Those are the phone uplink frequencies, back to the cell towers. So if Block A, 518-523MHz is occupied by a downlink from the cell towers, then any phone that is 5G capable will use the uplink block A, 663-668MHz back to the tower. Note that in most urban areas, "cell towers" are actually the roofs of nearby buildings.
Is there a way to reduce cell phone interference in DIY KM-84?
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That Pin-1/Tab connection has been bugging me through this whole thread. I wonder if you would see an improvement if you used Pin-1 as the central point instead of the tab. So Pin-1>PCB pad, and, Pin-1>Tab. You could just cut the PCB-Tab connection at the tab end and move the wire to Pin-1 to test. Shorten that lead to exact length required while you're at it. And, just FYI, I believe that you have reversed Pins 2 & 3 in your connections to the board. I would leave them as they are, but make note that these mics may have opposite polarity to other mics in your kit.
One last thing...if you DO try replacing the FET, I wonder what would happen if you made the gate lead as short as possible by positioning the part over that hole, and then used the longer leads on the source and drain.
joulupukki
Well-known member
I previously tried Pin-1 directly to PCB and Pin-1 directly to the tab with the shortest possible wires ... with the exact same result. In fact, I have two of these mics and the one not pictured still has that arrangement.
The two Neutrik EMC Female connectors will arrive tomorrow. If, after trying those, it still has the problem, I will try an alternate FET (with as short of gate connection as possible). I ordered a couple of backup trimmer resistors just in case I have to fully remove the old one (and end up not being able to get it out cleanly).
If I knew what kind of ferrite bead was on the circuit @jp8 posted here (https://groupdiy.com/threads/is-the...-interference-in-diy-km-84.86707/post-1136804) I would try one of those first.
Edit: Maybe one of these?
https://www.mouser.com/c/passive-co...-cable-cores|~Outside Length|~Inside Diameter
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joulupukki
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Excellent eye! You're right. Because of the way the XLR pins are physically laid out I reversed these pins to keep the length of wire shorter from PCB to the pins. I compensated by reversing the secondary leads of the transformer so the polarity matches my other mics. I'm not sure if that's "best practice", but it seems to work.
Cqwet Dbdfte
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...a ground connection to the AC outlet is not meaningful for UHF, (if that is indeed the issue).
May work for 60Hz.
A perfect ground would have infinite size and zero resistance and impedance, so as to not develop any voltage drop, at any freq or current intensity. As such "ground" is more of a concept than a reality.
Looking at the original Neumann schematic, the circuits ground system is connected to the case via a resistor (10R?)at the XLR ground (shield) return.
That would lift the circuit above ground (potential) of the cable shield.
Hard grounding can create current paths where you don't want them.
The cable shield can be bonded to the XLR case at the remote end. Its purpose is only shielding, and not carry a signal.
May work for 60Hz.
A perfect ground would have infinite size and zero resistance and impedance, so as to not develop any voltage drop, at any freq or current intensity. As such "ground" is more of a concept than a reality.
Looking at the original Neumann schematic, the circuits ground system is connected to the case via a resistor (10R?)at the XLR ground (shield) return.
That would lift the circuit above ground (potential) of the cable shield.
Hard grounding can create current paths where you don't want them.
The cable shield can be bonded to the XLR case at the remote end. Its purpose is only shielding, and not carry a signal.
Cqwet Dbdfte
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The length of the gate lead is a moot point as it is connected to the capsule with a long wire working as an antenna. Shortening it ~2mm won't affect the total length much.
You could try shorting the gate to circuit ground and see if you still get RFI.
Next try a resistor in the gate lead close to the FET body, i.e. leave only enough to solder on a small resistor, tell me if 1k reduces RFI and if the noise increases.
Problem solved.
If you carefully take a 3 foot length of stranded (silver plated) 16ga wire (the finer stranding the better - this helps with both higher and lower freudian frequencies) and strip both ends. Take one end and attach a spade lug for a #6 screw. Attach this spade lug to your good earth/ground at an outlet. Take the other end and solder it to a precicsly measured Trapezoid of Reynolds Heavy Duty Aluminum Foil (it must be that brand or you will get sideband interference around the Karmichael crossover points) which I have already compensated for in your wire length. This trapezoid should be 17 inches in height with the narrow top at 14 inches, and the bottom and exactly 17 inches (500-600 megahertz, right?). Now after the solder joint cools, turn OFF your cell phones and carefully wrap your them in the foil. Now you may continue recording and mixing in peace and quiet. NOTE! This does not work with metric measurements, only imperial.
