About Hum ... shielding for input and or output transformer.

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Adky

Well-known member
Joined
Jun 14, 2004
Messages
175
Location
Hong Kong
Any suggestions on how to block off Hum...... from power transformer or any hum... that got got transmit to the input or output transformer thur air... well... I have to move it atleast 6 inch or a foot away of the box to stop the hum.... also tried making shield or blocking plates out of Aluminum , Copper , Metel.... still not too much help , and aluminum does no effect at all .......
any suggestions?
 
Make sure that is where the hum is getting in first.
Jumper the sec of the input xfmr and listen for noise.
If it disappears, you are getting hum through the input transformer.
I would give up on shielding the pwr trans.

Instead, focus on the input trans.

You can:

Get a better one as far as hum rejection
Move the one you have
Add a mu/copper/mu/copper/mu shield around the input area.
Fiddle with the taps and shield lugs on the input transformer
Give up and try the 1950's way: cancel it in the line amp by installing a hum balance pot on the heaters.
This works, but not always that good.

What Not to do:

Add a 0.1 uf cap from input transformer to ground.
 
well... it is just at testing stage.(PTP) box in a 19" case.. and basically now it lead me to think that the input transformer are just junk right now... it has a very bad hum rejection is a 150ohm to 15K input trans ...
as sson as I move it closer to the circut or any near to the power trans it start to hum.... closer the louder ................ maybe just dumpster item :D
 
I just finished my dual G-Pultec and I also got a litte hum.
Not that noticeable, but it is still there.
Here are some picts that I snapped when I was building. It shows the transformer placement.
PT_PT2.JPG


Will it help to cover/shield them with a metalbox to reduce the hum?
Or do you have any other suggestions?
 
after all the trial and error , I will think move the power supply into a external case and move them 10 feet away are the best , I also experienced the hum some time will come thur from another device which contain a larger power supply turned on when rack together in a rack ....... maybe I need to make some Mu-Metel Can shilelding in the future ...... are mu metel the only metel that can block the hum or any other stuff can also do the job? 1mm copper ?
 
And , it doesn't hurt to shield the inductors in a passive eq circuit ,
because of course whatever they pick up get amplified by the
make up gain
 
Move the transformers away as far as possible from the audio transformers and inductors - in your picture they're much too close to the audio boards.

Also it may be helpful to stack the two transformers - that way it's possible to cancel at least a little of the stray field.

Jakob E.
 
As the designer Gyraf says, move the transformers from the audio board. Also, clean up that wiring. Put Power supply and transformers in one corner, audio in another. If you still have hum put a chuck of steel between the Power Supply and Audio area. Or, if you don't want to do much work, keep the power transformers in an outside box.

I know how hard it can be to be clean with wires, but those fancy outside knobs deserve a clean inside.
 
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