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Constructional Project
Super-Ear
Audio Telescope
Tom Merryfield
Listen more clearly to those distant sounds
UPER-EAR has been designed to
incorporate a home-constructed par-
abolic element which boosts the
sensitivity of an electret microphone for
picking up sound at a distance. For exam-
ple, as utilised in wildlife studies and, dare
it be said, for eavesdropping on conversa-
tions from afar!
Because the microphone is securely held
in copper tubing, impact and vibrational
sounds through a barrier can also be
detected to a certain extent. It was found
with the prototype, that so long as a few
precautions are followed, the results are
comparable to those from a commercially
produced device.
Parabolic Theory
Most readers will be familiar with
satellite dish antennas in the shape of a
parabola. Whereas a true parabola has a
precise mathematical definition, most
items approximating to this shape and
with a reflective surface can be used to
“catch” sound.
As shown in Fig.1, sound waves travel-
ling more or less parallel from a distant
source strike or “illuminate” the parabolic
element. These in turn are re-directed to a
focal point,
X
, at which the microphone is
placed. In effect, this captures the targeted
audio.
Although the focal point varies with dif-
ferent parabolas, the pick-up power of the
microphone is considerably boosted since
more sound waves are available from a
particular source. The received input signal
is then amplified as smoothly as possible
by a sensitive circuit.
via resistor R1, which behaves as its load.
The signal produced by MIC1 is a.c. cou-
pled via capacitor C1 to the base of
npn
transistor TR1.
The network around TR1 forms the pre-
amplifier stage. Resistors R2 to R5 bias it
for linear amplification. The BC109C cho-
sen is ideal for low noise audio applica-
tions such as this, offering more than ade-
quate gain, although other general purpose
high gain
npn
transistors will work in this
design.
Any instability at this stage could distort
signal processing throughout, hence the
inclusion of capacitors C2 and C4.
Capacitor C6 provides thermal compensa-
tion in the emitter circuit. Capacitor C3
and resistor R6 decouple the stage from
disruptive power supply variations.
Audio Amplifier
Capacitor C5 couples the preamplified
signal to level (Volume) control VR1, from
where it is fed via C7 to IC1 input pin 3.
The circuit around IC1 forms the audio
amplifier stage. Capacitor C8 acts as an
audio filter and the value quoted can be
varied up to several nanofarads.
The LM386N audio amplifier i.c. has
been chosen for IC1 because it is relative-
ly easy to use and provides a smooth gain
of over 200 (set by capacitor C9).
Fig.1.Properties of a true parabola
Circuit Description
In order to adequately amplify the input
signal, the circuit consists of both pre-
amplifier and audio amplifier stages.
Referring to the full circuit diagram for the
Super-Ear Audio Telescope in Fig.2, the
electret microphone, MIC1, is powered
Fig.2. Complete circuit diargam for the Super-Ear Audio Telescope. This circuit is built on two circuit boards; preamplifier
and audio amplifier
388
Everyday Practical Electronics, June 2005
S
Table 1: Perception of Sound Intensity
Source
Decibels*
Perception
–
0
Silence
Rustling Leaves
10
Barely Audible
Soft Whisper
20
Quiet
Conversation
60
Audible
Factory
80
Fairly Loud
Construction Noise
110
Very Loud
Rock Concert
120
Pain Threshold
* Decibels, perceived level varies with frequency
Note that the human ear tends to perceive
higher frequencies more easily
Ensure that the electrolytic capacitors
and the semiconductors are inserted the
correct way round as shown. Use a socket
for IC1, but do not insert the i.c. until the
assembly has been completed and checked
for accuracy.
Note that the electret microphone insert
is also a polarised device. Its case is inter-
nally connected to one of its pins, to which
the 0V connection should be made. The
wiring to the microphone should not
exceed about 140mm in length.
Tes t i ng
The circuit should be fully tested before
housing it in a metal case. It is suggested
that the main amplifier is tested first, with-
out it being connected to the preamplifier.
When the amplifier is powered, touch-
ing the middle lug (wiper) of VR1 (or pin
3 of IC1) should produce a coarse buzz at
COMPONENTS
C12
100
µ
radial elect. 25V
The two prototype circuit boards, with
the tape-insulated Mic.between them.
Differs slightly from the final unit
C14
100
µ
axial elect. 25V
Semiconductors
TR1
Resistors
R1
BC109C npn
transistor
Most of the external components for
IC1 are needed for stability. Capacitor
C13 caters for variations in the supply
voltage and is used on the main amplifier
board, as opposed to the preamplifier
board.
