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In Reply to: Instantaneous hum posted by breadhead on April 5, 2007 at 04:08:33:
That's an easy one- you're inducing magnetic hum from the power transformer.
Follow Ups:
I built the single ended EL34 amp form the Angela site (as a mono amp) but I used a found power transformer and it hummed from turn on. When I rotated the OPT 90 degrees and moved it as far from the power trans it went dead quiet.
Cheers Steve
...in some creative newbie way. I've seem some wonderous things out there!
Possible, and of anyone, you'd sure know! But I think that if he can't find a wiring error around the power transformer, if he moves it and/or rotates it 90 degrees, the problem will diminish- I learned this one the hard way.
The OPT are as far from the power transformer as I can get them and they are rotated 90 degrees.
Thanks,
Jim
Just turning, or rotating an OPT is not always turning the field linkages 90 deg out of phase. If both power and output are mounted on the same core side, assuming both are E/I construction for illustration, then rotating either will still leave the coils of both linked. You need to turn one or the other on to the short side of the core to unlink the coils. You would need to mount the power or output, the one not on it's short side, belly down on the chassis to unlink both coil and core. You can of course fiddle with this scheme at length, but if a three axis rotation does not reduce the hum and you have also picked the power transformer up off of the chassis to break that potential link, then you have a problem elsewhere.If the noise is a buzz it could just as easily be radiation from the rectifier circuit wires or B+ transformer wires.
Bud
To my surprise it happened to me even with a toroid powertransformer. I thought they had no stray field but a Lundahl 1660 interstage on a distance of about 10 cm was sensitive enough to get the hum into the amp.
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