This is only a preview of the July 2008 issue of Silicon Chip. You can view 30 of the 104 pages in the full issue, including the advertisments. For full access, purchase the issue for $10.00 or subscribe for access to the latest issues. Items relevant to "DSP Musicolour Light Show; Pt.2":
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By JOHN CLARKE
Microphone preamp for
PCs & MP3 players
Want to connect a professional microphone with balanced
outputs to the line input of your PC’s soundcard or an MP3
player, for high-quality voice recordings? This microphone
preamp circuit lets you do just that. It features a balanced
input, has a clipping indicator LED and can be powered from
a USB port or from an external DC source.
Main Features
•
•
•
•
•
•
Balanced input
Stereo output
USB or plugpack powered
Level control
Peak indicator
Line level output
68 Silicon Chip
A
LTHOUGH MOST PCs have a
microphone input for recording,
these inputs are for basic electret microphone types only. Electret microphones are typically used in headsets
and are generally of low quality.
Similarly, some MP3 players include an internal electret microphone
for recording but again the quality is
limited. These players often also in-
clude a line input, to accept external
audio signals.
Using an electret microphone will
generally be satisfactory for recording
brief announcements and reminders.
However, if you want really good
sound quality, a professional microphone will be required. This type of
microphone will also be necessary
when the microphone needs to be
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ALTERNATIVE
DC INPUT
OUTPUT
TO COMPUTER
VIA 3.3mm
STEREO
PLUG
MICROPHONE
INPUT
VIA
XLR PLUG
AUDIO
OUT TO
COMPUTER
POWER
IN VIA
USB 'A'
PLUG
XLR MIC
INPUT
DC IN
USB
LEVEL
SWITCH 1
PEAK
POWER
POWER
SWITCH
SWITCH 2
NEON 3
Fig.1: this diagram shows how the Balanced Microphone Preamplifier
is connected to a computer. Power can come either from a USB port or
from an external DC plugpack supply (see text).
more than just a metre or two away
from the computer or MP3 player.
Why are they better
So why do the professional microphones give better sound than lowcost electret microphones? There are
several reasons.
First, professional microphones use
a high-quality microphone element
that has a smooth frequency response
plus low noise and low distortion.
Typical low-cost electrets do not have
a smooth frequency response but a
response that peaks around the mid
audio frequencies. And while low-cost
electret microphones readily detect
handling and breathing noises, professional microphones are designed to
minimise this problem.
Professional microphones also have
a tailored pickup response that is more
sensitive towards the front of the microphone than to the rear. This lack
of sensitivity towards the rear helps
prevents unwanted noise pickup.
Another advantage of professional
microphones, particularly for voice recordings, is that they give more depth
to the sound. That’s because the bass
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response is more pronounced when
the microphone is brought close to the
mouth. A headset electret microphone,
on the other hand, usually has a poor
bass response.
Taken together, these refinements
mean that a professional microphone
will produce a recording that sounds
far crisper and cleaner than one from
an electret microphone – all without
extraneous noises masking the wanted
sound.
Of course, electret microphones
are ideal for many applications. In
fact, high-quality electret microphone
inserts are often used in professional
microphones and can produce excellent sound quality when placed inside
a professional microphone housing.
It’s just that if you want high-quality
recordings, a professional-quality
microphone is the way to go.
Balanced outputs
While professional microphones
can come in many forms (eg, dynamic
and electret types), they all have one
thing in common and that is a balanced
output. A balanced output provides
two signals that are 180° out of phase
with each other. These signals are fed
out via a 3-pin XLR plug that connects
Specifications
Signal-to-noise ratio: 80dB with respect to 1V output and 50mV input & with
600W input loading impedance (this measurement includes a 20Hz - 22kHz
bandpass filter).
Frequency response: within 0.25dB from 20Hz to 20kHz.
Total harmonic distortion: less than 0.01% at 1V output and 50mV input
for all frequencies from 20Hz - 20kHz.
