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

[Cqwet Dbdfte]

Current, very low.
Freq?
Impedance? Higher is better, 2k is possible

 

[Cqwet Dbdfte]

5G cellular starts at 618MHz in North America.

Not sure how much band n71 is rolled out. Uplink 663 - 698MHz.

 

[Big Valley Audio]

I’m very late to this but here’s a couple quick things to try.

-First download cell phone tower locator app on your phone
-The mic will make the most noise NOT when the capsule is pointing at the tower but when the most surface area of the mic body is facing the tower.
-Using tinfoil, wrap the body of the mic but extend it to cover the female XLR on the mic cable. This should drop the noise considerably.
-Connect pin 1 inside the female XLR to pin 0 (zero) inside the female XLR. Just use a small loop of wire. Pin 0 being the tab that has the screw in it or on Neutrik style connectors it is a tab that eventually makes contact with the shell of the XLR itself.

The RF rejecting Neutrik that you have on order ( that Ike recommended) will reduce the noise even greater than all of the above. You only need to replace the female XLR on the mic cable.

Neumann KM184’s were the most susceptible to this noise.

 

[Big Valley Audio]

I forgot to add that after connecting pin 0 to pin 1 on the female XLR on the mic cable or adding the Neutrik RF female on the mic cable, you will no longer need the tinfoil.

 

[Cqwet Dbdfte]

The tower is not the issue, it would be constant interference, right below s tower you may get -40dBm, in this guys basement with a weak tower, maybe -100dB, his cellphone is screaming at max. The tower app can find the band used, or it may use several bands, aggregating, or switching towers.

 

[Big Valley Audio]

That might be the case but I have had cell RF that was at -100dB when measured in a studio (that a KM184 microphone was set up in) that was still causing interference.
The (demodulated) noise that the mic picked up was down at approximately -60 dBm when the output of the mic pre was measured (with 35 dB of gain on the mic preamp). Which depending on what you are recording is obviously bad.
When multiple mics were used on a quiet source ( quartet, etc) the recording was unusable.
a combination of the Neutrik XLR and orientation to the tower solved the problem.
That’s my experience.
Mostly pencil mics but also Sony C48, U87.

 

[Cqwet Dbdfte]

Low input capacitance JFETs means wide bandwidth, adding decoupling capacitance to the gate means killing the already weak charge coupled input.
Possible solutions:
shielding, to a level functional for relevant RF.
Feeding output signals thru a shield with feedthru caps,
decoupling output LF signals, (many SMT type filters for this),
Use RF absorbing material inside the mike cavity.
An experiment with a direct injected RF signal into the FET to determine susceptibility level.
That FET is a wideband RF detector. 1960's paradigm may render the proposition useless.
A vacuum triode may be a better alternative for that charge coupled amp.

The shielding effectiveness of that mike body should be measured in a TEM cell.

EDIT:
Adding a series resistor close to the gate is a very common wide band (aid).
A few kOhms should cut the gain down at the highest freq.

 

[joulupukki]

What is the purpose of 10 Ohm R13 in the original Neumann KM-84 schematic? http://recordinghacks.com/pdf/neumann/KM83-84-85-SM84_Schematic.pdf

I’m just curious if it would do any attenuation of high frequency noise coming in through the XLR connection?

 

[Matador]

To give a path for phantom return current, and to provide a measure of isolation between analog 0V and the shield.

 

[ricardo]

Can you post your EXACT DIY circuit?

Show ALL the connections between the PCB and the mike body and try to get them in the approximate physical positions on the circuit. Also the wiring of the XLR in the mike. What route does XLR p1 take before it gets to the mike body ?

Then can you do the same for the MP item?

 

[Cqwet Dbdfte]

Put the mike inside a metal bag or metal can, and see if RF get thru the cable.
A cookie tin may work.
Being the interference is a 100Hz harmonic series dont exclude Power supply issues.

 

[joulupukki]

Can you post your EXACT DIY circuit?

I'm using the PCB created by Graeme Woller. Attached is the schematic.

Circuit board:

Show ALL the connections between the PCB and the mike body and try to get them in the approximate physical positions on the circuit. Also the wiring of the XLR in the mike. What route does XLR p1 take before it gets to the mike body ?

