[sdiy] Magnetic Bug squashed !

[harry]

The deal with my center tapped mains transformer... is balanced power. the
idea is that the 120V is actually a center-tapped 60V and that noise will tend
to cancel out.  I read some claims about this and decided to try it. It worked
a little bit... but my studio is usually very quiet anyway (AC power on the
floor...
signals on the ceiling).

I would NOT do this if this was not a captive studio where I am the only one who
may TOUCH a wiring point !   The transformer I use is big enough to run the whole
studio... but if this was a commercial venue there would be bigger transformers, or

many of them, or both...

The "other" consideration for the hum removal would have been properly designed
differential inputs and outputs... which would eliminate all that bullsh!t and work
better. But this was another "rush job".

Beware: some "commercial" manufacturers are hackers like ourselves, with poor
knowledge about safety and proper wiring practices. I have been injured more
than once with stupid shit like "hot" chassis and two wire (non-polarized) plugs.
And SOME idiots (360 systems pro midi bass, for one...) think that their unit will
never ne in a rack, and tied to other units... even though they provide rack ears
and
jacks seemingly for this purpose. They used the chassis as the heatsink for the 5V
regulator... with NO isolation. Needless to say that didn't work well on a system
level.

I laughed for hours while cursing them and anyone they ever rode in a elevator with
!!!

H^) harry



Magnus Danielson wrote:

> From: harry <harrybissell at prodigy.net>
> Subject: Re: [sdiy] Magnetic Bug squashed !
> Date: Sat, 26 May 2001 23:54:44 -0400
>
> > Hi Magnus:
>
> Hi Harry,
>
> > Thanks for the comments.
>
> Anytime!
>
> > I'm using the nylon shoulder washer trick almost all the time now... The idea
> > is that I control whan and where a ground occurs... not the random chance
> > of the jacks themselves.
>
> Certainly a good and valid point - in audio!
> I've seen people trying the same idea in RF environment (say signals
> of several 100 MHz) - not a good idea!
>
> > They make nylon 1/4 jacks but I hate them. They strip out way too
> > easy. So I'm using a flat and a shoulder washer, and the steel hardware.
> > Only a problem on a really thick panel...
>
> Yeah, right. It takes some time to find a solution which is both
> sturdy as well as electrically functional.
>
> > The chassis actually gets its ground now from the rack itself... I've found
> > that eliminating all but one ground in the system is usually the lowest noise
> > solution...
>
> Why do you think this is the prefered way of doing things in say
> telephone stations? ;o)
>
> Well, actually, I think that you should not fiddle with the ground
> connections on the power side at all. But, you should not rely on
> those grounds for providing a quite ground for your audio signal.
>
> > I prefer the audio cables for that point... since you usually want those
> > grounds anyway. But IF you are using AC mains powered equipment its
> > a real good idea to have a resistive or capacitive ground for
> > safety.
>
> Indeed! Fiddeling with the way the safety wireing in gear works just
> shows that you are not looking at it with a professionals eyes. You
> avoid the problem but create new ones. There should be a DC path and
> there should be means of prohibiting buildup of large voltages and
> currents. All this is really a field of its own. There are good books
> written about it.
>
> > I know that a major fault will weld the audio cables in place (I
> > don't care... they will conduct enough current to
> > trip the breaker... eventually).
>
> This only shows another missconception people have, it is not the
> overvoltage which kills many things, its the overcurrent which kills
> them! In the end you really must consider the amount of energy
> exposure (leading to overtemperatures) at points.
>
> You don't want to learn how ridiculously small currents it takes to
> make the heart give up.
>
> My point, as well as Harrys, is that don't compromise the safety of
> gear just to reduce noise, there are usually much better ways of doing
> things.
>
> > The shielded transformer is a good thing too... I run my AC mains through a
> > shielded
> > Toroid isolation transformer... a 120V input / 120VCT output. The center tap
> > goes
> > to a separate ground rod for the studio. VERY quiet... BUT you must not run
> > equipment on the normal 120V (hot-neutral) and connect THAT to the system...
> > or
> > you BLOW it (literally)
>
> Indeed. Indeed. When you mess with such things, you are out on a very
> dangerous path.
>
> What you can do is to use an isolational transformer to restore a
> sound relationship between the ground and the live/zero. You don't
> actually have to replace the ground since you now have isolated away
> the potential diffrances between them. Thus, the ground-to-zero
> voltage becomes zero again (as it should be from your main fuse).
>
> Unluckilly there exist no real standard for how to do all this and all
> kinds of solutions exists. That people tend to run unbalanced signals
> all over is also not much of a help.
>
> I wrote a little thing on grounding over at SAS, maybe it's time to
> translate it to english and drop it on this list...
>
> Cheers,
> Magnus - connecting grounds