Dealing with RFI or How to Turn Off the Radio Station in Your Mics and Guitars

[HCMarkus]

In my 1,000 SF studio, I’ve been fighting RFI in my longer mic cable runs, thanks to an AM radio station with its three antennas just across the valley from my location. The station frequency is 910 kHz, and daytime transmission levels are high enough to give me undesired "listening experiences” (i.e. very audible radio interference) when recording. Some mics and mic preamp combinations don't exhibit audible issues, but others have really horrible interference, particularly a transformerless Neumann TLM103 into a Focusrite ISA Two mic pre. Even with Star-quad cable and additional grounding.

Got a 10 Pack of Mix 75 Ferrite Rings from Palomar Engineers today. Palomar sells rings that handle specific frequency ranges; Mix 75 is the one best suited for dealing with AM radio interference. Priced (about $10 each)and packed well, they arrived quickly via US Mail; almost as quickly, I headed to the studio to try them out.

I put a number of turns of a mic cable thru the 1.4” opening in one of the Rings and fired up the problematic Neumann/Focusrite combination. The level of interference was dramatically reduced, but still barely audible. So, per Palomar's suggestion, I put another Mix 75 Ring on another short cable with multiple turns. Complete and total success was achieved. Interference below the level of mic preamp noise, tested with extreme gain with no impact on audio quality.

By tightly wrapping the cable thru a Ring, I was able to fit enough cable turns that a single Ring does the job for me in all but the highest gain situations; if needed, I can just add one of the several spare short Ring-Equipped cables I now have on hand. And let me emphasize how dramatic the change is! From "in-your-face obnoxious" AM radio interference to silence. As noted on Palomar's website, it is important to place the Ring near the Male (mic preamp) end of the cable for best results.

I’ve occasionally had issues with certain guitar amps and radio interference. I will utilize the Ferrite Rings to deal with problems like that should they arise in the future. I am so happy to have found such a simple, effective and inexpensive fix for this vexing problem. I can now use any combination of Mic and Mic Pre I wish without worrying about RFI.

https://palomar-engineers.com.

Informative page: https://palomar-engineers.com/wp-conten ... -RFI-1.pdf

Example of Mic Cable with Ferrite Ring: https://palomar-engineers.com/tech-supp ... p485296192

I have no affiliation with Palomar other than that of a happy customer.

[cuttime]

Glad this worked for you. I've heard of studios having to resort to complete Faraday cage metal screening in the walls to alleviate this problem, but since your studio is already constructed, I assume that is not an option.

[HCMarkus]

The Faraday cage gets mentioned often. Apparently, not many are aware of this far simpler and less costly option.

[mikehalloran]

Ferrite rings were one of many solutions that I tried to remove the electrical noise in my studio. Though helpful, the result wasn't good enough.

What did the trick was going to star quad microphone and interconnect cables. Here's a basic primer:
https://www.cabledatasheet.com/faq-abou ... and-cores/

Having them made by SESCOM through Markertek is far less expensive than purchasing so-called boutique cables from anyone. Also far less expensive than going to SESCOM directly.
https://www.markertek.com/product/sc25x ... en-25-foot
SESCOM will substitute Belden or Mogami star quad cable if you want for an additional charge but I find Canare to be dead quiet and durable using cotton twine under the shield to keep them round and add durability. There are over 1.500 "standard" skews so, if you can't find the right combination of cable ends, give Markertek a call. For those who 'roll their own', Markertek also sells the bulk cable and Neutrik connectors. SESCOM will do custom lengths but that is an additional cost. For example, I needed a 5 foot pair of cables with a straight TRS on one end and a 90° on the other—upcharge was higher than expected but I really needed that pair to be shorter than 6'.

Mogami star quad cable has a much smaller diameter (and less capacitance) than the others and that can come in handy in a few situations where durability is not an issue. I have a couple of 1/8" TRS interconnects with Mogami star quad cable in a fixed installation.

With star quad cables, there's no mystic mumbo-jumbo: Either you hear a noise reduction or you don't. If there's no difference, you did not need them. It really is that simple. Amazon has some star quad cables—much more expensive but you can return those easily if testing to see if you'll benefit.

My first star quad set were interconnect cables to quiet down my monitors. Holy cow, what a difference! After the success of that, my other interconnect and microphone cables followed.

