Wire mesh shielding on DIY mic?

[rubinstu]

I'm working on a DIY dual-ribbon Blumlein mic. My weapon of choice is a laser cutter with thin plywood, so that's what I'm making most of the parts from. Please see the rendering below.
The obvious problem is how to shield this thing from EMI "hum". I'm trying to keep costs down and the building simple, but obviously not at the cost of noise.
Do you think it would be sufficient to line the inside with stainless steel mesh and ground it?
The mesh idea is nice since it can also act as a mechanical and pop protection for the ribbon and hopefully be acoustically transparent. I should also be easy to work with, conform to weird bends, etc.
Something like this from Amazon: Ovsor 2Pcs 60 Mesh Stainless Steel Mesh16×50 inches (40.6×127cm), Woven Wire Mesh Not Easy to Rust Mesh Roll, Steel Screen for DIY Project, Floor, Hole, Garden, Vent Cover ?
Thanks

 

[k brown]

If you use stainless steel, be sure get a type that's non-magnetic; some types are not.

Otherwise copper or brass.

 

[gyraf]

Are we sure that ribbon mics actually needs shielding other than for mechanical-protection services?

I mean, condenser mics sure need it - but I'm not sure at all that ribbons does: ribbons work well and noisefree when free-hand-testing while repairing, where condensers need a faraday cage type of setup to be listenable at all because of electrostatic noise..?

If anything, I would suspect that ribbons sometimes would benefit from magnetic screening - but that's all another and more complicated (heavy) story

perhaps wood-and-silk is all that's really needed - and this could surely be formed in nice ways..

/Jakob E.

 

[rubinstu]

@J Brown, thanks for the tip. I was originally planning to use copper foil tape, much like one would use for shieling the cavity of an electric guitar, but somehow I got it in my head that I would need a magnetic material, like steel. Perhaps my theoretical understanding is not correct.

@Jakob, thanks for your comments. It's very useful for me to know that we should not need EMI shielding.

In my OP, I think I was using the term "shielding" a little to vaguely, though. What I can specifically is that when we used the mic connected to Focusrite Scarley 2i2, we get a good bit of audible "buzz". Analyzing the signal, there is definitely a huge spike at 60 Hz. The mic motors currently have NO shielding at all. They are just naked as a jaybird on a clamp.

With this info, what do we think? Copper tape? Steel mesh? Aluminum?

Thanks again

Stuart

 

[ccaudle]

The mic motors currently have NO shielding at all. They are just naked as a jaybird on a clamp.

How are the motors wired? The way you usually see ribbon motors wired with two wires from the top connection down each side is to help with hum cancellation. I do not quite follow the interference path which makes that work, but it seems pretty effective, and pretty consistent that most commercial ribbon motors use that scheme.

What turns ratio transformer are you using? Torroid or bobbin? Shielded? Since the motor is very low impedance it is not as susceptible to electrostatic interference (what conductive shielding prevents). Since the motor is only a "single turn" it is not as susceptible to magnetic interference as the transformer, so my intuition is start at the transformer and make sure that is optimize first, then the wiring to the motor, and only then worry about electrostatic shielding.

 

[rubinstu]

@ccaudle, thanks for the thoughts.

The motors are pretty crude with the upper side having the two wires shorted wires coming down to the bottom like you described. (But my hand-built versions are fairly ugly!)

I will definitely play with shielding just the transformers to see what happens. I read somewhere that someone uses a steel coffee can for testing!

Here is the exact transformer: Geistnote Gōst™ Series Transformer GN-C36 1:36

 

[Mark mth001]

I made a Blumlein ribbon pair with a plastic tube, cut out to take the motor frames. Where it isn’t cut out, the tube carries the transformers and wiring. It’s weak like that but I wound copper mesh round the outside of that tube which shields the motors and wiring and then the whole assembly fits inside a slightly larger perforated tube. Works really well, looks cool and it’s physically robust. I use it all the time! See my vid below. I have a connector at the bottom which then goes to 2 XLRs and fetheads. You could have the fetheads anywhere but I like them near to the mic so it’s amplified before a long cable run.

Ideally you could 3D print this but this one I made around 15 years ago. If you 3D printed it you could have a mechanism to turn the mics so they are in the same plane (to set the mic gains accurately together) and then turn them back to being 90 degrees.

I love this mic because it makes it really easy to position as it’s one unit, and it’s a lovely natural sound for (say) instrument and voice. I have in the past clipped an omni to the top of the mic and used it as a horizontal ambisonics mic.

I’ve attached a photo of the inner tube during construction. It’s this one that has the mesh wound completely around it and then it’s all put inside the second tube. The metal base at the bottom of the inner tube that carries the multipin is earthed and in the final assembly process, I wound some copper tape around the bottom of the tube before pushing on that metal base, and that copper tape then contacts the mesh which is wound around the entire tube - so once it’s put together, all the motors / transformers / wiring are basically inside a faraday cage, and the whole thing slides into the perforated outer tube to keep it trapped. I had it apart today as one lead had come off and annoyed that I should have taken photos - it’s very tightly packed so I don’t want to undo it again. This video from a few days ago shows the mic but you can hear a buzz which I have now cured.