Isolation transformer hum

[Koolaide]

Well I completed the enclosure and wiring for the IT. It is a wooden enclosure primaries attached direct to power cord (with wire nuts)and secondary's attached directly to a standard 110v outlet. The outlet is mounted to 2 metal L brackets bolted to wood base. No grounding. I connected to variac today and as I turn up the voltage hum is there and increases. Note the variac is grounded. Shop has florescents overhead. I did not notice this hum when I tested the IT - plugged into variac with wires and jumpers and connected to meter with jumpers?????
Should I be concerned?

[Koolaide]

Just to clarify above post the output voltage is about 1.5% of input voltage just as in original test. Test were without load. Hum is not bad but it is there. I suspect all is OK.

[Phil_S]

Line the enclosure with tin foil. Apply a light layer of ordinary glue. Lay foil on the glue. Rub with a rag. This will shield the IT. Of course, it could easily be something else picking up the florescent light. When I am finished with an amp, I attach foil to cardboard to cover the open side during testing. Then I line the bottom of the cab with foil. It's cheap and it's easy to do.

[xtian]

Are you talking about hum you're hearing from the speaker, or mechanical noise emitted by vibration?

[Koolaide]

Are you talking about hum you're hearing from the speaker, or mechanical noise emitted by vibration?

No speaker connected. Great question!
I would assume or mechanical noise emitted by vibration. It does change slightly when I touch the transformer, Seems to lessen. I turned off the lights, and the hum (if you will) is still there. Again it is slight, but clearly audible. Should I be concerned?
Thanks for the input Aaron.

[martin manning]

Maybe your wooden enclosure is amplifying the normal hum coming from the transformer?

[Koolaide]

You could be right Martin. Think I'll pull the enclosure off and see. The transformer is mounted to a wooden base and the enclosure fits over the top.

[Koolaide]

pulled enclosure, it helped slightly. But still there. Shield with a metal plate no change. The question remains.... to be continued. I'll work on it some more tomorrow.

[xtian]

All transformers vibrate mechanically, some worse than others. Sometimes you can mitigate by applying more torque to the fasteners.

[Koolaide]

I tightened the laminations (very little)- decided this is normal transformer noise. Thanks all for the input.

Now on to installing a digital meter on my variac. The saga continues.

[didit]

One can strip away mounting and bells, and pot the core in a can. Most effective hum reduction though adds costs and makes any repairing an ugly mess.

Best .. Ian

[Koolaide]

Thanks Ian,
I considered potting it, but as you note it could get messy. As for end bells, there aren't any.

On a side note, I finished the Variac mod, and an on to building an amp cradle. Think I'll finish that later today or tomorrow.

[Phil_S]

For mechanical vibration, you might try mounting on rubber grommets.

[Koolaide]

hadn't thought of that one. Thanks Phil.

[Phil_S]

Those cone shaped plumbing washers, relatively firm black plastic/rubber, should do nicely if you have nothing else. You can probably get them for $1 at the big box store.