Effects of metal enclosures on circuits

DNM, NVA and others use acrylic for their enclosures.

Other than aesthetic considerations, what do people feel are the benefits of using this or other non-metalic enclosures, and are there any negatives?

Perhaps we can try again

 

One fairly obvious advantage of a metal enclosure is its EMI/RFI rejection, and also its compliance of radiation of either.

 

There is far more field generated inside an amp then outside it. A steel case doesn't allow that field out (or not in significant quantities). So what happens to it, it doesnt just dissapear, it reflects off the case and bounces around, cutting the field that is newly created and mixing with the audio signal to partly turn it to mud, i.e. dirt on the window. Alluminium is much better but still retains some field and is conductive so field currents can run around it so it needs an earth strap, and that earth strap is also 0v so it gets on there. Next stage is insulate the panels of the case from each other, so now we are getting somewhere, but the back panel still has to be earth strapped. Next stage is a case that doesn't conduct or block field so you can move to a much easier and better sounding form of construction known as double insulated and you no longer need the earth strap, so 0v can float. There two good generally available materials for this, acrylic (or as most people call it Perspex) or wood. Wood has the downside of being inflamable where as acrylic just warps and at extremes just starts to melt, it takes almost as much temperature to catch fire as alluminium does, which is well outside the possible temp rise in a amp frying.

 

I remember back in 90s seeing Denis Morecroft debunk the commonly held view that aluminium is 'non-magnetic'.

He had two V shaped highly polished runners and rolled a small cylindrical magnet down each in turn. One was fashioned from aluminium and the other was acrylic.
The magnet hit the floor far quicker when rolling down the acrylic shaft, while there were clearly magnetic forces being induced into the aluminium by the spinning magnet.


<edit> Crossed with Richard's post - but nicely compliments the points.

 

That is quite often impurities, but as with all metals pure alluminium is next to impossible to create.

How many people realise the Space Shuttle fuel is alluminium dust (modified) as a slurry and oxygen. If you let it catch fire then like sodium and potasium you can't watch it.

 

I remember reading about this when a youngster and thinking it odd at the time.

Are we talking of electric or magnetic fields, or both? Is the supposition that noise is induced back into the critical circuits? If this happens is it likely to be at detectable levels?

Do boxes with switch mode power supplies suffer more?

Is the effect more critical when digital and analogue circuits share the same space?

Has any serious scientific research been done in this area, or is it just one of those potty HiFi fancies?

 

Yer! I am going to write a paper on it to the AES why should I waste my bloody time, people hear it or they don't (shrug!)

 

What's all that about? I posed a pertinent series of questions. Another might be:

Do thermionic valve circuits respond differently to semiconductors?

There is so much pseudo-science and just plain daft quackery about that I would be very pleased if a serious study produced a reliable answer to the question.

 

Rob you can do the same thing dropping a disc shaped magnet down an ally tube, I forget the name of the scientist who discovered it. Aluminium in none magnetic but it does reflects field, very different properties and well understoodif somewhat harder to find on wikipedia.

 

It's a long time since I read physics, but I remembered correctly. Aluminium is magnetic, just not very magnetic:

http://info.ee.surrey.ac.uk/Workshop/advice/coils/mu/#diamag

See paramagnetic

Edit:

But look at the relative strength of the effect in Fe and Al.

We could properly ask the question "Does the earth wobble in it's orbit round the sun when I jump up and down?" Well Newton's laws will allow me to predict by how much - but the much is so tiny it is effectively zero.

Are these effects on HiFi performance (if they exist) of similar insignificant magnitude? I really don't know.

 

Why when you have a pair of ears one each side of your head.

 

I don't think my ears are that reliable, and there are behavioural experiments that show them not to be so.

But the question about the effects of case material on the performance of the electronics in it is not question about human perception: it is about the behaviour of electrons.

I am quite prepared to accept there is an effect, but show me using a rigorous scientific method. I sit here musing that the particles in the solar wind might affect the performance of computer chips if they strike the semi conductor junctions at just the wrong time; and I wonder if there have been computing errors because of the (poosible) effect; but I have never gone looking for any research.

 

If a piece of equipment is acrylic cased what is done to make it pass EMI/RFI regulations?

 

Then do it yourself, as all I want to do is use my ears. I have no interest in proving it to anyone. I have proved it to myself and that is enough. Having done the work nearly 30 years ago I really can't be bothered about other peoples opinions. They can believe or not believe as is their want.

And if any of you have read any AES papers and the replies they tend to prove nothing, but just create even more egotistical arguments. So just listen to music, if it sounds better it is better, and you can enjoy the process, not this bizarre mental self masturbation.

 

I will throw another seemingly silly thing at you. Do any of you own an amp (or CD player) with an anodised alluminium case. So now spray the outside of the amp with WD40 and wipe it around thinly, listen to music before you do it, listen again to the same music after, leave for an hour and listen to that music again. Let me know your results.

 

The effect is very real - and I should think easy to demonstrate (measure) that there is some effect on audio circuits when placed inside a metal case. Induction certainly exists when we want it, and therefore it will often be present when we don't.

The question boils down to 'can you hear it?'
If you can, under what circumstances does the effect become audible, ie with power amplifiers with high circulating current, line level circuits, with all metals etc etc.

How you listen is up to the individual. If you want something rigorous, just place the amp in an adjacent room using long speaker cables and get someone to play it alternately inside and outside the casing. Do it enough times and you should see a pattern emerge.
Can't say I've ever done any detailed listening for the effects.

 

Might try it!

Easy enough to remove the case on my 72. Might see if YNWOAN can hear the difference. I can record it too but my recording skills might not be able to show any difference.

 

Wow! that really works! John Bonhams squeaky bass drum pedal on "Since I've Been Loving You" is completely quiet now!

 

Well lets put it this way no valve amp ever made would pass it or even CE, nor would a large percentage of the other audio industry products. So virtually everyone who is a bit dubius self certificates and lets others bother about it.

I approach the problem by being a double insulated design and using a ground plain, on which I self certificate.

But if they decide to get awkward you wont be able to buy any new valve amps, along with quite a few others in the industry, especially imports, including some large companies.

Thankfully the government and its "jobs worths" couldn't seem to give a shit either.

 

I will give you a clue WD40 is anhydrous so it drives away any moisture, even the most minute amounts, up to a few years ago it stayed that way even when dry until it wore off, then they changed the formula so now when it dries it stops being anhydrous.