John Curl's Blowtorch preamplifier

[PMA]

They changed the area where the two parts connected to some tongue and groove-like system, giving a kind of "overlap". Originally they had this small joint, but got it covered whit the tongue and grove. Maybe EMC gaskets could be used also. ..... Lots of bolts joining the parts would also help.

RK

This all togetether makes sufficient Al connection. I have 40 screws along my box, tongues, grooves and overlapping.

 

[Vogue]

Firstly one could use bare aluminium - this does solve the problem with anodizing of the sheets.

Another idea is to make eletrical connections between the sheets using wire and copper screws. This should create one electrical unit from all the sheets used for the chassis.

Lastly one could improve the fit/connection of the sheets by using copperwire or foil in the overlaps - his should give a better connection when the sheets are bolted together (large bolts and high pressure).

Vogue

 

[AndrewT]

Hi,
what about the potential developed between dissimilar metals?
Even different alloy proportions between nearly the same metals will corrode at the junction due to this electrical potential.
Where is copper in relation to pure aluminium in the potential regime?
Where is pure aluminium cf aluminum alloy? What alloy does one have to hand? Are the exact alloying constituents declared exactly for each sheet/blank you have?

Use the same metal for the whole case and do not add another metal to the joint without knowing what is about to start corroding the instant you make the connection.

 

[ken.berg]

Hi,
what about the potential developed between dissimilar metals?
Even different alloy proportions between nearly the same metals will corrode at the junction due to this electrical potential.
Where is copper in relation to pure aluminium in the potential regime?
Where is pure aluminium cf aluminum alloy? What alloy does one have to hand? Are the exact alloying constituents declared exactly for each sheet/blank you have?

see: http://www.engineersedge.com/galvanic_capatability.htm for some context about galvanic corrosion

 

[Terry Demol]

Has anyone thought of using non magnetic Stainless Steel sheet.

It has very good RF shielding properties and is a lot cheaper than
machining Al.

Check here for comparison to Al

http://www.ramayes.com/EMI_RF_Foil_Shielding.htm

cheers

Terry

 

[PMA]

Would be good to know resistivity, rel. permeability, Bs and Hc, these parameters count.

 

[PMA]

Scientific equipment often uses ‘‘nonmagnetic’’ stainless steel, relying on the steel’s nonmagnetic
behavior to leave external magnetic fields unaltered. However, stainless steel’s permeability can rise
significantly when it is welded or machined, possibly perturbing an external field. Such
perturbations will diminish well above the stainless steel’s saturation point. The authors measured
the permeability of both welded and machined 304 stainless steel as a function of an external
magnetic field, and found that both saturate at fields of approximately 0.25 T. © 1998 American
Institute of Physics. @S0034-6748~98!00110-5#

 

[MikeBettinger]

Mike, could you please start a new thread, and shed dome light on this topic? I think there are quite a few out there whom would listen to your experience with great interest.
Magura

I doubt that I would be a good choice to start a thread on the topic. My experience is based on needing to solve problems in order to stay employed. I'm good at watching for clues and pattern recognition. After 30 years I solve problems quickly and have my own understanding of what works and doesn't. From past experience my way of describing things is more conceptual and, although I know what I'm saying, it frustrates others (and me) to discuss things in two (effectively) different lanquages and without the eye contact.

I'm not trying to dance around your suggestion, it's just that I don't mind attempting to discuss any topic at hand, which is why I throw out comments on things that I feel differently or strongly about, but I seriously doubt I could convey a bigger picture effectively.

It might help if Mike defined the frequencies that were of interest to him in the professional sense. Blocking RF if you're also interested in RF is one thing. Blocking RF if you're interested in AF is another.
Grey

In a professional sense my focus is primarily on accurately logging low frequency sensor data. It's the environment that they operate in that is challenging. So my applications would be similar to the audio spectrum and the need is to ignore the noisy world around them.

Regards, Mike.

