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

[Cqwet Dbdfte]

This aluminum tape didn’t improve it at all…and it definitely has electrical contact with the mic body (tested it with a DMM):

View attachment 123547

If RF enters thru the top screen, shielding elsewhere wont help. Put the foil over the "screen" area, at least to locate the RF entry point.
Those slots at the screen have a length for some perfect 1/4 wave RF frequency, working as slot antennae .
The metal screen(s) behind the slots should have very good metallic contact to the body, and no glue, paint, or insulators between metal parts. If the capsule fine mesh screen does not have good metallic connection to the mike body, and/or the threads do not provide for an RF tight interconnection (with very short gaps) a floating metal part may work as an antenna.
Some (woven) metal screens have poor interconnections, or may develop it over time. Screen rooms for RF testing are notably worse for screen material, solid sheet-metal is preferred. That not to say wire-screen material could not be made better. Expanded material would be better for shielding.
I cannot find any pic on how the screens in the mike are bonded together.

 

[Cqwet Dbdfte]

...adding a ferrite bead to the Source lead of the JFET, then the audio bypass cap "below" it, would add some negative feedback , reducing gain at HF/VHF/UHF.

 

[joulupukki]

If RF enters thru the top screen, shielding elsewhere wont help. Put the foil over the "screen" area, at least to locate the RF entry point.
Those slots at the screen have a length for some perfect 1/4 wave RF frequency, working as slot antennae .
The metal screen(s) behind the slots should have very good metallic contact to the body, and no glue, paint, or insulators between metal parts. If the capsule fine mesh screen does not have good metallic connection to the mike body, and/or the threads do not provide for an RF tight interconnection (with very short gaps) a floating metal part may work as an antenna.
Some (woven) metal screens have poor interconnections, or may develop it over time. Screen rooms for RF testing are notably worse for screen material, solid sheet-metal is preferred. That not to say wire-screen material could not be made better. Expanded material would be better for shielding.
I cannot find any pic on how the screens in the mike are bonded together.

I think I’m less suspicious of this being the problem because if I slip the DIY circuit board into the MP body, it has the same exact problem.

Vishay-Siliconix (logo is actually Siliconix's own); last row is production fab / batch / date code.

https://www.mouser.fi/ProductDetail/Vishay-Siliconix/J305-E3?qs=E5eJzWX2jIA6qyExkH0nCg==

Ah, thanks for helping me learn that!

 

[Cqwet Dbdfte]

Not sure about your circuit, but here are some ideas to reduce RF response on the JFET.

 

[Migs 31]

Km84's were sensitive to these high frequency noises,see u87A schematics with LCR filter after the input. That can be adapted inside a xlr female connector, it helped a lot, I had the same situation. Of course mic needs proper grounding.

@joulupukki said: " I’m less suspicious of this being the problem because if I slip the DIY circuit board into the MP body, it has the same exact problem."

Am I wrong or a valid answer and easy way to solve this RF problem was mentioned in posts:
#41 #52 and #59 by fellow member @TLRT?

Neumann and AKG both used this technique to solve the issue. I mean, they know a thing or two about building Mics. See Schematics attached.

 

[joulupukki]

It’s this:

@joulupukki said: " I’m less suspicious of this being the problem because if I slip the DIY circuit board into the MP body, it has the same exact problem."

Am I wrong or a valid answer and easy way to solve this RF problem was mentioned in posts:
#41, #52 and #59 by fellow member @TLRT ?

Neumann and AKG both used this technique to solve the issue. I mean, they know a thing or two about building Mics. See Schematics attached.

I’m willing to give it a shot. I tried ordering the Neutrik EMC XLR connectors but I’m guessing maybe I didn’t order from a legit place because they still haven’t sent them. So I’ll probably end up cancelling that order.

I guess there are two things going on here in my brain: 1) What can I do with the current mics to improve them (which is what most of this thread has been about) and 2) What is significantly different between these and the MP build that makes the MP build noticeably better in RF rejection? The MP build doesn’t use an RLC filter.

I don’t think there’s enough room to fit these inside the current PCB I’ve got (nor the XLR connector’s mounting mechanism) so I assume I could order these parts and just build that into one of my XLR female connectors?

https://www.mouser.com/ProductDetail/80-PFR5331J100J11L4

https://www.mouser.com/ProductDetail/542-78F470-RC

 

[Cqwet Dbdfte]

You have a link to the "MP build'?

 

[joulupukki]

You have a link to the "MP build'?

https://microphone-parts.com/products/sdc-84-microphone-kit-neumann-km84-style

They don’t publish the schematic they use but they are very similar to the original KM84. My guess is the biggest difference is the layout of their PCB. They use 22uF electrolytics where the DIY one uses 4.7uF tantalums, etc. They also have a “Snare” version of this mic, which is shown in this build:

 

[TLRT]

It’s this:

I’m willing to give it a shot. I tried ordering the Neutrik EMC XLR connectors but I’m guessing maybe I didn’t order from a legit place because they still haven’t sent them. So I’ll probably end up cancelling that order.

I guess there are two things going on here in my brain: 1) What can I do with the current mics to improve them (which is what most of this thread has been about) and 2) What is significantly different between these and the MP build that makes the MP build noticeably better in RF rejection? The MP build doesn’t use an RLC filter.

