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

[joulupukki]

After purchasing a stereo pair of MP SDC-84 mics, I wanted to dive deeper and try my hand at sourcing all the parts to see if I could build some DIY mics that performed as well as the MP mics.

I bought some circuit boards from Graeme Woller, used some inexpensive Takstar CM-60 mics as donor bodies, 3U cardioid capsules, and 3U GZT-84 transformers. The project has turned out extremely well so far and, for the type of acoustic instruments I'm recording, I have the mics sounding virtually identical.

But, there's one big difference in performance and that's the resilience to cell phone RFI. The MP mics are definitely susceptible to it, but these DIY mics are at least 10 times more susceptible. I've tried re-routing the wires on the transformer and to a small degree different routings have helped some.

Here's a couple of pictures of the mics (the top is one of the MP mics). On the middle one I shortened the transformer leads to see if it'd help (didn't seem to). On the bottom one I twisted some of the transformer leads without shortening. That maybe helped marginally.

At this point, I'm wondering if the main difference is the board layout in the DIY? Or would there be something else I should consider that I haven't thought of yet?

 

[Ike Zimbel]

I can't tell from the pics what's going on electrically but you can see that mechanically the MP mic has a very solid connection between the circuit board and the chassis. If that mechanical connection also included an electrical connection to a ground plane on the circuit board, I could see that making a difference.

 

[joulupukki]

Interesting. True, the MP board has a square pad that the screw for the XLR connector housing clamps down on. The Graeme PCB board just has XLR Pin 1 connecting to the ground pin on the board, which is connected to the copper ground running throughout the board. The capsule definitely has a ground connection through the body or otherwise, I wouldn't be getting good clean output from the mic otherwise, right?

 

[RuudNL]

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

Switch off your mobile phone!

 

[Ike Zimbel]

Interesting. True, the MP board has a square pad that the screw for the XLR connector housing clamps down on. The Graeme PCB board just has XLR Pin 1 connecting to the ground pin on the board, which is connected to the copper ground running throughout the board. The capsule definitely has a ground connection through the body or otherwise, I wouldn't be getting good clean output from the mic otherwise, right?

It's really the metal case of the mic body that gives you your RF shielding, so make sure the XLR screws are tight, and that there's a good connection when the body is fully assembled.

 

[ccaudle]

The Graeme PCB board just has XLR Pin 1 connecting to the ground pin on the board

That sounds like classic "pin 1 problem" where the noise currents on the shield are injected directly into the audio circuitry. XLR pin 1 should connect to the housing as directly as possible, and then the circuit reference node should have a connection to the conductive housing to complete the phantom power return circuit.

 

[joulupukki]

So if I’m understanding this correctly, the issue could be this wiring from the XLR pin to the XLR1 pad on the board and to the ground wire that connects to the mic body chassis via the phillips screw? …and more specifically, that the wire there may be too long?

 

[joulupukki]

The good news is I'm not picking up any AM/FM radio stations with the mics. The bad news is, if someone's smart phone or smart watch is too close, I can hear it. So, unless I'm recording myself or I can guarantee whoever's in the sound booth doesn't have any cellular devices (at least within 1 meter of the mics), these DIY mics are "safe." Otherwise, I'll reach for the MP mics every time.

 

[Ike Zimbel]

Maybe try taking your DMM and reading between the Pin-1 connection and the casing? It should read close to 0 ohms. But, if the paint/plating is non-conductive it could be messing with that. If you do get a questionable reading, take the XLR screw out and sand down the plating in the countersunk hole and try it again. If that makes a change, have a look at where the tube meets the XLR housing, rinse and repeat. And, ALWAYS make sure the screws are tight on mic bodies.

 

[joulupukki]

0 ohms between Pin1 and the casing. Screws are always tight. When a cell device isn't within about 1 meter, the DIY mic has quieter self noise than the MP SDC-84. I'll have to experiment later with making sure the tube and housing are nice and clean.

 

[ccaudle]

How do the PCB's of your DIY mics attach to the housing? There are usually screws, but your mics (the red PCBs, correct?) do not have brackets and do not have screws.


