Constructional Project
Super-Ear
Audio Telescope
Tom Merryfield
Listen more clearly to those distant sounds
UPER-EAR has been designed to Although the focal point varies with dif- via resistor R1, which behaves as its load.
incorporate a home-constructed par- ferent parabolas, the pick-up power of the The signal produced by MIC1 is a.c. cou-
Sabolic element which boosts the microphone is considerably boosted since pled via capacitor C1 to the base of npn
sensitivity of an electret microphone for more sound waves are available from a transistor TR1.
picking up sound at a distance. For exam- particular source. The received input signal The network around TR1 forms the pre-
ple, as utilised in wildlife studies and, dare is then amplified as smoothly as possible amplifier stage. Resistors R2 to R5 bias it
it be said, for eavesdropping on conversa- by a sensitive circuit. for linear amplification. The BC109C cho-
tions from afar! sen is ideal for low noise audio applica-
Because the microphone is securely held tions such as this, offering more than ade-
in copper tubing, impact and vibrational quate gain, although other general purpose
sounds through a barrier can also be high gain npn transistors will work in this
detected to a certain extent. It was found design.
with the prototype, that so long as a few Any instability at this stage could distort
precautions are followed, the results are signal processing throughout, hence the
comparable to those from a commercially inclusion of capacitors C2 and C4.
produced device. Capacitor C6 provides thermal compensa-
tion in the emitter circuit. Capacitor C3
Parabolic Theory
and resistor R6 decouple the stage from
Most readers will be familiar with
disruptive power supply variations.
satellite dish antennas in the shape of a
parabola. Whereas a true parabola has a Audio Amplifier
precise mathematical definition, most Capacitor C5 couples the preamplified
items approximating to this shape and signal to level (Volume) control VR1, from
Fig.1. Properties of a true parabola
with a reflective surface can be used to where it is fed via C7 to IC1 input pin 3.
 catch sound. The circuit around IC1 forms the audio
As shown in Fig.1, sound waves travel- Circuit Description
amplifier stage. Capacitor C8 acts as an
ling more or less parallel from a distant audio filter and the value quoted can be
In order to adequately amplify the input
source strike or  illuminate the parabolic
signal, the circuit consists of both pre- varied up to several nanofarads.
element. These in turn are re-directed to a The LM386N audio amplifier i.c. has
amplifier and audio amplifier stages.
focal point, X, at which the microphone is been chosen for IC1 because it is relative-
Referring to the full circuit diagram for the
placed. In effect, this captures the targeted ly easy to use and provides a smooth gain
Super-Ear Audio Telescope in Fig.2, the
audio. of over 200 (set by capacitor C9).
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
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.
Testing
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
C12 100� radial elect. 25V
The two prototype circuit boards, with
C14 100� axial elect. 25V
COMPONENTS
the tape-insulated Mic. between them.
Differs slightly from the final unit
Semiconductors
TR1 BC109C npn
Resistors
transistor
Most of the external components for
R1 3k6
IC1 LM386N audio
IC1 are needed for stability. Capacitor
R2 470k See
amplifier i.c.
C13 caters for variations in the supply
R3 4k7
SHOP
voltage and is used on the main amplifier
R4 100k
TALK
Miscellaneous
board, as opposed to the preamplifier
R5 1k
page MIC1 electret microphone
board.
R6 10k
insert
With IC1 s output impedance being 64
R7 18&!
S1 min s.p.s.t. toggle
ohms, Walkman-type headphones are
R8 22&!
switch
preferable to speakers. This also avoids the
All 0�25W 5% carbon film or better.
SK1 3.5mm jack socket
problem of  telephony which can hamper
B1 9V battery (PP3
performance.
Potentiometer
type), with clips
VR1 10k rotary
Construction
carbon, log
Stripboard 25 holes � 9 strips; strip-
The prototype was built on two separate
board 30 holes � 14 strips; control
stripboards for several reasons. Firstly,
Capacitors
knob; parabolic dish (see text); metal
both stages are isolated, which helps min-
C1 1� radial elect. 25V
case (110mm � 75mm � 60mm); cop-
imise problems with spurious feedback,
C2, C7 10n polyester (2 off)
per tubing, standard type, approx.
whilst making fault-finding easier.