Signal handling: 2.8V RMS output
Sensitivity for 1V out: 9mV
July 2008 69
Parts List
1 PC board, code 01107081,
102 x 83mm
1 diecast aluminium box, 119 x
94 x 34mm
1 3-pin small size female XLR
panel socket
1 Type-A PC-mount USB socket
1 2.5mm PC-mount DC socket
1 3.5mm PC-mount stereo jack
socket
1 Ultra Mini SPST rocker switch
(S1)
1 1kW 16mm linear pot (VR1)
1 50kW multi-turn top-adjust trimpot (code 503) (VR2)
1 knob to suit potentiometer
4 M3 tapped x 6.3mm standoffs
8 M3 x 5mm screws
2 M3 x 10mm screws
2 M3 nuts
4 M3 flat washers
1 solder lug
17 PC stakes
1 150mm length of red mediumduty hookup wire
1 75mm length of green mediumduty hookup wire
1 75mm length of 2-core shielded
cable
1 25mm length of 6mm diameter
heatshrink tubing
4 rubber feet
Semiconductors
2 TL072CP dual op amps (IC1,IC2)
1 LM393N dual comparator (IC3)
1 MAX232CPE RS232 line driver
(IC4)
1 LM336-2.5 2.490V reference
(REF1)
1 BC327 PNP transistor (Q1)
1 5.6V 1W zener diode (ZD1)
to a matching 3-pin XLR socket on the
microphone lead.
As a result, the two balanced microphone signals are fed down the
microphone cable via separate leads.
These leads are shielded to help prevent them from picking up noise and
mains hum. In addition, this arrangement effectively removes any noise
that is picked up when connected to a
balanced input on an amplifier.
In operation, the balanced leads
each pick up the same noise signals
along the length of the microphone
2
MICROPHONE INPUT
(XLR CONNECTOR)
lead. That’s because these leads are
run very close to each other, often as a
twisted pair. When fed into a balanced
amplifier, the signal from each lead is
subtracted and this removes the common noise signal in each lead (ie, the
noise signals are cancelled out by the
amplifier because they are in phase).
The wanted audio signal from each
microphone lead is also subtracted
but because these are in anti-phase,
the signal level is actually doubled as
a result of the subtraction. This means
that balanced microphone leads can be
BALANCED INPUT
AMPLIFIER
OUTPUT
(3.5mm STEREO
SOCKET)
1
3
IC1a,IC1b,
IC2a
VR1 LEVEL
POWER
INPUT
PEAK DETECTOR
POWER
SUPPLY
(IC4)
–
+
POWER
LED
IC2b, IC3
PEAK
LED
Fig.2: the block diagram of the Balanced Microphone Preamplifier. The
balanced signals from the XLR socket are amplified by IC1a & IC1b and
summed in IC2a to give an unbalanced output which is fed to the output
socket. IC2b, IC3 and the associated LED provide peak level indication,
70 Silicon Chip
1 1N5819 1A Schottky diode (D1)
1 3mm green LED (LED1)
1 3mm red LED (LED2)
Capacitors
1 100mF 25V PC electrolytic
2 22mF NP electrolytic
1 10mF 16V PC electrolytic
6 1mF monolithic ceramic
2 100nF MKT polyester
4 220pF ceramic
2 100pF ceramic
Resistors (0.25W 1%)
1 220kW
1 180W
3 100kW
2 150W
2 22kW
1 68W 1W (R1)
1 20kW
1 39W 1W (R1)
12 10kW
2 33W
1 2.2kW
1 10W
2 680W
1 10W (R1)
many metres long without any noticeable increase in noise pickup.
In addition, the output impedance
of professional microphones is usually
very low and this also minimises noise
pickup. Impedances are often well
below the standard 600W, with some
high-quality microphones having an
output impedance as low as 150W.
Of course, a balanced amplifier is
also required in order to use a balanced
microphone and this is always found
on professional audio gear. We also
published a Balanced/Unbalanced
Converter in the June 2008 issue of
SILICON CHIP (it can convert signals
both ways).
In operation, the balanced amplifier
correctly subtracts the balanced signals and provides the gain required to
bring the signal level up to line levels.
