2. amps-preamps
  3. 1367679356

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

Showing 3 responses by zd542

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?
zd542
5,339 posts
 
I'm not 100% sure, but it sounds like a gain issue to me. I don't know what kind of speakers you have but this type of thing often occurs if they are very efficient. If you can, try a different source that has a lower output than what you currently have. It doesn't have to be anything expensive. A mass market CD or DVD player is fine. If the noise is reduced or disappears, too much gain is most likely the problem.
zd542
5,339 posts
 
There should be no problem confirming that the problem lies in the preamp by removing it from the system and just using the volume control on the dac. Other than that, I think the OP is right in that the preamp is the issue. After reading Onemugs post and finding that the only IC's used were Blue Jeans and Anti Cables, I feel there is a small chance the problem may be the IC's. I've has several situations where noise was due to IC's picking up noise.

Another resource you may want to try is The Cable Company. A lot of people think of them for only cables, but they sell a lot of tubes. I don't want to start a war, but I feel Upscale is very overrated. Cable Co. and maybe one or two other dealers, I feel is the most reputable I know of. They'll send you some tubes to try first so you don't have to buy anything. Also, as I mentioned above, I think there is a small chance the IC's may be at fault. I find AQ cables to be extremely well shielded. If you can try a pair of those, it wouldn't hurt.

I just thought of one last thing. Are you sure its not a ground issue?
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