2. tech-talk
  3. 951880219

Hum and Hiss with Tube pre-amp

I recently got a McCormack DNA1 Deluxe and initially used the pre-amp section of my Arcam Alpha 9, and then a Creek OBH 12. No problems, but no excitement!!! I decided to try a tube pre-amp. I have tried a Melos MA 111 and a Cary Ae3 kit that was up-graded in many ways by it builder. Both of these units are reportd to work fine by their owners. When I put them into my system (front end of CAL DX 1 cd and Rotel -entry level- tuner) I get very audible hum (60 cycle) and hiss(like FM hiss). I have tried cheater plugs on both amp and pre-amp to no avail. It was suggested that I disconnect the inputs to the amp and turn it on to check for noise- I did this and there was no noise. This suggests the problem is elsewhere. Since the tube pre-amps worked fine in other systems, they do not seem to be the culprit. But then again, with the ss pre-amps there was no problem, apparently ruling out the front end. But that leaves NOTHING causing the obvious problem!!! Any ideas?? Thanks for any and all help. Rich
drrnc2
cshuhart
2 posts
 
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.
drrnc2 OP
30 posts
 
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
redkiwi
1,092 posts
 
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).
wcvalve
1 posts
 
Maby there is too much gain in your preamp?
More to discover