Hum and Hiss with Tube pre-amp

Maby there is too much gain in your preamp?

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).

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

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.

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

Problem could be caused by cable TV hookup. Try disconnecting the cable feed to your TV if it's connected to your system. Also, my Audio Research LS-1 used to whistle and make all kinds of fun noises. Actually, it was the tube in the LS-1. Try changing yours to low-noise type.

I get hum in my system because of TV cable. If your stereo in connected to your TV system in any way via VCR, DVD etc try disconnecting the TV cable at the wall. I have never solved this problem and will probably to to DirectTV to "solve" it. Its the cable companies fault-bad grounding. The hiss might be due to several sources. 1. Bad tubes - Replace with low noise tubes. 2. Impedence mismatch between the amp and pre amp. If the amp has a lower input impedence then the output impedence of the preamp, that's your problem. Solution is a new amp or preamp. Its in the design. 3. Did you test the hiss under normal listening levels? All tube preamps will have some hiss and if you crank up the volume you will hear it.

I forgot to mention my experience with white noise hiss from tube preamps. My experience is that tube preamps produce a low level white noise hiss that is barely audible (you have to put your ear right up the speaker to hear it). The noise is not noticable to me when music is being played. I have never owned a solid state preamp; but, my solid state amp does not produce any noise when my pre-amp is muted. This suggests to me that the noise floor is lower for solid state devices in gereral. I have read complaints that solid state preamps sound harsh; but,I am not sure if this is true. It sounds like the solid state preamp has worked better for you. Perhaps you should stay with it and not go to tubes. One last comment. The problem may be with the design of the pre-amps that you are using. Maybe they are susceptable to external AC interference. The reason that the other users didn't notice the problem is because the noise level in their listening area is lower than yours.

60 Hz hum is a undereported problem. With your pre-amp connected to your amp try disconnecting the sources to the pre-amp. I had a problem with 60 Hz hum. Trouble shooting determined that the source of the hum was from a cable TV coax (I was taking audio outs from the VCR to my pre-amp).