[sdiy] Magnetic interference: opinions ???

Happy Harry paia2720 at hotmail.com
Thu May 24 17:36:38 CEST 2001


Mind yer elders now...  ;^)  inline


>From: Magnus Danielson <cfmd at swipnet.se>
>To: harrybissell at prodigy.net
>CC: synth-diy at node12b53.a2000.nl
>Subject: Re: [sdiy] Magnetic interference: opinions ???
>Date: Thu, 24 May 2001 17:35:25 +0200
>
>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.

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.
>
>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...
>
>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)


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)


Also, consider that powerline is part of the
>curcuit.

Ahhh... so I need batteries !!! ;^)

H^) harry
>
>Cheers,
>Magnus
>

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