RF Interference in Speaker Cables??? (video)

[solderdude]

The radio reception is caused by semiconductors in the amp working as an AM detector.
All tube (so no hybrids) do not have diodes in them so chances are you will not be bothered by this as much.
You could end up with some hum instead.
Best option would be to contact the radio station and ask them for assistance.

 

[tvrgeek]

OK, plug a cable into the input of a scope with a 100M impedance, and surprise! It picks up RF. Plug it into a n 8 Ohm speaker and hyess what? Nada. Can you measure it at -80 dB? Who cares.

Well, with an exception I had. Back in Colorado, a yahoo put a 1000 W amplifier on his CB radio. ( 2.5W legal limit) He blew one of my tweeters. So I switched to Kimber cable. No difference in sound, but resolved the issue temporally. As the truck owner was a jerk and laughed at me, I solved the problem with a strait bin. Through the coax. No more 1000 W Lafayett "footwarmer"

I took the opportunity to replace the T2000 tweeters with some Seas and ended up in a better place.

Tube amps may not have diodes, but they have selenium or tube rectifiers. Besides, it is the the feedback loop where noise is injected into the amp. A less than prefect connector can act as a detector for AM.

If you had an amplifier with a diff output, like a tube transformer, I guess you could get some 16 gauge shielded cable if you really need to. Seems ot be another "intuitive problem" that basically does not exist.

 

[milosz]

In my case, the RFI results in a particular kind of hum. It is 60 Hz hum with lots of harmonics, the most audible seems to be 240 Hz. When the FM station shuts down their transmitter briefly for weekly maintenance, the hum goes away.

Only one of my solid state amplifiers is susceptible to this - an Adcom 555 MK II. Also, when I listed to LPs it seems the vinyl setup is susceptible, I am assuming it is coming though the cartridge / arm / phono stage here. Some other stuff - some $15 powered computer speakers - also suffers, but none of my headphone amps, other solid state power amps, or things like line preamps, DACs, cassette decks, a higher-quality amplified 5.1 computer speaker setup, etc seem to have any problem.

I have put tiny chokes and small capacitors at the input to my phono preamp, which only helped a little. I rarely listen to vinyl, as I don't care for the noise and distortion inherent in vinyl playback.

But the Adcom 555 MK II drives the bass panels in a 4-way amplified system with my Magneplanar MG 3.6's, this is my "main" system, and I plan to try some thing to try and eliminate this issue in that amp. The amp is only fed audio between 45 and 200 Hz, so when I hear these harmonincs higher than 200 Hz coming out of the bass panels, I know it's the Adcom that is bedeviled by RFI.

The tube amplifiers that I have used / tried do not pick up the RFI that some of my solid state amps suffer from. I believe that, in general, this is because the tube circuits don't have as wide a bandwidth as solid state circuits.

I had read somewhere that in solid state amplification, things like this 100 MHz FM radio energy causes saturation of certain solid state devices, slew rate limiting, etc. The solid state stage may be perfectly capable of dealing with strong signals all the way up to a few KHz, but less and less able as you move up in frequency- and at 100 MHz the device will saturate even with a very low level signal.

Why the RFI that I hear is a 60 Hz hum (+harmonics) is something that puzzles me. Maybe the FM station's signal has a little 60 Hz AM modulation of their carrier? I can check this, I have a spectrum analyzer that will tell me how much AM is on that carrier.

Or some differential stage in the amplifier that normally rejects all residual 60 Hz power supply hum in the amplifier has it's common mode rejection ability thrown off by the RF?

