[Arzela]

I'm considering buying an Elpac
wall wart to power my Meta.
They seem to sell thier models in either
a standard grade or a medical grade.
Is the medical grade worth the additional
price (aproximately twice as much as the standard)?
In particular, would the medical grade be
better at filtering out RF/line noise?

 

[jeffreyj]

Medical grade PSUs usually let more noise from the power line in because they have have to meet extremely rigid leakage current specs. The upshot of this is that no capacitors can be connected from line to ground, only line to line. Common mode noise, then, will sail right past the line to line capacitor where it will be wholly up to the common-mode inductor to suppress.

Another potential problem is the requirement for extra insulation between primary and secondary on the transformer. This degrades regulation, increases leakage inductance (no relation to leakage current) and generally makes it harder to reject noise (spikes, harmonic trash, etc.) from the line.

In other words, don't go medical!

 

[Arzela]

Thanks Jeffrey, I'll go non-medical.
From your response, I gather that
Medical grade power supplys
are designed so that they do not
introduce noise into the AC line?


At the risk of imposing, could you
suggest a source for info on how
to suppress line noise? The AC
in my house is extremely dirty

I'm hoping there is a simple
circuit that I could put in between
the DC source and my amp to get rid
of line noise (or perhaps from the wall AC
to the transformer).

 

[jeffreyj]

Quote:

Originally posted by Arzela Thanks Jeffrey, I'll go non-medical. From your response, I gather that Medical grade power supplys are designed so that they do not introduce noise into the AC line?

You're quite welcome, Arzela.

The idea behind medical-grade supply design is to eliminate current flowing in the ground under any condition (except faults, of course). Conflicting assumptions are made that ground is "sacred", yet it absolutely cannot be relied on to conduct leakage currents away. If the ground connection at the wall socket is open in a regular supply, for instance, and line-to-ground capacitors are installed for noise suppression, then a voltage divider will be formed with "ground" at the midpoint. Not much current can be drawn through a 0.01uF capacitor at 60Hz, but it's definitely enough to kill you if the connection is through an EKG electrode applied with conductive jelly to your chest! Because line-to-ground capacitors are verboten and rules concerning allowable EMI from power supplies are getting stricter worldwide, the medical supply is usually an already-quiet linear one, not a switcher (though it is certainly possible to make a switcher quiet enough to pass EMI specs without a line noise filter - it's just expensive to do so!)

Another common feature of medical supplies is Faraday shielding of the transformer. This is very useful in preventing the transformer's interwinding capacitance from allowing fast rise time spikes to pass through it. Now that I've thought about it some more, I've realized that a blanket recommendation to not use medical-grade PSU's might not be useful or even accurate - they are still more expensive, though, and mainly because of the leakage-current specification (and the testing required to secure agency approval of said spec!).

Quote:

At the risk of imposing, could you suggest a source for info on how to suppress line noise? The AC in my house is extremely dirty

Oh, you don't really want to know this, do you? One of my books on switching power supply design has a whole chapter dedicated to the line noise filter - about 75 pages or so! I highly recommend buying one of Corcom's power entry modules with EMI/RFI filtering built-in. Select one for a load current as close as possible (without going under!) - this way you'll get the most common mode inductance (ie - noise suppression) for the amount of current allowed.

added the following links:

Core Selection for EMI Filters


Common Mode Filter Design Software

 

[Voodoochile]

Electrical characteristics aside, the medical-grade stuff is just plain more durable, and that might be the principal difference on some goods. Meant to hold up to severe intern use.

Nice, but not neccesarily the best for audio use.