A few years ago, we upgraded from 2-wire aluminum to 3-wire copper (house was built in 1960). No idea how standard cabling would sound nowadays and I'm not curious to find out. I still live 1–3 miles away from E=MC2 (SCSI over microwave), HP, Apple, Microsoft, Nvidia, Facebook, Google etc. all using massive amounts of power and many with microwave towers and dishes.

[HCMarkus]

Star Quad made no discernible difference in my situation in which RF was a leaking into the Focusrite via the shield; hence the Ferrite Ring solution's superior effectiveness in this case. I expect the fact that the ISA mic pres have extended HF response is a likely reason they were more susceptible to my local AM Station's broadcasts than my other pres:

frequency response at maximum gain is 16Hz to 118kHz (-3dB) and at unity gain extends up to 200kHz.

Star Quad Cable, Faraday Cages and Ferrite Rings may each be the best solution for some situations.

[HCMarkus]

Another Ferrite Ring "Problem Solved" Story

I always take a DI when tracking electric guitars; makes editing a lot easier (transients are obvious on the DI track, a great reference when editing saturated guitar chord tracks) and allows me to change the character of tracked guitars, often going cleaner than the guitarist's amped sound for definition in mixes or setting up a complimentary virtual amp for stereo spread.

My local AM Station has not ceased transmitting since I last wrote about the problems they create for me, and setting up for today's session (before sunset) with a guitarist who loves to use pretty heavy distortion, I had a lot of AM RFI leaking into the Marshall JMP-1 in my studio. A little experimentation and I was able to deduce the long hard-wired cable run from the guitar DI to its mic pre was allowing RFI to get into the JMP-1. I know it wouldn't be a problem once the sun. went down, but we were starting well prior to that hour. What to do?

A short ferrite ring-equipped XLR cable placed where the DI connects to the cable feeding the mic pre solved my problem. No more RFI in the Marshall. The session went great!

The End.

[mikehalloran]

Thanks for reporting back.

I wish I knew about this stuff in high school. My house was 1/2 mile away from a 20kW AM station and avoiding radio leakage was a struggle; My Jordan Boss Tone Fuzz was a booster antenna for that station.

I own that house now and rent it to SJSU music students; station is still there. If my tenants have RF issues, I've not heard about it but if they mention it to me, I still have that box of ferrite rings and clamp-on beads and will say, "Try this."

There was a pizza parlor a block away where I would hear bands get their start including The Tubes, Fritz w/ Stevie Nicks and Lindsay Buckingham, Pablo Cruise, Journey and the Doobie Brothers among many others. My bands played there, too but none of them went anywhere. If Monday wasn't a school night, my dad would let me walk over and sit in on Chet Baker's Jazz Jam after getting home from Youth Symphony rehearsals—he was living in Milpitas with his mother during rehab 1970–'71. Trying to get San Jose to put a historical marker on the building; a number of members of those bands have offered to chip in if it happens.

[HCMarkus]

If my tenants have RF issues, I've not heard about it but if they mention it to me, I still have that box of ferrite rings and clamp-on beads and will say, "Try this."

Here's how I do it:

[bayswater]

Will that work with other sources of interference, e.g. the noise created in a guitar when you get too close to the equipment rack?

[HCMarkus]

Will that work with other sources of interference, e.g. the noise created in a guitar when you get too close to the equipment rack?

RFI caused by any source within Ferrite Ring's effective frequency range is probably fair game as long as you are dealing with cables. Ferrite cores/rings reject Common Mode interference, i.e. noise induced on a group of conductors. From Palomar:

When you slip a ferrite bead over a wire and there is RFI (also called common mode current) conducted down the wire (in addition to the “normal” AC or DC signal), it is just as though you put a resistor in the wire which only reduces the common mode current.

Unfortunately, I don't think there is a way to wrap a guitar pickup around or thru one. Could small Ferrite cores or rings be effectively used inside a guitar? I just don't know!

[daniel.sneed]

Beating a long dead horse: in case of high gain guitar amp noise, humbucking pickups may help.
In case of balanced and unbalanced line noise, isolation transformers may help too.

[mikehalloran]

Beating a long dead horse: in case of high gain guitar amp noise, humbucking pickups may help.
In case of balanced and unbalanced line noise, isolation transformers may help too.

Symmetrically Balanced Power has +60V on one leg and -60V on the other. It’s dangerous since normal grounding is not possible—the shield is detached. Recommended for studio and laboratory use only. Not cheap but the only dead quiet solution i know for single coil pickups,

https://www.sweetwater.com/store/detai ... -p-2400-it