Tape or foil for mic transformer shield

[rubinstu]

In my DIY ribbon mic project, I'm getting a significant amount of hum. This is expected since as of now, I have zero shielding at all! I think (hope) that encapsulating the transformers will be sufficient. You can see the photo of the preamp / transformer below.

I am considering one of these two options:
Magnetic Shielding Film MCF5 3 Linear Feet
or
SICCOMA Faraday Cloth Tape Double Conductive Cloth Adhesive Tape for EMI Shielding, Wire Harness Wrap, Grounding, Guitar Interference Signal Blocking, Electrical Repair (1 in x 66 FT)
from Amazon.

I'm looking for cheap and easy but good enough.

Do you think these would get the job done? Which would be better?

Does the magnetic shield need to grounded (electrically attached to the signal ground, the case of the XLR, etc.)?

All thoughts are appreciated!

Thanks!

 

[rubinstu]

No takers on commenting on this? I'm sure someone has something to say.... Thanks!

 

[TimG]

I’m not a mic expert but I don’t know how you can measure the hum without a body on the mic. The body can be part of the circuit or shielding that manages the hum.
Do you have a body you can enclose it in for measurements?

 

[dmp]

I'm looking for cheap and easy but good enough.

Do you think these would get the job done? Which would be better?

Does the magnetic shield need to grounded (electrically attached to the signal ground, the case of the XLR, etc.)?

You can think of shielding as two different things - RF and magnetic. They are both shielding EMF, but use either conductive or magnetic shield.
For RF shielding you can use the conductive mic body or conductive metal film (I have copper). This should be grounded to act like an antenna, picking up the RF junk and sending it to ground. Shielding cable is similar, with the shield around the wires, and should be ground to chassis on one end.
Almost always, the metal mic body does this. You often will get a lot of buzz/hum from lights on a dimmer, for example. Shut the lights off and it goes away.

Magnetic shielding is sometimes important for reducing induced hum in a transformer. Better transformers have some amount of magnetic shielding (i.e. -30dB).
The expensive magnetic tape with high permeability is for magnetic shielding.
Magnetic shielding has to be a continuous loop without sharp edges, to redirect the magnetic field and prevent it from going into the transformer core. It would also be connected to the chassis ground.
I would address RF shielding first, and then decide if magnetic shielding around the transformer is necessary.

 

[rock soderstrom]

All thoughts are appreciated!

First of all, I would like to express my deepest respect for anyone who builds their own Ribbon motor! I could not.

To your problem the following: Faraday cage would be good for the whole mic but it's not as imperative from my point of view as it is for a condenser mic, as member Gyraf also writes in the other thread on this topic.

Shielding the transformer I think is a good idea because it is essentially a very sensitive pickup.

Here is a video of exactly your setup and also the same problem.

Check the video description, there he gives an update on the noise problem with the Geistnote transformer - grounded shielding cures the hum! He uses some layers of tinfoil! I think the MCF5 stuff would be better, but I have no realworld experience.

 

[Екатерина]

Generally speaking, there will be a lot of buzzing noise if there is no shell to shield all components. At least the condenser microphone is like this. The shell does not necessarily connect to the ground wire, but the shell should exist. Maybe tinfoil and paper tube are a good choice. I wish you success

 

[rubinstu]

@rock soderstrom, thanks for that video. I'm actually using the same transformer (the Geistnote) that he had trouble with. Our projects are pretty similar!
@TimG, I did not show it in the photo, but the mic has sort of a "frame" more than a body. Eventually, maybe, I'll add some mesh for EMI and acoustic "xplosions" for the ribbon. I don't have a great way to measure the noise...
@dmp I'm pretty sure it's good-old magnetic interference a la guitar pickup, like you said. For among other reasons, spectrum analysis shows it dead nuts on 60 Hz (I know, that could still be from electrical equipments like computers, LED lights, whatevers, but it's pretty consistent whether if I turn off lights, move it away from things, etc. Plus, it just "acts" like a cheap guitar pickup!)
I'm going to try the shielding tape and see what happens. Thanks, everyone!

 

[rock soderstrom]

I'm actually using the same transformer (the Geistnote) that he had trouble with. Our projects are pretty similar!

Yeah, right. Did you read that from the description?

"Update the day after uploading initial video....I have discovered wrapping the transformer in a few layers of tinfoil then grounding that and the rest of the microphone to the ground wire of the xlr dramatically improves any background noise ultimately cleaning up the signal. Normally in metal enclosures the casing itself acts as a faraday cage but, since this was a wooden box, I ended up using the metal mesh puff screens as grounding."

Keep us posted about your project and show more photos and a sound demo when finished, please

 

[TromNek]

rubinstu,
Any luck with your hum problem ?

I have hum problems in my apartment with the ribbons. It's magnetic field related (rotate the mic and you can make it almost disappear).
As such is can only be deflected (as dmp described).
Interesting that all my ribbons experience it except an sE VR1 that is very immune to it. I had 2 other VR1s that were not immune. So it makes me wonder why one of them is immune.

So I'm interested to know if you came up with any solutions.

 

[rubinstu]

rubinstu,
Any luck with your hum problem ?

I have hum problems in my apartment with the ribbons. It's magnetic field related (rotate the mic and you can make it almost disappear).
As such is can only be deflected (as dmp described).
Interesting that all my ribbons experience it except an sE VR1 that is very immune to it. I had 2 other VR1s that were not immune. So it makes me wonder why one of them is immune.

So I'm interested to know if you came up with any solutions.

