[asterisk-users] Line noise/hiss on Openvox A400P card on FXO

Sebastian Arcus shop at open-t.co.uk
Fri Mar 2 00:24:29 CST 2012

On 01/03/12 19:07, Dave Platt wrote:
>> 5. Placing ferrite cores on the phone cables.
> Do either of the phone lines in question have DSL on them?
> If so, a ferrite core (which will block common-mode RF
> signals) probably won't help much, if at all.  DSL is a
> differential-mode signal, and its frequency content starts
> down in the tens of kHz.  Ferrite cores are usually intended
> to block much higher frequency interference, and won't have
> enough inductance to help much with DSL signals.
> What I would suggest, is that you get yourself a couple
> of DSL "microfilters"... plug them into the A400P FXO
> ports, and plug the lines into the filters.  These sorts
> of filters are designed specifically to block DSL differential-
> mode signals from getting into analog-phone circuits, and
> they will also be fairly effective against other forms
> of low-frequency-RF noise.

Hi Dave and thanks for the suggestion. Although the lines don't have 
ADSL on them (we have a separate line for that) - I've tried ADSL 
filters on the them anyway. I even chained two filters to see if it will 
have any effect. I'm afraid the noise is still the same.


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