Steel RACO boxes and magnetically induced hum

[zpyder]

What are standard RACO boxes made of?   Steel?  I read here: http://www.hubbellcatalog.com/raco/RACO_boxes.asp?FAM=RacoBoxes that they are indeed made of steel.  Anyone know better?

I recently read in R.G.'s article "Star Grounding in Tube Amplifiers" (http://www.geofex.com/Article_Folders/stargnd/stargnd.htm), that not only is steel not the best conductor, but that it is also ferromagnetic - that is, it absorbs magnetic fields.  This seems to present a hum threat... I've been using RACO boxes to house my pedals since they're cheap, already have some holes, and are heavy frickin duty - but now I'm wondering if this is bad practice in general to house in a RACO or any other steel chassis...

I wonder if anyone has had hum problems with a circuit in a RACO box and been able to solve it by putting it into a different enclosure...?

I wonder if anyone wants to tell me "Chill out moron!  That article was about tube amps which carry very large voltages and produce a lot of gain.  Your pedal is well outside the range of......"

cheers,
zpyder

[R.G.]

Ferromagnetic materials  - iron, nickel, cobalt, and most of their alloys like especially steels - are best thought of as conductors of magnetic fields.

Copper is much more than ten to the 13th power a better conductor of electrons than air is.That's more than ten trilliion times. Steel is only a factor of thousands to tens of thousands of times better conductor of magnetic fields than air or vacuum. There are no magnetic insulators in the same sense that there are electrical insulators. Free space "conducts" a magnetic field fairly easily.

What this means is that steels form an easier place for M-fields to "flow" than air or vacuum; the field would rather be in the steel than in the air, but it's not perfect. In tube amps, where there is an output transformer with many turns of wire on a nice, magnetically soft iron core, a steel chassis can conduct in hum from the stray fields caused by AC power lines and we can hear it as hum.

If there is no strong source of AC magnetic field, like in effects powered by batteries or DC, then a steel shell tends to "conduct" any stray field around the effect circuit, and actually does some shielding from external fields.

If the effect has a big coil of wire - like a wah inductor - then a steel chassis can conduct M-field in if there are strong AC fields caused by the effect. It can also help shield against external fields. Which it does best depends on the details of the field and where it comes from. If an effect has only the normal circuitry, no many-turned coils, then a steel shield is overall more of a shield than a channeler. It's not likely that a Raco box would make hum worse. The worst case would be an AC power line power supply inside a  wah chassis. Then aluminum would be a better choice.

[Paul Perry (Frostwave)]

If anyone actually needs to sheild something from a magnetic field, a Raco box - or any steel box - isn't likely to make them happy. Back in the Old Days, we had mu-metal shields that were a special alloy that was very good at this (if you didn't bend it or otherwise disrupt the grain structure). No wonder people like to design out coils & chokes..

[R.G.]

Dead correct, Paul.

Mu-metal is a special alloy concocted especially for magnetic shielding. For those who are not accustomed to EE-speak, "mu" is the Greek letter used to represent the magnetic multiple of permeability of a material over that of free space, it's M-field "conductivity". Vacuum and gasses have a mu value of one. Ferromagnetic materials have mu values up to several thousand. I believe that mu-metal is the highest mu material that is not a special laboratory conconction.

From Wikipedia:

Mu-metal is a nickel-iron alloy (75% nickel, 15% iron, plus copper and molybdenum) that has a very high magnetic permeability. Permeability is represented by μ, the Greek letter mu.

The high permeability makes mu-metal very effective at screening static or low-frequency magnetic fields, which cannot be attenuated by other methods.[1]

Mu metal requires special heat treatment — annealing in hydrogen atmosphere, which reportedly increases the magnetic permeability about 40 times. The annealing alters the material's crystal structure, aligning the grains and removing some impurities, especially carbon. Mechanical treatment may disrupt the material's grain alignment, leading to drop of permeability in the affected areas, which can be restored by repeating of the hydrogen annealing step.

You REALLY have to need shielding to be willing to jump through the hoops needed to use mu-metal effectively.

Soft iron, preferably with up to 4% silicon, is about the cheapest high-mu material. Not surprisingly, this is what transformer and motor laminations are made from. High mu and low hysteresis losses seem to be associated with physical softness, as annealed irons have higher permeabilities and lower hysteresis losses than the same material tha has been hardened. Iron and its alloys like most metals will work-harden. That is, if you bend or even hit it, the material hardens a bit. I was told by a grizzled old verteran transformer guy that it's possible to detect a difference in magnetic properties in a transformer that's been dropped on the floor even once.