Trafo Shielding inquiry

sr1200 · Feb 25, 2013

I have a unit whose transformer is a little "too close for comfort". At higher gain settings there is an audible hum on channel 2 (closest to the tx) I ordered some shielding from Don Audio, but the shield is a little too high to fit in a 1u case. Would there be any problem with trimming the mu metal down length wise to bring the height down? (or should i go tuna can?)

zayance · Feb 25, 2013

It's not Mumetal but oriented steel sheets, and i think it's ok to cut bend or whatever compared to Mumetal or even Permalloy.

zebra50 · Feb 25, 2013

... to be honest Mumetal gets cut, bent and soldered here! It may in theory reduce its screening ability a little - but in practice it still works!

CJ · Feb 25, 2013

if it is pwr transformer hum, you could go old school and inject more hum out of phase to cancel it out,

usually the pwr trans will generate a constant amount of hum, so we can use this trick,

that way you could keep the mu metal for another project without messing it up,

zayance · Feb 25, 2013

What Don Audio sells is not Mumetal so he's ok, it's

two layer "grain-oriented steel with 3% silizium" - "DIN46400"

sr1200 · Feb 25, 2013

Thanks for the replies. Aluminum snips ahoy!

zebra50 · Feb 25, 2013

That's the spirit! CJ stylin!

sr1200 · Feb 25, 2013

CJ, the hum is caused by the proximity between the output TX of the channel and the power TX (unfortunately, the only other option would be to get a bigger case at this point) but would what you're talkin about work for this situation? (id love to know how to do that in general lol)

CJ · Feb 25, 2013

just make a voltage divider from a pot and a a few resistor, wire both ends to your 6.3 heater winding or other source of 50/60 cps, inject the wiper lead into the input of the signal xfmr and turn the pot till the hum is gone, better use a cap so the dcr does not get seen by the xfmr,

sr1200 · Feb 25, 2013

Frightening, but cool... Ill probably try this out on a "test subject' before i try it out on a semi-working unit.

CJ · Feb 25, 2013

you only need a few millivolts to cancel hum, so make the divider maybe 100:1 to be safe and work your way up,

alexc · Feb 27, 2013

Can this be done with say a line level output transformer ?

sr1200 · Feb 27, 2013

I think thats what we're talkin about here.

Trafo Shielding inquiry

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