The ideal connection scheme would break this direct connection from pin 1 to the PCB:



Have the shortest possible connection between pin 1 and housing, and then connect the pad for PCB ground to another point on the housing. That is often easier to arrange in a large diaphragm microphone, there is just not much space in a typical end address small diaphragm mic to do that without building in contact spring fingers in the housing design.
These papers give some good background information:
http://www.audiosystemsgroup.com/AES-RFMicrophonesASGWeb.pdf
http://www.audiosystemsgroup.com/AESPaperNYPin1-ASGWeb.pdf

 

[joulupukki]

How do the PCB's of your DIY mics attach to the housing? There are usually screws, but your mics (the red PCBs, correct?) do not have brackets and do not have screws.
View attachment 123277


The ideal connection scheme would break this direct connection from pin 1 to the PCB:

View attachment 123289

Have the shortest possible connection between pin 1 and housing, and then connect the pad for PCB ground to another point on the housing. That is often easier to arrange in a large diaphragm microphone, there is just not much space in a typical end address small diaphragm mic to do that without building in contact spring fingers in the housing design.
These papers give some good background information:
http://www.audiosystemsgroup.com/AES-RFMicrophonesASGWeb.pdf
http://www.audiosystemsgroup.com/AESPaperNYPin1-ASGWeb.pdf

Yeah, this is a bit of a hack to make a PCB designed for an MXL-style pencil mic work with a more simple XLR connector only (a cheap Takstar CM-60 donor body). Unfortunately, those outer holes of the PCB near the XLR connector do not seem to be solder pads with copper but are just through holes.

The PCB is attached to the XLR connector by wedging the PCB in between XLR pins 1 and 2 on one side and XLR pin 3 on the other side with short jumper wires soldered from the PCB's XLR pads directly to the XLR connector's pins.

I'll have to study up on those papers you included links for. Looks like as an experiment, if I could somehow connect the PCB's Pin 1 to a different location of the housing, that may be ideal?

 

[ccaudle]

if I could somehow connect the PCB's Pin 1 to a different location of the housing, that may be ideal?

Definitely worth a try.

 

[Cqwet Dbdfte]

The PCB looks to have possibly a JFET input, some of which are great at UHF.
This end handles the smallest signals and the gate of it would be most susceptible for interference. Common RF practices like using RFI power line filters, ferrite beads, feed thru caps, proper screening should be employed to effect.
The housing and the screen may need to be revised. A single piece metal screen (etched, laser cut, punched) rather than bonded wires which may have poor intermetallic connections.
The problem could be as easy as a few ferrite beads in the XLR conductors, or something more fancy, with a proper SMT PCB focused on RF properties.
Filling the mike body with (RFI type absorber) carbon foam may do the trick.
Try the simple stuff first.

 

[rmburrow]

It's really the metal case of the mic body that gives you your RF shielding, so make sure the XLR screws are tight, and that there's a good connection when the body is fully assembled.

This appears to be another case of audio rectification where an external signal is being demodulated within the mic, and the connections and case shielding are good. Ferrite beads may be the answer. You should get ferrite beads that are optimized for the UHF frequencies; these beads slip over the conductor. First, try a bead over the connection to the gate of the input FET, test with the cell phone, and compare the audio with/without bead. Next, try two beads, one on the lead to pin 2 of the XLR and the other to pin 3 of the XLR. This is a low impedance connection and should not affect the audio. The output transformer should stop RF but treating connections with ferrite beads to the "outside world" should help. I make my living with RF. Hopefully this works.

 

[Matt Nolan]

I see a possibly unpopulated cap position by the JFET - maybe part of the problem? Also, you could put very small caps to ground from your hot and cold outputs, there's room.

 

[Khron]

The PCB looks to have possibly a JFET input

It most definitely DOES have a JFET input.

Filling the mike body with (RFI type absorber) carbon foam may do the trick.

That sounds kinda risky, with exposed components/terminals...

Also, you could put very small caps to ground from your hot and cold outputs, there's room.

Often it's recommended for those to be soldered directly to the (inside) XLR pins.

 

[mrgrooves666]

There's gotta be something more at play here. I have a pair of Banzai's wich have Graeme's 84 pcb in a slightly different layout. Just for sakes I just grabbed one and played around with my android phone Only when I actually place the phone on the desk and lay the mic over it, very random ocasionally I will get some little interference , nothing like you mention at all...This layout does allow XLR pins to be soldered directly to the PCB board, so that could be related as mentioned then...?

 

[joulupukki]

@mrgrooves666 That's interesting you're not getting much interference. I wonder if something significant changed on the layout of his MXL-style boards that would make that much of a difference? Or, I wonder if the type of metal used in the Takstar CM-60 body (the donor mic) isn't as good for shielding? Grasping for straws here.

One big difference between the layout of the MP board and this one is that the JFET gate has a significant length to it. The MP location of the JFET is right next to the isolated terminals. But, it seems the most interference comes when I've got my cell phone near the XLR connector so maybe this is a non-issue?

DIY:


MP:


Just so it's clear what kind of interference I'm experiencing, I recorded a quick little video of it:



...and I have tried different XLR cables (all have the same effect).

 

[Khron]

the JFET gate has a significant length to i

Slipping a ferrrite bead onto that gate lead is that much more worth a shot, then.