Ground loop problems - looking for opinions

[Glass_painter]

Hello everyone,

Just finished building this 6BM8 amp, sounds very nice when every channel is tested separatly, but when connected together one channel sounds 40% louder then the other and the lower one is distorted. I suspect the grounds to be the problem because when I disconnect the ground wire it is OK. I have builet separate power supplies but the ground is still connected via the tuner I am using. Decupling the ground solves the problem.

What I do not understand is why connecting the R and L grounds creates the problem and how can I solve it without using a transformer in the input.

http://www1.odn.ne.jp/~ces58160/tubeamp/6bm8/6bm8pp.html

Thank you very much.

Glass_painter

 

[EC8010]

There's nothing in the circuit to diagram to suggest that there should be a problem, so I think you are looking for a wiring error. Check your DC voltages when you connect the grounds together.

 

[Glass_painter]

Ground loops?

Hi,

Yes, this was my guess as well so I looked at the DC volages, they do change but very little and all values are in spec (connected or not). I even took the power supply apart and built a dubble PS separate for aech side, now the only mutual wire is ground.

Any other ideas?

Thanks.

Glass_painter

 

[EC8010]

If you have an oscilloscope, remove the NFB and apply a sinewave from an oscillator or test CD to both channels and look to see if it's the input stage or just the output stage that is distorting/changing level.

 

[Glass_painter]

Thanks, I will try it tonight and let you know.

Glass_painter