Communications Audio Equalizer
The audio levels required at the microphone input of transceivers
are not the same from model to model or from brand to brand. Moreover
the transceiver and microphones frequency responses are different and
one would wish to correct the audio spectrum before applying it to the
transmitter input. A commercial audio equalizer can be used, but the
available units, generally aimed to the HiFi market, are expensive and
not well suited to the 300-3000 Hz amateur use.
Generally a radio operator wants a simple device to roll off or accentuate the lows and/or highs of its voice characteristics.
I have recently built the equalizer described below (front panel designed with the F5BU GALVA software).
1. Schematic diagram.
(click on the picture to enlarge)
The
overall maximum gain is around 20 dB and the equalizer uses a Baxendall
structure. While the schematic is quite conventional (I have been
inspired by a D. DeMaw description in QST August 1989) I have spent
some time running Pspice simulations to obtain an 800 Hz crossover
frequency well suited to my transceivers frequency response and my
voice characteristics : The corresponding C4, C5, C7 and C8 values were
selected from batches of 47nF and 4.7nF capacitors. The operational
amplifier is a Microchip low voltage and low power MCP602 and the
equalizer overall supply current is less than 0.5 mA at 3V. The two AA
1.5 V batteries powering the unit should last almost as long as their
normal shelf life. 2.Measurements and use.
The measured equalizer response curves for maximum, minimum and no correction settings appear below.
During
adjustments, be careful and don't exceed the maximum audio level
allowed by your transceiver : too much audio will produce overdriving
of the transmitter and distortion! It is a good idea to monitor the RF
level with an oscilloscope or a peak powermeter in order to prevent
overdrive.
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