2. amps-preamps
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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
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
3,340 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.
onemug
784 posts
 
I was thinking it could be the combo of high gain and hi-eff speakers but I see you mentioned yours are 87db.

Are your interconnects shielded? Have you tried changing them anyway or maybe rerouting them? It may come down to reducing the gain as your only choice.

btw, how close to the speakers are your ears when you hear this hiss?
mechans
3,048 posts
 
It is really frustrating when it acts just like a bad or simply noisy tube would almost exactly. I think his only choice is to check every component in the signal scheme that he mentions. He knows the noise is coming from a section of the circuit and see if any of those parts are bad or out of spec. What else can he do?
He didn't mention who sorted the tubes, factory sorting e.g., is next to useless. Some labs are much more sophisticated than others with differing strigent rules determining noisy or not tubes. RAM testing , for example is very stringent and is very expensive, even if he sometimes uses cheap tubes to start with. nI am not sure what his tube supplier is now for 6922s.
BTW really good NOS 6922s are painfully expensive, sold as is usually with just gain, transconductance, short and gas testing, but with no noise waranty !
grannyring's avatar
grannyring OP
6,645 posts
 
The tubes came from Upscale Audio and they do a great job testing. Great points all, but I have been there and done that. I use unshielded ic's by Anti Cables, but also tried the well shielded IC from Blue Jean cables and the same hiss was present.

I have done all the needed testing on sources, amps to be sure it is the pre and not the source or IC or anything else. It is the preamp.

As I mentioned above, the hiss does not change with volume and is always present even at very, very volumes.

The parts all test fine in the preamp. I know it is a circuit design thing and I will need to mod. Hoping someone would have an idea of where to try a mod.

I will continue to read all I can on this issue as I like the challenge! Love learning and trying .
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