Ferromagnetic materials in audio connectors

[pilau]

As far as my experience goes, negligible conductor resistance is the most influential factor on an audio connector's signal transmission quality, followed by a tight mechanical connection and a coating that is resistant to oxidation over time.

But I recently read this interesting IEEE research paper that challenged my experience. It calls attention to the magnetic qualities of connectors, which introduce passive intermodulation, aka 3rd-order harmonics distortion to the signal! The paper focuses on RF connectors, but details principles that can be applied to audio connectors as well. Here are some key points from the paper:

1. Passive intermodulation (PIM) in coaxial connectors results from ferromagnetic material in the coatings and in the elemental iron in the base brass of conductors.
2. PIM increases as the signal frequency goes down, due to a decrease in skin effect.
3. PIM in connectors with nickel layers is much higher than in connectors without nickel layers.
4. PIM in connectors with nickel layers can be effectively reduced to desired levels using thicker surface coatings and/or higher conductivity coatings, from non-magnetic materials (gold, silver, copper, ternary alloys).
5. For connectors without nickel layers, the iron content in the base brass should be reduced especially when the connector is used in low-frequency applications.

Further digging brought up a research paper from 1975, by the NRL (US Navy Research Lab), titled The Danger of Intermodulation Generation by RF Connector Hardware Containing Ferromagnetic Materials. Their key message corresponds with the above:

It is imperative that the communication community be again and quickly alerted to the harmonic and IMG interference danger of RF connectors and associated components which contain ferromagnetic materials iron, nickel, cobalt, and their alloys. In a recent search for improved RF connector designs it became evident that the body structure of many of the newer type coaxial connectors and adapters are being manufactured from stainless steel, type 303, a low permeability 2 ferromagnetic alloy. That such a material could be considered for fabricating RF connectors is difficult to understand because of the known non-linear effects of even small quantities of ferromagnetic contaminants in RF systems. To cut cost, nickel plating, another ferromagnetic material, is apparently also being widely substituted for previously used silver or gold plating of brass stock connectors. [...] [such interference generating materials must be eliminated in all transmission elements of the systems.

Interesting that by 1975, these effects were already "known", to paraphrase the above.

Testing a bunch of different TRS connectors from well-known manufacturers (Switchcraft, Neutrik, Rean, Hicon, D'Addario/Planet Waves), I was astounded by the substantial and evident differences between them: some connectors were strongly magnetic, some weak, and some none at all. In some connectors, only the ground conductor was magnetic. In others, the pin of the male TRS connector (the part that plugs into the jack) was not magnetic at all, while the conductors you solder the cable to were strongly magnetic.

Very interested to hear from electrical engineers about the meaning of this.

 

[kongwee]

Majority of ASR do not like GR Research Danny on non magnetic materials on connectors and bindings post. Even screws on binding post should be non magnetic.

 

[MaxwellsEq]

Interesting. My initial read-through shows the measurements were performed using two frequencies at 935,000,000Hz and 960,000,000Hz with an intermodulation product at 910,000,000Hz.

 

[SIY]

For audio, in some edge cases, one can measure distortion resulting from ferromagnetic materials. To put it in perspective, it can get as high as 1000 times lower than audible distortion thresholds. IOW, totally irrelevant for actual component use even in the worst cases.

I do love seeing people fretting about the ferromagnetic leads on resistors used in their tube amps. I have made enemies by holding magnets near the tubes, not to mention the transformers...

 

[MaxwellsEq]

The navy paper seems to be based on signals injected between 240,000,000Hz to 310,000,000Hz.

 

[HarmonicTHD]

There is this recent and well documented case (it is solved now btw) and as you can see this is more a technical/reputational problem but not a sound quality problem as it is still by orders of magnitude bellow audibility.

Buckeye 3 Channel Purifi Amp Review

This is review and detailed measurements of the Buckeye 3 Channel Purifi amplifier based power amp. It was kindly drop shipped to me by a member and costs US $1,699. As you see, the case is utilitarian. The amplifier is extremely light making you think there is nothing in the box! The back...

www.audiosciencereview.com

 

[Sokel]

We're talking about the same thing that messed up Buckeye's measurements and made him halt his orders until it was found that the reason was the steel connectors?

Edit: @HarmonicTHD got it first

 

[dualazmak]

Just for your possible reference,,,

I wrote/shared the similar topics here (post #250 on my project thread) and here (post #13 on the remote thread) for SP high-level cable wiring/connectors.

 

[dualazmak]

If you take a look inside some rather high-end HiFi amplifiers, you'll see that the SP output wiring (and power wiring?) uses non-magnetic terminals and screws made of brass (no iron at all) or pure copper. However, this is also a common-sense measure to prevent sound quality deterioration in HiFi amplifiers. I remember it being pointed out and explained in interviews with a Yamaha amplifier designer and a Rotel engineer.

It is frustrating when working with magnetized screwdrivers (screwdrivers) because you can't catch the screws, though.

Yamaha's and Rotel's amplifier designers had a hard time persuading the assembly workers at the amplifier factories, but in the end they convinced them to use non-magnetic terminals and screws, giving priority to sound quality; I've also heard that the screwdriver, which uses a chuck to fix screws and bolts to the tip, was devised so that it could be used in factories. In my DIY audio setup, I have the same thing; I strictly/completely eliminate/avoid any magnetizable metal/screw in my SP cabling/connecting.

