[sdiy] Magnetic Bug squashed !

[harry]

Hi Magnus:

Thanks for the comments.

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. 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...

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...
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. 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).

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)

H^)  harry

Magnus Danielson wrote:

> From: harry <harrybissell at prodigy.net>
> Subject: [sdiy] Magnetic Bug squashed !
> Date: Sat, 26 May 2001 01:38:58 -0400
>
> > Thanks all for the tips. The magnetic bug is now dead. As some pointed
> > out... it could be a grounding bug. It was BOTH... the magnetic hum was
> > masking a more subtle ground loop problem.
>
> I think I hinted you towards that. In my experience it is typical, you
> have a full suite of problems and you need to figure out the worst
> ones before you go ahead and learn about the others...
>
> I've gotten a little bit out of practice, but I used to be pretty good
> at it once... (naturally in my own view).
>
> > This was a rack mounted "360 Systems Slavedriver"... a semi-useless
> > Guitar/CV interface. I use it for the string select logic, with mods...
> > to get
> > a low note priority input to a custom octave dropper / bass synth. As
> > practice
> > starts in 10 hours (morning)... I had to fix it right away.
> >
> > I removed one power supply and used a wall-wart. Then I opened the 360
> > box... it had a 723 based dual tracking supply... with a shitty little
> > 24VCT
> > E-I core tranny... that was the major source of mag field... much more
> > than
> > the potted block supply. So I used the potted block (+/-15) to supply
> > the
> > 723... eliminating half the AC in the box.
> >
> > The I isolated the PCB from the box... I use nylon shoulder washers to
> > insulate
> > the 1/4" jacks... XLR connectors with the shield floating. The only
> > ground to the system is now at the output lead. Hum gone.
>
> Depending on your transformer this might or might not be
> good. Transformers have capacitive leakage which will make the
> secondary coils to experience a signal insertion relative to primary
> coil 0/chassi. This could be a source of trouble. Fortunately, this is
> at a very high impedance so tossing in a 250 kOhm resistor between
> chassi and signal ground can be sufficient to reduce this noise alot.
> I've seen boxes where this noise would reprogram the DSP from FIR to
> IIR and have the whole thing selfoscillate ;o)
>
> It took some searching to find, but was obvious when I figured it out.
> I really like all transformers to have double individual layers of
> conductive isolation foil since then, since that will give a very
> effective tool on handling these problems.
>
> > This also eliminated an isolation transformer in the bass output...
> > which SCREAMS
> > "I've got a ground loop problem" don't it. Should have listened
> > earlier....
>
> You only learn just too late what your actual problems where, don't you?
>
> > Still, its nice to kill that bug after 20 years of buzzing...
>
> Right.
>
> > Thanks again all....
>
> You are most wellcome!
>
> Cheers,
> Magnus