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Coping with the stray-field monster

7/5/2000 9:14 AM
Dominik Coping with the stray-field monster
Hi there,  
I have a problem with my Dumple preamp clone. I use an American Standard Fender Strat and I get some nasty stray-fields when I play next to the clone. The noise gets better when I move around some, but even at a distance of 3m I get those darn noises when in overdrive mode. The chassis is all-steel (3mm) and the tranny is situated inside the 19" chassis. The tranny has no shielding covers and it is bolted to the chassis by means of those star-shaped washers and security washers plus scre and nut. I have the same phenomenon when I use a different Strat. Switching to in-between pickup position helps a lot. The noise is the almost gone. I know that this problem is fairly normal. But I wonder what I could do to reduce the amount of background noise. The amp itself is almost noiseless when I use a CD-player for signal purposes. Seems like the tranny emits a strong electromagnetic field. Maybe I could shield the tranny with mu metal or aluminum??  
Grateful for any input on this!  
Best Regards,  
7/5/2000 11:19 AM
Ray Ivers

On page 4-16 of 'The Ultimate Tone' Volume 1, Kevin O'Connor recommends screening the inside of the amp box with steel mesh screen to minimize the radiation of stray fields. He says to cover the back panel as well as the remaining five interior panels of the box, making sure to have the screen section overlap and contact chassis ground. I've never done this, but I imagine it will improve your hum situation.  
On the other hand, if your amp is just a chassis... you could try shielding the transformer, but if your transformer is now INSIDE the chassis, it might be easier to just make a steel bottom panel for the chassis and see what that does; also, if the chassis has holes in it you might want to try covering them.  
And, of course, the best fix is humbucking pickups or EMG's, provided they give you the tone you want.  
Ray Ivers
7/5/2000 12:31 PM
Stephen Conner

Single-coil pickups are stupidly sensitive to magnetic fields and will buzz when they are anywhere near anything with a transformer in it. There's not a lot you can do about it, especially if your transformer is already inside a steel chassis.  
You could try adding a belly band to the transformer: a thick strip of copper that wraps right round the whole transformer, parallel to the windings. The ends are soldered together and it acts as a shorted turn, forcing stray flux back inside the core. It's difficult to describe, but a lot of stereos have them.  
Steve C.
7/5/2000 12:50 PM
Thanks to Ray and Steve!  
I have seen the copper-band solution. I will think about going that road, but I guess that it would be difficult to get the copper onto the transformer. Maybe I could build a box of copper around the tranny inside the chassis.  
Anyway, thanks for the input!  
7/5/2000 1:43 PM

The belly band needs to be as close to the transformer as possible, and also needs to be reasonably thick.  
Once you have a belly band on, a second shield layer of soft steel will help even more. It is very, very difficult to shield out low frequency magnetic fields. Input transformers sometimes use as many as seven layers of alternating copper and iron to cut down on induced M-field.
7/5/2000 4:47 PM
Stephen Conner

I pulled a power transformer out an old IBM green monitor that had a belly band and then three layers of steel sheet. I guess they needed it to stop the stray field making the screen shake.  
Steve C.
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