Help ! on reducing transformer hum !

[fikri]

I'am designing a hybrid distortion box project that including a SS and a tube circuit running on high voltage (somekind like a H&K Tubeman or a Mesa Boogie V-Twin). It is almost finished when i start to put on the DPDT switch, and it hums. The problem is, i'm using two transformer (back to back) put all in one enclosure, and the switch is positioned between the two transformer. I think this is the best positioning that i can do, since the enclosure has a limited space. And i dont want to change the position (because i have done all the drilling !). Can you suggest something to reduce the transformer humming ?. O, and i have use a shielded cable too.

Thanks !

[puretube]

try rotating 1 xfmr by 180 degrees... - maybe you can get some cancellation ("outphasing") -

[fikri]

Thanks man ! i will try your suggestion.

[puretube]

err, note: maybe some cancellation...

[frickecello]

Make an aluminum enclosure for the transformer and put it inside, dont forget to ground this enclosure...

[R.G.]

Rule number one: Don't put leaky line voltage transformers inside tight, small boxes with low level audio signals and a lot of amplification.

Rule number two: if you have to use two leaky line voltage transformers inside a tight, small box with low level audio signals and a lot of amplification, don't put all of your audio switching directly between the two transformers.

There may not be a whole lot you can do. Getting immunity from high level magnetic fields at close proximity is very, very difficult. The best positioning for those transformers is in another box, a good distance away. You have a bad situation set up there.

[fikri]

There may not be a whole lot you can do. Getting immunity from high level magnetic fields at close proximity is very, very difficult. The best positioning for those transformers is in another box, a good distance away. You have a bad situation set up there.

Yes it is  Finally i throw away the box and the transformer, and get myself a bigger box and a high voltage toroidal transformer, what a nice (and expensive) way of getting rid of the hums !  But it works VERY NICE !!!

[ubersam]

How about some mu-metal, as in magnet shields - http://www.lessemf.com/mag-shld.html - I used a combination of magnetic shielding foil and magnet shield to contain the EMF in my Furman AR1215 which had a huge toroidal xformer. Worked very well.

[R.G.]

Finally i throw away the box and the transformer, and get myself a bigger box and a high voltage toroidal transformer, what a nice (and expensive) way of getting rid of the hums !  Grin But it works VERY NICE !!!

I'm sorry it was expensive, but you could have worked a long time to get the hum out of what you were describing.

I used a combination of magnetic shielding foil and magnet shield to contain the EMF in my Furman AR1215 which had a huge toroidal xformer. Worked very well.

Having a toroidal transformers already cut the emitted M-field by a couple of orders of magnitude, so it's a much smaller problem to start with.

Mu metal and other shields are good but they are generally not something you can put on after the fact and get big reductions in hum. They help modest problems.

I have seen engineers try to shield out M-fields where they could not change the enclosure for other reasons. Some of them succeeded. Some never did.