 

[milosz]

I am one of the referenced few people to experience FM radio station interference with modern equipment (in an extreme manner). See this thread I posted here about a month before Amir's video: ASR - Radio interference (FM) and buzzing in speakers without input source. The thread has more details, but here is the gist:

• I live less than a mile from a massive FM radio broadcast tower.
• As soon as I turned on my last amp for the first time, and before I started playing any music from the actual source, you could clearly hear FM radio playing through the speakers (although one speaker was much louder than the other). Nothing in the system has an FM transmitter. Increasing the volume knob didn't really affect the volume of the interference.
• My speaker cables are twisted (Belden 5000UE). Moving the cables in the room didn't really seem to make a difference. Neither did switching to another cable of a different length (I went from 10ft to 15ft and also tried coiling near the speaker).
• I ended up switching from the Cambridge Audio amp to an Emotiva BasX and that appeared to fix the problem.

However, and what I haven't posted in my original thread, is that when I used the headphone jack in the Emotiva (rather than through my headphone amp) for the first time the other day, I could hear the FM radio station coming through my headphones very clearly. I assume this means that the issue isn't totally gone. I have no idea how to test/measure to see if this is still an issue with the speaker output (as well as the headphones, which is clearly audible).

In any event, I really want to avoid snake oil, but it seems like an RF / Ferrite clamp or donut is a very cheap way to possibly just take this off the table as an issue. I really don't feel like buying a new amp as it has now presented in two of them.

My questions:

1) I see literally 574 choices on https://www.mouser.com/c/passive-components/filters/ferrite-clamp-on-cores/ and more in google, which is overwhelming. @solderdude mentioned some have better results with different frequencies. Does anyone have a recommendation for something to take care of an issue FM interference, specifically to address this antenna: https://radio-locator.com/info/WIHT-FM.

2) Would I only place the chokes on the speaker cables? Would I put one on my headphone cable if that is where I am getting interference? If it is the AMP itself picking up the signal, is there another option?

Thanks so much in advance!

I think you would need to try to put ferrites on the speaker cables as a first step, if this helps the speaker audio it should also help the headphone output.

100 MHz RF is a funny thing. It can get in to things you wouldn't expect it to get in to, it is hard to "ground away" as the reactance in most paths to ground will appear as fairly high impedances at 100 MHz, etc etc. 100 MHz RFI is a devil to deal with.

 

[milosz]

I found some ferrite clamp-on chokes that eliminated an FM broadcast RFI problem I was having. Laird 28a2024-0a0 ( see THIS LINK ) little rectangular clamp on jobs. I was having an RFI issue - a particular kind of hum audible through the speakers which I recognize as being caused by RFI pestering an amplifier - I put these chokes on all 4 leards going to the speakers ( the - and + leads, right at the output of the amplifier) and also put one on the unbalanced (RCA) signal cables close to the right and left RCA inputs of the amplifier. The RFI went away.

The Laird 28a2024-0a0 are available for $2.48 from Mouser, $4 from Digikey, etc.

 

[Lambda]

I put these chokes on all 4 leards going to the speakers ( the - and + leads, right at the output of the amplifier)

This is not how they are suposed to be used! + and - from one Speaker must go troug the same choke!

 

[solderdude]

And preferably 3 to 4 'windings' through the center hole, but this is hard to do with most speaker cables.
It also must be as close as possible to the amp.
read this !!

 

[milosz]

So, when the RFI buzz stopped after putting on the chokes, you are saying that even though the chokes fixed the problem, it's still somehow wrong? If the chokes fixed the problem, what is wrong here?

These chokes are rated at 220 ohms impedance at 100 MHz for one wire going through the center of the choke. Yes, more windows would greatly increase that, but my #12 speaker wire will not do any windings through the choke, it's too thick.

But my point is, since they fixed the problem, how is it wrong?

 

[milosz]

This is not how they are suposed to be used! + and - from one Speaker must go troug the same choke!

Why? Explain the physics behind that.

 

[Lambda]

Why? Explain the physics behind that.

It is a high permeability high loss material that goes in saturation as soon as some current goes trug it.

They are also some times called "current compensated choke" because the positive current from the + line compensates for the negative current from - line and vice versa.
The magnetic fields cancel out and the net magnetization from the wanted ( in this cases audio) current signal is zero.
This way they only affect the noise in differential mode.

If you do it like you did they affect noise and the audio signal. but the audio signal is manny times stronger "overwhelming" the core and making it thereby inefficient for noise.