C3 47� axial elect. 25V
15mm diameter � 100mm; mounting
Secondly, because the project is handheld,
C4 10n ceramic disc
clip for tubing (see text); nuts and bolts
compactness is an issue. Two smaller cir-
C5 220n radial elect.25V
as required; connecting wire; solder
cuit boards can be used to make the most
C6 47� radial elect. 25V
pins, solder, etc.
of the available space.
C8 20p ceramic disc
The component layouts and track cut-
C9 10� radial
ting details for the two stripboard
Approx. Cost
elect. 25V
assemblies are shown in Fig.3. Assemble
Ł10
Guidance Only
C10, C11, 100n ceramic
the boards in the usual order of ascend-
C13 disc (3 off)
excl batt &
ing component size, having first
hardware
correctly cut the tracks where required.
Everyday Practical Electronics, June 2005 389
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 ą0�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
surface free. The microphone can then be
carefully to avoid damaging its pins.
Electret Mounting
Solder the connecting cable to the pins eased into the tubing to around 3mm to
Because the circuit is extremely sensi-
and, as shown in Fig.5, wrap tape around 5mm short of the rim. For a snug fit and to
tive, anything picked up by the micro-
each of them. prevent loosening, apply further rounds of
phone can result in an ear-splitting whine
Push the cable through the copper tub- tape.
dominating the output. This can be
ing until it emerges from the other end.
resolved by thoroughly insulating the
Now apply further rounds of tape over the
microphone s bare metal surface using Parabolic Matters
microphone s case, leaving the pick-up
insulating tape. Handle the microphone 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
satellite dish antenna, for example,
In Use
increases six decibels simply by dou-
In use, some degree of experimenting is
bling its size.
required providing ambient noise does not
Because holding the parabolic element
block out the targeted audio.
can muffle sound pick-up, it is worthwhile
Point the reflector dish in the general
mounting it on a simple support via strong
direction of interest. As shown in Fig.6,
adhesive pads or glue such as Araldite.
hold the Super-Ear unit in front of the
Avoid using fixtures which involve
reflector. For optimum pick-up deter-
drilling holes, except for the 10mm centre
mine the focal length for the chosen
hole mentioned earlier. For bigger and
 dish , i.e. the distance at which the
heavier parabolic elements, the suggested
microphone is held from the centre of the
method of home assembly as shown in
reflector.
Fig.6 can be used rather effectively.
A trial and error method sufficed for the
prototype, with interesting results. Using
Acknowledgements
this method, low level sounds (only just
The author offers many thanks to Ralph
audible but unintelli-
Turner and Dave Moran at
gable) were
Bradford College, for
ampl i fi ed
kindly assisting with
usi ng
this project.
an
aluminium
bowl of diame-
ter 200mm as the par-
abolic element, and having
Drill a hole at the centre of the element
a focal point of 90mm.
equivalent to the diameter of the microphone,
Of course, the larger the dish
in this case 10mm. In terms of picking up
diameter (typically up to 800mm) and
sound this helps redirect the sound  illumina- the more parabolic the shape, the
tion relative to the size of microphone.
