Hum and Hiss with Tube pre-amp

Folks-- Thanks for the rapid feedback. Let me add the following, which may help: The input impedance of the DNA 1 is 100K ohms and I don't know the output impedance of the two pre-amps. Could they be that high? Keis had suggested that this might be a problem. There is no TV hooked to the system, just sources of FM and cd. The hiss/hum is not low level. It can be clearly heard at the listening position at normal listening levels. An increase of the volume knob does not lead to an increase in the volume of the hiss/hum. I turned on the pre-amp and amp and disconnected the source interconnects from the pre-amp as 7p62mm had suggested. I did this one at a time, and then both at the same time. There was no change under any of these conditions: the hum/hiss was strong and obvious. Any more ideas? Thanks again for any feedback. Rich

My DNA-1 Deluxe does a great job of amplifying baseline tube "hiss" from SFL-1 Signature, just audible 2 feet from Thiel 1.5's at idle when the house is quiet. Steve McCormack told me that I should live with it, get less sensitive speakers, or think about a different solid-state preamp. Some techhie said I could switch out the input resistors for 200K, but McCormack thought this would degrade sound quality. So I'm doing like the man says, and enjoying the amp a lot.

Once again I thank you for the feedback and add the following. I tried a shielded interconnect between the pre and the amp: no change; I also moved the pre-amp about 2.5 feet away from the amp: no change. These measures were designed to deal with AC interference and impedance mismatch respectively. Any new ideas? Thanks again. Rich

I presume you have tried disconnecting the FM input? Trying another cable or two might even be worth it. An item you might check (or leave to a qualified person) is whether signal earth is connected to chassis or electrical earth or not on each of the units. You can do this by looking inside them or measuring for a short between the RCA socket's earth (outer) connection and the chassis or electrical earth. Sometimes the signal earth is connected directly to the chassis or electrical earth and sometimes the signal earth is floated by connecting to the chassis or electrical earth through a resistor and capacitor. For the shield on your interconnects to be effective it is nice if at one end there is a component that has signal earth connected directly to electrical earth. But having this at both ends causes an earth loop - but this should probably have gone away when you tried the cheater plugs. The point is that the earthing scheme is a likely culprit - perhaps none of your components have a direct connection from signal earth to chassis earth. I once struck this very same situation and eventually got Plinius' chief designer to visit my house and check out the total system's earthing arrangement. Eventually we settled on making a change to the earthing arrangement of the pre amp and the problem went away - but I was very glad of his advice on the solution that would not end up compromising the sound (not to mention safety).

Maby there is too much gain in your preamp?