Dec 162017
Text file that explains how to make a very effective line noise filter.
File FILTER.ZIP from The Programmer’s Corner in
Category Tutorials + Patches
Text file that explains how to make a very effective line noise filter.
File Name File Size Zip Size Zip Type
FILTER.TXT 4096 1606 deflated

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Contents of the FILTER.TXT file

Gary, here's the pass-through filter info I promiced. You can get all parts
at the Radio Shack.

Overall parts needs.
Two Modular line cords
Two Passthrough modular jack mounts. Sometimes called an extension adapter,
it allows two cords to be direct plugged togather to form a longer cord.
An experimenters box. Plastic top with metal bottom is ideal. A 2"x2"x4" is
ideal for a three stager.

Parts needed per filter. (Normally three)
One 1.0 uf capacitor, non-polorized. rated at at least 100VDC
One 5K Potentiometer. Long splined. Make sure it goes to zero ohms. Check
it in the store, they will let you if you explain why you are doing it and
that there is a 13% failure rate.

Open the experimenters box. Notch the sides for access of the jack mounts.
Use permabond or simular glue to peg them down.
Drill three holes in the top of the box for the long splines of the pots.
Equally space them. Make sure the pots main bodies have room to wire too. Do
not allow them to touch.
Take one of the modular cord, cut off the connectors, with about 1.5" of wire
attached. Strip them back 1". Note the red and green wires, cut the others
back to the nub. These connectors are going inside the box on the inside side
of the connectors. Don't do it yet.
Take the pots, zeroize each. To zeroize, take a multimeter and place one
lead on the center lug of the pot and the other on the third lug, counting left
to right (lugs towards you). Turn the spline clockwise, check the ohmage
reading. Make sure the reading is zero. If it zeroized, solder a cap to
the center lug.
Now solder wires stripped out of the remaining wire connect the stages
togather. The red goes to the capasitor end not soldered to the pot. The
green goes to the lug that zeroized. Now solder in the connectors using the
same points.

XXX !-cap- !-cap- !-cap- XXX Connector
XXX | | | XXX
XXX--Green- | | | ------Green--XXX
| -Pot- -Pot- -Pot- |
| | | | |

Now, put the splined through the holes, and screw them down. Then push the
jacks inside the inside sides of the of the modular pass through jacks.

Now put the unit inline with the phone line. Normally, the
capasitors will do the job. If noise still gets through tweeking the resisters
off zero will 'dial in' the unit to maximum protection. Tweek only the first
one, then mark the optimum setting, then and only then go to the second one.
No, maxing the resisters does not work.... Its a magic combination. I also
suggest you lock them down once you find the 'magic' combo.

Other notes. I have seen and have used small amperage pop-up fuses put in
line prior to the first stage. This has saved my system at least twice in my
You may also try two caps in parrellel in place of one. This effectively
doubles the stages, but makes the tweeking much more sensitive, almost too
sensitive. It is easier to just add stages. Theory says that there will be no
loss of signal no matter how many caps you use. The resisters are bleed off of
any major spikes, they really have no other purpose.
On another of my units I do have nothing but caps and pop-up's. Does the job
very well, but required over 30 caps to do the same job as a three stage with
the pots.

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