.e., the mean sensitivity is independent of the direction of the incident sound, and the frequency characteristic is nearly so. This is, in part, achieved by the use of a spherical case and a porous front screen designed according to the principles disclosed by F. F. Romanow in British patent 436508.
Because of its wide frequency range, robustness and reliability of the 4021 microphone, it is suitable for standardization and measurement work. For this purpose it is possible to supply free field calibrations at particular frequencies. Further details of this service are available on application. The mic is practically distortionless in all normal sound fields. The total harmonic content is of the order of one-half to one per cent at sound intensity levels approaching the threshold of pain.
In certain applications it is desirable to have some rise in response and some directional properties at high frequencies. To provide such characteristics a 4001B acoustic baffle may easily be fitted in place of the Romanow front screen. The curves of figures 1 and 2 show the effect on this response. The microphone is suitable for use in tropical or marine atmospheres. It is, however, essentially an indoor instrument and should not be used out-of-doors in severe wind. The standard finish is stove black shrivel enamel and satin-chrome plating. The outlet of the mic is a three-pin connector located inside the die-cast case, and a 4069A jack is required to make connection thereto. The two outer pins connect to the coil, and the centre pin to the body of the microphone. The microphone incorporates a locking device to prevent it becoming accidentally detached from the 4069A jack.
Text above is quoted from the STC Sound Reproduction Equipment booklet No. 1620, produced in 1961.
The reverse side of this sheet reads as follows: IMPORTANT: Coil Resistance and Breakdown Measurements. Care must be exercised not to pass more than 1 milliampere DC through the coil, and if it should be desired to check breakdown to case, the voltage should not exceed 80 volts applied through a protective resistance which will limit the current to 1 milliampere.
The following seven images were provided via the courtesy of Greg Thompson.
The following seven images were provided through the courtesy of John and Rachel.
Please visit the lower section of this page for additional STC 4021 technical specifications.
For those of us who use the old STC mics, it can be a challenge finding a cable to fit. Below are three images of an adapter that converts these mics for use with an XLR-equipped cable. The same adapter works with STC mics as well as with Coles Electroacoustics mics. These adapters are not available at this web site.
A few vendors offer these adapters on the Web.
These convert the old WE, Coles, and STC connector to XLR.
Obviously a contemporary XLR-equipped cable can then be used.