This means that the recording can be
made using the line input rather than
the microphone input at the computer.
Alternatively, for an MP3 player, you
can again use the line input and forget
about the internal microphone.
Recording at line levels also helps
to minimise noise. That’s because the
signal does not have to pass through an
internal preamplifier in the computer
or MP3 player.
How it’s connected
Fig.1 shows how the SILICON CHIP
Microphone Preamplifier is connected
siliconchip.com.au
POWER
BC327
B
E
C
A
POWER
LED1
C1
1 F
2
6
1
4
IC4
MAX232
3
K
ADJ
+
K
16
C3
1 F
680
+5V
LM336-2.5
–
TYPE 'A' USB SKT
+5V
33
A
13
11
14
S1
100 F
25V
D1
1N5819
C4
1 F
10
USE ONE
ONLY
R1
C2
1 F
5
12
ZD1
5.6V
1W
1 F
K
+7V
OPTIONAL
MUTE
SWITCH
XLR INPUT
SOCKET
2
100pF
22k
8
2
NP
3
180
3.5mm STEREO
JACK SOCKET
150
150
10k
10k
10k
7
IC1b
5
22k
68 1W
20k
6
+7V
1
4
10k
OPTIONAL
THROUGH
XLR PLUG
+5V
IC2a
3
220pF
220pF
LEVEL
1k LIN
VR1
2
10
39 0.5W
10k
10k
100k
R1 VALUE
10k
1
IC1a
2
22 F
1
9V
12V
220pF
3
220pF
100k
DC INPUT
VOLTS
6V
IC1, IC2: TL072
100nF
+7V
22 F
NP
3
DC
IN
33
15
1
A
4
10 F
–6V
100pF
LEDS
K
A
+5V
1N5819
A
100k
2.2k
+2.5V
K
TP1
6
+
REF1
LM336-2.5
A
SC
2008
IC2b
10k
–
10k
ZD1
8
5
100nF
2
TP2
10k
7
+5V
ref
3
IC3a
220k
1
5
6
TP3
1 F
10k
E
B
Q1
BC327
C
IC3: LM393
VR2
50k
TPG
8
IC3b
7
4
10k
PEAK
LED2
A
K
680
K
MICROPHONE PREAMP FOR COMPUTERS & MP3 PLAYERS
Fig.3: the full circuit. IC4 functions as a charge-pump converter to provide +7V & -6V supply rails for op amps IC1 &
IC2. IC1a & IC1b are wired as non-inverting amplifier stages, while IC2a sums their outputs. IC3a & IC3b function as a
window comparator. They compare a sample of the output signal with reference voltages set by REF1, IC2b & VR2.
to a computer. It includes the 3-pin
XLR connection for the microphone
lead and a stereo 3.5mm jack socket
output. The connection is made to
the computer using a 3.5mm stereo
jack-to-jack lead or a 3.5mm stereo
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jack to RCA plug lead if the computer
has RCA inputs.
Power for the unit can come from
either a DC plugpack supply or from
the computer’s USB port. In the latter
case, the USB port provides a +5V rail
to the preamp via a standard ‘A’ Male
to ‘A’ Male USB connector lead.
In operation, the preamp is switched
on and off via a power switch on the
side of the case and a “Power” LED
lights when the unit is on. The output
July 2008 71
XLR INPUT SOCKET
DC
INPUT
D1
10k
TPG
100k
REF1
LM336-2.5
1
2
22 F
NP
2-CORE
SHIELDED CABLE
signal level is adjusted using a pot and
a peak indicator LED lights when the
signal exceeds line level.
If the peak indicator LED lights,
then the signal level is too high. This
will cause clipping and distortion in
the recording. In practice, it’s simply
a matter of adjusting the level to avoid
any peak indication during microphone use.
Fig.2 shows the block diagram of
the Microphone Preamplifier. The
balanced microphone signal is fed in
via the XLR input socket and the two
anti-phase signals are then amplified
by op amps IC1a & IC1b. The signals
are then subtracted in IC2a and fed to
the output jack socket.
