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This simple adaptor
allows commonly
available electret lapel
and headset microphones
to be used with public
address systems. It
features a balanced
output and is built into
a compact case that can
be clipped to a belt or
slipped into a pocket.
By JOHN CLARKE
Lapel microphone
adaptor for PA systems
W
HILE STANDARD HANDHELD
microphones are generally used
for most public address (PA) applications, there are times when a lapel
microphone is much more convenient.
A lapel microphone not only frees
up a user’s hands but also allows the
wearer to roam about easily. They are
ideal when giving talks and lectures,
and for certain types of theatre work.
Another advantage of lapel microphones is that they provide a reason54 Silicon Chip
ably consistent output, even when
the person speaking turns their head.
That’s because a lapel microphone is
usually clipped to the user’s clothing
around the chest area and so remains
at a similar distance from the mouth
regardless of head movement. By contrast, hand-held microphones must
always be held close to the mouth,
otherwise the signal level will vary
drastically.
Lapel microphones are generally
available in two forms. By far the
most common form for PA use at the
present time is the radio microphone.
This consists of the lapel microphone
itself plus a small radio transmitter
which is worn by the user – eg, inside
a shirt pocket or by attaching it to a
belt. The signals from the transmitter are picked up by a corresponding
receiver which then feeds the signal
to the PA system.
The big advantage of the radio
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Fig.1: the circuit uses op amps IC1a & IC1b to provide a balanced output signal, while relays RLY1 & RLY2 shunt
the signal to ground when activated, to provide muting.
microphone is that it allows the user
to roam freely over several tens of
metres without being tethered to a
lead. However, this freedom comes
at a high cost, with complete radio
microphone systems typically costing
around $600.
Despite its advantages, this high
cost cannot always justified, especially
when full use of the radio transmitting
feature is not exploited. This particularly applies to applications where
the user doesn’t need to roam too far.
In those situations, a much cheaper
solution is to dispense with the radio
system and instead use a tethered lapel
microphone – ie, one that’s tethered to
the PA amplifier via a lead.
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However, obtaining such a wired
lapel system is quite another matter.
Music shops are keen to sell the wireless microphones but are usually at
a loss when asked to supply a wired
type. The older-style dynamic lapel
microphones simply no longer appear to be available, while the smaller
electret microphones require a power
source.
So why can’t you simply use an
electret microphone and power it from
the phantom supply that’s sometimes
available in PA mixers? Unfortunately,
it’s not as simple as that, for a couple
of reasons.
First, many mixers do not have
phantom power and if they do, the
Main Features
•
•
•
•
•
•
Uses standard electret lapel
microphone
Adaptor attached to belt or in
pocket
Battery powered (9V)
Balanced output
Muting facility
Battery indicator
current available is well in excess of
that required for an electret microphone. Electrets require only 0.5mA
or less for correct operation, whereas
January 2004 55
Parts List
1 PC board, code 01101041 (86
x 64mm) for Jaycar and DSE
cases; or code 01101042 (81
x 61mm) for Altronics case
1 case measuring 135 x 70 x
24mm with battery compartment (DSE Cat. H 2949
(grey), Jaycar Cat. HB 6510
(black), Altronics H 0342
(grey))
2 panel labels, 59 x 16mm and
114 x 50mm
1 belt/pocket clip (Farnell 353
6294 (grey) or 353 6282
(black))
1 lapel microphone (Jaycar AM4092 or Altronics C 8907 or C
8913)
2 5V reed relays (RLY1, RLY2)
(Jaycar SY 4036)
1 double-pole 3-position (DP3W)
slide switch (S1) with 2 x M2.6
mounting screws (Altronics S
2030)
1 3.5mm PC board socket (Jaycar PS 0133) or 3-pin chassis
male miniature XLR connector
(Altronics P 0891) – see text
1 right angle stereo 6.35mm jack
plug to 3-pin XLR line plug
lead (Altronics P 0902 XLR
line plug and P 0047 jack)
5 metres of dual-screened
microphone cable (Altronics
W 3028)
1 stereo 6.35mm metal line
socket (Altronics P 0080A,
Jaycar PS 0194))
1 9V battery clip lead
1 9V battery
3 M3 x 6mm screws
1 M3 x 10mm countersunk screw
1 M3 x 20mm countersunk screw
1 M3 x 10mm tapped spacer
1 50mm cable tie
13 PC stakes
the phantom power from a PA mixer
is usually between 14mA and 60mA
– enough to destroy an electret unless
precautions are taken.
