AC Line Filter and/or Regulator....What do you use?

[ajm]

12-14-2025

I looked at the forums and hope I have this posted in the best spot.
I did a brief search as well and didn't see anything.

I have all the makings of a bad stew.
An old house with old wiring, assume all outlets ungrounded.
Half a block from several buildings with God knows what antennas on their roofs.
Half a mile from a major international airport.

Details:
I have grounded the house AC outlet that I'm using to a water pipe. AFAIK all of the other outlets in the house are ungrounded.
I have a Furman PL8 rack mounted type power strip plugged into the outlet.
I run my amp and effects (none digital) off of the Furman.
Using a Kill A Watt AC current meter I'm drawing less than 2 AAC total.

Over the past year or so I am getting strange noises coming through the amp.
It's hard to describe but it's sort of a bleeping type sound if that makes any sense.
No CB radio voices.
I have tried several things to try and isolate the problem.
I have even unplugged the guitar and the effects from the amp, and it still comes through the amp alone.
The more overdrive there is the more it comes through.
I have tried other amps, same thing.
I have a wall wart powered old answering machine in another room, and occasionally it comes through faintly on phone messages.

I'm not sure if it's the AC service, or if it's an over the air thing.
I have a feeling it's the service.
I don't know if any of the neighbors have issues, but none of them play guitar.

I have a friend with a battery powered generator that I can borrow.
That is on the to-do list.
In less than a minute I'll probably know if it's an AC service issue, or an over the air issue.

So...........

Does anybody else have a similar problem?
What has been the solution?
An AC filter better than the Furman?
An AC regulator?
Any part numbers?
If it's a service issue, call the electric company?

Thanks in advance.

Search terms:
AC
Line
Regulator
Supply

[Larry Dering]

Are you laying your cell phone anywhere near the amp? Try turning cell phone off and see if it happens. Turn the amp at different angles and listen.

[Dave Grafe]

Are you laying your cell phone anywhere near the amp? Try turning cell phone off and see if it happens. Turn the amp at different angles and listen.

Definitely sounds like cell phone interference, a phone can induce noise in a pickup or electronics circuit if it's close enough. If the noise was in the electrical service your Furman would catch it.

If you have a ham radio operator broadcasting nearby it is conceivable that airborne interference could affect your pickups but not likely the amp by itself. Go talk to your antenna-laden neighbors. If they are not transmtting RF your problem lies elsewhere.

[ajm]

Cell phone nearby (in the house)?
Nope.

HAM transmitting neighbors?
That could be tough to nail down.
Besides, I don't get voices coming through.

Computer nearby?
Nope.

The noise/interference that I'm getting is tough to describe.

[Dave Grafe]

Okay we have ruled out the cell phone but not ham radio interference as sideband interference is the most common kind.

https://electronics.stackexchange.com/q ... y-problems

If you experienced the same issue with other amps and speakers the amp is ruled out. If your second amp used the same speaker for the test the speaker is NOT ruled out. Nearer to home "chirps" can happen with bad patch cables, speaker cables, voice coils, really anything that intermittantly shorts or opens the electron flow.

Keep us posted

[Bob Sykes]

Interesting problem. Seems obvious but worth saying that all EMI problems have 3 elements. Source, Coupling mechanism(s), and victim. The victim is the only known here. It appears the evil cell phone bleeps have been eliminated.

Sounds a lot like my old house (including proximity to the airport). I also have some outlets grounded to the copper water pipes. These pipes should be bonded to the ground rod, probably near the meter/service entrance. My water piping has a solid plastic feed pipe before it goes into the ground, making it electrically "floating" without that bonding wire.

Everything you have done so far are good steps to isolate the problem. I'm sure you know overdrive will boost the audio noise floor. Are these tube amps or solid state? Tube amps are usually more susceptible to radiated RF noise.

The interference (also) on the telco line points to radiated noise coupling into your house. This landline should also be grounded at the service entrance.

Could be both radiated noise coupling into the AC wiring and/or directly illuminating the amps. House wiring makes an excellent longwire antenna. It may just be your service or could be the neighbors don't have anything with vacuum tubes

The generator should be helpful if its output waveform is "clean" (Nice 60Hz sine wave). This doesn't sound like an AC regulation problem.

You can try calling the power company. They intentionally inject RF signals into the grid for various purposes.

