Firstof a short introduction of myself...I am from Hungary and I've been bulding electronics since the 80s, but haven't touched them in the past 20 years. Anyway, one of my friend who is collecting old radios, recently brought me a diy tube preamp without any documentation. So, I had to do a little reverse engineering, since I wanted to finish the product. It was so unfinished, that even the power supply was not complete. Finally I have managed to make a regulated power supply using the schematics from this page (figure 4) : https://audioxpress.com/assets/upload/files/bicknell2890.pdf
After some modification I was able to get the RIAA corrector working too, but there was a "small" problem with it. It hummed like crazy. I finally was able to minimize the hum (50 Hz) to a lower level, using several techniques, like the star grounding, virtual filament ground and DC filament heating, but the hum is still there and sometimes quite annoying . Am I missing something here to eliminate the hum or it's unavoidable at the this low signal level ? Fyi, here is the schematics of the RIAA preamp :
After some modification I was able to get the RIAA corrector working too, but there was a "small" problem with it. It hummed like crazy. I finally was able to minimize the hum (50 Hz) to a lower level, using several techniques, like the star grounding, virtual filament ground and DC filament heating, but the hum is still there and sometimes quite annoying . Am I missing something here to eliminate the hum or it's unavoidable at the this low signal level ? Fyi, here is the schematics of the RIAA preamp :