 

[Gordy]

My idea was 20 mm overlap with the top and bottom, as the side and front should be 40 mm thickness.

But still this is proberbly not ideal.

Anyhow the Blowtorch chassis is made from 3 parts - better than my idea - 6 parts.

Well will be checking the price of a chassis like the Blowtorch - made from 100-120 mm. aluminium sheet.

I would be stupid not to go all the way!

Vogue

I intend to use these people for my next enclosure:

http://www.emachineshop.com/
http://www.emachineshop.com/users/personal.htm

Not cheap, but neither am I !

 

[Terry Demol]

Scientific equipment often uses nonmagnetic stainless steel, relying on the steels nonmagnetic
behavior to leave external magnetic fields unaltered. However, stainless steels permeability can rise
significantly when it is welded or machined, possibly perturbing an external field. Such
perturbations will diminish well above the stainless steels saturation point. The authors measured
the permeability of both welded and machined 304 stainless steel as a function of an external
magnetic field, and found that both saturate at fields of approximately 0.25 T. © 1998 American
Institute of Physics. @S0034-6748~98!00110-5#

Yes I too found that link

http://scitation.aip.org/getabs/ser...00069000010003695000001&idtype=cvips&gifs=yes

Sheet metal enclosures are easy to fabricate without welding or
machining. The only part subject to heat are the thin edge where
laser cut or work hardened when punched or sheared.

I'm still not convinced it's a real problem. There are many
advantages.

cheers

Terry

 

[john curl]

Sheet metal is nonlinear.

 

[Terry Demol]

Sheet metal is nonlinear.

In what way?

 

[PMA]

Hysteresis, B = f(H), and you seen that Bsat = 0.25T.

 

[Vogue]

Found this ... (dont know if its correct).

http://www.cebik.com/trans/hardware.html

Se the table on the original page ...

"The rate of corrosive effects between dissimilar metals depends on their "nobility." The more distant the metals on the chart (see Table 1), the greater the potential between them, even in the most weather-protected conditions. As the table notes, a difference of only +/-0.3 volts between the atomic potential of two metals at a junction indicates the strong possibility of significant corrosion at the junction."

Here is another link ...
http://www.rfi.com.au/wireless/tech-tips/dissimilar-metals.html

If correct ... then Aluminium bolts/screws should be used to avoid corrosion and ensure the best electrical connection between the aluminium sheets.

Other materials as iron and especially copper and silver will result in corrosion of the aluminium.

Vogue

 

[reinhard]

Are we here for Hifi preamp

or military application for nuclear wars?

Reinhard

 

[AndrewT]

Hi,
there are at least two possible solutions, but I don't know how effective they are.

1.) coat the joint with grease or similar to keep air and water vapour away from the junction between the dissimilar metals (just as one coats battery terminals in an automobile).

2.) bolt a block of zinc to the chassis to act as a sacrificial anode. Call it a vibration damper so that others never hear about the alledged problem.

 

[courage]

Not really related to the last posts, but I either missed something or my reading is bad , but I have not seen John or anyone else mention the perfect type of Mosfets (second stage Blowtorch) absolete or not.

Although John mentioned the IRF 530/9530 as a suitable replacement, it can hardly be considered as the best replacement equal to or perfectly matching the performance of the 2SK389/2SJ109.

The Mosfets originally used in the second stage of the Blowtorch may still be available..... or not?

So suggestions John or anyone?

Thanks,

Courage

 

[john curl]

The parts used in the Blowtorch second stage are the Fairchild IRF510 and IRF9510. Nothing special. The main factor is to be able to heatsink the device effectively.
Supertex once made a better part pair for this, but no more.
Same with Hitachi.
I am now designing with Hitachi 2SK214, 2SJ77 parts for the same service, but they are hard to find as well, unless you have your own personal stock.

 

[jacco vermeulen]

The TO220 Sutex VN/VP02 series you mean ?

 

[john curl]

That is one good example.