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Old 16th March 2013, 09:01 AM   #1
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Default PCB and transformer placing and shielding

Hey guys.

I am working on the enclosure i plan for the Octal CCDA kit i ordered and i have a couple of question.

Click the image to open in full size.

as you can see in the picture, the main PCB seats dead center and the power transformer seats to its left, with a metal barrier between them for shielding. the Pot and selector switch will be placed on the panel to the right of the PCB with shafts running to the front.

is this configuration "dangerous" given the distance between the transformer and PCB? am i just creating HUM problems for myself even with the barrier? would a Faraday cage on the transformer be enough to solve it or do i HAVE to move the PCB all the way to the other side of the enclosure?

thank you for your help
Tank out.
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Old 16th March 2013, 11:51 AM   #2
Loren42 is offline Loren42  United States
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Looks fine. I did the same thing for my preamp. I actually purchased a metal box from Digikey to shield the inputs and it holds a relay selector PC board. Belts and suspenders.

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Old 16th March 2013, 12:27 PM   #3
jcx is offline jcx  United States
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Default air space and Iron

low carbon steel (cheap Iron) is an OK magnetic "conductor" ~ 100-1000x better than air- you can "short out" the leakage mag field lines that want to complete loops in air external to your transformer by making a box around the PS parts - perforated sheet lets air flow for heat management

a single flat sheet not much taller than the transformer won't be perfect - but closed 6 sided box also isn't absolutely necessary - U shape may be OK

and use air gaps too - mount the xfmr on non-magnetic standoffs if mechanically attached to the Iron shield sheet metal - may need mechanical damping if the xfmr leakage field makes the sheet vibrate = direct sound output

real paranoia/severe environments would use multiple Iron boxes - a 2nd around the amp board - but audiophiles worry about running signal near ferromagnetic material - lead dress, standoffs, and good pairing, twisting of wiring helps

hobbyist's often jump on Mu Metal - it is seldom competently applied - it saturates quite easily and should only be used where mag fields are already very low - cheap Iron is the 1st layer of defense - and usually all that is needed

very thick Al, Cu is needed to "reflect" 50/60 Hz mag field - wil cost lots more than Fe

Last edited by jcx; 16th March 2013 at 12:37 PM.
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Old 16th March 2013, 12:34 PM   #4
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by jcx View Post
.................. closed 6 sided box also isn't absolutely necessary - U shape may be OK

and use air gaps too - mount the xfmr on non-magnetic standoffs if mechanically attached to the Iron shield sheet metal

real paranoia/severe environments would use multiple Iron boxes - a 2nd around the amp board ..........
A Faraday cage must completely enclose the emitter/receiver to be effective.

Does the same apply when considering magnetic fields?
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Old 16th March 2013, 01:00 PM   #5
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my initial plan was to just throw a square piece of Fe in there, make sure it mechanicaly touches the upper pannel and floor (to close a circuit) and thats it.. but rust is a concern, so can i use stainless steel for that?

also, using U shape? how about this:
Click the image to open in full size.

and what about the wood panel? should i cover it with metal to shield it as well?
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Old 16th March 2013, 01:05 PM   #6
jcx is offline jcx  United States
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physics notions of ideal or prefect aren't required, are never practically attainable

"effectiveness" has dimensions, thresholds in engineering - knocking a 60 dB SPL 50/60 Hz hum down by 20-30 dB will usually be enough in practice for home listening - even if the signal would still look poor by instrumentation standards

magnetic fields are all dipole (or higher) and fall off ~ 1/r^3 - so you can say mag fields really don't "like" traveling in air
this means you can get pretty effective "shadowing" with good mag field conductor and air spacing - just avoid the edges where the field is concentrated
the field lines from the open sides of the box will be really attenuated by the longer air path, much preferring to short through the mag conductive shield rather than loop around it



popper: are the top and bottom box covers Iron or Aluminum?

and leakage field will pass right through wood sides, thin non-ferrous metal - if your preamp is right against that side of your amp box then it could see the leakage

stainless may be poor or even completely non-magnetic - just use paint, plating if you are after some asthetic that prevents painting

Last edited by jcx; 16th March 2013 at 01:15 PM.
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Old 16th March 2013, 01:39 PM   #7
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well, i was planning to make this from stainless steel (laser cut)..

it seems that covering the inside of the wood pannels with Fe will be a good idea also...
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Old 16th March 2013, 02:00 PM   #8
jcx is offline jcx  United States
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probably wouldn't want to couple mag field to mag conductive cover sheet that stretches over your amp board too

better to have a complete sub box for ps

"old school" linear ps:
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Last edited by jcx; 16th March 2013 at 02:27 PM.
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Old 16th March 2013, 05:24 PM   #9
gootee is offline gootee  United States
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At least as important as shielding the transformer will be making sure the pairs of conductors to and from it are not good transmitting antennas, and that the input pairs are not good receiving antennas. To do that, keep each pair always close together, and tightly twisted. Input pairs could also be shielded twisted pairs, with shield connected to chassis ground at input end only.
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Old 16th March 2013, 10:24 PM   #10
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Hi Tankpopper - PT orientation can also help. The magnetic lines of force exit/enter the transformer in-line with the laminations in an E-core transformer. The magnetic field is lowest off the ends of the laminations, and highest along the face of the windings.

Turing your PT so that the laminations "point" towards the rest of the amplifier, if you can. This will minimize the magnetic coupling. It is especially important to rotate the OPT so that the laminations are oriented 90 degrees to the PT laminations - again, to minimize magnetic coupling between the transformers.

Good luck! (and kudos on the chassis drawings - looks good!)
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