The peak detector circuit monitors
the output signal level and flashes the
Peak Indicator LED when the signal
exceeds the threshold level. This circuit comprises IC2b & IC3, plus the
LED itself.
Circuit details
Refer now to Fig.3 for the complete
circuit details.
As shown, the balanced input signals from the microphone are coupled
in via 22mF non-polarised capacitors
and fed to the non-inverting inputs
72 Silicon Chip
VR1
LEVEL
680
LED2
100pF 100pF
2.2k
IC4
MAX232
TP2
TP1
Q1
220pF
3
1 F
1 F
TP3
1 F
1 F
1 F
10k
1 F
VR2 50k
10 F
IC3
LM393
10k
33
680
22k
22k
NP
100 F
ZD1
5819
10k
100k
180
220k
33
10k
10k
10k
220pF 100nF
20k
IC2
TL072
IC1
TL072
10k
100nF
10k
E
1
4
10
150
220pF
10k
10k
220pF
2
R1
10k
100k
1
3
150
22 F
USB TYPE A
SOCKET
3
2
3.5mm STEREO
JACK SOCKET
Fig.4: install the parts on the PC
board & complete the wiring as
shown in this assembly diagram.
The pot is secured by soldering
its body to four PC stakes, then
soldering its terminals to another
three PC stakes (see photo).
LED1
K
A
PEAK
A
DE C NALA B
CI M
P MAERP
K
18070110
S1
POWER
of op amps IC1a & IC1b (pins 3 & 5).
These input signal lines (ie, pins 2
& 3 on the XLR socket) are each tied
to ground using a 100kW resistor, to
prevent them from floating with no
input connected.
The 220pF capacitor across the two
inputs shunts radio frequencies while
the 100pF capacitors at pins 3 & 5 also
shunt RF signals to ground.
Pins 3 & 5 of the op amps are each
tied to ground via a 22kW resistor,
again to prevent spurious operation
in the absence of an input signal. Note
that the signal ground and the supply
ground are isolated in this part of the
circuit to reduce earth loops and this
is the reason for the different earth
symbols shown.
Op amp IC1a amplifies the pin 3
microphone signal, while IC1b amplifies the pin 2 signal. Both are configued
as non-inverting amplifiers with 10kW
feedback resistors, while a 180W resistor and 1kW pot (VR1) are connected in
series between their inverting inputs.
The 220pF capacitors across the 10kW
resistors roll off the high-frequency
response above 70kHz.
VR1 is the level control and this
varies the gain of IC1a & IC1b between
about 9 and 56. The outputs from these
op amps appear at pins 1 & 7 and are
summed in unity gain differential
amplifier IC2a.
For signals coming from IC1a, IC2a
functions as an inverting amplifier
with a gain of -1, as set by its 10kW
feedback resistors. However, for signals on its pin 3 input, IC2a operates
as a non-inverting amplifier with a
gain of 2. Because of this, the signal
from IC1b is divided by two using a
10kW resistive divider before being
applied to IC2a.
This means that each signal path
from IC1a & IC1b has overall unity
gain through IC2a. However, IC2a
inverts the signals from IC1a, so that
they are now in phase with the signals
from IC1b. As a result, both signals
add to provide an overall gain of 2 for
the stage (ie, IC2a sums its two input
signals).
The resulting unbalanced signal appears at pin 1 of IC2a and is fed to the
left & right terminals of a 3.5mm stereo
jack socket via 150W isolation resistors. The 220pF feedback capacitor
across IC2a rolls off the high-frequency
response of this stage.
Peak detector
IC2a’s output also drives the peak
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INSTALL EITHER
THE DC SOCKET OR
THE USB SOCKET
BUT NOT BOTH
detector circuit. This consists of op
amps IC3a & IC3b which are wired
as a window comparator, plus IC2b
and REF1 which provide an accurate
reference voltage for the comparator.
As shown, the signal level is first
attenuated using a 20kW and 10kW
resistive divider and then coupled to
pins 2 & 5 of IC3a & IC3b respectively
via a 10mF capacitor.
