|[AMRadio] Hint for shielded audio cable|
John Coleman ARS WA5BXO
wa5bxo2005 at pctechref.com
Sun Apr 9 02:57:44 EDT 2006
I am speaking of high gain audio chassis here Bob. A ground
loop can create all kinds of hum in audio circuitry and you will never
get it right until you get rid of all ground loops especially if a power
XFMR, heater XFMR or power choke is on the same chassis. These are not
transmission lines for moving power.
An example would be, to use these lines to connect a microphone
gain control in between preamp stages. You could have three lines
wrapped by a forth. A orange wire that connects between the plate output
coupling capacitor to the top side of a gain control and a green wire
that connects from the wiper of the control to the grid of the next
stage. The third wire lets say brown, goes from the bottom side of the
control to the ground buss. (See "ground buss" in the next paragraph)
The three wire group is wrapped with a blue wire which is connected to
the ground buss as well but is not connected at the control end. If the
line is relatively short then it is OK to use the wrap wire shield as
the ground for the control. It is also OK to do this if it is a true
coaxial cable and not a wrap shield. If the wrap shield is used then it
is best to run a separate wire for the ground on the control else you
will find a RF circuit present in you audio circuitry. The separate
ground wire is run inside the wrap rather than outside strictly for
appearance. It's neater that way.
RF circuitry is entirely different. I prefer to ground ever
thing with real short wires to a single point near the socket of the RF
stage. As for the heaters, one must consider the type of RF circuit.
If it is a common cathode circuit where the cathode is directly grounded
then nearly any type of heater wiring is OK and RF bypassing is better.
If it is a cathode follower or grounded grid circuit then filament
chokes should be used even with an indirectly heated cathode. The
filament chokes should be very close to the heater connection and should
be bypassed on the side that is not connected to the heaters so as to
leave the RF on the heater. In RF output circuits with directly heated
cathodes and common cathode circuit the filament XFMR for the tube or
tubes should be very close to the tubes underside and heavily bypassed.
The primary of the filament transformer should be RF choked and
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!!
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