Low frequency noise in a 12B4 preamp

[astouffer]

Have you tried connecting 0.1uF film caps across the main filter caps? This made a difference with random noise on a low voltage supply I was building. You could also try 0.01uF across each diode or tube plate.

 

[stigla]

Stigla, you mentioned there might be resonances between the inductor and capacitors, and that was my thought too. Where is the best place to place this series resistor? Between the inductor and the second 33uF capacitor? Also what is the minimum value that could be used and still be useful? Thanks in advance.

Well, I have only tried it with choke input supplies. I normally have a resistor in series with the choke to get a lower Q for the LC filter (Q shold be under ~0,7 to get a good damping of the resonance).

Thats why I suggested moving the 100uF so that the resistor is directly a part of the LC filter. I'm no expert, but you can try... 🙂

Sonically I've had good luck with damping the resonances like this, with choke input supplies. Especially with Phono amplifiers.