REF1, an LM336-2.5, is used to
provide a 2.5V reference and this is
applied to the pin 5 input of op amp
IC2b. This stage operates as a noninverting amplifier with a gain of 2
and provides a 5V reference at its pin
7 output.
This reference voltage is fed to a
voltage divider network consisting
of a 10kW resistor, trimpot VR2
and a second 10kW resistor
to ground. As a result, two
different reference voltages
are applied to pins 3 & 6 of
IC3a & IC3b, with VR2 used to
set the voltage between these
inputs.
These two reference voltages
are labelled as TP2 and TP3 on
Fig.3 and are equally spaced either
side of 2.5V. Pin 3 of IC3a is set to
the TP2 voltage, while pin 6 of IC3b is
biased to the TP3 voltage. The pin 2 &
pin 5 inputs of IC3a & IC3b are biased
to the 2.5V reference set by REF1 via a
100kW resistor. As a result, the signal
from IC2a swings above and below this
2.5V reference.
Note that IC3’s outputs are open
collector and so the outputs can be
tied together. They are connected to
the +5V rail via a 220kW resistor and
so are normally held high.
Provided that the signal level at
pins 2 & 5 does not exceed the reference thresholds (ie, doesn’t go above
or below these levels), the outputs of
IC3a & IC3b will remain high due to
the pull-up resistor. Conversely, if the
signal exceeds one of these reference
threshold voltages, the corresponding
op amp will switch its output low.
Thus, if the voltage on pin 2 of IC3a
goes above the reference voltage on
pin 3, IC3a’s output will switch low.
Table 2: Capacitor Codes
Value
1mF
100nF
220pF
100pF
mF Code
1mF
0.1mF
NA
NA
IEC Code
1u0
100n
220p
100p
EIA Code
105
104
221
101
Similarly, if the voltage on pin 5 of
IC3b goes below the voltage on pin 6,
pin 7 of IC3b will switch low.
This output low from either comparator then turns on PNP transistor
Q1 and lights the peak indicator LED
(LED2). The associated 1mF capacitor between the op amp outputs and
the +5V rail ensures that the outputs
remain low for 200ms after the comparator switches off, so that very fast
overload transients aren’t missed.
In practice, trimpot VR2 is set so
that the TP2 & TP3 reference voltages
are 442mV above and below the 2.5V
reference respectively. This corresponds to a 1V RMS sinewave signal
at IC2a output.
Note that for a 1V RMS sinewave,
the peak voltage is 1.414V. This is divided by 3.2 using the resistive divider
on IC2a’s output and the 100kW resistor at pins 2 & 5 of IC3a & IC3b. As a
result, the 1.414V peak is reduced to
441.8mV which is why VR2 is adjusted
for 442mV above and below 2.5V at
TP2 & TP3 to give peak indication
when IC2a’s output signal goes above
the 1V RMS level.
Power supply
As mentioned previously, the unit
can either be powered from a USB
Table 1: Resistor Colour Codes
o
o
o
o
o
o
o
o
o
o
o
o
o
o
siliconchip.com.au
No.
1
3
2
1
11
1
2
1
2
1
1
2
1
Value
220kW
100kW
22kW
20kW
10kW
2.2kW
680W
180W
150W
68W
39W
33W
10W
4-Band Code (1%)
red red yellow brown
brown black yellow brown
red red orange brown
red black orange brown
brown black orange brown
red red red brown
blue grey brown brown
brown grey brown brown
brown green brown brown
blue grey black brown
orange white black brown
orange orange black brown
brown black black brown
5-Band Code (1%)
red red black orange brown
brown black black orange brown
red red black red brown
red black black red brown
brown black black red brown
red red black brown brown
blue grey black black brown
brown grey black black brown
brown green black black brown
blue grey black gold brown
orange white black gold brown
orange orange black gold brown
brown black black gold brown
July 2008 73
Above: the prototype
was fitted with both a DC power
socket and a USB socket but you
should fit one or the other – see text.
