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Alternator noise filter

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New Member
Good day All!
I'm looking for a circuit to filter out alternator noise. I've searched and searched but I cannot find anything that will provide specifics.

I've heard of joining 2 1N4001 diodes, one from ground, the other from +12V and installing a capacitor from the output. What value capacitor should I use? Thanks for your help!


Active Member
Filtering out alternator noise can be simple or a challenge - it all depends on a bunch of things. If it's radiated to other systems in the car you might treat that differently than noise that gets in via the 12 volt connection.

Step #1 (in my opinion) - make sure a diode in the alternator hasn't failed. It's easier to fix that problem than filter the rather profound "noise" is puts on to the system.

Step #2 - check to see that existing filters, if any, have not failed. Sometimes they are right on the alternator.

The current requirements will have some impact on the filter. Any component in the main current path, such as a choke, will have to carry the current required. I've put capacitors in parallel with the load which is quite effective however there is some risk that the capacitor could fail since it's essentially across the line and can act as a momentary current source whenever system voltage dips - it takes a lot of abuse. On a lower current requirement (2 amps) a choke with sufficient current carrying capability was sufficient - viewed noise reduction on a scope. The choke or capacitor only affects noise brought in via the power connection.

FYI - Radio Shack sells a fairly low cost, high current filter for probably less than commercially made components.


New Member
r u taliking about the alternator noise being caried out by your sound system...
seems to me a ground loop problem with ur sound system..

try to relocate your ground wires to a different grounding point...
poor ground connectivity causes ground loop problem. causing your sound system to pick up alternator noise....

you'll observe upon turning off the engine the noise is also gone...

hope this helps.


New Member
Usually the cause of alternator noise is your signal cables(RCAs) too close to a power wire.If your RCA cables have a built in amp turn on wire that could be the cause too, if amp turn on current is too much.If it is not your signal cables then a ground is usually at fault(as stated above), it could be an internal ground inside of one of the system components.Filters are a band-aid that don't fix the problem, they just cover it up.Ground Loop Isolators(filters) in most cases will reduce signal output.Just make sure your signal cables are ran down the opposite side of the vehicle as the power wire, double check the integrity and location(solid metal, no brackets, braces, or reinforcements) of your grounds and it should eliminate your noise.


New Member
Thank you all for your valuable input. I appreciate your time! :D

The alternator noise that I am speaking of is in an audio system, just to answer previous questions.
I've thoroughly researched the alternator noise issues, and the common fixes:
1. running power away from signal lines.
2. good solid grounding, seperating grounds etc.
3. Checking alternator capacitor, filters etc.
and the suggestions you guys gave. Thank you!

I'm still in need of a circuit that will filter this alternator noise. I'm interested in developing. :shock:

Does anyone know of any circuits?

Thank you, thank you, thank you, thank you!!! :lol:


Active Member
I'll describe what I did to address noise in one instance.

The problem: observations by fellow ham radio operators that my VHF transmitted signal had a lot of alternator whine. The transceiver is a handheld battery powered unit that I plugged into the cigarette lighter. It does not have the filtering capability that is normally part of a mobile radio to save weight and because it's designed to run on a battery pack.

I put a scope on the cigarette lighter and observed waveforms - no load and transmitting full power - and with a friend who was listening to my transmitted signal. There was a lot of "stuff" riding on top of the DC but also a profound depression. That depression turned out to be the result of a failed diode in the alternator - presumably voltage bucking the system DC.

I replaced the alternator which helped but some noise remained. I added an inductor that I had on hand - in series. Wire size was sufficient for the 2 amps of current. The high amplitude/short duration spikes disappeared but some noise remained. I added a filter capacitor in parallel with the load - about 10,000 mf at 50 volts. The inductor and capacitor solved the problem.

It certainly is possible to design a filter then build it however you'd need to know what's going in and what you want out - at a given current flow.

I hope this helps.


New Member
Ahh, so it's more or less, analyzing the signal, and filtering with a cap or small inductor. Interesting.

Hey thanks a bunch for your help guys! You're awesome!
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