source selector / volume hum in Aikido

[cbutterworth]

While still awaiting the arrival of my heater regulated supply, I did some tweaking to my Aikido.

Things had been going well. I was using a single input directly to the chassis mounted Goldpoint stepped attenuator. Hum was very low with the right combination of tubes.

OK, so I decided to connect the remaining inputs via a chassis mounted Elma selector. I ran a ground buss between signal GNDS of all inputs. This went to the GND on the Goldpoint attenuator. The input signals went to the source selector, then to the Goldpoint Input.

From the Goldpoint attenuator, I connected the GND to circuit signal input GND and the Output from the attenuator to the circuit signal INPUT.

Anyway, I now get hum. Except that it is dependent upon the attenuator position. When volume is all the way down - full attenuation I get no hum. When the attenuator is all the way open - no attenuation at all, I get no hum. As the volume is cranked-up, the amount of hum increases.

I hadn't noticed this before, although previously, the volume was so high that I needed maximum signal attenuation. I since installed 40db attenuation using a voltage divider between signal input and GND of the attenuator.

Any ideas?

Charlie

[Pano]

Originally posted by cbutterworth
When volume is all the way down - I get no hum. When the attenuator is all the way open - I get no hum.

That is the classic symptom of a ground problem. That's the 1st place to look. Bad ground or ground loop. Wish I cold be of more help - someone here probably will be.