This also works for mic pre's, microphones, and artists.
Problem solved.
If you carefully take a 3 foot length of stranded (silver plated) 16ga wire (the finer stranding the better - this helps with both higher and lower freudian frequencies) and strip both ends. Take one end and attach a spade lug for a #6 screw. Attach this spade lug to your good earth/ground at an outlet. Take the other end and solder it to a precicsly measured Trapezoid of Reynolds Heavy Duty Aluminum Foil (it must be that brand or you will get sideband interference around the Karmichael crossover points) which I have already compensated for in your wire length. This trapezoid should be 17 inches in height with the narrow top at 14 inches, and the bottom and exactly 17 inches (500-600 megahertz, right?). Now after the solder joint cools, turn OFF your cell phones and carefully wrap your them in the foil. Now you may continue recording and mixing in peace and quiet. NOTE! This does not work with metric measurements, only imperial.
This also works for mic pre's, microphones, and artists.
Problem solved.
Cqwet Dbdfte
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Yes, RF prefers Imperial measurements.
joulupukki
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Right. The DIY and the MP circuits do not use the 10R at all. I tried it just for kicks to see if I'd notice any difference – and did not.
Wait, do I also need to make and wear a tin foil hat or will that just cause more interference?
Cqwet Dbdfte
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The tin foil hat has to be grounded, and trapezoidal, no yodler hats plz.
Correct
No- the capacitance between the foil hat with your hair as an insulator and your body itself will then create a capaictor turning the whole thing into an antenna - so no hats.
I think theres something fundamentally different about the 0V arrangement in the two mics ,
Use copper foil to approximate the ground plane /case connection found on the lower noise mic .
pin 1 and its associated wiring is gone from the pcb .
Use copper foil to approximate the ground plane /case connection found on the lower noise mic .
pin 1 and its associated wiring is gone from the pcb .
joulupukki
Well-known member
I’m not sure that I understand.
Here is the MINIMUM to get rid of RFI coming in via the cable.
Solder 2 x 22nF or 10nF CERAMIC capacitors between p2 and the Ground Tag and p3 and the Ground Tag. The leads MUST be as SHORT AS POSSIBLE.
I only show the CERAMIC on p2 cos your p3 is under the PCB. You'll have to work out how to connect the p3 CERAMIC to the Ground Tag with short leads.
Next put Ferrite Beads on the leads (or better still lossy inductors) between p2 & p3 and the PCB.
This is the scheme I used on the Calrec stick mikes and Dip. Ing Wuttke used for Schoeps.
The CERAMICS are C12 & 13 on the CMC5 circuit
https://groupdiy.com/threads/schoeps-cmc5-inductor-values.73321/
and the lossy inductors are Dr3 & 4
They MUST be AT the XLR. Check that the Ground Tag makes a REALLY GOOD CONNECTION to the mike body. Try to shorten the transformer output leads or at least twist them tightly together.
You may or may not need extra work on the grille, FET etc that various people here have suggested, but without the above mods, you WILL have RFI problems.
Solder 2 x 22nF or 10nF CERAMIC capacitors between p2 and the Ground Tag and p3 and the Ground Tag. The leads MUST be as SHORT AS POSSIBLE.
I only show the CERAMIC on p2 cos your p3 is under the PCB. You'll have to work out how to connect the p3 CERAMIC to the Ground Tag with short leads.
Next put Ferrite Beads on the leads (or better still lossy inductors) between p2 & p3 and the PCB.
This is the scheme I used on the Calrec stick mikes and Dip. Ing Wuttke used for Schoeps.
The CERAMICS are C12 & 13 on the CMC5 circuit
https://groupdiy.com/threads/schoeps-cmc5-inductor-values.73321/
and the lossy inductors are Dr3 & 4
They MUST be AT the XLR. Check that the Ground Tag makes a REALLY GOOD CONNECTION to the mike body. Try to shorten the transformer output leads or at least twist them tightly together.
You may or may not need extra work on the grille, FET etc that various people here have suggested, but without the above mods, you WILL have RFI problems.
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Can you also check the MP PCB to see if there are 10n or 22n CERAMICs between the p2 & p3 pads and the ground/shield p1 pad.