With IC1’s output impedance being 64
ohms, Walkman-type headphones are
preferable to speakers. This also avoids the
problem of “telephony” which can hamper
performance.
Construction
The prototype was built on two separate
stripboards for several reasons. Firstly,
both stages are isolated, which helps min-
imise problems with spurious feedback,
whilst making fault-finding easier.
Secondly, because the project is handheld,
compactness is an issue. Two smaller cir-
cuit boards can be used to make the most
of the available space.
The component layouts and track cut-
ting details for the two stripboard
assemblies are shown in Fig.3. Assemble
the boards in the usual order of ascend-
ing component size, having first
correctly cut the tracks where required.
3k6
IC1
LM386N audio
amplifier i.c.
R2
470k
See
SHOP
TALK
page
R3
4k7
R4
100k
Miscellaneous
MIC1
R5
10k
electret microphone
insert
R7
18
Ω
S1
min s.p.s.t. toggle
switch
Ω
All 0·25W 5% carbon film or better.
22
SK1
3
.
5mm jack socket
B1
9V battery (PP3
type), with clips
Potentiometer
VR1
10k rotary
carbon, log
Stripboard 25 holes × 9 strips; strip-
board 30 holes × 14 strips; control
knob; parabolic dish (see text); metal
case (110mm × 75mm × 60mm); cop-
per tubing, standard type, approx.
15mm diameter × 100mm; mounting
clip for tubing (see text); nuts and bolts
as required; connecting wire; solder
pins, solder, etc.
Capacitors
C1
1
µ
radial elect. 25V
C2, C7
10n polyester (2 off)
C3
47
µ
axial elect. 25V
C4
10n ceramic disc
C5
220n radial elect. 25V
C6
47
µ
radial elect. 25V
C8
20p ceramic disc
C9
10
radial
elect. 25V
µ
£10
Approx.Cost
Guidance Only
C10, C11,
100n ceramic
C13
disc (3 off)
excl batt &
hardware
Everyday Practical Electronics, June 2005
389
R6
1k
R8
the output. There should also be a low level
hum at the output, confirming that the
amplifier is working. Once the amplifier
has been proved, the preamplifier can be
connected to it.
Key test voltages are shown in Table 2
and can be measured with a multimeter.
Initially check if the microphone is picking
anything up by gently tapping it and hear-
ing the output. The prototype easily picked
up ambient sound with VR1 set to less than
a quarter of a turn.
Casing It
One problem encountered with circuits
such as this is that they can easily pick up
electrical interference, including odd (but
strangely untraceable!) vibrations.
This was remedied by housing the circuit
in a metal case, with the microphone situat-
ed in a copper tube (see later).
Alternatively, plastic tubing with metal
tape wrapped around it will suffice.
Before mounting the boards in the case,
first drill the necessary holes in it and
secure the copper tubing. As Fig.5 shows,
the latter can be attached fairly robustly to
the case with the help of a mounting clip as
used in plumbing installations.
Table 2. Key Test Voltages
Pre-amp supply line
3·8V
Voltage across microphone 3·1V
TR1 base
0·2V
TR1 collector
3·1V
IC1 pin 5
4·5V
Assuming a 9V supply (error ±0·1V)
Fig.3.Super-Ear Audio Telescope stripboard component layouts, interwiring and details of breaks required in the copper tracks
of the preamplifier and amplifier boards.The wiring and positioning of the two boards within the two-piece aluminium case is
shown in the above photograph
390
Everyday Practical Electronics, June 2005
Fig.5. Insulating and wiring the microphone insert (a), end
view showing insert “plugged” into the copper pipe (b) and
the Mic.insert recessed in the copper tubing (c)
Fig.4. Securing the copper tubing, housing the mic. insert,
inside the metal case (a) and (b) position of off-board com-
ponents on one side panel
Electret Mounting
Because the circuit is extremely sensi-
tive, anything picked up by the micro-
phone can result in an ear-splitting whine
dominating the output. This can be
resolved by thoroughly insulating the
microphone’s bare metal surface using
insulating tape. Handle the microphone
carefully to avoid damaging its pins.
Solder the connecting cable to the pins
and, as shown in Fig.5, wrap tape around
each of them.
Push the cable through the copper tub-
ing until it emerges from the other end.