There are always better filters available but I'd hold off on the spend until the noise source and/or coupling mechanism(s) are better understood. Does the noise get louder if you bypass the Furman? A RF spectrum analyzer would tell the story if you can get your hands on one. Is the noise constant or intermittent? The generator would eliminate the power company question. If that doesn't work, my SWAG is airport telemetry. Airports (and planes) can radiate significant RF fields as data transmissions as well as voice communications. You'll have to wrap your house in tin foil to fix this

HAM radio operators also can transmit data, so the lack of voice modulation doesn't completely rule them out. Most HAMs are fairly knowledgeable about electronics and interested in resolving interference issues that their equipment is causing. They are however under no legal obligation to do so in this case. IF the interference is affecting "over the air" T.V. or radio reception, you could complain to the FCC. Guitar amps don't get any protection from them so it's back to the tin foil shielding unfortunately.

As Dave said, keep us posted.

[Dave Grafe]

One more time: "chirps" are not from interference, most likely a bad connector on the amp's speaker cable, especially if it's plastic molded one.

[Tim Rose]

My electric company sends a 12.5khz ping to read the meter.. so they say. Happens every 30 sec of so.....can hear thrrough poorly wired systems....drives me nuts

[Lee Baucum]

My electric company sends a 12.5khz ping to read the meter.. so they say. Happens every 30 sec of so.....can hear thrrough poorly wired systems....drives me nuts

Gone are the days (for many municipalities) of the "meter readers" walking around reading meters. (I used to be one!)

The meter readers have been replaced with "smart meters". Electric meters and water meters. Don't know about gas meters.

~Lee

[ajm]

Update 12-18-2025.

I suspected that the issue was coming from either:
- Over the air radiated RF junk, or
- Through the AC power lines.

I have a friend that has a portable battery powered AC converter/generator.
I suspected that this is an AC power issue.
Yesterday while waiting to use the generator, I decided to cross one possibility off of the list.
My music room is on the north side of the house.
I took an amp to the south side and hooked it up.
None of the noise/blips/fluttering/etc issues!!!!!!!!!!!!!!!

I took the amp back to the music room on the north side.
The problem was still there.
Assuming it's not an AC power issue any longer, I started messing around.
Picking up the amp (a small combo) and rotating it and moving it around the room made the problem come and go.
Then...........
I sat the amp in its normal spot and noticed by accident that walking between the amp and a north facing window made the problem come and go.
So, what's directly to the north?
Gee, not much. Only:
- A 5 story hotel, with what looks to be some kind of antennas hanging off the side, about 100-200 yards away.
- To the west of the hotel, about another 100-200 yards, what can best be described as a "tree" looking tower with all kinds of antenna looking stuff on it.
- To the north of the hotel, across the street, a major international airport.

Just to be thorough, today we tried setting the amp up to pick up the noises.
When connected to the house AC the problem was still there.
Then we tried using the generator to isolate the amp from the house AC.
The problem was still there when using the generator.
I'm going to conclude that the problem is NOT with the house AC wiring.

So for grins, I got a couple of rolls of tin foil.
I started running strips of foil covering up the window.
By the time I got the whole window covered this problem is basically completely gone.
Note that the foil is taped over the window to the wall and is NOT grounded.

Now for the next step.
Some options are:
1) Leave the foil covering up the window.
It seems to work, but it's nice looking out the window when practicing.
2) The window has an outside screen.
Right now it's nylon or whatever.
So, try replacing the screen with metallic (Home Depot has aluminum screen material).
Then get rid of the foil.
I won't know if it works until I try it.
Thoughts?
Will the RF still get through the screen?
Would it be helpful to put 2 layers of screen up, one over the other, for better coverage?
Would I need to ground the screen? (Note that the foil is not grounded).
3) Any other (legal) ideas? ;>))

Thanks in advance to all who have chimed in.

[Bob Sykes]

Sounds like you have figured it out. Copper/bronze screen will work best if you can find it. Not cheap. It's used in some commercial Faraday cage constructions. The screen does not need to be grounded. 2 layers of screen works better but they need to be electrically isolated from each other.

Sounds like the interference is over 1GHz. Once you get into that region, the signal become more "line of sight" and can be attenuated with free-floating barriers (your foil) in the window. I'd look into increasing the shielding in the amp. What kind is it?

I find that wearing a KFC bucket covered with foil on my head clears things right up (for the unfamiliar - google Buckethead)