Room Recorder

My wife was working on a
doctoral dissertation and needed to do some field work involving
personal interviews in various settings. What would be the best way,
technically speaking, to record the interviews? To pass a tape recorder
or microphone back and forth seemed too awkward and clipping wired
microphones to interviewees didn’t make for a particularly informal
atmosphere. Radio microphones seemed overly expensive, too. After some
thought, I can up with the “Room Recorder”, an add-on microphone
preamplifier circuit for use with a tape recorder. While I don’t make
any great claim to originality for the circuit, it has produced first
class results over one year of interviews and might prove useful to
anyone doing similar work.

The preamplifier was plugged into a Sony Cassette-Corder (any similar
device will work) by means of a long, screened microphone cable and
placed in a central location in a room or on a bench. The circuit will
pick up every whisper, so background noise should be considered when
choosing a location. A 2-terminal electret microphone picks up the
sound, which is then amplified by a TL071CN low-noise op amp. Note that
the microphone’s negative terminal is connected to its case. Negative
feedback is applied to the inverting input through a 10kO resistor.
Increasing the value of this resistor will increase sensitivity, and
vice versa. For ease of use and quietness of operation, the circuit is
powered from a 9V battery. The power switch is mounted on the case. The
circuit draws about 2mA and would therefore give about 10 days
continuous service from a 9V alkaline battery.
Author: Thomas Scarborough – Copyright: Silicon Chip Electronics

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