How Common Mode Chokes Work | Würth Elektronik: Electronic & Electromechanical Components > News > Blog: World of Electronics

Examine the function and operation of a common mode choke and the way it reduces unwanted interference.

www.we-online.com

it only works if the differential mode curter get canceled out.
If you play any audio trough it you Saturate the core (and distort the audio signal)

 

[milosz]

It's not a common mode choke.

The Nickel-Zinc material in the choke has low permeability and high volume resistivity, which yields high impedance in the 100 Hz range but near zero impedance at audio frequencies. ( Refer to the chart, the material in the chokes I'm using is Mix 28)

The presence of the ferrite around the speaker cable adds lossy inductive reactance at high frequencies (100 MHz and up) but adds almost no impedance at audio frequencies. It's not designed to couple common-mode phases to each other in order to chancel noise out.

I must point out again that the way I'm using the chokes WORKS, if they were wrongly used they WOULDN'T work. But since they have completely removed the FM broadcast RFI (89.7 MHz) then the only conclusion that can be drawn is that I have used them correctly. And there is no distortion in the audio, at least not according to the sweeps I've done with OmniMic set to record THD.

 

[solderdude]

So, when the RFI buzz stopped after putting on the chokes, you are saying that even though the chokes fixed the problem, it's still somehow wrong? If the chokes fixed the problem, what is wrong here?

These chokes are rated at 220 ohms impedance at 100 MHz for one wire going through the center of the choke. Yes, more windows would greatly increase that, but my #12 speaker wire will not do any windings through the choke, it's too thick.

But my point is, since they fixed the problem, how is it wrong?

There will be some attenuation and when that is enough it is enough. Depending on how high the RF is and how sensitive the devices in question are the more attenuation you may need.
When it fixes the buzzing all is O.K.
In the end it is all about lowering RF to levels where AM detection does not occur anymore.

 

[Lambda]

It's not a common mode choke.

Not how your using it and this is the problem.

dds lossy inductive reactance at high frequencies (100 MHz and up) but adds almost no impedance at audio frequencies.

it adds the same amount of Inductance but highly non linear.

I must point out again that the way I'm using the chokes WORKS

Sure as long as no music is playing or at low volume.
and at higher volume you don't here it.

if they were wrongly used they WOULDN'T work.

If this is your definition of "work"

. And there is no distortion in the audio, at least not according to the sweeps I've done with OmniMic set to record THD.

Not that you can hear maybe. but there is differently an small effect that don't need to be there if you would use them properly.

You can use them "Wrong" an and get away with it because it works still good enough and distortion/attenuation is low enough.

 

[DonH56]

I would not call not running both wires through the choke "wrong"; it may have lower common-mode attenuation, but that may not matter for this specific case. I would normally loop both cables a loop or three through the core, but have found that is not always needful, and as said above can be difficult unless you have small wires and/or a big core.

The ferrite cores will serve to attenuate HF noise on the wire, single-ended or common-mode. To some extent the amplifier's output topology and grounding scheme influence the extent to which using the choke in common-mode fashion will help the circuit. I have done it both ways in the past and found single-ended often works fine. Running both wires through and/or wrapping a few turns may help but, again IME, is only needed in cases where the interfering signal is very strong or the amp's output very sensitive to it (or both). One obvious exception is amplifiers having bridged (or "balanced") outputs so both terminals are floating ("hot"). In those cases the common-mode coupling would seem to make the most sense.

FWIWFM - Don

 

[aerochrome2]

I think you would need to try to put ferrites on the speaker cables as a first step, if this helps the speaker audio it should also help the headphone output.

100 MHz RF is a funny thing. It can get in to things you wouldn't expect it to get in to, it is hard to "ground away" as the reactance in most paths to ground will appear as fairly high impedances at 100 MHz, etc etc. 100 MHz RFI is a devil to deal with.