Now apply further rounds of tape over the
microphone’s case, leaving the pick-up
surface free. The microphone can then be
eased into the tubing to around 3mm to
5mm short of the rim. For a snug fit and to
prevent loosening, apply further rounds of
tape.
Parabolic Matters
As mentioned earlier, almost anything
concave and reflective to sound will boost
the pick-up power of the microphone, it
does not have to be a true parabola. The
shape includes items such as aluminium
bowls, old style car hub caps, and disused
satellite dishes.
Fig.6. Suggested method for constructing a cradle/support for mounting a bigger
more robust parabolic element (dish)
Everyday Practical Electronics, June 2005
391
In Use
In use, some degree of experimenting is
required providing ambient noise does not
block out the targeted audio.
Point the reflector dish in the general
direction of interest. As shown in Fig.6,
hold the Super-Ear unit
in front
of the
reflector. For optimum pick-up deter-
mine the focal length for the chosen
“dish”, i.e. the distance at which the
microphone is held from the centre of the
reflector.
A trial and error method sufficed for the
prototype, with interesting results. Using
this method, low level sounds (only just
audible but unintelli-
gable) were
amplified
using
satellite dish antenna, for example,
increases six decibels simply by dou-
bling its size.
Because holding the parabolic element
can muffle sound pick-up, it is worthwhile
mounting it on a simple support via strong
adhesive pads or glue such as Araldite.
Avoid using fixtures which involve
drilling holes, except for the 10mm centre
hole mentioned earlier. For bigger and
heavier parabolic elements, the suggested
method of home assembly as shown in
Fig.6 can be used rather effectively.
Acknowledgements
The author offers many thanks to Ralph
Turner and Dave Moran at
Bradford College, for
kindly assisting with
this project.
Drill a hole at the centre of the element
equivalent to the diameter of the microphone,
in this case 10mm. In terms of picking up
sound this helps redirect the sound “illumina-
tion” relative to the size of microphone.
an
aluminium
bowl of diame-
ter 200mm as the par-
abolic element, and having
a focal point of 90mm.
Of course, the larger the dish
diameter (typically up to 800mm) and
the more parabolic the shape, the
stronger the illumination. The gain of a
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. . . . . . . . . . . . . . . . . . . .
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£10
MICRODYNE CORP
. Receiver . . . . . . . . . . . . . . . . . . . . .
£60
VARIAN V2L-6941F1
Travelling Wave Tube Amplifier . . . . .
£50
MOORE READ SFC500/1
AUXR Static Frequency Convertor
120 Volts 400 HZ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
£50
DRAGER 21/31
Multi Gas Detector . . . . . . . . . . . . . . . . . .
£10
PHILIPS PM8237B
Multipoint Data Recorder . . . . . . . . . .
£20
ENDEUCO 4417
Signal Conditioner X 2 . . . . . . . . . . . . . .
£10
PULSETEK 132
DC Current Calibrator . . . . . . . . . . . . . . .
£30
PM1038-D14
Display with 1038-N10 Network Analyser. No
Heads . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
£50
MEGGER MJ4MK2
Wind Up 1000V M0hm . . . . . . . . . . . .
£30
METROHM
250V Pat Tester . . . . . . . . . . . . . . . . . . . . . . .
£15
SULLIVAN AC1012
4 Decade Resistance Box 0.05% . . . .
£10
BRANDENBURGH O20
Static Freq Convertor 110/240V input
50/60HZ Output 115V 400HZ 20VA . . . . . . . . . . . . . . . . . .
£40
NARDA 706
Attenuator . . . . . . . . . . . . . . . . . . . . . . . . . . .
£10
ANALOGUE ASSOCIATES X800
Audio Amplifier 800watt (400w
per Channel no DC Protection) . . . . . . . . . . . . . . . . . . . . .
£60
W&G PCG2
PCM Channel Generator . . . . . . . . . . . . . . . .
£30
SIVERS LAB
12400 - 18000 MHz . . . . . . . . . . . . . . . . . .
£10
SIVERS LAB 5212
2500 - 4000 MHz+C264 . . . . . . . . . . .
£10
CROPICO VS10
DC Standard 10V . . . . . . . . . . . . . . . . . .
£30
EXTRA SPECIAL OFFERS
HG.P. 8560A
Spectrum Analyser, 50Hz-2·9GHz. . . . . . . . .
£3.250
H.P. 8569B
Spectrum Analyser, 10MHz-22GHz . . . . . . . . .
