Compressor For Electric Microphone


The ‘FM Remote Control
Receiver’ (available on this website in Infra-red circuits section) has a
connector where an analogue output is made available. To make a simple
intercom or P.A. system the associated transmitter needs a microphone
pre-amplifier that outputs a signal at the correct level. And that is
exactly the function of this circuit. Actually, this design is adapted
from a circuit published last year (‘AM Modulator for Intercom’). A few
things have been changed so that it can work with the 5 V supply from
the transmitter module. The OTA (IC1) used here is the single version (CA3080), which has slightly different characteristics from the dual CA3280.

Compressor for Electret Microphone

Compressor for Electret Microphone

The quad opamp is the same rail-to-rail TS924IN, made by ST. The
turnover frequency of the filter (3rd order 1 dB Chebyshev) has been
increased slightly to improve the intelligibility of speech and is now
about 5.5 kHz. The filter now amplifies the signal by a factor of 10. In
practice it is possible that due to various tolerances and the fact that
the opamp is not perfect, the filter characteristic shows some deviation
from that required. In our prototype it was necessary to change R15
into 2k7 to straighten the response curve. The DC current variation at
the output of the OTA and the resulting offset
variation at the output of current/voltage converter IC2d is such that
the gain of IC2d has to be substantially smaller than in the ‘old’
design.

Otherwise the output could easily rise to the supply voltage at low
signal levels. The value of R6 has therefore been made smaller by a
factor of 10. This has reduced the gain of the circuit by 20 dB, which
is compensated for in the filter. The amplitude of the signal from IC2d
is fed back as a control current to the OTA by
peak rectifier D1/C3 and inverting amplifier IC2b. R7 limits the loading
on IC2d. P1 can be used to adjust the amplifier between a fixed gain and
maximum compression. Figure A shows clearly what effect the circuit
has. 0 dBr corresponds to 100 mV. The maximum gain, with P1 set to
maximum compression, is about 48 dB (250 Ω) for small signals.

The minimum gain is about 20 dB (10 Ω). The OTA
is then slightly overdriven and the distortion becomes several percent!
With a fixed gain selected (P1 shorted) the gain is about 42 dB (125 ×).
The middle curve was measured with P1 in its central position. The
curve drawn for a fixed gain (the straight line) doesn’t finish at the
edge of the graph because the end of the line corresponds to the maximum
possible output level, which is 25 dBr (≈1.76 V or 5 / 2√2). Figure B
shows the frequency response. The low turnover frequency is mainly
determined by C8 (and to a lesser extent by C1) and is about 120 Hz.

The current consumption is about 7 mA When the circuit is battery
powered we recommend the use of three AA cells, because the circuit
still works perfectly at 4.5 V. If you want to use a higher supply
voltage (maximum 12 V for the de TS924IN and 30 V for the CA3080, but
you should also think of the voltage across the electret microphone!)
you have to keep in mind that the maximum current through R9 (which is IABC)
is only 2 mA. When we consider a maximum chosen current of 1 mA and the
maximum output voltage of IC2b (half the supply voltage, which is 2.5
V), then the value of R9 should be (2.5 – 0.7) V / 1 mA = 1.8 kΩ. The
value of 0.7 V corresponds to the potential between pin 5 and earth.

For a larger safety margin R9 is calculated with the full supply
voltage and a current of 2 mA: (5 – 0.7) V / 2 mA = 2k2 (rounded
upwards). Of course the regulation will then be different (a little less
gain). This circuit and the transmitter module can therefore be fed
from the same 5 V supply. Because the transmitter requires a DC offset
at its input, a resistor is connected to +5 V via a jumper, which biases
the output to half the supply voltage. With the jumper open R17
functions as a load resistor when the output is not connected, because
C9 still has to charge up even without a load. If you’re designing a PCB
for this compressor then it makes sense to include the transmitter
module as well. The current consumption then increases by about 10 mA.
Author: T. Giesberts – Copyright: Elektor Electronics


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