I ended up buying a kit (https://palomar-engineers.com/ferri...rol-RFI-Range-20-250-MHz-13-filters-p73792382) - it was probably more expensive than I needed, but not badly priced. It contained enough to handle all my power cords, speaker wires and RCAs throughout my system. I am listening to headphones out of the amp now and it is dead silent. Subjectively, I think the rest of the system sounds more clear as well, but I am aware that part may be very well in my head without the ability to do measurements. Thanks all!

 

[mmuetst]

I have a question, how does this piece of document compare to The Video of @amirm?

speakers or headphones. There is always feedback around that output stage, so RF present at the output will follow the feedback network to the input of a gain stage, where it will be detected and amplified. This problem is made much worse when parallel wire cable (zip cord) is used to feed the loudspeakers or headphones, and can usually be solved simply by replacing the zip cord with a twisted pair of POC (plain ordinary copper). [Pseudo-scientific advertising hype for exotic cables notwithstanding, it was shown nearly 30 years ago that #12 copper twisted pair (or #10 for very long runs) is a nearly ideal loudspeaker cable.] [R. A. Greiner, "Amplifier Loudspeaker Interfacing,” JAES Vol 28 Nr 5, May 1980] As we will discuss later, the twisting of a pair greatly reduces the level of RF that the wiring couples to circuitry.

source: http://www.audiosystemsgroup.com/RFI-Ham.pdf (page 3)

 

[Cbdb2]

I have a question, how does this piece of document compare to The Video of @amirm?

speakers or headphones. There is always feedback around that output stage, so RF present at the output will follow the feedback network to the input of a gain stage, where it will be detected and amplified. This problem is made much worse when parallel wire cable (zip cord) is used to feed the loudspeakers or headphones, and can usually be solved simply by replacing the zip cord with a twisted pair of POC (plain ordinary copper). [Pseudo-scientific advertising hype for exotic cables notwithstanding, it was shown nearly 30 years ago that #12 copper twisted pair (or #10 for very long runs) is a nearly ideal loudspeaker cable.] [R. A. Greiner, "Amplifier Loudspeaker Interfacing,” JAES Vol 28 Nr 5, May 1980] As we will discuss later, the twisting of a pair greatly reduces the level of RF that the wiring couples to circuitry.

source: http://www.audiosystemsgroup.com/RFI-Ham.pdf (page 3)

Speaker cables are not great antena or we would use them as antena. Anyone measure the RF level at an amps output? Most amps have an output zobel that will filter the RF. There confused about feedback. The output is feedback to the error correcting (-ve) input so the RF on the output is reduced by the feedback, if the audio amp actualy has any gain in RF which they incorecctly assume.

 

[restorer-john]

Speaker cables are not great antena or we would use them as antena. Anyone measure the RF level at an amps output? Most amps have an output zobel that will filter the RF. There confused about feedback. The output is feedback to the error correcting (-ve) input so the RF on the output is reduced by the feedback, if the audio amp actualy has any gain in RF which they incorecctly assume.

Speaker cables and some amplifiers have acted as AM radios since forever. Radio Breakthrough it was called and the various junctions in the feedback loop acted as a crude detector and in the presence of a nearby radio station, you got a free single channel tuner.

Less common these days, as most radio stations are FM and even the AM stations aren't as powerful as they once were.

 

[Speedskater]

source: http://www.audiosystemsgroup.com/RFI-Ham.pdf (page 3)

Jim Brown at the Audio Systems Group is the go-to interference expert. He is the retired Audio Engineering Society, EMI/RFI group co-chair.

 

[Cbdb2]

source: http://www.audiosystemsgroup.com/RFI-Ham.pdf (page 3)

Jim Brown at the Audio Systems Group is the go-to interference expert. He is the retired Audio Engineering Society, EMI/RFI group co-chair.

But this is wrong. "There is always feedback around that output stage, so RF present at the output will follow the feedback network to the input of a gain stage, where it will be detected and amplified."

Experts that don't know how negative feedback works.
The RF at the output goes to the negative input and is reduced.