Description:
A
microphone
amplifier
that may
be used
with
either
Electret
Condenser
Microphone
(ECM)
inserts
or
dynamic
inserts,
made
with
discrete
components.
Notes:
Both
transistors
should
be low
noise
types.
In the
original
circuit,
I used
BC650C
which is
an ultra
low
noise
device.
These
transistors
are now
hard to
find but
BC549C
or
BC109C
are a
good
replacement.
The
circuit
is self
biasing
and will
set its
quiescent
point at
roughly
half the
supply
voltage
at the
emitter
of the
last
transistor.
The
electret
condenser
microphone
(ECM)
contains
a very
sensitive
microphone
element
and an
internal
FET
preamp,
a power
supply
in the
range 2
to 10
volts DC
is
therefore
necessary.
Suitable
ECM's
may be
obtained
from
Maplin
Electronics.
The 1k
resistor
limits
the
current
to the
mic.
This
resistor
should
be
increased
to 2k2
if a
supply
voltage
above 12
Volts DC
is used
and is
not
needed
if the
Mic
insert
is
dynamic.
The
output
impedance
is very
low and
well
suited
to
driving
cables
over
distances
up to 50
meters.
Screened
cable
therefore
is not
necessary.
The
frequency
response
measured
across a
10k load
resistor
is
plotted
below
simulated
using a
12V
power
source:
The
noise
response
of the
amplifier
measured
across
the 10k
load is
shown
below.
Please
note
that
this
plot was
made
with the
mic
insert
replaced
by a
signal
generator.
This
preamplifier
has
excellent
dynamic
range
and can
cope
with
anything
from a
whisper
to a
loud
shout,
however
care
should
be taken
to make
sure
that the
auxiliary
equipment
i.e.
amplifier
or tape
deck
does not
overload.
