[sdiy] Magnetic interference: opinions ???

[Magnus Danielson]

From: "Happy Harry" <paia2720 at hotmail.com>
Subject: Re: [sdiy] Magnetic interference: opinions ???
Date: Thu, 24 May 2001 15:36:38 -0000

> Mind yer elders now...  ;^)  inline

But WHY? ;o)

> >A good way to check if you have a electrostatic or electromagnetic
> >major field is to change the impedance in the receiving loop. If the
> >disturbance goes up as you reduces the sumimpedance you have a
> >predominantly electromagnetic field where as if it goes down you have
> >a predominantly electrostatic field. You can get similar results by
> >increasing the sum impedance in the ring. If you know what kind of
> >field you are dealing with you can then work on the appropriate
> >measure as such. Changing the impedance of an loop can be one valid
> >solutions at times.
> 
> Electrostatic would be an inefficient method of coupling, as each
> unit is in a separate, complete closed box... only very high frequencies can 
> pass here.

Right... but please recall that I started of in a VERY generic
discussion as I worked by discussion down to your problem. I've
noticed that sometimes this joggles the senses and ideas spurs off to
think in a diffrent maner on the issue.

> >I'd guess that you have troubles with electromagnetic fields when
> >considering the frequency (60 Hz) and that you have large lumps of
> >transducers (transformers) that operate at that frequency.
> 
> Yes that's right
> 
> >By the sound of it, it seems like your curcuit is a very sensitive
> >receiver since you seem to pick up disturbances from not only one
> >source but many. This would lead me to look into where in that curcuit
> >you pick up the disturbance.
> 
> As a typical "fuzzbox" it has very high gain. Input wiring could be
> slightly better... but I want a quick solution as I'm designing the
> replacement circuit now... it will not be ready for quite some time
> so the old one is being pressed into more service...

Hmm... high gain you say... then you really should care about having
things in the propper order. If you have electromagnetic fields
causing troubles, reducing the loop area of your receiving antenna
might do the trick. If your input wiring take diffrent paths, then
they will form part of the open loop. Coaxial mounting is one solution
since both conductors will have the same center position. However,
that is sensitive to ground loops which can't be canceled out. Another
method is twisting the pairs tightly together so that the laternating
polarities of each little loop area causes cancelation. This is a
trick that works well for low frequency electromagnetic fields.
Yeat again ground loops should be avoided.

Oh, have you measured the chassi ground to signal ground voltage?
Use a high impedance voltage meter!
If you've got high potential there you might consider bringing it down.

> >Another things is what happends on the way things are grounded. Ground
> >loops can be a major killer aswell.
> 
> I can get the hum with the other units on, and no electrical connection at 
> all. As you unpower the AC transformers in the
> other units... hum level drops (to acceptable levels)

How do you monitor that? By earphones or? Battery powered earphone amp
can be a real big help in these situations, since you can monitor
signal levels without upset the curcuit as much as you may do with AC
powered equipment.

> Have you tried running with all
> >transformers in and out?
> 
> I don't understand in and out... like power on and off.. or do
> you mean audio isolation transformers ?  (did both of those)

Yeah, isolational transformers on anything going in and out - and
cutting the ground cords!

> Also, consider that powerline is part of the
> >curcuit.
> 
> Ahhh... so I need batteries !!! ;^)

Maybe that could help you, maybe not!

Cheers,
Magnus