I don’t think there’s enough room to fit these inside the current PCB I’ve got (nor the XLR connector’s mounting mechanism) so I assume I could order these parts and just build that into one of my XLR female connectors?

https://www.mouser.com/ProductDetail/80-PFR5331J100J11L4

https://www.mouser.com/ProductDetail/542-78F470-RC


View attachment 123787

You can add this filter inside the xlr connector easily(with ceramic or polystryrene caps)

 

[Cqwet Dbdfte]

The length of the lead from the JFET to the capsule thru the white insulator and whatever impedance to ground the capsule provides, forms an antenna, only shielded by the long slots at the top, plus whatever shielding the wire mesh behind the slots gives.
If not done well that mesh may be ineffective.
Cover it up with aluminum foil wrap and measure results
This mike was designed long before the cellphone made its debut.

 

[joulupukki]

The length of the lead from the JFET to the capsule thru the white insulator and whatever impedance to ground the capsule provides, forms an antenna, only shielded by the long slots at the top, plus whatever shielding the wire mesh behind the slots gives.
If not done well that mesh may be ineffective.
Cover it up with aluminum foil wrap and measure results
This mike was designed long before the cellphone made its debut.

If this is the main problem why would there be no difference with the DIY circuit board slid into the MP body?

 

[Cqwet Dbdfte]

Diff between between DIY and what?

I can only go by known principles and my observations.

It seems like the FET is an RF detector, with an antenna on the gate, else RF would make its way up thru a shielded lossy audio cable, a lossy PCB, work across the audio transformers leakage capacitance, couple across the 1uF cap that would be crap for RF, then conduct from the drain to gate get rectified and make the trip back as audio at 100Hz from some cellphone RF modulation, or possibly radiate from the XLR leads,
or
be picked by a poorly shielded too long gate lead.

 

[joulupukki]

Diff between between DIY and what?

Sorry. I should have been more clear. If I slide the DIY circuit board into the micparts mic body, the DIY circuit board still exhibits just as much RF noise as before so I'm suggesting that the difference between the Takstar CM-60 donor mic body (used for the DIY circuit board) and the micparts body are equal in that respect. And I think that means that the difference is specifically something to do with the differences in the circuit board layouts and/or components used.

It seems like the FET is an RF detector, with an antenna on the gate, else RF would make its way up thru a shielded lossy audio cable, a lossy PCB, work across the audio transformers leakage capacitance, couple across the 1uF cap that would be crap for RF, then conduct from the drain to gate get rectified and make the trip back as audio at 100Hz from some cellphone RF modulation, or possibly radiate from the XLR leads,
or
be picked by a poorly shielded too long gate lead.

Understood. I am going to try and order parts to build a RLC filter into an XLR connector and/or try out a Neutrik EMC XLR connector. If that doesn't end up being *the* issue, then I'll go about trying to shorten the JFET's gate wire.

 

[Cqwet Dbdfte]

The gate lead cannot be shortened to any meaningful length, unless you place the JFET against the capsule connection, i.e. the entire wire between the JFET body and the capsule is the "gate lead". Shorten this to a 32nd wavelength, or some such, or
add inductance in the gate lead, or prevent RF from entering the grill to begin with.
To exclude the top grill as the entry point simply cover it completely with metal foil. If that does not work, suspect another entry point.

 

[joulupukki]

The gate lead cannot be shortened to any meaningful length, unless you place the JFET against the capsule connection, i.e. the entire wire between the JFET body and the capsule is the "gate lead". Shorten this to a 32nd wavelength, or some such, or
add inductance in the gate lead, or prevent RF from entering the grill to begin with.
To exclude the top grill as the entry point simply cover it completely with metal foil. If that does not work, suspect another entry point.

Yeah, I’ve covered the entire top half of the DIY mic with foil and it still has the problem.

 

[rmburrow]

Interesting. Can you send me a link or manufacturer/part number to something that may work?

Amidon made ferrite beads but a lot of these companies may no longer be in business. Check with Mauser Electronics on line.

 

[joulupukki]

Amidon made ferrite beads but a lot of these companies may no longer be in business. Check with Mauser Electronics on line.

Yes. They have some, but there are multiple values/etc. I’m wondering what specific one(s) would work.

 

[Cqwet Dbdfte]

In the absence of correct RF frequency the correct value would be a cut and try proposition. The freq would most likely be above 700 MHz, but could go up to 3.8 GHZ.
Lower freq penetrate better, so my guess is lower.
Ferrite selection would be going thru data sheets/catalogs from several mfgs.

 

[joulupukki]

178 options, for example, looking at SMD options marked as EMI/EMC: https://www.mouser.com/c/passive-co...|~Filters&termination style=SMD/SMT&instock=y

 

[Ike Zimbel]

In the absence of correct RF frequency the correct value would be a cut and try proposition. The freq would most likely be above 700 MHz, but could go up to 3.8 GHZ.
Lower freq penetrate better, so my guess is lower.
Ferrite selection would be going thru data sheets/catalogs from several mfgs.

5G cellular starts at 618MHz in North America.