Shielding for Power Transformer Induced Hum

[Bret <echoplex@yahoo.com>]

Kim is right on with the defense for his position.  

Regarding the hum influence of one rack device's transformer to
another, I agree with Kim that this is bad design. 

However, you may find some mitigation by using so called mumetal, a
soft iron alloy with high nickel content (85%), iron (15%) and other
metals.  I have used this type of material for electro magnetic
interference with some success on in reducing emi in inductive tape
heads (in cassette players) from electrically noisey automobile
dashboards and ignition systems.  It is useful for a broad range of
frequencies of emi.
read more at
http://www.magnetic-shield.com/faq.html
and read about a developmenter/experimenters kit at
http://www.magnetic-shield.com/labkit.html
You can demonstrate to yourself the ability of nickel to divert
magnetic fields.
Take 2 same size magnets that will repel each other when oriented
against each other horizontally, and that you can cover pretty well
with 1 or 2 USA nickel coins.  These coins don't have nearly the nickel
content, as mumetal, but they will show the base effect.

Place one magnet on a non iron table top.  Take the other same size
magnet and orient it so that it will repel against the magnet on the
table.  You will feel the force against the magnet in your hand as it
nears the magnet on the table.  Now, place 1 or 2 nickels on the magnet
on the table and bring the repelling hand magnet near the table magnet.
 You will notice that the repelling force between the 2 magnets feels
much weaker.  

It feels and sounds is as though it 'blocks' the magnetic field, but I
my understanding is that it actually 'dirverts' the field.
bret
--- Kim Flint <kflint@loopers-delight.com> wrote:
> At 03:25 PM 1/25/2002, Alex Stahl wrote:
> >Just to play devil's advocate, my goal for some thirty years has
> been to 
> >run my entire studio off of one or a couple DC voltages, supplied at
> home 
> >by a single power supply resembling a wall more than a wart (PV
> panels in 
> >the cabin) and on the road by car batteries or whatever. Equipment
> which 
> >already has DC input jacks makes it slightly easier to pretend I'm
> ever 
> >going to complete this transition.
> 
> A fine idea, and in fact that is often how telco and data center
> racks 
> work. You generally have 48V DC running to each device. The DC supply
> is 
> generated from a power supply shelf where you'll have fully redundant
> power 
> supply modules converting the AC supply from the power company into
> the 48V 
> DC. That's the sort of environment the networking system I'm
> currently 
> designing goes into, so we don't need to deal with AC conversion. One
> key 
> reason that is possible is that a standard exists for this power, and
> every 
> device follows it and there are many vendors for the external
> supplies. 
> Such a standard doesn't exist for music gear, so you'll probably
> never 
> easily accomplish your goal. Of course, in the stuff I'm doing at the
> 
> moment, the power shelf alone probably costs far more than most home
> studios...
> 
> 
> >It's also just not true that wall warts necessarily radiate more hum
> into 
> >audio paths than internal power supplies.
> 
> I think your statement should read "internal power supplies don't 
> necessarily radiate less hum than wall warts".
> 
> If the internal supply is badly designed, than sure it will cause a 
> problem. However, the problem is "bad design", not "internal". A well
> 
> designed internal supply will not have such a problem, and will
> always beat 
> a wall wart for performance.
> 
> 
> >The transformer inside one very expensive piece of gear I own is
> directly 
> >beneath the mic preamp in another unit which I would like to have
> mounted 
> >in an adjacent rack space. Oh well, not much I can do about that.
> 
> that's bad design. You the customer shouldn't tolerate that
> designer's 
> choice any more than a choice to use a wall wart.
> 
> 
> >The internal power supply in another very popular multi-effects
> processor 
> >is a horribly noisy piece of crap which not only generates more 60Hz
> than 
> >a PG&E substation but only puts out more RF than Sutro Tower. Oh
> well, not 
> >much I can do about that either.
> 
> again, bad design. what you do about it is don't buy crap like that,
> and 
> tell the manufacturer the reason why.
> 
> My low-end Mackie 1202 mixer has an internal power supply and it
> causes no 
> hum in the audio that I can hear. In fact, it's my belief that the
> internal 
> supply is part of the reason the Mackie is as clean as it is for the
> price. 
> It clearly didn't add too much to the cost, since the mixer was cheap
> 
> enough to be an impulse buy for me. That's because the engineer who 
> designed it did a good job.
> 
> The power supply is an incredibly important part of any audio gear.
> You the 
> customer should realize that and it should carry much more weight in
> your 
> buying decisions than it apparently does for most people.
> 
> 
> >At least with a wall wart it's really simple to relocate the hum 
> >generator, keep all the AC in the bottom of the rack, or even
> replace the 
> >power supply with something cleaner than any manufacturer puts
> inside 
> >their gear if it's still a problem.
> 
> I don't find that simple. I find it to be a pain in the ass actually
> that I 
> should ever have to consider these options.
> 
> 
> >Oh yeah, there's a decent chance that the first thing to fail in
> your rack 
> >will be a power supply, and it's kinda cool to be able to carry
> cheap 
> >spares and replace them in 30 seconds.
> 
> That's also design. A well designed supply will take reliability into
> 
> account, and it will will outlast most other parts of the system. The
> worst 
> thing you have to worry about with a good internal supply is finding
> a 
> standard IEC power cable or a fuse, either of which can be found 
> practically anywhere in the world on short notice.
> 
> All of the power problems I ever have are wall warts either breaking
> or 
> getting lost, or a device getting fried because the wrong wall wart
> got 
> plugged into it. Wall warts are designed for sitting behind desks
> where 
> they never move. They usually have wimpy wires and wimpy connectors
> and not 
> very rugged construction that was never intended for the real world
> that a 
> musician will put it in. So they break easily. Plus, every company
> seems to 
> use a different wall wart, so when it breaks it's often hard to find
> a 
> replacement. Oftentimes you can only get it by special order from the
> 
> manufacturer! You don't have those headaches with proper internal
> supplies.
> 
> >Like I said, just playing devil's advocate. My only real pet peeve
> is the 
> >Yamaha stuff which has an internal supply but a hardwired, way too
> long, 
> >too stiff, unrepairable AC cord.
> 
> that's also bad design, because in addition to what you mention it
> also 
> means you can't switch plugs when you take it to another country with
> 
> different power standards. bad, bad, bad. all supplies should use the
> 
> standard IEC connector and support worldwide power voltages.
> 
> kim
> 
> 
> >At 10:17 PM -0800 1/25/02, max valentino wrote:
> >>Absolutely right, Kim....thanks for continuing to say it.  Who
> knows 
> >>maybe someone will listen!
> >>Max
> >>
> >>
> >>  > there is no excuse for wall warts in products intended for
> >>>professional
> >>>audio use.
> 
>
______________________________________________________________________
> Kim Flint                     | Looper's Delight
> kflint@loopers-delight.com    | http://www.loopers-delight.com
> 


__________________________________________________
Do You Yahoo!?
Great stuff seeking new owners in Yahoo! Auctions! 
http://auctions.yahoo.com