[RFI] Big Farad Audio Caps & RFI?

Christopher E. Brown cbrown at woods.net
Tue Apr 13 01:36:44 PDT 2010

On Mon, 12 Apr 2010, Eric - VE3GSI wrote:

> Is there an advantage using those 'big caps' when mobile? Perhaps a positive
> noise reduction from the car electrical/electronics?
> Courious,
> Eric - VE3GSI

Those 'big caps' have too low a SRF to be useful as an RF bypass.  *IF* 
you have low frequency ripple making it through to the radios audio stage 
it could help but this would normally indicate a failing battery, and 
could be filtered with a *MUCH* smaller cap.

The only really useful role would be the same one they serve in a large 
stereo install.  As a current reserve.

In a audio system they keep the voltage from sagging (causing distortion) 
during current peaks (bass strikes mainly).

In a mobile install, there is very little reason if only running a radio. 
Proper size wire will do fine for the 20 - 25 amps a 100watt mobile draws.

If you are running say a SG500 amp...  A 500watt mobile amp cat hit 70 - 
90 amps on voice peaks.

You can run 1/0 cable, you can run #4 or #6 from the main batt to an aux 
batt and 18 inches of #6 to the amp and be OK for AM/FM or dead carrier...


you can run smaller cable with a large audio cap by the amp.  The audio 
amp will do nothing for a 70amp+ continious draw, but it will level out 
the supply voltage when transmitting uncompressed SSB (average draw < 
25amps, peaks to 90amps).

Personally I favor 1/0 cable and a aux battery, but if one is stuffing 
things into a compact car, or say a jeep wrangler and only runs SSB a cap 
could be viable.

The main issue is when someone installs an 500watt mobile amp at the end 
of a 12 foot #6 run and attempts to drive it to 500watts.  The voltage 
drop on audio peaks leads to severe overdrive on peaks and distortion.

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