Second, an electret microphone
provides only a single “unbalanced”
output. This means that there are just
two output connections – ie, the shield
and the signal wire. However, any
leads that are several metres long or
more in a PA system can readily pick
up 50Hz mains frequency hum which
is then amplified and fed through to
the loudspeakers as an annoying buzz.
In this case, both signal leads still
pick up mains frequency hum but
because the lines are balanced, the
hum signal can be rejected to just leave
the wanted microphone signal. This
is done in the PA mixer – it receives
the balanced signal and subtracts the
non-inverted microphone signal from
the inverted microphone signal. This
removes the mains hum signal, since
the same signal will be present in both
Balanced output
The way around this problem is
to use what’s known as a “balanced”
output. This type of output has two
signal outputs plus a shield lead, with
one output inverted with respect to
the other.
56 Silicon Chip
Semiconductors
1 TL072 dual op amp (IC1)
1 BC328 PNP transistor (Q1)
1 4.7V 1W zener diode (ZD1)
1 1N5819 Schottky diode (D1)
2 1N4148, 1N914 diodes
(D2,D3)
1 3mm green LED (LED1)
Capacitors
1 470µF 16V PC electrolytic
4 100µF 16V PC electrolytic
1 22µF 16V PC electrolytic
2 10µF 16V PC electrolytic
1 100nF MKT polyester
1 1nF MKT polyester
Resistors (1% 0.25W)
1 100kΩ
1 680Ω
1 22kΩ
2 560Ω
6 10kΩ
1 220Ω
2 6.8kΩ
2 100Ω
1 1kΩ
1 22Ω
Specifications
Frequency response: 16Hz to
16kHz (actual response depends
on the microphone used)
Output level: typically 100mV
Current consumption: 4mA when
on, 11mA on mute, 0.1µA when off
leads. By contrast, the microphone
signal is doubled, since subtracting an
inverted signal from the non-inverted
signal gives twice the signal level.
Lapel microphone adapter
That’s where the Lapel Microphone
adapter comes in – it not only provides power to a standard electret
microphone but also includes all the
necessary circuitry to provide balanced output signals. In addition, it
also includes a muting facility which
shorts the signal output to ground, so
that sound is no longer heard through
the PA system. This muting function
is completely silent in operation – ie,
there are no clicks and pops in the
sound when the muting is switched
in or out.
As shown in the photos, the unit is
housed in a small case which contains
a separate battery compartment. The
lapel microphone plugs into a socket
at the top of the case, while the output
lead plugs into a 6.35mm stereo socket
on one side.
A single 3-position slide switch is
used to switch the power on/off and to
select the muting. An adjacent green
indicator LED flashes when the power
is switched on and this can also be
used to indicate the battery condition.
A bright flash indicates a good battery,
with the LED becoming increasingly
dim as the battery goes flat.
In addition, the LED serves as an
indicator by glowing faintly when the
switch is in the Mute position. It also
flashes brightly and decays when the
unit is switched off, to acknowledge
the switch selection.
Circuit details
Fig.1 shows the full circuit details
of the Lapel Microphone Adaptor. It
includes a dual op amp package (IC1)
to do the audio signal processing, plus
two relays to shunt the signal on each
balanced line to ground during muting. Power for the circuit is derived
from a 9V battery and is applied via
reverse polarity protection diode D1
and power switch S1.
The electret microphone is plugged
into a mini XLR male socket or a
3.5mm jack socket, depending on the
type of electret used. It is powered
from the 9V battery via 1kΩ & 22kΩ
resistors and a 100µF filter capacitor.
This decoupling is necessary to keep
supply noise and ripple from degrading the microphone signal.
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This is the view inside the completed prototype. The 6.35mm jack socket has its
outer cover removed and is secured to the PC board using a cable tie. The socket
is then further secured by its threaded boss when the lid is fastened down.
The output signal from the microphone is fed to the pin 5 (noninverting) input of op amp IC1a via a
100nF capacitor. This capacitor and its
associated 100kΩ resistor roll off the
low-frequency response below 16Hz
Note that IC1a’s pin 5 input is biased at half-supply (ie, Vcc/2) via the
100kΩ resistor which is connected to
a voltage divider consisting of two
10kΩ resistors across the 9V rail. This
allows the op amp’s output to swing
symmetrically above and below Vcc/2.
