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Old 10th January 2013, 10:20 AM   #1001
FdW is offline FdW  Netherlands
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Originally Posted by PMA View Post
A small correction, 1M and below
Also, it is a good indication for the need to put the prereg in a separate metal enclosure The question is, how far are you (the builder) prepared to go .
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Old 10th January 2013, 10:32 AM   #1002
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My sim has not included any additional passive filtering. In case you have some RC's in a final design, it will help a lot.

Regarding metal enclosure, I support you totally. I almost cry when I see the diy-builders put the pcb boards into wooden boxes, or free in the air without any shielding. Such high gain and sensitive circuits. I cannot understand it, for the reason that all the efforts may be lost when using unshielded case or wrong wiring... Such is DIY
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Old 10th January 2013, 10:41 AM   #1003
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Is anything gained by putting prereg and xformers in a seperate space within same box with only aluminum plate dividing the two spaces?
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Old 10th January 2013, 10:41 AM   #1004
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And floating DC at the output is also gone?

Chris
I wish It's a separate issue altogether.

So far it seems possible to significantly reduce it by keeping the input transistors' temperature very steady. Thin copper foil thermally glued to all of them may do the job. It's just a pity they are not facing each other in a more compact formation on the board. Not sure if using MAT02/03 will be possible but may be worth investigating.

As for the suggestions to encase the boards airtight, it would be great if it wasn't for all the heat dissipated by the shunts. Ventilation is a must.

If it was just a question of slowly drifting offset i would just ignore it, but this is more akin to infra low frequency noise. It just cannot be good for sonics.
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Old 10th January 2013, 10:57 AM   #1005
FdW is offline FdW  Netherlands
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Originally Posted by buzzforb View Post
Is anything gained by putting prereg and xformers in a seperate space within same box with only aluminum plate dividing the two spaces?
Yes, use a box in the box for the prereg. Do not ground it (the prereg box) (not needed) just keep it isolated from the rest of the electronics and mechanical parts, mount it using Teflon bolts/nuts. It will be a cage-of-Faraday and it will stop most HF leaking from the prereg to the rest of the circuits.

Also, you should put the transformer in its own box, and if possible this box should be placed in the prereg box, so now we have a box-in-a-box-in-a-box This box also should be isolated from all other circuits and/or mechanical parts.

The mains connector should be mounted directly on the transformer box and the cord should have a metal shield that may be attached to the transformer box and sticks out of the main enclosure not touching any other parts of the construction.

Any way, you can go as far as you like, implementing a box-in-the-box-in-the-box schema goes pretty far I have never seen it done (commercially).
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Old 10th January 2013, 11:09 AM   #1006
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Any way, you can go as far as you like, implementing a box-in-the-box-in-the-box schema goes pretty far I have never seen it done (commercially).
That being said, shielded transformer in a power supply having its own enclosure is a common sight. More so for they noise they are generating than picking up, though.....

Comments about the arrangements are very good. Generally, it is good practice to put an EMI filter right at the power cable input of the power supply, and these usually come in metal packages so nothing unshielded can propagate towards the inside of the case. A shielded pre-regulator in a separate box is your best bet for high-gain, low-noise amplifiers (PMA your comments are very true....)

In my pre-reg, I built my own EMI filter, it is two-stage (higher damping for RF than one-stage), component values adjusted for the characteristic impedance to have good impedance matching and no ringing, and not too big components to have high self-resonance frequency. In other words, there is more to EMI filter design than meets the eye

Building your own EMI filter also allows you to separate the grounds right at the input, I use two back-to-back power diodes (and Nelson Pass and a few others do similar things), to reduce / avoid contamination through the ground - that way, you can control the ground loops rather than trying to fight them.
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Old 10th January 2013, 11:12 AM   #1007
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Perhaps a layout of your idea would be helpful, if you have the time. I am in the midst of both the paradise and Salas Jfet RIAA and want to help them both achieve full potential. It might help others as well.
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Old 10th January 2013, 11:19 AM   #1008
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Perhaps a layout of your idea would be helpful, if you have the time. I am in the midst of both the paradise and Salas Jfet RIAA and want to help them both achieve full potential. It might help others as well.
Stay tuned for an update of the assembly guide, then
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Old 10th January 2013, 12:26 PM   #1009
PMA is offline PMA  Europe
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Quote:
Originally Posted by FdW View Post
Yes, use a box in the box for the prereg. Do not ground it (the prereg box) (not needed) just keep it isolated from the rest of the electronics and mechanical parts, mount it using Teflon bolts/nuts. It will be a cage-of-Faraday and it will stop most HF leaking from the prereg to the rest of the circuits.
Are you sure .....
Normally the shielding box should be connected in one point with board ground, reference 0V or something like this. In case you leave metal box floating, it will NOT serve as a Faraday cage. It will act as an antenna and it will couple the EMI noise received on its surface thru coupling capacitance (just geometrically defined) into the circuit board (do not forget you have the wires going IN and OUT of the box). If you connect the box with board reference ground, then it WILL act as a Faraday shield.
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Old 10th January 2013, 12:26 PM   #1010
Waly is offline Waly  United Kingdom
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Originally Posted by analog_sa View Post
I wish It's a separate issue altogether.
Get used with it, or AC couple the pre.
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