The rear panel carries the XLR socket and has access holes for the USB
connector (left), the adjacent DC power socket and the line output jack.
port or via a DC plugpack. Diode D1
provides reverse polarity protection
if a DC plugpack is used, while series
resistor R1 depends on the plugpack
voltage (see table on circuit).
In practice, any 300mA DC plugpack
with an output voltage of 6V, 9V or
12V can be used.
A 100mF capacitor filters the incoming supply rail, while S1 is the power
on/off switch. Zener diode ZD1 en74 Silicon Chip
sures that the resulting supply rail is
limited to 5.6V to prevent damage to
IC4, while R1 is necessary to prevent
excessive current through ZD1.
Note that no reverse supply protection is provided for the USB supply
since this uses a polarised connector
that cannot be reversed. Note also that
only one type of supply should be used
with this preamplifier. DO NOT install
both a USB socket and a DC socket on
the PC board, as damage to the USB
port could occur if both supplies were
used at the same time.
IC4 is a MAX232 RS232 line driver
IC but the line driver section is not
used in this circuit. Instead, we are
simply using it to generate the necessary plus and minus supply rails for
the rest of the circuit.
Basically, the MAX232 includes
two internal charge pumps that convert the +5V supply to nominal
unloaded ±10V rails. The first
converter switches capacitor C1
and dumps its charge into C3 to
double the supply to derive the
+10V rail. Similarly, the second
converter inverts this +10V rail
by switching C2 at a rapid rate
and dumping the charge into C4,
to provide the -10V rail.
This switching of C1 and C2 takes
place at a nominal 400kHz rate.
In practice, the resulting supply
rails are loaded down to about +7V
and -6V by op amps IC1 & IC2. Note,
however, that the LM393 op amps
(IC3a & IC3b) are powered directly
from the +5V supply rail, to prevent
excessive loading on IC4.
The positive and negative supply
rails generated by IC4 appear at pins
2 & 6 respectively and are decoupled
using 33W resistors. In addition, these
rails are bypassed using a 100nF capacitor to minimise supply noise. The
power LED (LED1) is driven from the
+5V rail via a 680W resistor.
Finally, note that the signal earth for
the preamplifier and the earth for the
power supply are isolated via a 10W
resistor. This reduces any current flow
in the ground when the preamplifier
is connected to a computer using both
USB power and the stereo 3.5mm jack
to feed in the signal. This is necessary because in this case there would
be two earth paths between the unit
and the computer – one via the USB
connector and the other via the audio
connection.
Construction
Construction is straightforward
with most of the parts mounted on a
PC board coded 01107081 and measuring 102 x 83mm. This is housed in
a diecast aluminium box measuring
119 x 94 x 34mm.
Begin by checking the PC board for
any defects such as shorted tracks and
breaks in the tracks. Check also that
the hole sizes are correct by test fitting
siliconchip.com.au
19
13
30
8
3MM
11.5
13
16
22.5 MM
6
HOLE DIA.
6MM
HOLE DIA.
7MM
3MM
REAR PANEL DRILLING DETAILS
15
19
5
This close-up view shows how the
Level Control potentiometer (VR1)
is mounted on the PC board.
siliconchip.com.au
SWITCH CUTOUT -- RIGHTHAND SIDE
10.5
94
10.5
7
3MM
DIA. HOLE
3MM
DIA. HOLE
76.5
the major parts, ie, the 3.5mm stereo
jack socket and either the DC socket
or the USB socket. The holes for the
four-corner mounting screws should
be 3mm in diameter.
Finally check that the PC board fits
into the box and that the cutout has
been made for the XLR socket.
Fig.4 shows the parts layout on the
board. The resistors can be installed
first. Table 1 shows the resistor colour
codes but a digital multimeter should
also be used to check each resistor
before soldering it in place.
Follow these with the ICs, taking
care to ensure that they are correctly
oriented. Make sure also that the
LM393 goes in the IC3 position. We
used sockets for IC1 & IC2 but this really is unnecessary and you can solder
the ICs straight in.
