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A Gain
Controlled
Microphone
Preamp
By JOHN CLARKE
Designed for use with PA systems, this gain
controlled microphone preamplifier will
provide a constant output level for a wide range
of input levels. It ensures that the amplified
sound level is always the same, regardless as to
how loudly (or softly) a person speaks.
How often have you heard a public
address system where the sound level
varies all over the place? This problem
occurs because different people speak
at different sound levels.
For example, if the person using
the microphone speaks loudly, then
the gain control has to be reduced
to bring the amplified sound back
to the correct level (or to prevent
overload). Conversely, if the person
speaks quietly, then the gain control
has to be advanced to maintain good
audibility.
Indeed, a very quiet talker may not
provide enough signal to ensure an
adequate sound level, even if the amplifier is set to maximum gain.
Level fluctuations can also be
30 Silicon Chip
caused by people who turn their
heads from side to side as they speak,
and by people who alternately move
closer to and further away from the
microphone.
Main Features
•
•
•
•
•
•
•
Suitable for dynamic microphones
Balanced input
Constant output over 50dB
input range
Powered by 9VDC plugpack
Low input impedance
Low distortion
Fast response
A sound system operator can compensate for some of these problems by
riding the gain control on the amplifier.
However, there is always a delay in
the response because the operator first
has to hear the incorrect level before
making changes. Another approach is
to use a less directional microphone
to reduce level variations from people
who move around while speak
ing
but this greatly increases the risk of
feedback.
A gain controlled microphone preamplifier, such as the unit described
here, will help to solve these problems.
It automatically varies its gain in response to the microphone signal to
ensure that a constant level is fed to the
PA amplifier. As the output signal level
from the microphone goes down, the
gain of the microphone preamplifier
goes up, and vice versa.
As a result, the amplified audio level
is essentially constant for virtually all
people, regardless of their speaking
style or how they move about in front
of the microphone.
In effect, it’s just like having a person
constantly riding the gain control on
the amplifier, except that it’s all auto-
C1
2
4
INPUTS
Fig.1: block
diagram of the
Plessey SL6270DP
voice operated
gain adjusting
device (VOGAD).
It contains two
amplifier stages &
an AGC detector
block.
V+
7
3
GAIN
CONTROLLED
AMPLIFIER 1
10k
680
8
5
OUTPUT
AMPLIFIER
2
2k
AGC
DETECTOR
SL6270
1
RT
6
CT
matic. And of course, an electronic circuit responds far quicker to any level
changes than a human operator, so that
the adjustments are imperceptible.
In technical terms the preamplifier
has a gain of around 50dB for low input signal levels (ie, 70µV) but limits
once the input signal reaches about
1mV. The output signal level remains
virtually constant at 100mV for input
signals ranging from 1mV to beyond
100mV.
As can be seen from the accompanying photograph, the unit is housed in
a small low-cost plastic case. A 3-pin
XLR socket fitted to one end of the case
accepts a balanced microphone input,
while a 6.5mm phono socket on the
other end provides the single-ended
(or unbalanced) output signal. Power
for the circuit comes from a 9V DC
plugpack supply.
Block diagram
Refer now to Fig.1 – this shows
a block diagram of the Plessey SL
6270DP voice operated gain adjusting
device (VOGAD) which forms the
heart of the circuit. Let’s see how it
works.
Inside the SL6270DP IC are two
amplifier stages and an AGC detector
block. The AGC detector monitors
the output level from amplifier 2 and
provides a DC control signal which
sets the gain of amplifier 1.
+8V
0.1
BALANCED
MICROPHONE
INPUT
2
SHELL
3
2.2
3
100
2
OUTPUT
47k
1
1M
6
47
9V
INPUT
I GO
10
8
IC1
SL6270
1
K
R1
.0033
5
100 4
A
7
IN
10
REG1
7808
GND
OUT
10
+8V
1k
A
LED1
K
GAIN CONTROLLED MICROPHONE PREAMPLIFIER
Fig.2: the final circuit of the Gain Controlled Microphone Preamplifier. Resistor
R1 allows the gain (& thus the AGC range) to be adjusted – see text & Table 1.
Power comes from a 9V DC plugpack & is regulated to 8V by REG1, while LED 1
provides power on/off indication.
