Simple noise reduction filter for ICOM 756
	I use Heil Pro-Set Plus headsets with my Icom IC-756 radio. A small problem is quite high noise floor
	generated by the radio itself.
	There seems to be two different mechanisms for the "unnecessary noise":
		1) Noise generated by the audio power amplifier. You can hear it even audio volume control
		is in counter clock position
		2) Wideband (3kHz) noise floor when narrow CW filter is in use.
	Those two problems indicate that from S/N point of view ICOM has not done a perfect job in their system design.
	The radio is good but could be even better..
	My simple solution to the problems is as follows:
		1) add extra attenuation to headphone line. With resistive attenuator we get broadband loss.
		I have 9 dB extra attenuation in this device. This tackles the audio amp problem on SSB and CW.
		2) add low-pass filter on CW, cutting frequency 800Hz.
		This attenuates the wideband noise generated by all the circuits after the CW mode filter and
		which is above the CW pass-band.
	The schematics is very simple. Passive filter do not need a power supply. The challenge may be to find
	the inductors. Mine are VOGT 39mH common mode chokes, where the two windings are connected parallel
	decreasing losses. Inductance value do not change.
	The nominal impedance of the LC-filter is 100ohms. It should see 100ohms at both ends.
	Therefore there is 47 ohm in input and 82 ohm resistor in output. The same resistors provide the extra 9 dB
	wideband attenuation.
	After using this external device for some time there is no return to the old.
	If this device is applied to other rigs, matching resistor R1 may need to be changed.
	A receiver without CW filter becomes a bit better on CW with this filter too.
	OH1TV
	1.3.2008
	Schematics
	Frequency response
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	PS. My IC756 is the older version, not any Pro x version.