[sdiy] Magnetic interference: opinions ???

[Magnus Danielson]

From: harry <harrybissell at prodigy.net>
Subject: [sdiy] Magnetic interference: opinions ???
Date: Thu, 24 May 2001 00:14:02 -0400

> Hi Kids:

Hi Ya Ol' dude!

> I have a Hex Fuzz that I just put in a rack. It gets magnetic induced
> hum from other AC powered units in the rack.  The rack is totally
> electrically isolated from the hum sources... it IS magnetic
> induced.  Power down those units and the hum goes away...
> or move the receiver... hum goes away..
> 
> Should I
> 1) Shield the receiver
> 2) Sheild the sources
> 3) change the AC power supplies to offboard units or other types (toroid
> ???)
> 
> All boxes are aluminum... I can't find any steel enclosures...
> 
> The power supplies in question are potted brick supplies +/- 15V @ 100mA
> 
> made by Intronics  (new old stock... 20 years ???)
> 
> Thanks for the tips

The first rule of thumb in ANY EMI/EMC engineering is:

1) There's a source
2) There's a transmission path
3) There's a receiver

You can work on making the source a less efficient one,
You can work on making the tranmission path a less efficient one and
You can work on making the receiver a less efficient one.

Shielding is a very crude way of solving things. It may be worth it,
but it could just do minor assistance.

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.

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.
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.

Another things is what happends on the way things are grounded. Ground
loops can be a major killer aswell. Have you tried running with all
transformers in and out? Also, consider that powerline is part of the
curcuit.

Cheers,
Magnus