I hope to do some testing tonight. It's been a little crazy here and there are workers banging around which makes is not too conducive to audio testing!

 

[C12VR]

The transformer is essentially the sound of the ribbon, geometry aside. I would recommend going with a cinemag, lundahl, or similar transformer (NOT edcor, which isn't good enough imo to justify the price), all of which are no more than 30-40 more than the Geistnote OEM tx. If you really can't afford it, I understand, but if it is in reach I would go with one of these from the get-go next time. It's not worth skimping like with condenser capsules where the difference is a week's pay.

 

[JW]

I have a question about shielding with tape as well. Slightly off topic. I remember liking the sound of an output transformer (Cinemag CMOQ-2) used as a step down input transformer in a 1176. I was told not to do this at some point, but is the only reason the lack of shield that input transformers have? It can certainly handle the signal level. Can shielding tape be a way to do this?

 

[C12VR]

I have a question about shielding with tape as well. Slightly off topic. I remember liking the sound of an output transformer (Cinemag CMOQ-2) used as a step down input transformer in a 1176. I was told not to do this at some point, but is the only reason the lack of shield that input transformers have? It can certainly handle the signal level. Can shielding tape be a way to do this?

You are confusing two types of interference. Magnetic interference is distinct from the type that you can remove with copper. Copper is not really permeable like mumetal is. It provides no magnetic shielding. The best vintage input transformers had both types of shielding---K241D had three nested mumetal cans with copper between.
Also transformer ratio is designed for impedance matching. I doubt that it wants a lower impedance than 600 ohms, rather higher. EDIT NVM 1176 is step-down input

 

[Newmarket]

Low frequency hum is often H-field related and you need a ferrous material to attenuate this. Preferably an appropriate grade of properly annealed mumetal. But any ferrous metal like steel will have some effect. And it's more effective if you can have two or more isolated layers. You don't need to "Ground" it for H-field shielding but if you do so for eg the outer layer that will also take care of E-field shielding eg rfi from computers, phones etc.

 

[ach]

I am considering one of these two options:
Magnetic Shielding Film MCF5 3 Linear Feet
or
SICCOMA Faraday Cloth Tape Double Conductive Cloth Adhesive Tape for EMI Shielding, Wire Harness Wrap, Grounding, Guitar Interference Signal Blocking, Electrical Repair (1 in x 66 FT)
from Amazon.

Coincidentally, I bought both of these tapes from amazon. I was experimenting with ways to reduce EMI from a mini switch mode supply that would be near to audio transformers.
The first is legitimately a form of mumetal foil with a high nickel content (strong magnetic attraction) and had a significant & measurable attenuation of EMI and low frequency interaction. Note that this is thin but non-ductile foil that is not easy to conform to sharp corners. I highly recommend this for low level magnetic interference.
The second tape is pretty useless for magnetic interference and has no measurable benefit over copper foil.

 

[rock soderstrom]

Coincidentally, I bought both of these tapes from amazon. I was experimenting with ways to reduce EMI from a mini switch mode supply that would be near to audio transformers.
The first is legitimately a form of mumetal foil with a high nickel content (strong magnetic attraction) and had a significant & measurable attenuation of EMI and low frequency interaction. Note that this is thin but non-ductile foil that is not easy to conform to sharp corners. I highly recommend this for low level magnetic interference.
The second tape is pretty useless for magnetic interference and has no measurable benefit over copper foil.

Thank you for your real world experience with this MCF5 foil! I'm going to order a roll.

 

[TromNek]

Coincidentally, I bought both of these tapes from amazon. I was experimenting with ways to reduce EMI from a mini switch mode supply that would be near to audio transformers.
The first is legitimately a form of mumetal foil with a high nickel content (strong magnetic attraction) and had a significant & measurable attenuation of EMI and low frequency interaction. Note that this is thin but non-ductile foil that is not easy to conform to sharp corners. I highly recommend this for low level magnetic interference.
The second tape is pretty useless for magnetic interference and has no measurable benefit over copper foil.

Thanks ! I think I'll pick up a roll of the MCF5 foil too.
I saw it a while ago, but wasn't sure if it was 'truely' the real thing.

I want to try and make some a couple magnetic field 'shields' for use with any of my ribbon mics.
I'm able to deflect the magnetic field by placing a piece of sheet steel directly below my mic and bending it upwards around the about 75% of the body.
That deflects the field around my mic and removes the hum.

I'm hoping I can make something smaller with real mu metal. So maybe the MCF5 foil will work.
Probably a three or more layers separated with some other ferrous metals.

rock soderstrom, please leave feed back on you this works for you and details on what you do.

 

[Mr. Moscode]

I haven't built a ribbon mic but where needed I've use Mu-metal as both a magnetic and electrostatic shield in the same place in a mic pre. If you reorient the mic and it reduces or goes away you have EMF. You may be too close to a power transformer.

RF noise, dimmer buzz, radio noise, etc should go away when the housing is closed up. If the body is large enough, like a RCA77, get some mu metal, roll it up, stuff it in the body of the mic, let it expand, ground it and see what happens.

 

[rubinstu]

Out of the gate, I did not have much luck with the MCF5 foil. I may try a little more with various ways to wrap, seal, etc.. I was expecting a least little attenuation of the hum, but I barely got any, if any at all. I'll give it another pass tonight to see what I come up with.
I also bought some cheap steel electrical junction boxes. I'm going to put the whole pre-amp assembly (which includes the transformer) in the box and see what happens.