Nuts and Volts - December 2008

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T EH_AE B_RIG

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.

Circuit Description

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.