I did some further tests on my circuit board. I first removed
the battery for the negative supply and jumpered across it's
contacts. The only change I could see was that the circuit
output with no signal went from approx 0.2mv to 0.6mv (about
1/2 division on the meter). I then clipped the Zener diodes
and hooked the unit to my dual output supply. With a DC voltage
to the metering circuit to make the meter read about 2/3 scale,
I varied the input voltages from 12 volts down to 5 volts with
NO change in the meter reading. My conclusions are that having
both batteries reduces the no signal offset voltage and that
the voltages do not have to be regulated. This saves about 18ma
from each battery!
73, Roger
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