 

[DonH56]

PIM has been known and understood for decades. The biggest issues were for high-power transmitters and particularly antennas where exposed connections would oxidize and lead to high distortion (among other problems, like fires). Similar to the aluminum-copper interface problems homeowners and businesses faced as Cu wires were replaced with Al feeders to service panels. It has been getting more attention recently because higher dynamic range systems are becoming more coming, so distortion formerly well below the threshold of caring is now routinely achieved (or not). I also suspect it is being exposed more in the audio world (among others) as the cost of copper rises and alternative materials are being used. It has always and continues to be a problem for high-reliability systems where the desire for inexpensive materials that can withstand exposure to the elements, like passivated stainless steel, wars with the need for low-resistance connections that avoid PIM.

Disclaimer: This is not my field so I do not know all the details. I have read somewhat conflicting evidence that the problem may be more due to dissimilar materials in contact than their magnetic properties (e.g ferrous or not) per se. It has been years since I was working on designs for very high-rel environments (airframe, shipboard, space, etc.) and I have not kept track.

Note creating tight (preferably sealed) connections using similar metals is still the primary solution AFAIK.

 

[Speedskater]

I find it interesting that my cable TY signal enters my home thru a Copper Clad Steel (CCS) coax cable. That coax cable carries hundreds of TV channels at the same time. While all those channels are now digital, in the old days they were analog.

 

[GXAlan]

We're talking about the same thing that messed up Buckeye's measurements and made him halt his orders until it was found that the reason was the steel connectors?

Edit: @HarmonicTHD got it first

Kenwood made a big deal about anti magnetic construction in the 1980s and have superb measurements to show for it.

The problem is that in the 1980s, anti magnetic material meant building everything out of plastic!

Kenwood L-08M Monoblock Amplifier (Vintage Review)

Kenwood L-08M Monoblock Amplifier Review (Vintage Gear; QuirkAudio restored) Where were you in 1980? In Tokyo, Japan, a small team of engineers released the most sophisticated amplifier of its time... A bit of history In the 1970's, an elite team of veteran Trio-Kenwood engineers left the...

www.audiosciencereview.com

Kenwood L-08C Pre-amplifier Review (Vintage Audio, QuirkAudio restored)

Kenwood L-08C Pre-Amplifier Measurements (Vintage Audio) Part 1: Kenwood L-08M Monoblock Amplifier Review https://www.audiosciencereview.com/forum/index.php?threads/kenwood-l-08m-monoblock-amplifier-vintage-review.37756/#post-1327414 If you haven't read Part 1, please give that a read...

www.audiosciencereview.com

 

[SSS]

Haha, PIM seems nothing to do with audio frequencies. With high frequencies the skin effect dominates and there the outside materials may be a problem because it is the place with the highest current.
At audio there is almost no real skin effect. Audio Precision should be asked how their XLR connectors are and if there is PIM. I think PIM will be below the measurement capability of the instruments.

 

[DonH56]

Haha, PIM seems nothing to do with audio frequencies. With high frequencies the skin effect dominates and there the outside materials may be a problem because it is the place with the highest current.
At audio there is almost no real skin effect. Audio Precision should be asked how their XLR connectors are and if there is PIM. I think PIM will be below the measurement capability of the instruments.

The recent Buckeye Amps test and debug effort begs to differ. At high power it is a problem at low frequencies too if you are looking at very low distortion levels.

 

[SSS]

The recent Buckeye Amps test and debug effort begs to differ. At high power it is a problem at low frequencies too if you are looking at very low distortion levels.

Hi, I could not find any hint regarding magnetic induced distortion in the Buckeye tests which I found. Would be great if you please may provide a reference link for this Buckeye amp test. With Class-D amps there are anyway inductors involved, also with ferrite cores.

 

[DonH56]

Hi, I could not find any hint regarding magnetic induced distortion in the Buckeye tests which I found. Would be great if you please may provide a reference link for this Buckeye amp test. With Class-D amps there are anyway inductors involved, also with ferrite cores.

Buckeye 3 Channel Purifi Amp Review

Looks as though Buckeye's buffer designer also had a dummy load issue. I talked to him today and found he was using some PE dummy loads. I had bought the same ones and returned them. I found they could not be used for measuring low distortion situations. Might have found this sooner but we had...

www.audiosciencereview.com

 

[SSS]

Buckeye 3 Channel Purifi Amp Review

Looks as though Buckeye's buffer designer also had a dummy load issue. I talked to him today and found he was using some PE dummy loads. I had bought the same ones and returned them. I found they could not be used for measuring low distortion situations. Might have found this sooner but we had...

www.audiosciencereview.com

There was the snap-on lug mentioned being of steel which may cause distortion. On the other hand it seemed to be something wrong with the board design.
Whatever, the simple cheap lugs for automotive applications are to my experience not perfect regarding contact resistance. So with high current drawn this can be an issue. Maybe the magnetic property too, so using brass should help. Personally I prefer soldered contacts or banaplugs.

 

[AnalogSteph]

Those who have read Douglas Self's power amplifier book should remember that he ran into this issue with speaker binding posts, too (which turned out to be steel internally). When you are chasing the triple zeros behind the decimal point kind of distortion that a Blameless amplifier design can deliver, this stuff can really mess you up.

 

[egellings]

There can be an effect on S.Q. when using steel in connectors, but I think the effect would be microscopic when listening to music. I suspect that once you get to 0.01% in distortion, going lower will likely not be audible. I do laud the engineering that makes such low figures possible, though. It just that there's no reason to order a drop forge when all you need is a tack hammer.

 

[DonH56]

There was the snap-on lug mentioned being of steel which may cause distortion. On the other hand it seemed to be something wrong with the board design.
Whatever, the simple cheap lugs for automotive applications are to my experience not perfect regarding contact resistance. So with high current drawn this can be an issue. Maybe the magnetic property too, so using brass should help. Personally I prefer soldered contacts or banaplugs.

Read the post and maybe some around it. It was a materials issue, not the board.