In greater detail, amplifier 1 is a
DC-controlled balanced input amplifier which accepts signals from the
microphone. This stage in turn drives
amplifier 2 via a 680Ω resistor and
external capacitor C1 which rolls off
the low-frequency response.
Amplifier 2 has a gain of about 15,
as set by the 10kΩ feedback resistor
and the 680Ω input resistor. Its output
appears at pin 8 and is also fed to the
AGC (automatic gain control) detector
block which provides the DC control
signal. The 2kΩ resistor and external
capacitor CT set the AGC attack time,
while RT provides a discharge path
for CT.
Finally, the control voltage across
CT is applied to amplifier 1, which
adjusts its gain accordingly and thus
sets the output level on pin 8.
Circuit details
Refer now to Fig.2 for the final circuit. In addition to the SL6270 (IC1),
there’s just a 3-terminal regulator, a
power indicator LED and a few resistors and capacitors.
As shown, the balanced inputs from
the microphone are coupled to pins 4
& 5 of IC1 via 100µF capacitors. These
capacitors are necessary to prevent
DC current from flowing in the micro
phone. The 2.2µF capacitor between
pins 2 & 7 sets the low fre
quency
roll-off to 300Hz, while the .0033µF
capacitor between pins 7 & 8 (ie, in
the feedback path of amplifier 2) sets
the high frequency roll-off to 5kHz (R1
open circuit).
Resistor R1 has been included to
tailor the AGC range. This resistor is in
parallel with the internal feedback resistor between pins 7 & 8 of IC1 and so
reduces the gain of amplifier 2. Table 1
shows the effect of different values of
R1 on the sensitivity and the resulting
affect on the signal-to-noise ratio.
Note that as R1 decreases (ie, the
gain goes down), progressively higher input signal levels are required to
maintain the -3dB output level. It is
this reduction in gain that gives the
improved signal-to-noise ratio.
The output at pin 8 is AC coupled
to the output socket via a 10µF capacitor. Note the associated 47kΩ resistor
to ground. This provides a charging
path for the 10µF capacitor when no
load is connected, to prevent large
thumps when the unit is subsequently
plugged in.
The 47µF capacitor and the parallel
August 1995 31
This view shows how the PC board & the various sockets fit inside the case.
Take care to ensure that the supply polarity is correct before soldering the leads
to the DC power socket.
1MΩ resistor on pin 1 of IC1 set the
time constant components for the automatic gain control. The 47µF capacitor
sets the attack time to 18ms, while the
1MΩ resistor sets the decay rate the
20dB per second.
Power for the circuit is derived from
a 9VDC plugpack. This feeds 3-terminal regulator REG1 which delivers
an 8V rail to power IC1. The 10µF
capacitors at the input and output
of REG1 are for stability and supply
ripple rejection. LED 1 provides power
indication and is driven from the 8V
rail via a 1kΩ limiting resistor.
Construction
The Gain Controlled Microphone
Preamplifier is built onto a PC board
coded 01207951 and measuring 49
x 48mm. Fig.3 shows the wiring
details.
Begin the assembly by installing PC
stakes at the external wiring points.
The remaining parts can be installed
in any order but take care with the
OUTPUT
SOCKET
47uF
1M
XLR PANEL
SOCKET
orientation of the electrolytic capacitors and the IC. Resistor R1 can be left
off the board at this stage, as it may
not be necessary with your particular
microphone.
The 3-terminal regulator is installed
with its leads bent at right angles to
mate with its mounting holes and
is bolted to the board using a screw
and nut.
Take care with the orientation of the
LED – its anode lead will be the longer
of the two. It should be mounted with
its top 25mm above the board surface,
so that it will later protrude through
a hole in the lid. Note that it may be
necessary to extend its leads in order
to obtain the correct height.
That completes the PC board assembly. It can now be installed inside a
plastic zippy case measuring 82 x 54
x 32mm.
First, drill and cut out the holes
for the XLR, phono and DC sockets
at either end of the case – see Fig.3.
You will also need to shave back the
ribs in the side of the case so that the
PC board can sit directly on the base.
Next, attach the front-panel label
to the lid and drill out the four corner
mounting holes and the hole for the
LED. This done, fit the PC board inside
the case and mount the XLR, phono
and DC sockets. Finally, complete the
wiring as shown in Fig.3. Note the link
between pin 1 of the XLR socket and
its earth terminal.
