The circuit shown is that of a microphone compressor i.e.
an amplifier circuit which exhibits gain, but continually adjusts that gain,
to maintain a reasonable constant amplitude output signal
regardless of the amount of signal coming from the microphone itself.
Its uses are many, in the audio field, probably the most common
is in a broadcasting studio (etc) where a constant audio line level is required
regardless of whether the announcers voice be loud of soft.
This simple circuit is easy to "get working" and uses readily available components.
The circuit is based on a well known design;
except I have chosen for the active circuit element
to be a single supply op-amp (LM324)
as opposed to the more conventional types which require
both a +ve and -ve supply voltage for their operation.
This means that the circuit can be minaturized construction
and may be incorporated into the microphone body itself.
It requires an extra core within the mic' cable to run a +12V DC supply for the electronics.
The circuit action is as follows:
I have chosen an "electret" type microphone insert because of their small size,
excellent frequency response, high output, availability and cheap cost.
These small capsules usually require some form of foam shroud
to suppress microphone "pop"
I have not tried any other mic types i.e. magnetic, ceramic/crystal etc
as the electret has all the above advantages.
The microphone is AC coupled to the op-amp
the LM324 has 4 seperate op-amps within the package,
only one is req'd the others are simply left disconnected.
See manufacturers data sheet for full pinout.
The amplified output is sampled, rectified and used to charge a capacitor
which sustains a smoothed DC voltage which is representative
of the average signal amplitude.
This voltage is used to control the gate of the "N" channel JFET
and hence its "on" resistance.
(types MPF102/105, J310 and 2N3819 all work)
Because the JFET is connected in the feedback path of the op-amp
it can be easily seen that the overall circuit gain can thus be controlled
I have used a shottky type diode(s) for the voltage doubling rectifier
simply because of their low on-state voltage drop (290mV)
However common small signal silicon diodes like 1N4148 should work fine as well.
The circuit performs best with the on-board local +5V voltage regulator
suitably decoupled with 2.2 µF electro's.
Higher supply voltages seem to cause distortion and paralysis on voice peaks?
The two 47KΩ resistive voltage divider is used to establish
an operating point; connected to the non-inverting input.
Additional relevant information gleaned from manufacturers data:
The electret microphone diaphram is an insulating plastic film, polarized with a permanent electric charge. It is made by heating the film in the presence of a strong magnetic field. The microphone is effectively a small capacitor, with a high impedance at audio frequencies. The o/p is buffered by a FET source follower inside the microphone capsule. This acts as an impedance changer with o/p impedance of several hundred ohm.
This is why you must supply DC voltage through an external
10KΩ "drain" resistor, and AC couple off the audio signal
The LM324 is a true single supply op-amp.
To reduce the power supply current drain the amplifiers have a class A output stage
for small signal levels which convert to a class B in a large signal mode.
For AC applications where the output load is capacitivley coupled
to the amplifier a resistor (10K Ω) is placed to ground
to increase class A bias current and prevent crossover distortion.
With DC coupling there is no crossover distortion.
I have had good results with this design.
The attached picture shows a prototype constructed on a piece of 'veroboard"
for inclusion into a microphone encasement.
Update May 2017
From Dave Miller Princeton N.J. USA.
From experimentation an alternative position for the 1MΩ bleed down resistor,
is from gate to gnd. This will recover the circuit quicker after lock-up
due to sudden loud noise......reports Dave.
© 2Q 2020