John Curl's Blowtorch preamplifier

[JPV]

We have explained this round and round, but people do not care. Thick aluminium or copper also shield against LF magnetic field, skin effect - eddy currents. Depends on thickness and frequency, of course. Thick conductive, non-magnetic material does not saturate.

Plastic with metal powder sprayed cover is almost useless, just better than nothing.

Of course there is always attenuation but at what level:

1 inch thick of aluminium has an absorption loss of less than 20db and 2 inches gives 40 db absorption loss at power frequency. This is low in presence of high lf magnetic field.

To achieve high attenuation it is good design to use layers: first a good conductors and then high permeability. You can then achieve good attenuation with reasonnable thickness without saturating the magnetic material but it remains tricky and it is geometry dependent
This is my experience at least.



JPV

 

[john curl]

I once had a car with only 32 horsepower. I went 5 years and 95,000 miles with it. That is the equivalent of tin foil or conductive spray on plastic. It was barely enough, but it sure beat walking.
Now, should WE all revert to 32 HP, or even to 2CV in order to save this planet some needed resources? It is the same argument. Perhaps any hp over 32hp or 5CV should need special authorization from the government.

 

[Edmond Stuart]

Dauphine?

 

[JPV]

For RFI ( and that is the subject of my comments) a plastic case spray coated but without openings is better than a thick aluminium box with slits.

JPV

 

[PMA]

I have been designing optical fibre transmission based instruments for this and similar customers

http://www.zku.cz/index_a.html

for more than 20 years. Transmitters work near to VHV and UHV circuit breakers and measure switching phenomena. Shielding is the most important issue. Thick aluminium or copper outer box + thinnner wall Al or Cu inner box was found as the best solution. 2 years ago customer insisted to try inner iron housing. It failed in comparison, due to non-linear Fe magnetic behaviour.

 

[john curl]

Thanks for bringing up Fe nonlinearity, PMA. For some reason, many here don't think it possible or important. That is what separates mid fi from real hi end.

 

[scott wurcer]

Sorry to hit any hot buttons, I was simply commenting on several hundred years of accumulated experience helping customers with applying low power circuits. Telling them to turn up the input bias is not really an option.

 

[JPV]

Thanks for bringing up Fe nonlinearity, PMA. For some reason, many here don't think it possible or important. That is what separates mid fi from real hi end.

Are you saying that using high permeability materials in shielding against low frequency is mid fi because on non linear variation of permeability with field intensity?

JPV

 

[PMA]

Non ferromagnetic metal housing is better for audio.

 

[hitsware]

Why do they still use (I think) steel chassis for tube amps ?
(As opposed to aluminum for SS)

 

[JPV]

Non ferromagnetic metal housing is better for audio.

Any explanation ?

JPV

 

[Wavebourn]

Why do they still use (I think) steel chassis for tube amps ?
(As opposed to aluminum for SS)

I use both, and hear no difference.

 

[john curl]

It has been found that Fe chassis or connectors sound bad. We use a magnet to evaluate. There have been papers written about it, but it is difficult to measure. My only REAL measurement was a muMetal connector cover that I had hoped would help me with hum shielding. I COULD measure it when used. SY turned me on to simple aluminum foil, which worked pretty well in my test bed, BUT I was using a 5KHz tone, measuring up to 50KHz, and had an 18dB hi pass filter at 400Hz. I would not need any hum reduction.
Of course, you don't have to believe me, but others tend to regard my best efforts with enthusiasm.

 

[john curl]

PMA, you were right about the JC-2. Oversight. Your fix is our fix now. Thanks.

 

[hitsware]

I use both, and hear no difference.

Helluva lot easier to drill a hole in aluminum
You use aluminum for tube circuits ?

 

[john curl]

Stick with steel, amateurs!

 

[Wavebourn]

Helluva lot easier to drill a hole in aluminum
You use aluminum for tube circuits ?

Yes. It is easier to punch and drill. Also, it is lighter. For hardness ribs were invented long time ago, so no problem with heavy iron trannies on thin perforated aluminium chassis.

 

[hitsware]

>heavy iron trannies on thin perforated aluminium chassis.

You think that's the reason for the steel with tubes tradition?
SS xfmrs are just as heavy (but you've usually only 1 as opposed to output xfmrs and maybe a choke also)

 

[Wavebourn]

>heavy iron trannies on thin perforated aluminium chassis.

You think that's the reason for the steel with tubes tradition?
SS xfmrs are just as heavy (but you've usually only 1 as opposed to output xfmrs and maybe a choke also)

I think, the keyword is "Tradition". Like since Lansing made first speaker for the Cinematograph the majority of PA speakers today have the same design, almost millennium later.

 

[hitsware]

Stick with steel, amateurs!

REAL Amateurs cut up circuit board into panels and solder them together as the walls of the 'chassis'.