2. AC System 
eliminated with the balance control 
on the front panel of the chopper 
amplifier which inserts a small DC 
signal in series with the input cir- 
cuit. When the line balance poten- 
tiometer and the thermal EMF balance 
control are properly adjusted, there 
should be no steady AC signal at the 
AC monitor point but only a fluc- 
tuating signal. These adjustments 
must be made at the lowest detector 
signal level to prevent overloading 
of the amplifier at high gain settings 
(iii) Outputs - Two outputs are pro- 
vided on the amplifier, a low imped- 
ence balanced output for directly 
driving the milliammeter movement of 
a strip chart recorder, and a high 
impedance single-ended output for 
driving another amplifier or an active 
bandpass filter. 
2,3 
The AC amplifier system, cap- 
able of accepting signals in the 2 
to 40 cps range, was designed to 
operate on the 4000 foot circumfer- 
ence air-cored detector coil which 
has been described. Such a detector 
should yield 14.8 millivolts per 
gamma RMS at 10 cps. Because the 
Signal level at this frequency is 
likely to approximate 10~™* gamma, or 
less, a sensitive amplifier is re- 
quired, 
(i) Input filter - The input filter 
is designed to pass frequencies from 
DC to 40 cps and to reject all higher 
frequencies. In addition to a 60 cps 
rejection section and line balance 
potentiometer similar to that used in 
the DC system, a low-pass section is 
required to reduce power line har- 
monics and RF interference which is 
usually of comparable amplitude to 
the geomagnetic activity between 2 
and 40 cps. The line balance poten- 
tiometer is used to check for a satis- 
factory ground in the same way as 
with the DC system. The filter also 
incorporates a one ohm precision 
resistor for injection of a calibra- 
ting voltage. 
A separate 30 cps rejection filter 
has been found necessary in the 
vicinity of Victoria but has not 
been required at remote sites. 
(ii) Preamplifier - The preamplifier 
has two stages of amplification 
separated by a 60 cps rejection 
filter. The input stage is a vacuum 
tube amplifier, and a special low- 
level high step-up ratio input trans- 
former matches the detector to the 
329 
grid of the first tube of the input 
Stage. The second stage is a tran- 
sistor amplifier. The B+ and tran- 
sistor stages are supplied from 
self-contained batteries. A 6-volt 
storage battery is used to supply 
the heater voltage. Both stages 
are plug-in units. 
With an input transformer primary 
impedance of 40 ohms and a generator 
of internal impedance of 40 ohms, 
the amplifier gain from generator to 
output is 270,000 times. If the 
input signal should be large enough 
to overload the second amplifier 
stage this can be removed and re- 
placed by a unit having no amplifi- 
cation. 
The preamplifier noise depends to 
a large extent on the characteristics 
of the input tube, which operates 
under special low-noise conditions. 
It has been found on test to be as 
low as 0.004 microvolt for an input 
impedance of 40 ohms and a bandwidth 
of 2 to 30 cps, which is approxi- 
mately the thermal noise level. A 
noisy tube Can easily raise this 
value by a factor of ten to 0.04 
microvolt which is still acceptable 
although regular checks should be 
made to avoid a further increase in 
noise. This is accomplished by 
removing the detector from the input 
filter and plugging in instead a 
dummy source which contains the resis 
tive input impedance plus a calibra- 
tion attenuator. The system noise is 
then compared with a small known 
Signal. 
(iii) Postamplifier - The transistor 
postamplifier provides the extra gain 
necessary to amplify small signals to 
the level required by the recorders. 
The maximum gain is about 35 db and 
can be reduced to zero db in 5 db 
steps. The maximum overall gain fwm 
a 40 ohm source to the postamplifier 
output is now about 1.6 x 107, times 
or 144 db. The postamplifier power 
supply is run from the 115V, 60 cps 
supply. 
(iv) Output - The postamplifier is 
followed by a commercial active band- 
pass filter which serves to attenuate 
the frequencies persisting above 40 
cps, primarily 60 cps, and its har- 
monics. The bandpass filter output 
can be monitored on an oscilloscope 
or recorded on a high-speed paper 
recorder or tape recorder. 
In order to record the average 
level of activity in this frequency 
range continuously a rectified and 
integrated signal can be fed to a 
