Is there a way to reduce cell phone interference in DIY KM-84?

[joulupukki]

My parts for my U87 build came from Mouser today. I ordered a couple of these with it: https://www.we-online.com/components/products/datasheet/74270001.pdf


I've installed it on the gate of the JFET in one of the DIY84 mics. Sadly, it has absolutely zero effect.




One thing I did notice while testing (with some cables that aren't the Neutrik cables) is that if I touch the cable's XLR housing that's metal, the RF noise goes WAY down vs when I just have it sit standalone. Does that give any more of a clue as to what's going on and/or how to improve the situation in this specific application?

 

[jp8]

Welcome to the magic world of RF .

To me, your latest observations are another indication that the RFI is predominantly, or even only, entering from the cable side. Not through leaks in the body (except for the leaky XLR connector), nor through the capsule.

A human body has a rather high lumped capacitance to earth (~150 pF) with very low inductance due to the relatively high width/length ratio of the "conductor". This capacitance has an impedance of less than a few Ohms to earth at 600 MHz and up. OK, you'll have to add the skin resistance (RF travels through the surface of your body), but even the capacitance of only your hand to earth is a low impedance to earth. Try "shorting" the RF signals travelling along the cable shield to earth by means of a wire, and you'll notice it won't work. The wire has a rather low capacitance to earth, but a high inductance. Let's assume you took 5 feet of wire, then it has over 4500 Ohms of impedance to earth at 600 MHz and up. Maybe one of the RF Gurus here is able to calculate the attenuation of the RF signal on the cable shield, but the wire will for sure attenuate way less than the human body shorting the RF to earth. When I studied electronics, I always hated the antenna design classes. And I continued to do so in the 40 years after that, so the small amount of knowledge I had has died away.

Jan

 

[MicMaven]

Mr. @joulupukki -- have you tried installing any clamp-on ferrite beads from your HF radio station on the cable? Most ops have a supply of hollow or clamp-on Mix 31 or Mix 43 clamp-on ferrite RF suppressors hanging around in the shack. Shoot some stations are festooned with hundreds of 'em on each cable, because they are too lazy to determine the real cause of RF in their shacks - quite the opposite of the Herculean effort you are giving to solving your issue.

I don't necessarily recommend them as a cure - only to provide additional clues and I expect they may confirm Mr. @jp8 's take the cable is the antenna. I also anticipate they will provide substantially more clamping power than the little beads you installed today.

Just my take ... at least I mean well . . . / James

 

[Matt Nolan]

I expect they may confirm Mr. @jp8 's take the cable is the antenna.

I think the success with the Neutrik EMC connector has already confirmed that.

 

[joulupukki]

Mr. @joulupukki -- have you tried installing any clamp-on ferrite beads from your HF radio station on the cable? Most ops have a supply of hollow or clamp-on Mix 31 or Mix 43 clamp-on ferrite RF suppressors hanging around in the shack. Shoot some stations are festooned with hundreds of 'em on each cable, because they are too lazy to determine the real cause of RF in their shacks - quite the opposite of the Herculean effort you are giving to solving your issue.

I don't necessarily recommend them as a cure - only to provide additional clues and I expect they may confirm Mr. @jp8 's take the cable is the antenna. I also anticipate they will provide substantially more clamping power than the little beads you installed today.

Just my take ... at least I mean well . . . / James

I bought some of these years ago to solve some other completely non-related problem (https://www.amazon.com/gp/product/B015RCYICQ). I just tried a couple of them clamped around the XLR cable. Zero effect. Probably the wrong kind of beads.

----

So explain this ... if I use the same "bad" cable, the MP mics don't have nearly the same issue So something is clearly significantly different inside the microphones that make the CM-60 bodies/boards not perform as well. Ideally, I'd like to figure out if there's anything I can do inside the DIY CM-60 KM-84 mic to fix the issue instead of be reliant on the Neutrik EMC connectors.

 

[Khron]

make the CM-60 bodies/boards not perform as well

Do you still have the stock CM60 boards at hand?

 

[joulupukki]

Do you still have the stock CM60 boards at hand?

Yes, but I destroyed the part where the XLR connector attaches (to pillage the XLR connector for these mics). I would have to buy another CM-60 to test if the original had the same issue. :-|

 

[MicUlli]

That is a really challenging problem. Perhaps try small caps (47pF or so) between JFET drain and source to GND..

 

[MicMaven]

I bought some of these years ago to solve some other completely non-related problem -- tried with zero effect ...Probably the wrong kind of beads.

Well shoot, they are possibly the wrong type, but I suspect something else explains it. The headline curiously announces 7mm, while the ad copy says they fit 11mm cable. Probably not a material fact, just an observation. The question remains what material they are made of. Amidon and Fair-Rite sell Mix 31 for HF, and Mix 43 for HF to UHF, and also 61 Mix for 6m and higher (as I best recall) - there is a chart with exact info on each manufacturer's web site, respectively. Occasionally it comes down to how many you use - as the effect is cumulative. But if you put enough of them on the cable, they should have a positive cumulative effect. A recognized expert is AES Member Jim Brown, K9YC, who published a number of papers on the Pin 1 problem and suppressing EMI/RFI in audio circuits with ferrite products. Maybe you could ask him directly if he has any ideas. He is generally quite helpful and responsive to particular inquiries.

http://www.audiosystemsgroup.com/publish.htm

So explain this ... if I use the same "bad" cable, the MP mics don't have nearly the same issue So something is clearly significantly different inside the microphones that make the CM-60 bodies/boards not perform as well. Ideally, I'd like to figure out if there's anything I can do inside the DIY CM-60 KM-84 mic to fix the issue instead of be reliant on the Neutrik EMC connectors.

