John Curl's Blowtorch preamplifier part II

[RNMarsh]

Originally Posted by fas42

What distance from steel reduces the effect to audibly zero? Find that, make a cube of thin steel with double those dimensions and use spacers to position the components to be in the middle of the cube.

May look awkward, but certainly cheaper ... !

Frank

The answere is about 2-3 cm distance -- signal carrying wires from ferrous metals. -RNM

[jneutron]

Originally Posted by gpapag

Jneutron
I really hadn’t noticed this part of your post.
I am surprised that I see H and B changing roles.

That's because I normally work air and non ferrous. Sorry bout that..

You are indeed being more accurate. For a magnetic circuit, such as a gapped core, H will indeed be constant through the loop, whereas B will be dependent on mu..

j

[john curl]

Well, it seems that no affordable 'breakthroughs' have been made with shielding. Kind of disappointing, but it might reinstate why the CTC Blowtorch preamp cases were so thick and made with aluminum. Copper or silver would cost much more, of course.
To understand or estimate shielding thickness, shielding articles or reference books by OTT are very useful, for people who want to understand WHY aluminum shielding has to be so relatively thick.

[Steve Eddy]

Originally Posted by john curl

To understand or estimate shielding thickness, shielding articles or reference books by OTT are very useful, for people who want to understand WHY aluminum shielding has to be so relatively thick.

Simple. The permeability of aluminum is about the same as that of air, so it can only shield against magnetic fields through eddy current losses. The thicker the material, the greater the losses.

se

[jneutron]

Originally Posted by Steve Eddy

Simple. The permeability of aluminum is about the same as that of air, so it can only shield against magnetic fields through eddy current losses. The thicker the material, the greater the losses.

se

Yabut...do you really understand...

It's only been stated about a bazillion times here..

j

[S.A.G.]

John,

for an "out of the box" approach to chassis and enclosure design you may want to have a look at what HP did in the 90's here:

http://www.hpl.hp.com/hpjournal/94aug/aug94a2.pdf

Should be affordable even to DIYers...

Regards

Giorgio

[bcarso]

Wavebourn mentioned the notion of multiple perforated nonferrous nested shields a while back. Has any one had experience with those, or is in a position to analyze their effectiveness?

Of course for magnetic fields the most important thing is loop area. After that is made as small as practicable, orientation and distance. With a given piece though, once the best job is done with the internals, one is at the mercy of the end user.

The sense of value conveyed by the mass of the equipment is also quite significant to the end user's perceptions. I recall a business associate hefting a small powered loudspeaker and openly scoffing at how light it was. He had not heard of neodymium magnets and judged things from his experience with ceramic or alnico magnet motors.

I will say that having heavier speakers does help to prevent them from bouncing around as much. I'm about to use double-sticky tape on some ones on my desktop in lieu of weighting them down with tungsten.

[Steve Eddy]

Originally Posted by jneutron

Yabut...do you really understand...

Don't need to understand it. Just have to have the right books sitting on my bookshelf.

se

[jneutron]

Originally Posted by S.A.G.

John,

for an "out of the box" approach to chassis and enclosure design you may want to have a look at what HP did in the 90's here:

http://www.hpl.hp.com/hpjournal/94aug/aug94a2.pdf

Should be affordable even to DIYers...

Regards

Giorgio

From the looks of it, that packaging doesn't shield magnetically, nor even electric field shielding.

Originally Posted by bcarso

Wavebourn mentioned the notion of multiple perforated nonferrous nested shields a while back. Has any one had experience with those, or is in a position to analyze their effectiveness?

Use ferrous nesting. Layers of ferrous material will behave anisotropically.

The layers closest to the source will try their darndest to divert the field through that layer. Gaps of mu=1 will tend to isolate the steel layers. If you had 50% packing factor of sheets mu=1000 with equal thickness paper sheets, then the effective mu along the sheets would be 500, but normal to the sheets it's very close to 1.(well, 2).

But one of the biggest offenders with respect to chassis is control of the current paths.

Originally Posted by Steve Eddy

Don't need to understand it. Just have to have the right books sitting on my bookshelf.

se

Got that right..

How's it goin steve?

jn

[Steve Eddy]

Originally Posted by jneutron

Got that right..

How's it goin steve?

Busy busy busy. You?

se