IC1a is wired as a non-inverting
buffer stage and provides an output
which is in phase with the microphone
signal. By contrast, IC1b is connected
as an inverting amplifier. It operates
with a gain of -1, as set by the two 10kΩ
input and feedback resistors.
IC1b is fed from IC1a’s output (pin
7) and provides a complementary out
of phase signal at its pin 1 output.
The 1nF capacitor across the feedback
resistor rolls the signal off above about
16kHz to ensure stability.
As a result, IC1a’s output provides
the in-phase signal while IC1b’s output
provides the out-of-phase (or inverted)
signal. The op amp outputs are then
AC-coupled to the output socket via
series 10µF capacitors and 560Ω resistors. The 560Ω resistors provide a
www.siliconchip.com.au
nominal 600Ω output impedance and
prevent the op amps from oscillating
(due to the extra capacitance) when
the balanced microphone cable is
connected.
The 10µF capacitors are necessary to
remove the DC levels that are present
at the outputs of IC1a and IC1b.
Muting
As previously mentioned, the outputs can be muted and this is achieved
using relays RLY1 and RLY2 which
short the outputs to ground when
powered.
In addition, the outputs are muted at
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switch-on. This is necessary because
when power is initially applied to op
amps IC1a & IC1b (via switch S1b),
their outputs quickly rise to half
supply (Vcc/2). Without muting, this
voltage would be coupled into the
PA system and cause large switch-on
thumps. To circumvent this, relays
RLY1 & RLY2 are switched on at power
up to short the signal outputs to ground
until the voltages settle.
The relays are switched via switch
S1a and its associated circuitry based
on transistor Q1. This works as follows.
Switch S1 is a double-pole 3-position switch and when S1 is in position
1, no power is applied to the circuit. In
position 2, S1b’s contacts feed power
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January 2004 57
This close-up view shows the wiring
details to the double-pole 3-position
slide switch. The three switch
terminals at the top connect to their
corresponding PC stakes via short
lengths of tinned copper wire.
Fig.2: here are the parts layouts for the two different PC board versions
(Altronics top, Jaycar & DSE bottom). Make sure that all polarised parts
are correctly oriented and that the correct component is installed at
each location. Note that the Altronics version uses an XLR connector for
the microphone (ie, there’s no provision for a 3.5mm socket).
to op amp IC1, while the corresponding contacts in S1a connect transistor
Q1’s 10kΩ base resistor to ground via
a 100Ω resistor. As a result, Q1 turns
on and applies power to the relays.
As shown on Fig.1, the relay coils
are connected in series, with one side
going to ground via a 470µF capacitor
and 680Ω resistor connected in parallel. Initially, the 470µF capacitor is
discharged and so the full 9V is applied across the series-connected relay
coils – ie, 4.5V for each relay. This is
quite sufficient to activate the 5V relay
coils and close the contacts.
As the 470µF capacitor charges,
the voltage across the relay coils de58 Silicon Chip
creases. However, the relays remain
closed because their dropout voltage is
much lower than the voltage required
to activate them. The 680Ω resistor
sets the minimum voltage across the
relay coils to around 2.7V per relay.
This resistor is included to reduce the
current drawn from the battery while
the relays are closed.
The resistor and capacitor also cause
LED1 to momentarily flash when the
power is switched on. Initially, when
power is applied and the 470µF capacitor is discharged, LED1 is fed
via a 4.7V zener diode (ZD1) and the
series 220Ω resistor. The LED will
glow brightly with a fresh battery but
as the battery voltage falls to around
7.2V, there will be insufficient current
to light it at full brightness.
It works like this: since there is 4.7V
across ZD1 and a nominal 2V across
the LED, this leaves only 0.5V across
the 220Ω resistor when the battery is
at 7.2V. As a result, the LED current
is only about 2.3mA and so the LED
will only glow dimly.
By contrast, if the battery is at 9V, the
resistor will have 2.3V across it and so
the LED current will be around 10mA.
As a result, LED1 will glow brightly.
However, the LED does not light for
long, as the 470µF capacitor quickly
charges via the relay coils and turns
LED1 off again.