From XLR P1, there's currently a solid wire that goes to the XLR screw terminal tab. from there, it connects with another solid wire to the circuit board's XLR1 pad. Here's how that looks:

Then can you do the same for the MP item?

Micparts doesn't provide the schematic, but it's nearly identical except they use 22uF electrolytic caps and a 0.022uF ceramic bypass cap across the electrolytic cap for the JFET's source to ground.

 

[joulupukki]

Put the mike inside a metal bag or metal can, and see if RF get thru the cable.
A cookie tin may work.
Being the interference is a 100Hz harmonic series dont exclude Power supply issues.

The only thing I could think of is my cast iron dutch oven. I've got the mic in there and it's still going nuts with my cell phone nearby almost as much as before.

 

[Cqwet Dbdfte]

...ehh
Not a airtight test.
The metal should have no openings.
The opening for the cable should be snug, not shure how tight a metalized chips bag would be

 

[joulupukki]

...ehh
Not a airtight test.
The metal should have no openings.
The opening for the cable should be snug, not shure how tight a metalized chips bag would be

The cable was in a notch. I don't really have anything else that'd work.

 

[Khron]

The only thing I could think of is my cast iron dutch oven. I've got the mic in there and it's still going nuts with my cell phone nearby almost as much as before.

View attachment 123868

Is there some way you could conveniently earth the container?

 

[joulupukki]

Is there some way you could conveniently earth the container?

I'll have to try tomorrow. Maybe I can tie it onto the ground rod that's grounding my ham shack.

 

[ricardo]

From XLR P1, there's currently a solid wire that goes to the XLR screw terminal tab. from there, it connects with another solid wire to the circuit board's XLR1 pad. Here's how that looks:

View attachment 123867

I don't understand this pic. Is this the back of the XLR pins on the mike? Can you label p1 p2 p3 & the 'screw terminal tab'? What is the Grey conical bit at the top of the pic?

The circuit you show has no EMI/RFI protection at all. I'll post some stuff for this but first we need to confirm the EXACT way the cable shield gets to

• the body of the mike
• and the circuit.

 

[cyrano]

Simple test: do you have another phone to test?

Just to eliminate the odd chance that your phone is defective.

 

[Cqwet Dbdfte]

I'll have to try tomorrow. Maybe I can tie it onto the ground rod that's grounding my ham shack.

Ground at 700MHz+ would require a low inductance conductor, think as wide as it is long. Getting a low contact resistant connection to Terra firma use a couple of square yards of copper plate.
Not a viable option, or useful in any practical situation.
Ground at ~700MHz + is not a useful concept, better look at signal differences from a reference plane, in this case the FET, but anything that acts as a semiconductor junction can rectify the signal into the audio range.
Because you suspect one cause another may be overlooked. Confirmation bias is real, so divide and conquer. Exclude other causes, use another mike, pre amp, power supply, another mike cable etc etc.
Order shielded power cords from Amazon and throw out all old standard non shielded types.

The mike should be a Faraday cage, with a continuous metallic contact around any susceptible point. Any opening should be a "waveguide beyond cutoff". Any metallic weave or screen should be cut from a flat sheet and expanded, then bonded to the mike cavity for continuous peripheral contact.
The mike body should be copper plated, then nickel plated, maybe tin plated for solderability.
Any insulation should not in any way stop the contact.
Add a 2k - 10k resistor in the gate lead, wirewound is OK, max 1mm from FET.
The leads from the XLR to the PCB could be (wirewound) resistors, 50-ish Ohms should be tried, followed by caps max ~100pF to case, with a low inductance connection, like BeCu fingerstock soldered to the PCB then circumferentially connected to the conductive mike tube's inside wall.
The easiest solution is the best, but without a real pinpoint understanding of cause, too many options exist.
RF does not practically behave in any way like 60Hz so methods for this can be put aside.
A test chamber for the mike could be a 6 foot long copper tube, capped at the end, and steel wool crammed around the cable.
Even a test with the
XLR cable alone, then terminated, then on the mike with the signal leads disconnected, then disconnect parts on the PCB until the noise stops.