Thanks for the answer. just built it like in the diy-pdf: double checked everything. If I disconnect main ground within the amp the hum still appears. .http://www.google.de/url?sa=t&rct=j&q=&esrc=s&source=web&cd=3&ved=0CD8QFjAC&url=http%3A%2F%2Fwww.diyaudio.com%2Fmedia%2Fbuild-guides%2Fdiyaudio-f5-build-guide.pdf&ei=TY9VUoT6NsfCswaEnYCoBQ&usg=AFQjCNE-w5IuwiFOgA40G6Eb6FRnRqWgTA&bvm=bv.53760139,d.Yms&cad=rja
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Have you tried it with a different source? (To rule out your source as the ground loop problem)
Are your input jacks isolated from the chassis? Disconnect the ground wires and measure the resistance from the ground connector of the jack to the chassis. It should be an infinite resistance. With the ground wires connected the resistance should be around 10R - the cold resistance of the NTC thermistor connecting system ground to the chassis.
Does it hum with only one channel connected to the source? You may be able to resolve the ground loop by disconnecting the ground connection from one of the input jacks so the source ground only contacts at one point.
Photos can allow others to find mistakes that the builder cannot see without taking some time away from the project. Like waiting a couple days to proofread important writings.
Are your input jacks isolated from the chassis? Disconnect the ground wires and measure the resistance from the ground connector of the jack to the chassis. It should be an infinite resistance. With the ground wires connected the resistance should be around 10R - the cold resistance of the NTC thermistor connecting system ground to the chassis.
Does it hum with only one channel connected to the source? You may be able to resolve the ground loop by disconnecting the ground connection from one of the input jacks so the source ground only contacts at one point.
Photos can allow others to find mistakes that the builder cannot see without taking some time away from the project. Like waiting a couple days to proofread important writings.
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Are you referring to the Safety Ground from Mains PE wire to Chassis?
This connection is permanent and should never be removed.
Or, are you referring to the Main Audio Ground (MAG) to Chassis connection that is put there to blow the mains fuse if a potentially fatal fault occurs in your mains wiring/transformer?
Again do not tamper with this, unless you are absolutely sure you can protect yourself while carrying out any measurements.
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