When S1 is placed in position 3, IC1
is still powered but Q1’s 10kΩ base
resistor is disconnected from ground.
As a result, the 22µF capacitor is now
left to supply Q1’s base current for a
short time as it charges towards the
9V supply rail via the two series 10kΩ
resistors. After about 1s, Q1 switches
off and the relays also turn off, thereby
releasing the shorts across the output
lines from IC1a and IC1b.
Diode D3 quenches the back-EMF
voltage that’s generated when the relay
coils are switched off. This back-EMF
voltage is further damped by the 100µF
capacitor at D1’s cathode.
Note that the muting can be reactivated at any time by switching S1
back to position 2, so that the relays
are switched on again. In addition,
when the power is fully switched off
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(S1 switched to position 1), the relays
remain on for one second while the
22µF capacitor charges. This ensures
that IC1 is fully powered down before
the relays are switched off, to prevent
loud switching thumps in the PA
system.
As a further precaution, the 100µF
capacitor that’s used to decouple IC1’s
supply rail is quickly discharged via
a 100Ω resistor and position 1 of S1a.
Diode D2 is included to ensure that
the 470µF capacitor also discharges,
so that the relays turn on if power is
quickly applied again.
The 22Ω resistor in series with pin 8
of IC1 limits the surge current through
the switch when power is applied. Similarly, the 100Ω resistor at position 2
of S1a limits the discharge current from the associated 22µF
capacitor when S1a switches
this contact to ground.
A separate battery compartment
accommodates the 9V battery
that’s used to power the circuit.
The screw in the back of the
case (just above the 6.35mm
jack socket) is used to secure the
10mm tapped spacer to the PC
board (see Fig.4).
Construction
The assembly is straightforward
since all the parts are mounted on a
single PC board. There are two board
versions: one coded 01101041 (86 x
64mm) to suit a Jaycar or Dick Smith
Electronics (DSE) case; and one coded
01101042 (81 x 61mm) to suit an Altronics case.
Note that the Altronics version assumes the use of a mini XLR socket for
the microphone. There’s no provision
for a 3.5mm socket on this board.
Regardless of its origin, the specified
case measures 135 x 70 x 24mm and
includes a separate battery compartment. A small panel label measuring
59 x 16mm is affixed to the top panel
of the case.
Begin by checking the PC board for
any possible shorts between tracks or
you can use a PC-mount 3.5mm socket
instead. In that case, you won’t need
to make the cutout.
You should also check that the two
front corners of the PC board have
been cut out to the shape shown.
These cutouts are necessary so that
the board clears the internal pillars
in the case.
breaks in the copper pattern. Check
also that the hole sizes are correct. Note
that a cutout will need to be made in
the board to provide space for a mini
XLR panel-mount socket if you are
using a lapel microphone fitted with
a mini XLR (female) plug.
The XLR cutout is shown as an outline on the PC board. You also need to
file the edge of the PC board slightly
where shown, to allow room for the
XLR securing nut to encroach into the
PC board space.
Alternatively, if you are using a
microphone with a 3.5mm jack plug,
Table 2: Capacitor Codes
Value µF code IEC Code EIA Code
100nF 0.1µF 100n
104
1nF .001µF 1n0
102
Table 1: Resistor Colour Codes
o
o
o
o
o
o
o
o
o
o
o
No.
1
1
6
2
1
1
2
1
2
1
www.siliconchip.com.au
Value
100kΩ
22kΩ
10kΩ
6.8kΩ
1kΩ
680Ω
560Ω
220Ω
100Ω
22Ω
4-Band Code (1%)
brown black yellow brown
red red orange brown
brown black orange brown
blue grey red brown
brown black red brown
blue grey brown brown
green blue brown brown
red red brown brown
brown black brown brown
red red black brown
5-Band Code (1%)
brown black black orange brown
red red black red brown
brown black black red brown
blue grey black brown brown
brown black black brown brown
blue grey black black brown
green blue black black brown
red red black black brown
brown black black black brown
red red black gold brown
January 2004 59
Fig.3: here are the full-size etching patterns for the two versions of the PC board (Jaycar & DSE left; Altronics right).
screws and by fitting the securing nut
to the 3.5mm jack socket.
That done, the LED’s leads can be
bent at right angles about 4mm from its
body and the LED slipped into position
so that it protrudes through the front
panel. Adjust its leads as necessary
and make sure that it is oriented correctly before finally soldering it into
position.
