Yesterday I finished making changes to my all-differential PP amp. One of the changes involved increasing the grid stopper on the input (non-grounded) grid of the 6SL7 LTP splitter, from 10k to 200k, to take advantage of the Miller effect and limit the high-frequency response. The intention is to have -3dB at 40kHz.
Another change was to add 1Meg local feedback resistors from the 6SN7 driver plates to the splitter plates and to reduce global feedback by 50%.
The ovedrall gain is now a bit higher but 50Hz hum has become audible, whereas before it wasn't. I haven't changed the layout significantly. My power transformers are toroids and shouldn't be radiating significant hum, AFAIK. I wonder if giving the IP tube a big stopper may have increased its senstivity fo hum? Or maybe I should have increased the stopper on the grounded grid too, to preserve PSRR? Any ideas?
BTW, there is no connection to ground at all, because I don't have a 3-core power cable. That sounds pathetic, I realize, and I intend to remedy it but, up until now, it hasn't made any audible difference.
Another change was to add 1Meg local feedback resistors from the 6SN7 driver plates to the splitter plates and to reduce global feedback by 50%.
The ovedrall gain is now a bit higher but 50Hz hum has become audible, whereas before it wasn't. I haven't changed the layout significantly. My power transformers are toroids and shouldn't be radiating significant hum, AFAIK. I wonder if giving the IP tube a big stopper may have increased its senstivity fo hum? Or maybe I should have increased the stopper on the grounded grid too, to preserve PSRR? Any ideas?
BTW, there is no connection to ground at all, because I don't have a 3-core power cable. That sounds pathetic, I realize, and I intend to remedy it but, up until now, it hasn't made any audible difference.