Next on the list are PC stakes for all
the following: test points TP1-TP3, TP
GND, the GND terminal, the switch
terminals, the potentiometer mounts
and its terminal connections, and the
three input terminals (to terminate
the stereo shielded cable from the
XLR socket). Note that four PC stakes
are used to support the metal body of
VR1, which sits about 1mm above the
PC board (see photo).
Transistor Q1 (BC327) and the
LM336-2.5 voltage reference (REF1)
can now be installed, followed by
diode D1 and zener diode ZD1. Take
care to ensure that they are all oriented
correctly and don’t get Q1 and REF1
mixed up (they look alike). Note that
D1 and ZD1 face in opposite directions.
The capacitors can go in next. Be
6.5
3MM
DIA. HOLE
3MM
DIA. HOLE
BASE DRILLING DETAILS
Fig.5: follow this diagram to mark out and drill the holes in the metal case.
Alternatively, the diagram can be downloaded from the SILICON CHIP website,
printed out and used as a drilling template.
sure to orient the electrolytic types
as shown in Fig.4. That done, install
10-turn trimpot VR2, then solder potentiometer VR1’s metal body to its
four PC stakes.
To do this, first bend the pot’s three
terminals down at right angles, then
position the potentiometer vertically
on the board and push it’s metal body
down between the four PC stakes until
July 2008 75
LINE OUT
MIC INPUT
9-12V DC
INPUT
USB IN
SILICON
CHIP
BALANCED MICROPHONE PREAMPLIFIER
+
PEAK
POWER
+
+
POWER
ON/OFF
LEVEL
Fig.6: this full-size front panel artwork shows the hole locations for the
Level control and the two LEDs. Drill the Level control hole to 7mm & the
holes for the LEDs to 3mm
it sits about 1mm proud of the board.
Mark the solder points on the body,
then remove the pot and scrape away
the anodised coating at those points.
Next, cut the pot shaft off about
17mm from its threaded boss, then
reposition the pot on the board and
solder it’s body to the four PC support
stakes. The pot mounting can then
be completed by soldering its three
terminals to the adjacent PC stakes.
The two LEDs are mounted with
the tops of their lenses exactly 25mm
above the PC board. A 20mm-wide
cardboard strip slipped between the
leads of each LED makes a handy
“standoff” tool when soldering them
in position. Note that, in each case,
the anode lead (the longer of the two)
goes to the left.
Finally, you can complete the PC
board assembly by installing the
3.5mm stereo jack socket and either
the DC power socket or the USB
socket (but NOT both). This depends,
of course, on how you intend to power
the unit. As previously stated, you
don’t fit both because the computer
could be damaged if both supplies
were connected at the same time.
Internal wiring
If you buy a complete kit for this
design, it will probably be supplied
with all the holes pre-drilled and with
a screen printed front-panel label. If
not, you will have to drill the holes
yourself.
Fig.6 shows the front panel layout
and this can be used as a drilling
template. You will need to drill 3mm
holes for the Power & Peak indicator
LEDs, plus a 7mm hole for the Level
pot shaft. The latter is best made by
drilling a pilot hole and then carefully
enlarging it using a tapered reamer.
Next, you will have to drill holes
in the rear panel for the 3.5mm jack
socket, the XLR socket and either the
DC input socket or the USB socket.
Fig.5 shows the drilling details.
You will need to drill a 6mm hole
for the DC input socket, while the stereo jack socket requires a 7mm hole.
The square cutout for the USB socket
can be made by first drilling a row of
holes and then carefully filing to the
final shape.
The large hole for the XLR socket is a
bit trickier to make. This hole is too big
for most tapered reamers, so you will
have to drill a series of holes around
the inside circumference, then knock
out the centre piece and carefully file
it to shape. Its two mounting screw
holes are each drilled to 3mm.
Next, a square cutout for the power
switch must be made in the righthand
side of the case – see Fig.5. Again, this
is made by drilling a series of holes,
then knocking out the centre piece
and filing the hole to shape, until the
switch clips into position.