stronger the illumination. The gain of a
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LAMBDA 422FM 0-40V 0-1A Twice 4 Meters . . . . . . . . . .Ł50 RS 180-7127 Conductivity Meter . . . . . . . . . . . . . . . . . . . .Ł35 Heads . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Ł50 KENWOOD FL180A Wow and Flutter Meter. . . . . . . . . . . . . . Ł75
LAMBDA LK345A FM 0-60V 0-10A 2 Meters . . . . . . . . . .Ł75 EAGLE DC30 Probe 30Ku DC . . . . . . . . . . . . . . . . . . . . . .Ł5 MEGGER MJ4MK2 Wind Up 1000V M0hm . . . . . . . . . . . .Ł30 KENWOOD FL180 Wow and Flutter Meter, unused . . . . . . . Ł125
SYSTRON DONNER SHR40-2 0-40V 0-2A - 2 Meters . . .Ł25 AVO 100AMP Shunt for AVO 8 . . . . . . . . . . . . . . . . . . . . . .Ł5 METROHM 250V Pat Tester . . . . . . . . . . . . . . . . . . . . . . .Ł15 MARCONI 6960B Power Meter with 6920 head, 10MHz-20GHzŁ450
SORENSON SRL60-4 0-60V 0-4A . . . . . . . . . . . . . . . . . .Ł60 SOUTHERN CALFORNIA BTXRM-S-10 2000.0 MHz . . . .Ł50 SULLIVAN AC1012 4 Decade Resistance Box 0.05% . . . .Ł10 SOLARTRON 7150 DMM 6�-digit True RMS IEEE . . . . . . . . Ł75
GRENSON BPU4 +5V 2.5A & +/- 15V 0.5A . . . . . . . . . . .Ł25 MOTOROLA R2001D Communication System Analyser .Ł250 BRANDENBURGH O20 Static Freq Convertor 110/240V input SOLARTRON 7150 Plus As 7150 + temp. measurement . . . Ł100
RS 813-991 2 x 5v 2.5A or 2 x 12V 1.5A or 2 x 5V 1A . . . .Ł40 WESTON 1149 Standard Cell 1.01859 ABS Volts at 20C . .Ł10 50/60HZ Output 115V 400HZ 20VA . . . . . . . . . . . . . . . . . .Ł40 IEEE Cables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Ł5
RS 208-197 Line Voltage Conditioner - Output 240V 0.65A Ł40 MUIRHEAD A-6-B Resistance Box . . . . . . . . . . . . . . . . . .Ł10 NARDA 706 Attenuator . . . . . . . . . . . . . . . . . . . . . . . . . . .Ł10 H.P. 3312A Function Gen, 0�1Hz-13MHz, AM/FM
POWER CONVERSION PLC1000 Line Cond. 1000VA . . .Ł50 RACAL 9917A UHF Frequency Meter 10HZ-560 MHz . . . .Ł45 ANALOGUE ASSOCIATES X800 Audio Amplifier 800watt (400w Sweep/Sq/Tri/Burst etc . . . . . . . . . . . . . . . . . . . . . . . . . . . . Ł200
HARLYN AUTOMATION IPPS5200 System Power Supply Ł60 RACAL 9917 UHF Frequency Meter 10HZ-560 MHz . . . . .Ł40 per Channel no DC Protection) . . . . . . . . . . . . . . . . . . . . .Ł60 H.P. 3310A Function Gen, 0�005Hz-6MHz, Sine/Sq/Tri/
POWERLINE LAB807 0-300V AC 0.75A . . . . . . . . . . . . . .Ł40 RACAL 9915M UHF Frequency Meter 10HZ-520 MHz . . .Ł30 W&G PCG2 PCM Channel Generator . . . . . . . . . . . . . . . .Ł30 Ramp/Pulse . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Ł80
SIVERS LAB 12400 - 18000 MHz . . . . . . . . . . . . . . . . . .Ł10 RACAL 9008 Automatic Modulation Meter, 1�5MHz-2GHz . . . Ł60
SIVERS LAB 5212 2500 - 4000 MHz+C264 . . . . . . . . . . .Ł10 ISOLATING Transformer, input 250V, output 500VA, unused. . Ł30
CROPICO VS10 DC Standard 10V . . . . . . . . . . . . . . . . . .Ł30 RACAL 1792 Receiver . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Ł525
STEWART of READING
17A King Street, Mortimer, Near Reading RG7 3RS Used Equipment  GUARANTEED. Manuals supplied
Telephone: [phone]. Fax: [phone] This is a VERY SMALL SAMPLE OF STOCK. SAE or Telephone for lists.
www.stewart-of-reading.co.uk
Please check availability before ordering.
Open 9am-5.00pm Monday to Friday (other times by arrangement)
CARRIAGE all units Ł16. VAT to be added to Total of Goods and Carriage
392 Everyday Practical Electronics, June 2005