100uF
1
2
3
0.1
1
100uF
IC1 2.2uF
SL6270
01207951
R1
10uF
.0033
10uF
47k
10uF
5
REG1
4
1k
A
K
4
5
DC
SOCKET
LED1
Fig.3: install the parts on the PC board & complete the wiring as shown here.
Note that the LED is mounted with its top 25mm above the board surface, so that
it will later just protrude through a hole in the lid.
Fig.4: check your board carefully
against this full-size pattern
before installing any of the parts.
RESISTOR COLOUR CODES
❏
No.
❏ 1
❏ 1
❏ 1
32 Silicon Chip
Value
1MΩ
47kΩ
1kΩ
4-Band Code (1%)
brown black green brown
yellow violet orange brown
brown black red brown
5-Band Code (1%)
brown black black yellow brown
yellow violet black red brown
brown black black brown brown
Specifications
PARTS LIST
1 PC board, code 01207951, 49
x 48mm
1 Dynamark front panel label, 50
x 79mm
1 plastic case, 82 x 54 x 32mm
1 XLR 3-pin panel socket
1 6.5mm mono phono socket
1 2.1mm DC panel socket
3 3mm dia x 6mm long screws
& nuts
7 PC stakes
1 9VDC 300mA plugpack
Input impedance .................................. 150Ω unbalanced; 300Ω balanced
Supply current ..................................... 20mA
Supply voltage ..................................... 9VDC plugpack
Output level ......................................... 100mV nominal
Voltage gain ........................................ 52dB for 72µV input
Distortion ............................................. 2% <at> 90mV input
Signal to noise ratio ............................. see Table 1
Attack time .......................................... 20ms
Decay time .......................................... 20dB/second
Frequency response ........................... -3dB at 100Hz & 5kHz
Semiconductors
1 SL6270DP voice operated gain
adjusting device (VOGAD)
(IC1)
1 7808 8V 3-terminal regulator
(REG1)
1 3mm red LED (LED1)
Table 1: The Effect Of Changing R1
.0033µF
120µV
44dB unweighted
3.9k
.0082µF
200µV
52dB unweighted
2.2k
.015µF
540µV
57dB unweighted
1k
.033µF
900µV
63dB unweighted
680W
.047µF
1.2mV
66dB unweighted
Adjusting R1
+
Depending on the microphone, that
may be all there is to it. However, if you
now find that the microphone is now
noisy or too sensitive, or that unwanted background noises are audible, it
will be necessary to add R1 to reduce
the AGC range and the gain.
This will have to be done on a trial
and error basis using the values list-
DC IN +
Carefully check the polarity of the
DC plugpack connector before plugging it into the power socket. When
you are satisfied that it is correct, apply power and check that the power
indicator LED lights. Check also that
the output pin of the 3-terminal regulator (REG1) is at 8V.
If you don’t get the correct voltage,
switch off immediately and check for
OUTPUT
Testing
wiring faults. If the output voltage is
correct but the LED fails to light, then
it is probably incorrectly oriented.
Assuming that all is OK, you can
now test the unit with a microphone
and amplifier to verify that it is working correctly.
+
The wiring connections and the DC
sockets should be sufficient to secure
the PC board in position. However, if
necessary, you can further secure the
board to the bottom of the case using
machine screws and nuts.
Capacitors
2 100µF 16VW PC electrolytic
1 47µF 16VW PC electrolytic
3 10µF 16VW PC electrolytic
1 2.2µF 16VW PC electrolytic
1 .0033µF MKT polyester
Resistors (0.25W, 1%)
1 1MΩ
1 1kΩ
1 47kΩ1
R1 (see text)
ed in Table 1 (note that the feedback
capacitor in parallel with R1 must be
changed as well). For example, if R1
is 1kΩ, the gain and AGC range will
be reduced by 17dB, with a corresponding improvement of 19dB in the
signal-to-noise ratio.
There is a limit as to how far you
can reduce R1 though and this will
be determined by the sensitivity of the
SC
microphone being used.
+
open
BALANCED INPUT
Signal-To-Noise Ratio
+
-3dB Input Voltage
GAIN CONTROLLED
MICROPHONE
PREAMPLIFIER
Parallel C
+
R1
Left: the XLR microphone socket is mounted on one end of
the case, while the output & DC power sockets are on the
opposite end. Above is the full-size front panel artwork.
August 1995 33
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