In particular, note that anode lead
(A) is the longer of the two. This lead
goes towards the bottom edge of the
PC board as shown on Fig.2.
6.35mm jack socket
A right-angle stereo 6.35mm jack plug to 3-pin XLR line plug lead is used to
connect the balanced output signal from the Lapel Microphone Adaptor to the
PA amplifier.
Fig.2 shows the assembly details for
the two versions. Start by installing all
the PC stakes at the wiring and switch
terminal points, then install the resistors, diodes D1-D3, zener diode ZD1
and the IC. Make sure you place each
component in its correct position and
with the correct orientation.
Table 1 shows the resistor colour
codes but it’s also a good idea to check
the values using a digital multimeter
as some of the colours can be difficult
to distinguish.
The relays and transistor Q1 can
go in next, followed by the capacitors. Be sure to install the electrolytic
capacitors with the polarity shown.
The 3.5mm socket can also now be
60 Silicon Chip
installed if it is being fitted.
The 3-position switch (S1) is mounted on its side, with its top face aligned
with the edge of the PC board. Five
of its bottom terminals are soldered
directly to the previously installed PC
stakes as shown on Fig.2, while three
of the top terminals connect to their
PC stakes via short lengths of tinned
copper wire.
Drilling the front panel
The front panel can now be drilled
to accept the switch, LED and microphone input socket. That done, attach
the front panel label, then attach the
front panel to the PC board assembly
by installing the supplied switch
A hole is needed in the side of the
box for the 6.35mm jack socket which
is used without its outer cover. Mark
the hole location with the case clipped
together, noting that the socket sits
directly on the PC board and against
the battery compartment.
The mounting hole must be drilled
and reamed out to 10mm diameter,
which will not be large enough for the
threaded section of the socket. That
done, place the PC board in the case
and secure it in position using three
M3 screws (two at the top and one at
bottom right).
Next, position the socket in its
mounting hole and tighten down the
case lid with the four self-tapping
screws supplied. Now heat the socket
using your soldering iron until the
plastic case begins to melt, at the same
time pressing the case together so that
it forms a tight fit around the socket
and closes correctly.
Finally, remove the iron and wait
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Fig.4: this diagram
shows how the M3
x 10mm tapped
spacer is secured
to the PC board.
This helps secure
the 6.35mm socket
when the lid is
screwed down.
Fig.5: this artwork can be used as a
drilling template for the front panel.
for the heated case to cool.
The case will now have formed a
moulding around the threaded section
of the 6.35mm jack socket. It should
then be prised open again and the
socket secured in position using a
cable tie which passes through a hole
in the PC board and then around the
edge of the board.
To further secure the socket, a 10mm
M3 spacer is installed on the PC board
adjacent to it so that the lid can be
firmly screwed down at this point. To
do this, the mounting post in the base
of the case adjacent to the socket is
drilled out to 3mm and this hole goes
right through the case. In addition, you
have to drill out the post in the case
lid directly above this point.
That done, countersink the holes
and cut off the post in the lid using
a sharp utility knife. The 10mm M3
spacer can then be fitted in position
and secured using an M3 x 20mm
screw installed from the bottom of the
case as shown in Fig.4.
All that remains now is to complete
the wiring to the stereo socket and
connect the battery clip lead. Note
that the leads from the battery clip will
have to be fed through from the battery
compartment before soldering them to
the supply terminals on the PC board.
Testing
To test the unit, apply power and
check that the relays close and that the
LED flashes. If not, check that transistor Q1 has been installed correctly and
check its associated components. If the
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Fig.6: this is the full-size artwork
for the case label.
relays do close but the LED doesn’t
flash, check that the LED has been
installed with the correct polarity and
check the orientation of ZD1.
Finally, check that pins 1 & 7 of
IC1 are at about 4.5V (ie, Vcc/2). This
voltage should also be present on pins
3 & 5 (ie, the non-inverting inputs). If
everything checks out, then it is likely
that the unit is working correctly
and is can be tested by connecting
it to a PA system and plugging in a
SC
microphone.
H SILICON CHIP logo printed in
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Price: $A12.95 plus $A5.50 p&p.
Available only in Australia.
Just fill in the handy order form in
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or ring (02) 9979 5644 & quote
your credit card number.
January 2004 61
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