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Old 12th October 2004, 12:09 AM   #1
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Default Shielding preamp power supply

Hello,

I'm building an all tube phono preamp with power supply on separate PCB. I was told that putting everything in the same case would cause significant noise on the MC inputs, and that I would be advisable to use separate cases.

I was wondering where the noise would be from, the transformer or the whole tube rectifier/regulator PCB?

I would really like to put everything in the same case and would rather shield the noise source. What should I shield?

Also what kind of wire should I use from the low level MC inputs to the RCA jacks on the case? Should it be shielded or twisted?

Thank you,
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Old 12th October 2004, 02:32 AM   #2
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The biggest source of radiation is the transformer and (if you've got 'em) chokes. The mains cord and associated circuitry don't help either. It is quite worthwhile to move them outboard; for my preamp, I have the raw DC supplies outboard and the regulators in the preamp case.

Shielded versus twisted is a big "it depends" and a smaller "they'll both work fine."
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Old 12th October 2004, 06:21 AM   #3
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Nothing beats physical distance from the above mentioned noise radiators/generators too. I put all my preamps in big, non-magnetic cabnets (wood, aluminum or pure brass or copper) with the power supply components bunched at a rear corner and the circuit in the diagonally opposite one.
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Old 12th October 2004, 03:17 PM   #4
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Default Non-metallic

Why non-metallic, does it help not conducting the magnetic noise?

I was thinking of putting the power transformer in a metal box, inside the larger case, as far as possible from the circuit.

(PS:thanks for previous posts)
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Old 12th October 2004, 04:25 PM   #5
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Quote:
Why non-metallic,
You may get a bit of hum but the sound is also better
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Old 13th October 2004, 03:32 AM   #6
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Hi,

Quote:
Why non-metallic, does it help not conducting the magnetic noise?
Yes, it does.
Unfortunately sometimes it's just not that easy for a DIYer to find a layout where you can arrive at good enough compromise between very good sound not using metal enclosures and not picking up all kinds of noises either.

IME putting the MC headamp in a dedicated enclosure close to the PU leadout wires and shielding the powerxformers so they don't radiate into the amplification stages is a good compromise.

For the MM-level phonopreamp an outboard supply is often a good way to reduce noise pickup.
With careful layout and stargrounding it should be dead quiet even when used with the lid removed.
As for long umbillical cords, just make sure they're not the cause of resistive losses themselves as I sometimes see low heater voltages caused by too long cords.
If these cords only carry low currents slightly twisting the various conductors should suffice to keep the noise down.

I always use solid core wire (usually Tefzel or solid PTFE insulated silver) of the smallest diameter I can get away with_hence my remark on resistive losses_ for best sonic results and lowest noise.
Multistrand can be tricky (microphony) to use with very low level circuits such as MC stages and is harder to use especially with P2P wiring where you'll want the wiring harnesses to stay put.

Cheers,
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Old 13th October 2004, 05:33 AM   #7
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Quote:
Originally posted by Geek
.... non-magnetic cabnets (wood, aluminum or pure brass or copper)
I said non-magnetic, not non-metallic
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Old 13th October 2004, 02:13 PM   #8
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Default Shielding materials

Quote:
Originally posted by Geek


I said non-magnetic, not non-metallic
Geek,
I agree with you.But if you won't your T/X or/and chokes to radiate in their vicinity and induce some interference it is a MUST to shield it with a magnetic materials which are efective at EMI and for a RFI it is a non-magnetic material which is efective (Cu,Al) .Only a slight view in any tuner front end will make clear this issue.Weren't TI who designed alloy and produced metal sheets with one side with magnetic and other side with non-magnetic material which Counterpoint used many years ago for their top products!? Or am I missing smth.?

Regards,
Yugovitz
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Old 13th October 2004, 04:03 PM   #9
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Default Re: Shielding materials

Hi Yugo

Quote:
Originally posted by Yugo
Weren't TI who designed alloy and produced metal sheets with one side with magnetic and other side with non-magnetic material which Counterpoint used many years ago for their top products!?
You are right. I recall an issue of Electronics-Now reporting on someone that made such stuff. I don't remember the issue, but it was within a few issues of another similar announcement, when 3M made conductive tape that conducted along its thickness, not length or width. That'll give an indication of year at least.
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Old 14th October 2004, 07:18 AM   #10
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One way to minimize magnetic radiation from the power transformer is to use a toroid design. Admittedly, it transmits more noise from teh mains supply into the secondary windings but that can be suppressed.
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