Finally, four 3mm mounting holes
for the PC board must be drilled in
the base of the case. This is best done
using Fig.5 as a template.
Once the box has been drilled, the
next step is to insulate the threaded
ferrule of the 3.5mm jack socket with
a short piece of heatshrink tubing, to
prevent it making contact with the
case. This heatshrink tubing should
be shrunk on using a hot-air gun but
be careful not to apply too much
heat, otherwise you could damage the
socket’s plastic casing.
The PC board can now be installed
in the case. To do that, secure four
6.3mm tapped standoffs to the base
using M3 x 5mm screws, then place
the board in position and secure it
using another four M3 x 5mm screws
and four M3 washers.
With the board in place, you can
now fit the XLR socket and complete
the wiring as shown in Fig.4. Note
that 2-core shielded cable is used
for the connections between the PC
board and the XLR socket and that
the pin 1 terminal on the XLR socket
is the ground or shield pin. Note also
that a solder lug is fitted under one
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76 Silicon Chip
siliconchip.com.au
INSULATE SCREW
FERRULE OF JACK
SOCKET WITH
HEATSHRINK TUBING
ANTRIM
TRANSFORMERS
manufactured in Australia by
Harbuch Electronics Pty Ltd
harbuch<at>optusnet.com.au
Toroidal – Conventional Transformers
Power – Audio – Valve – ‘Specials’
Medical – Isolated – Stepup/down
Encased Power Supplies
Be sure to insulate the threaded ferrule of the line output jack socket with
heatshrink tubing. This ensures that it cannot make contact with the case
and cause an earth loop which would lead to hum.
of the XLR socket’s mounting screws,
to terminate the earth wire from the
PC board.
Testing
To test the unit, first apply power
and check that the power LED lights.
Now measure the voltage between
TPGND and pin 16 of IC4 – you should
get a reading of 5V for a USB supply,
or 5.6V if a plugpack supply is used.
Check also that pin 4 of IC1 is about
-6V, pin 8 of IC1 is at about +7V and
that TP1 is at about 2.5V (with respect
to ground).
If any of these voltages is incorrect,
switch off immediately and check the
supply wiring. Check also that IC4 has
been installed correctly.
Assuming everything checks out
so far, adjust trimpot VR2 so that the
voltages at TP2 and TP3 are 442mV
above and below 2.5V respectively (ie,
TP2 should be +442mV with respect
to TP1, while TP3 should be -442mV
with respect to TP1). This sets the peak
level indication.
Note that because of resistor
tolerances, you will not be able to
adjust VR2 so that TP2 and TP3 are
exactly the same value above and
below TP1.
Note also that if you intend using
this Balanced Preamplifier with a
computer, then it’s a good idea to set
the peak indicator so that it agrees with
siliconchip.com.au
the level indicator in your recording
software.
Alternatively, if using this preamplifier with an MP3 player (ie, via the
line input), adjust VR2 for the ±442mV
levels at TP2 & TP3, then check that
the sound is undistorted for all levels
unless the peak level is exceeded.
The assembly can now be completed
by fitting four stick-on rubber feet to
the underside of the box, then attaching the front-panel label, the lid and
the control knob. Make sure that the
two LEDs just protrude through their
holes in the lid.
The front panel label can be made
by downloading the artwork from
the SILICON CHIP website and printing it out on a colour printer. It can
be attached to the panel using either
double-sided tape or a thin smear of
silicone sealant.
Options
If you wish to have a through XLR
plug (so that you can feed through
the signal to another preamplifier
or mixer), then you will have to use
a diecast box measuring 119 x 94 x
57mm. Extra positions for PC stakes
have been included on the PC board
(at the front, left) for this wiring.
Finally, a switch can be added to
close the connection between pins 2
& 3 of the XLR socket for microphone
SC
muting.
Encased Power Supply
www.harbuch.com.au
Harbuch Electronics Pty Ltd
9/40 Leighton Pl, HORNSBY 2077
Ph (02) 9476 5854 Fax (02) 9476 3231
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This controller allows you to vary the
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We also have solar maximiser kits,
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July 2008 77
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