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

[joulupukki]

All done, and more. I've send boards KM84 boards with improved RFI filtering to joulupukki for evaluation and he's quite happy with them. Recording on youtube:

I screwed up the latest board revision, but once I have it ready, I will make it available to the public.

Jan

I can vouch for Jan’s design. It has lower RFI noise than the MicParts SDC-84 mics. It’s a clever design that’s very effective.

The mics in the video:

Mic A is my DIY Woller KM-84 with a 3U capsule in a Takstar CM-60 body.
Mic B is Jan’s KM-84 board with a 3U capsule in a Takstar CM-60 body.
Mic C is a Mic Parts SDC-84.

 

[Cqwet Dbdfte]

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

Ferrites are not conductive. They are ceramic.

 

[dai h.]

Ferrites are not conductive. They are ceramic.

Thanks. I tried to remember where I read the pertinent info, and I guess it was here (from David Josephson) :

(IIRC for whatever reason, some of the posts were deleted in the original source(website))

https://web.archive.org/web/20071107112108/http://recforums.prosoundweb.com/index.php/m/226658/0/

Ferrites, particularly when applied without engineering the whole solution, can cause more trouble than they solve. The problem is that most ferrite materials are partly conductive. If you put a ferrite bead on a wire and anchor it in place so it cannot touch anything else, that's fine. But if the bead touches anything else while it is also touching the signal lead, you now have a variable leakage path that can cause serious trouble, particularly at the input and output of a condenser microphone impedance converter (for the same essential reason -- a large DC potential superimposed on a small audio signal). Microphone companies who successfully use ferrites for RF suppression take particular precautions in this regard to avoid such problems. (David Josephson)

(so re-reading the post, can you have the bead touching (as long it's not touching anything else?))

 

[ccaudle]

I actually tried (and failed) to make a shield plate out of some copper sheet I have.

It would be nice if that were commercially available from one of the connector manufacturers. At a quick glance it looks like the difficulty would be getting the right metal alloy and right tempering so that it stayed springy to insure good contact to the body, and at least the Schoeps version is noted as gold plated, presumably to avoid corrosion. There are other plating options such as tin or nickel which would probably be nearly as good as gold and less expensive.

It would actually be nice if it were made as part of the connector insert during that manufacturing, so that the shield did not have to be inserted into the microphone after the connector.

 

[joulupukki]

It would be nice if that were commercially available from one of the connector manufacturers. At a quick glance it looks like the difficulty would be getting the right metal alloy and right tempering so that it stayed springy to insure good contact to the body, and at least the Schoeps version is noted as gold plated, presumably to avoid corrosion. There are other plating options such as tin or nickel which would probably be nearly as good as gold and less expensive.

It would actually be nice if it were made as part of the connector insert during that manufacturing, so that the shield did not have to be inserted into the microphone after the connector.

Jan experimented with a carefully-crafted plate like that and it didn’t have much, if any, effect at all. More important was to set up a filter right after the XLR connector, similar to what the Takstar CM-63 has. But he’s the expert on this. I just tested it out and found it to work well.

 

[Cqwet Dbdfte]

Thanks. I tried to remember where I read the pertinent info, and I guess it was here (from David Josephson) :

(IIRC for whatever reason, some of the posts were deleted in the original source(website))

https://web.archive.org/web/20071107112108/http://recforums.prosoundweb.com/index.php/m/226658/0/

Ferrites, particularly when applied without engineering the whole solution, can cause more trouble than they solve. The problem is that most ferrite materials are partly conductive. If you put a ferrite bead on a wire and anchor it in place so it cannot touch anything else, that's fine. But if the bead touches anything else while it is also touching the signal lead, you now have a variable leakage path that can cause serious trouble, particularly at the input and output of a condenser microphone impedance converter (for the same essential reason -- a large DC potential superimposed on a small audio signal). Microphone companies who successfully use ferrites for RF suppression take particular precautions in this regard to avoid such problems. (David Josephson)

(so re-reading the post, can you have the bead touching (as long it's not touching anything else?))

You could add some teflon tubing on the other legs of a thru hole JFET.
I've seen beads on transistor legs on high volume products.
The gate should be a ground potential, and the capsule case at high voltage.
Only small voltages between JFET terminals.

 

[jp8]

Jan experimented with a carefully-crafted plate like that and it didn’t have much, if any, effect at all. More important was to set up a filter right after the XLR connector, similar to what the Takstar CM-63 has. But he’s the expert on this. I just tested it out and found it to work well.

Indeed, contrary to my expectations, it did virtually nothing to suppress RFI. I made the plates from a stainless steel solder paste stencil, ordered from JLCPCB. See the attached picture. I carefully cut out the plates with household scissors under a microscope, so that all the teeth were the same length.

What did help was the combination of a ferrite bead in the Drain output, a GND ferrite bead, X2Y cap between pins 2 and 3 to XLR pin 1 GND and two more ferrite beads between X2Y cap and the transformer. Beads and X2Y cap are SMT parts, btw. There was no space between transformer and XLR connector for a Common Mode choke à la CM-63, but an X2Y cap works satisfactorily, provided you've got a good (or good enough) ground plane connected to the shell. This, I established using the same XLR insert as used in the Takstar CM-63, with a low impedance ground pin to the shell. I found those on Aliexpress here:

https://www.aliexpress.us/item/3294...main.5.75c879d2obEJKZ&gatewayAdapt=4itemAdapt

Jan

 

[Cqwet Dbdfte]

RF interference may mix with audio at low levels and produce IM products, some test to check for this may be in order. It may show up as an increased noise floor.
A ferrite bead in the JFET's source would work as negative feedback at some frequency range. In the drain it would starve the RF current.
Ferrites have a great many choices of effective frequency ranges. Pick something with high resistance in the cell band range, say 600MHz and up.

 

[ccaudle]

contrary to my expectations, it did virtually nothing to suppress RFI

I suppose the metal insert would only suppress RFI which entered through the "pin 1 problem" mechanism.
The XLR connector insert you found seems to have a reasonably low inductance path from pin 1 to case, so perhaps that is sufficient.

 

[jp8]

Ferrites have a great many choices of effective frequency ranges. Pick something with high resistance in the cell band range, say 600MHz and up.

I'm using the Murata BLM18HD102SH1D beads. Effective over a wide frequency range, including US and EU cell phone bands.

Jan

 

[Cqwet Dbdfte]

I'm using the Murata BLM18HD102SH1D beads. Effective over a wide frequency range, including US and EU cell phone bands.

Jan

Good choice.