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Lack of power supply shielding on high-end gear Equalisers (HW)
Old 4th March 2018
Gear Maniac

Lack of power supply shielding on high-end gear

Hi all,

Was taking a look through the summing mixer thread and it struck me.... almost none of these units have proper shielding on their power supplies! (Neve being an exception... maybe another I didn't catch).

What are your thoughts on this? Based on my limited knowledge, power supply shielding is kind of basic audio design.

Is this an oversight or maybe these power supplies are being designed in such a way that it's not needed?

Definitely googling the interior layout of any new stuff I get!

Old 4th March 2018
Gear Head

This is a bold statement, do you mean linear or switch mode?
You do understand PSU design and function?
Old 4th March 2018
Gear Maniac

well that's why I'm asking! Not claiming any grand amount of knowledge here... but I thought it always made sense to shield the PSU with some sort of cage to prevent RF leakage.

Am I wrong? Happy to learn!
Old 4th March 2018
Perform a noise vs frequency sweep on a graphical analyzer and you can see it if it's there. I usually see 120 and 250 hz when I use an Audio Precision. -100 db is common. Only a few boxes with on-board power supplies are clean to -125 dbu. The Meyer CP-10 EQ is one, it has over 1/4" of steel separating the power supply.

This applies to linear power supplies. Switchers can be all over the place too, usually with HF hash rather than hum.
Old 5th March 2018
Lives for gear

Equally important is local filtering, down to capacitor selection and placement near ICs, etc. Nearly a whole science, or at least a lot of empirical knowledge.

Power supply and power distribution down to component level must be considered all together as full part of the circuit design process and also from the very beginning (includes also carefully designed grounding schemes).

As such power supply and distribution don't enhance audio quality but design flaws can massively degrade performance.
Old 5th March 2018
You shouldn't assume that a design is lacking just because you don't see lots of sheet metal. Shielding is just one of several methods that can be used to exclude power supply noise from sensitive circuits. When applying it, one needs to distinguish between electric shielding and magnetic shielding, with the latter being much more difficult and costly to achieve. Personally, I consider shielding of either sort to be an expensive last resort to be applied only when other methods fail. These are my preferred techniques:
  • Careful design of the power supply
  • Physical positioning of supply relative to sensitive circuitry
  • Choice of switchmode over linear supplies
  • Use of shielded or torroidal magnetics
  • Clock synchronization, so ripple appears as DC
  • Proper layout of sensitive circuitry with special attention to loop areas

I routinely build CE-marked equipment with housings of injection-molded plastic. It is precision measurement gear intended for use in highly corrosive environments. It has really forced me to up my game in terms of design for electromagnetic compliance.
Old 5th March 2018
Gear Maniac

Interesting stuff - thanks for the education!
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