[AMRadio] Hint for shielded audio cable

W5OMR/Geoff w5omr at satx.rr.com
Sun Apr 9 07:26:54 EDT 2006

John Coleman ARS WA5BXO wrote:

>	If an RF chassis is a complete self contained rig, and has an
>analog modulator and microphone preamp built into it, then the modulator
>preamp circuitry should be on a separate sub assembly which is mounted
>via insulation and has only one ground connection to it.  Inside the
>audio sub assembly there should be a ground buss which is connected to
>the sub assembly chassis at only one place.  I generally will use some
>bare #12 wire and bend it so as to make small bridges across several
>insulated terminals spanning the bulk of the underside of the sub
>assembly.  If I need a ground point such as a cathode resistor or grid
>leak resistor or one end of a shielded cable then I use the buss for the
>ground connection.  In most preamp sub assemblies the single ground is
>at the microphone input.  This way the microphone connector is mounted
>directly on the chassis sub assembly.  The audio sub assembly is then
>mounted in such a way as to insure that its only ground is near or
>surrounding the microphone connection.  In some cases it is necessary to
>make the ground point somewhere else.  When this is the case the
>microphone connector should not be mounted on the chassis but on some
>insulating material and the ground connection for it should be made to
>the ground buss.   To be safe, it is best to rectify and filter and
>resistor limit the 6.3 volt AC for the heaters of the audio preamps.  Do
>this all outside of the audio preamp sub assembly.  Then ground one side
>of the heaters to the ground buss.  Although I have had good luck with a
>balanced AC and a filament hum bucking control it is just easier to do
>the DC filaments for a few tubes in the high gain preamps.
>I can't believe I typed up all the stuff!!

*This* is a homebrewer!

Good job, John!  Proud to know you as a 'friend'.


More information about the AMRadio mailing list

This page last updated 17 Feb 2018.