T EHAE BRIG
BY JIM STEWART
Ever wish you could build an “audio
telescope” that would let you hear
things that were faint or far away? Well,
this article shows you how to build
such a thing. We call it the Big Ear.
■ FIGURE 1
The Big Ear is shown in Figure 1. Its most obvious
feature is its parabolic dish reflector. The reflector
concentrates sound energy onto a microphone that
connects to a very high gain amplifier (see sidebar on
parabolic reflectors). And believe it or not, it’s all
assembled on a wood base you can find in a place that
sells concrete supplies and tools! More about the wood
base later; first we’ll discuss the amplifier.
The amplifier is built on a printed circuit board (PCB)
and mounted on stand-offs inside a small aluminum box
along with a nine-volt battery. Jacks for the microphone
and headphones — along with an on-off switch and a
volume control — are mounted on the box. Input jack J1
accepts a 2.5 mm mono plug that is wired to an electret
microphone. Output jack J2 is for a 3. 5 mm stereo plug
that is standard on many headphones.
Figure 2 is the amplifier schematic. Voltage to power
■ FIGURE 2
the microphone comes from the five-volt zener diode D3.
Resistor R11 limits the current in D3. The five volts are
applied through resistor R1 which sets the bias current for
the mic and the gain of the FET amplifier built into the
mic. Also, R1 and C2 form a low-pass filter to remove any
high frequency electromagnetic interference (EMI) that
might come in via the microphone wires.
The microphone is capacitively coupled via C1 to
inverting amplifier IC1. Because of C1, DC gain of IC1 is
1 while the AC gain is set by the ratio R5/(R2 + R1). C4
across R5 limits the bandwidth of IC1. With a wide-band
amplifier like the TL071, you should limit the bandwidth to
limit the noise (see sidebar on op-amp noise). For op-amp
IC1, the inputs need to be referenced to a level that is
between the positive and negative voltage “rails.” Since
the circuit is powered by a single 9V battery, R3 and R4
create a signal ground for the plus-input of IC1. C3 is a
bypass cap to connect AC signal ground to DC ground.
Since the mic will pick up loud sounds as well as faint
ones, we need to limit the output of IC1. That job is done
by R6 together with diodes D1 and D2. D1 and D2
are wired in parallel with opposite polarity to allow
current to flow both ways. Normally, the output of
IC1 is a lot less than the 600 mV or so needed to
put a diode into conduction. So, the diode pair acts
like an open circuit and R6 is isolated. A loud
sound drives the output of IC1 high enough to turn
on the diodes placing R6 in parallel with R5. That
lowers the gain of IC1 from 5 down to about 0.1.
D1, D2, and R6 form a limiter. Originally, I was going
to use an automatic gain control (AGC) circuit, but it
got too complicated. To keep things simple, I opted
for a limiter. You can use an oscilloscope to see the
output of IC1 at test-point TP.
Output of IC1 is coupled through C5 to the