Reply To: How to test universal preamp

[Dinx]

Hi Jim. Really do not want to be a bother but I am truly stumped. Still experiencing humming when Universal Pre-amp plugged into PRE-IN. However, when I plug my NODE into the pre-amp, and plug the pre-amp into an input RCA (CD input), it plays just fine, with totally silent background, no matter what volume level selected. The motorized volume pot (ALPS) is working just fine. Cannot get it to hum at allI tried different power supplies. A 12 VDC SMPS (measured 12.26 Volts.with the .5 ohm resistance gives 10.6 VDC at heater terminals when energized. Once I look up the heater resistances I will try and calculate the value of resistor needed to achieve the 6-7VDC at the terminals. I now have a power supply with variable outputs. With it set at 7.5 VDC I get 6.49 VDC at heater terminals when energized. I also changed the power bar plugged into Hubbel hospital grade receptacle and 2#12 back to panel. Ironically, I used to call on electrical engineering firms for a living. I have built numerous projects in the past (including 3 hybrid integrated amps using tube pre-amp and Class D amp combos) I buy all my parts from Parts Connexion (Hammond, Nichicon, Rubycon, Vishay resistors etc.) and Solen (Canadian sales/warehouse a few km’s from where I live). However, 2 pre-amps stacked like that surely cannot give optimum results. I just cannot believe I have been unable to get rid of the hum. So silent using line inputs… humming when connected through pre-in…The humming does attenuate at half volume with the music to noise ratio seemingly better. Why would it hum one way and not the other? Thank you for your patience. It is probably something silly and simple that I just cannot see. Getting old and tired. One of these days I will have to call it quits… but right now, it still gets may juices going. I like my Willsenton for tube sound and my Naim for SS.