Shielding with Mumetal and Copper

[Deke609]

Thanks Doc!

Exactly what are you trying to shield, and from what kind of noise?

I am coming at this from a place of curiosity, a wish to make something neat, and a double dose of ignorance: I neither know whether there is shield-attenuable causes of unwanted noise, nor, if there is, what kind    But with the knowledge that a combination of copper and mu metal should have me covered for a fairly wide spectrum of radiated interference, it's easy to make some copper and mu metal straws, see if they do anything, and if they do, then go about the hard part (for me) of trying to figure out in respect of what and how.  It's really just for fun.  All those images of thin mu metal blocking magnetic bands are pretty cool. And, although considerably more expensive than a roll of copper foil, a 1 meter roll of 2 inch wide mu metal foil is fairly cheap (I paid $30 CDN - probably costs a lot less in the States) and goes a long way when you're only building straws and maybe a little "hut" for the PT. It's made here in Canada, and the manufacturer claims it's EMI attenuating properties hold up well to bending. 

Re overkill: yeah, on further thought, a three layer mu metal straw shooting for 90dB's or so of attenuation of something that may not even exist is definitely overkill. I'll just do two layers: 1 mu metal, 1 copper.  And I'll ground the copper for EMI shielding as you suggest.

And I'll do the flux band that PB turned me on to.

@PB: yes, the top plate is wedged in against the copper and rests on top of a thin copper lip that runs the majority of the way around -- so lots of good contact.  The copper gets warm - although some of that heat must be coming from the bottom flaps of copper that are folded around the bottom of the base and up against the lower inch or so of the inside.  Bonus: I think the heat is accelerating the oxidation of the copper - it's starting to brown a bit, which is something I want. It's too shiny right now.   

cheers and thanks,

Derek

[Deke609]

How can it touch the chassis and still allow the chassis plate to drop into the base?

Without the copper, there's a bit of wiggle room between the chassis and the outer edge of the base's routered groove.  The copper is 30 gauge (approx 1/4 mm), and soft, and so works as a deformable shim that wedges the chassis into the base fairly tightly (see attached crappy pic). But I can still pop the chassis out with a two finger push of the chassis from below. If you used 26 gauge (0.016"), you might need to shave the top groove a tad wider - a few passes of a dremel with a sanding drum would probably do the trick, and then maybe a utility knife for the corners.  You wouldn't need to shave much.  But 26 gauge would be harder to cut to a nice edge.

cheers,

Derek

[Deke609]

I'll add, just in case someone is considering doing the copper cladding thing: the copper 90 deg bends and flat surfaces are a lot less lumpy in real life than the previous pic of the corner suggests.  The shiny copper catches the light in a way that accentuates small variations. Once the copper browns a bit, that won't be the case.

[Paul Birkeland]

Those solder terminals could be easily shorted if a DIY band migrated.

Yeah, you need to put the flux band on, then varnish over it and the transformer to keep it in place.

Also,you probably couldn't use the bell end without modification, not to mention the hole in the chassis would have to be resized or the trans would need to be elevated to keep the band from touching the chassis.

It isn't the end of the world if the band touches the chassis.  The bell end will work just fine, just take it off when you put the flux band on, then install the cover back over the band.

Sounds like a lot of work for very little improvement. I'm in! This wouldn't be the first time I signed up for something like this.

Don't forget that it will look cool!

[Jamier]

OK. So what gauge copper would be required? Could you unwind a 12 or 14 Ga foil inductor and use a strip of that? 1 layer, or more than? I must be crazy to be considering this.

Jamie

[Paul Birkeland]

Unwinding a foil inductor isn't a bad idea.

Copper slug and snail tape for your garden would work fine as well.  Soldering to it may not be all that fun, but it's possible (consider dipping one end in a solder pot if you have one.

[Deke609]

Something like this will be a good transformer to use for picking up hum:
https://www.ebay.com/itm/UTC-O-25-150-600-2K-Audio-MIC-LINE-Transformers-UTC-0-25/223173687434?hash=item33f630388a:g:-0sAAOSwJjNbtMBf
I tried to use a pair of similar transformers as LOMC step-ups and they hummed almost everywhere I tried them.

PB: I don't understand. Just a guess: Are you saying that I could use a transformer like this to check for EMI?  If so, does passing the transformer through a magnetic field cause it to hum?  All by itself, without being wired to anything?  Or did you mean that I could power the thing and then play around with ways of minimizing its hum?

Many thanks,

Derek

[Jamier]

PB, I have a 14ga. Inductor I can unwind. Is 1 turn enough? 2? That stuff is easy to solder. Would an earth ground be helpful? I guess I might as well if I'm going this far. I don't think this would be happening if Caucasian Blackplate was still around.By the  way, How much is a new PT if I screw this up?

Jamie

[Paul Birkeland]

PB: I don't understand. Just a guess: Are you saying that I could use a transformer like this to check for EMI?  If so, does passing the transformer through a magnetic field cause it to hum?  All by itself, without being wired to anything?  Or did you mean that I could power the thing and then play around with ways of minimizing its hum?

A step-up transformer hooked up to a moving magnet phono preamp playing through your system will pick up magnetic hum.  If you have no wires connected to it, you will have no way to measure hum. 

[Paul Birkeland]

PB, I have a 14ga. Inductor I can unwind. Is 1 turn enough? 2? That stuff is easy to solder. Would an earth ground be helpful? By the  way, How much is a new PT if I screw this up?

I can't ever remember seeing a shorting ring that was more than one layer.  Yes, solder a wire to it on the bottom side of the transformer and run that to the safety ground of the amp.  A new Stereomour power transformer is quite expensive, but you aren't really taking any risks unless you skip the varnishing step.

[Doc B.]

does passing the transformer through a magnetic field cause it to hum

No, but putting a small signal transformer in an alternating magnetic field can cause it to hum. Once again I want to emphasize that you have to deal with specifics regarding noise issues.

Is 1 turn enough?

By definition a shorting ring is one shorted turn of material.

[Jamier]

PB or Doc, Should the band run the full width of the Bobbin? I'm thinking yes.

Jamie

[Paul Birkeland]

I would keep it at least 1/4" away from each row of solder lugs.

[Deke609]

Thanks Doc. I was just guessing that the particular transformer linked to by PB might act as I imagine a really sensitive relay would if moved back and forth though a magnetic field. 

I am going to wait until I rewire my PTs for 8 ohms before playing with the shielding stuff on the outside chance that the added gain will make things more measurable. I currently have the PTs configured for 2 ohms with my LCD 4s. But I could use a bit of gain. I'm guessing that the switch to 8 ohms might produce some audible hum.  There is no hum at 2 ohms with the DC Filament.

In the meantime I will make some new power cords. I want to see if I can hear differences among a stock cord, the BH kit cord, and a BH-inspired cord with tinned copper braid shielding, with the shield wired to ground only at the mains power end.
cheers,

Derek

[Paul Birkeland]

Again, hum you will hear from an amp with directly heated tubes like the Stereomour will not be magnetic (unless there are major layout issues).  The noise floor of the tubes themselves will dominate what is audible and what is measurable.  Switching to 8 ohms will increase this noise.