I've done some consultancy work for MP but can't remember working on a KM84 type product. I think I have suggested these on their Schoeps clone
I've done some consultancy work for MP but can't remember working on a KM84 type product. I think I have suggested these on their Schoeps clone
joulupukki
Well-known member
There are no extra resistors or capacitors between the board and the pins in the MP build. I’ve built two of them. The pins from the XLR insert connect with wires directly to the PCB.
joulupukki
Well-known member
This seems similar to what the Neutrik EMC XLR connectors will do. UPS should be delivering those tomorrow. I'll give them a shot and if they have a significant positive change, I'll also see if I can rig up something that'd work inside the mic right there where you're suggesting. Or, if the Neutrik connectors work well enough, problem solved and I can just use those cables when using these mics (and they'll probably just be the cables I use for most everything).Here is the MINIMUM to get rid of RFI coming in via the cable.
Solder 2 x 22nF or 10nF CERAMIC capacitors between p2 and the Ground Tag and p3 and the Ground Tag. The leads MUST be as SHORT AS POSSIBLE.
I only show the CERAMIC on p2 cos your p3 is under the PCB. You'll have to work out how to connect the p3 CERAMIC to the Ground Tag with short leads.
Next put Ferrite Beads on the leads (or better still lossy inductors) between p2 & p3 and the PCB.
View attachment 124021
This is the scheme I used on the Calrec stick mikes and Dip. Ing Wuttke used for Schoeps.
The CERAMICS are C12 & 13 on the CMC5 circuit
https://groupdiy.com/threads/schoeps-cmc5-inductor-values.73321/
and the lossy inductors are Dr3 & 4
They MUST be AT the XLR. Check that the Ground Tag makes a REALLY GOOD CONNECTION to the mike body. Try to shorten the transformer output leads or at least twist them tightly together.
You may or may not need extra work on the grille, FET etc that various people here have suggested, but without the above mods, you WILL have RFI problems.
YMMV, but contrary to popular belief, I'm afraid those 22nF cercaps on the outputs just will not work to suppress UHF interference from cell phones. Not even when mounted directly on the XLR as I will show in the pictures below. ANY wiring from the caps to the chassis, to pin 1 or to whatever ground will be too long to make the caps effective at those frequencies. You'll need a circumferential ground connection from the XLR cable shield to the mic body, like the Neutrik EMC plugs have or an RFI filter like the Takstar CM-63 has.
I just did some measurements to prove these statements. Sharing all the pics and explanatory notes will be too much for this post, but I will publish them in due time on my website. Let me suffice now with some pictures I made using my RF jammer, which is a 100% AM modulated UHF transmitter. It is turned hard on/off at 217 Hz, so you'll see this frequency and all its harmonics displayed. The first RTA plot is of the MXL991 (aka MXL603), stock version, with 22nF on the PCBA. The 2nd RTA plot is the same mic with the caps on the XLR, as pictured between the two RTA plots below. The last two plots are a Behringer B-5 with regular XLR cable and the other RTA plot is with the same mic, but now connected to the jammer with a cheap t.bone cable fitted with Neutrik EMC connectors. With the MXL991, the 217 Hz is at approximately the same level. In the Behringer B-5 plots, the reduction achieved with the Neutrik EMC XLR is quite spectacular.
Let's wait and see if the Neutrik EMC plugs work equally well on joulupikki's mics. I'm very curious about the results...
Jan
I just did some measurements to prove these statements. Sharing all the pics and explanatory notes will be too much for this post, but I will publish them in due time on my website. Let me suffice now with some pictures I made using my RF jammer, which is a 100% AM modulated UHF transmitter. It is turned hard on/off at 217 Hz, so you'll see this frequency and all its harmonics displayed. The first RTA plot is of the MXL991 (aka MXL603), stock version, with 22nF on the PCBA. The 2nd RTA plot is the same mic with the caps on the XLR, as pictured between the two RTA plots below. The last two plots are a Behringer B-5 with regular XLR cable and the other RTA plot is with the same mic, but now connected to the jammer with a cheap t.bone cable fitted with Neutrik EMC connectors. With the MXL991, the 217 Hz is at approximately the same level. In the Behringer B-5 plots, the reduction achieved with the Neutrik EMC XLR is quite spectacular.
Let's wait and see if the Neutrik EMC plugs work equally well on joulupikki's mics. I'm very curious about the results...
Jan
What I meant was, "are there CERAMIC caps joining the p2 & 3 pads to the p1 pad ON THE PCB.
The CERAMICS are best on the pins but if the MP PCB is actually AT the XLR pins, this might be close enough.
The Neutrik cable addresses a different effect. It 'shortens' the route taken by the shield to the mike body. It may be the MP body has a better route from p1 to the mike body. IIRC, the Calrec mikes also used the fixing screws for the PCB to make further connections to the body near the XLR
