SEISMIC METHODS 



811 



circuits may be incorporated without the penalty of excess space or weight. 

 The first stage is controlled by a long time-constant circuit. A rectifier 

 feeding an RC type of circuit builds up a direct current potential propor- 

 tional to the incoming signal amplitude. A portion of this current is fed 

 through a diode to a buss capacitor tied to the first stage grid return. The 

 first break signals thus reduce the gain of the first stage to a low level. 

 When the input signal from the seismometer drops appreciably, the rectified 

 D.C. voltage also drops in value, and the voltage from the bias capacitor 

 tends to balance out by discharging through an RC rectifier system, which 

 reverses the original process, and increases the gain. 



Fig. 498. — Seismic amplifier unit. (Courtesy of Western Geophysical Co.) 



Maximum gain at no signal is set by the level control. Peak limiting 

 is accomplished by rectifying the output signal and controlling the second 

 cascade amplifier. This circuit is incorporated to limit the peak signal until 

 the long time-constant input gain control begins to function. Soon after 

 the first breaks arrive, the input gain control functions, and thereafter the 

 circuit action corresponds to the well-known expander-type circuit, employ- 

 ing a pre-charged condenser connected to the bias circuits. This condenser 

 discharges through an LR circuit at a time-constant commensurable to the 

 necessary time delay for the particular area under investigation. 



The output of the amplifier is applied to the galvanometer by means of 

 a transformer to make possible the proper matching of the impedance of 

 the output tube with that of the galvanometer. 