£1,500
MARCONI 2382
Spectrum Analyser, 200Hz-400MHz. . . . .
£1,250
TEKTRONIX 2465A
Oscilloscope, 4-ch, 350MHz . . . . . . . . .
£750
TEKTRONIX 2246
Oscilloscope, 4-ch, 100MHz . . . . . . . . . .
£375
H.P. 8640A
AM/FM Signal Gen, 500kHz-512MHz . . . . . . . . .
£175
SPECIAL OFFERS
OSCILLOSCOPES
LECROY 9400A
dual trace, 170MHz, 5G/S . . . . . . . . . . . . .
£500
Lecroy 9400
dual trace, 125MHz . . . . . . . . . . . . . . . . . . . . .
£400
TEKTRONIX 468
dual trace, 100MHz, digital storage . . . . . .
£300
TEKTRONIX 475
dual trace, 200MHz, delay sweep . . . . . . .
£250
TEKTRONIX 465B
dual trace, 100MHz, delay sweep . . . . . .
£250
TEKTRONIX 465
dual trace, 100MHz, delay sweep . . . . . . .
£175
PHILIPS PM3217
dual trace, 50MHz, delay sweep . . . . . . . .
£150
THURLBY PL320QMD
0-30V 0-2A twice digital PSU . . . . . .
£160
H.P. 66312A
0-20V 0-2A Communications PSU . . . . . . . . . .
£200
H.P. 6626A
Precision High Resolution PSU, 4 outputs,
0-7V 0-15A or 0-50V 0-0·5A twice; 0-16V 0-0·2A
or 0-50V 0-2A twice . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
£500
CIRRUS CRL254
Sound Level Meter with calibrator
80-120db LED . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
£95
WAYNE KERR B424
Component Bridge . . . . . . . . . . . . . . . .
£50
RACAL 9300
True RMS Voltmeter, 5Hz-20MHz usable to
60MHz, 10V-316V . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
£50
RACAL 9300B
True RMS Voltmeter, 5Hz-20MHz usable to
60MHz, 10V-316V . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
£75
AVO DA116
Digital Avometer with battery and leads. . . . . . . .
£20
FARNELL LFM4
Sine/Sq Oscillator, 10Hz-1MHz,
low distortion TTL output amplitude meter. . . . . . . . . . . . . . .
£75
FARNELL J3B
Sine/Sq Oscillator, 10Hz-100kHz, low dist. . . .
£60
HEME 1000
LCD Clamp Meter, 0-1000A, in carrying case . . .
£35
FLUKE 77
Multimeter 3½ digit handheld with battery & leads .
£45
KENWOOD VT176
2-Channel Multivoltmeter . . . . . . . . . . . . .
£50
KENWOOD FL140
Wow and Flutter Meter . . . . . . . . . . . . . . .
£50
KENWOOD FL180A
Wow and Flutter Meter. . . . . . . . . . . . . .
£75
KENWOOD FL180
Wow and Flutter Meter, unused . . . . . . .
£125
MARCONI 6960B
Power Meter with 6920 head, 10MHz-20GHz
£450
SOLARTRON 7150
DMM 6½-digit True RMS IEEE . . . . . . . .
£75
SOLARTRON 7150 Plus
As 7150 + temp. measurement . . .
£100
IEEE Cables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
£5
H.P. 3312A
Function Gen, 0·1Hz-13MHz, AM/FM
Sweep/Sq/Tri/Burst etc . . . . . . . . . . . . . . . . . . . . . . . . . . . .
£200
H.P. 3310A
Function Gen, 0·005Hz-6MHz, Sine/Sq/Tri/
Ramp/Pulse . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
£80
RACAL 9008
Automatic Modulation Meter, 1·5MHz-2GHz . . .
£60
ISOLATING
Transformer, input 250V, output 500VA, unused . .
£30
RACAL 1792
Receiver . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
£525
STEWART of READING
17A King Street, Mortimer, Near Reading RG7 3RS
Telephone: (0118) 933 1111. Fax: (0118) 933 2375
www.stewart-of-reading.co.uk
Open 9am-5.00pm Monday to Friday (other times by arrangement)
Used Equipment – GUARANTEED. Manuals supplied
This is a VERY SMALL SAMPLE OF STOCK. SAE or Telephone for lists.
Please check availability before ordering.
CARRIAGE all units £16. VAT to be added to Total of Goods and Carriage
392
Everyday Practical Electronics, June 2005
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