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DIY question on a preamp problem could use help...

Ok, love to hear from some of you who have good technical skill to help diagnose a problem. I purchased a used Lector Zoe preamp as an upgrade project. The unit has a lot of tube hiss/rush sort of noise through both speakers. The builder is no help as I have emailed them several times and they keep telling me to use a different tube. I have tried several tubes brand new and nos , but the same tube noise all the time. The preamp is very high gain as an FYI.

I am using two 6922 tubes. I have already modified the unit to sound far better with great results. None of the mods helped with the tube hiss that was present before and after my mods. I upgraded many caps and put in a nice Goldpoint stepped Attenuator with DACT remote. Lastly I bypassed the selector switch and went direct from the RCA input to the Goldpoint as I only use one source. I upgraded the signal wire and the end result is quite stellar. Problem is this tube hiss/rush noise that is constant through both speakers. I want to fix this and greatly reduce the noise. Where do I look?

Would reducing the gain help? Should I look at the resistors on the 6922 tubes? Change the value of any of these resistors to reduce gain? Not sure where to go next? The noise is not really impacted that much at all as I turn the volume up. The tube hiss stays pretty constant regardless of volume. The signal goes from the coupling caps right to the RCA outputs with no resistors after the coupling caps or on the RCA outputs to ground.

Any ideas?
grannyring
elizabeth
6,726 posts
 
You tried different tubes, but did not mention if you used tubes noted for 'low noise'.
Many tube dealers sort out special tubes which have very low noise. They cost more, sometimes a lot more, but are what you need. ONl ya few tubes in a big batch will have the quality of very low noise. And the effort of sortingthem out costs deaelrs time (money)
If you have simply been swapping ordinary tubes, then you are not addressing your actual problem: YOU need to use 'low noise' tubes. Period. And "very low noise' might be best for your application.
They may make all the difference you need.
grannyring's avatar
grannyring OP
4,637 posts
 
I have purchased low noise tubes specially sorted.
grannyring's avatar
grannyring OP
4,637 posts
 
My speakers are 87db and thus not very efficient. My dac has a volume which I use to cut the output level my preamp sees. I know the pre can be modified to help with this tube hiss or rush. Not sure how to do it?
zd542
5,339 posts
 
Did the low noise tubes help at all?

Another thing you may want to try is raising the gain on your DAC. Since your speakers are not super efficient, maybe you are pushing the preamp too hard. Also, how does it sound going from the dac to the amp?
stevecham
3,298 posts
 
You haven't told us what the noise is like when the preamp is simply idling without a source and the volume is at zero, so we don't know if it's the source or the preamp, or even the amp itself.
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