While I admire your logic, one should consider the entire microphone system from tip of the grill to the end of the cable, including body, connectors, cable, ferrite beads, and every other component a part of the RFI antenna. It is all subject to consideration, and any portion of the system might constitute the "antenna" to your RFI. Similarly, your radio station antenna system is comprised of all components from the tip of the farthest antenna element all the way back to the rig.

So far, it appears the Neutrik connectors are the only shot on target. For some reason the two circuits you are testing are simply not the same, so resonance in one does not mean resonance in the other. I regret I do not have the answer and cannot do more.

At least that is my take ... which is surely worth the price you paid for it. JHR

 

[ccaudle]

Amidon and Fair-Rite sell Mix 31 for HF, and Mix 43 for HF to UHF, and also 61 Mix for 6m and higher

For cell phone frequencies use type 61. Fair-rite recommends those for 250MHz-1000MHz range suppression.

 

[joulupukki]

My best guess right now is that the MP has a much better connection to the XLR shell vs this retrofitted Takstar CM-60 body. The PCB I've been rigging in here was designed for a mic body similar to the MP body (like the MXL 603s/804/991). If I can find one of those at some point, it'd be an interesting experiment to see if installing it into that body would result in a better outcome.

 

[MicMaven]

My best guess right now is that the MP has a much better connection to the XLR shell vs this retrofitted Takstar CM-60 body.

Can you devise another way to connect the board to the body, and etc? Perhaps you can reinforce or double up electrical continuity in some way? Example: I once killed hum in an electret capsule by soldering a wire from a point on the inside of the metal grill to the frame holding the circuit board, assuring continuity to Pin 1 and etc.

Serving Suggestion: I do not recall whether you tested the body-board combinations with different capsules or swapping the capsules from the quiet pair to the noisy pair. Who knows what that might tell you! (Shoot, you seem to have tried everything else!!)

Happy trails. James

 

[charlesaudio]

Haven't read the whole thread sorry. Have you tried to use a starquad cable with pin one connected to the shell of the connector on both end of the cable ?

 

[joulupukki]

Haven't read the whole thread sorry. Have you tried to use a starquad cable with pin one connected to the shell of the connector on both end of the cable ?

I haven't, mostly because I'm focusing on what could be done to the mic itself and not anything external – though the Neutrik EMC connector did wonders of improvement. I'll have to pick up some starquad bulk cable and switch my Neutrik EMC connectors to that at some point for a much better cable.

Can you devise another way to connect the board to the body, and etc? Perhaps you can reinforce or double up electrical continuity in some way? Example: I once killed hum in an electret capsule by soldering a wire from a point on the inside of the metal grill to the frame holding the circuit board, assuring continuity to Pin 1 and etc.

I could definitely experiment more with this type of thing. One thought is to also include one of these ferrite beads around the connection from PIN1/Shell to the circuit board ground.

Serving Suggestion: I do not recall whether you tested the body-board combinations with different capsules or swapping the capsules from the quiet pair to the noisy pair. Who knows what that might tell you! (Shoot, you seem to have tried everything else!!)

So the reference pair of microphones is a stereo-matched pair of Microphone Parts SDC-84 mics. They seem 95% immune to the RFI coming from my cell phone (if I have my phone right next to them, you can barely hear it, but in general, excellent resiliency). My two DIY KM-84 mics are using Graeme Woller's PCB designed to fit into MXL-style pencil mics. MXL pencil mics, as far as I can tell, have a similar XLR base connection to the board as the Microphone Parts SDC-84 mics.

I have the Woller PCB "rigged" to fit the Takstar CM-60 XLR connector in a very "rigged" way that I'd never be proud of ... but, it was all to experiment to see if I could make it work and stay in the cheap for mic bodies (two CM-60 mics cost me less than $45 on aliexpress).

In any case, yes, I've tried putting the MP capsules on the DIY mics and there's zero change.

If I ever see a "cheap" pair of used MXL mics, I'll snatch them up and try these exact circuit boards in those and it'd be interesting to see if the XLR base connection is the main difference between the two.

 

[joulupukki]

Hmm. I guess as another experiment I could disconnect the wires from the MP XLR assembly and disassemble the DIY XLR connector and wire up the DIY board with the MP XLR just to see if there's any improvement. Maybe a good little project for tomorrow. I just haven't wanted to disassemble the MP mic. But, it'll kill my curiosity.

 

[dai h.]

re: the ferrite bead on the FET gate (while it might make no difference to the negative outcome) my (novice) understanding was that they were supposed to be insulated from the lead.

Also, (unless you happen to already understand the issues really well) this (download) looks like good reading:

Tech Note on the RFI Shielding (Protection from RF interferences) in SCHOEPS microphones
Hans Riekehof, 2017

(on this page on the right side down around the middle) :
https://schoeps.de/en/products/colette/microphone-amplifiers/cmc-6.html#section-downloads

 

[joulupukki]

https://schoeps.de/fileadmin/user_u...traege_Aufsaetze/Papers/SCHOEPS_RFIShield.pdf

Interesting read. I actually tried (and failed) to make a shield plate out of some copper sheet I have. I don’t think I have the right tooling for crafting something that’d work. I don’t suppose something like that is available for purchase somewhere?

 

[joulupukki]

DIY board in the MP mic body (with the MP XLR connector) didn't really turn out that well. I'm not sure what I did wrong there, but there was some bad ground noise. I think maybe there may not have been enough clearance between the XLR housing and the pins of the DIY PCB board pins. I didn't yet try reinforced electrical connections - not really sure how I would do that yet. In any case, I'm gonna put this to bed for a while and focus on recording and editing. The Neutrik EMC connectors will function well enough for that. Thanks everyone who's helped out with troubleshooting this stuff. I've learned a LOT!