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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?
128x128grannyring
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?
grannyring's avatar
grannyring OP
6,530 posts
 
I did and do go direct from my dac to amps with dead silence both with the unshielded and shielded cables. Sound it fantastic direct also. I just love modifying gear to be the best it can be....thus this preamp project.

I will try another set of ic's and tubes.
clio09
3,360 posts
 
From my own personal experience I think Upscale's testing is a bit spotty. I have bought two separate sets of "low noise" tubes from them with the purchases several years apart. Both sets were noisy.

Try to contact Roger Modjeski. He has done preamp and phono stage designs using the 6DJ8/6922 and his designs are notoriously quiet. Getting a set from him would enable you to do a valid test as to whether the tubes are indeed the problem.
almarg's avatar
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almarg
9,670 posts
 
Bill, I took a look at some rear panel photos of the Zoe. Although it's hard to tell for sure, it looks like although the two pairs of main output RCAs and the two pairs of input RCA's that are adjacent to them are the type that are isolated from chassis, all of the other RCA's appear to be the type that is not isolated from chassis.

Which set of inputs did you wire to? If it was one of the non-isolated ones, it is conceivable to me that changing to one of the isolated ones might help. Or perhaps even vice versa.

Also, assuming you are using one of the main outputs, and not the "passive volume output," it might be worth disconnecting the internal wires that go to the "passive volume output" jacks, which appear to be non-isolated.

Just some long-shot guesses, but those are the only suggestions that occur to me aside from what has already been said.

Best regards,
-- Al
grannyring's avatar
grannyring OP
6,530 posts
 
Hi Al. I am using the main isolated CD input next to the two outputs. All of the non isolated inputs and the passive volume output have been disconnected. That total board is completely out of the picture now. I only kept that board in the chassis for cosmetic reasons.

This preamp has too much gain in my system and I have read others saying the same thing. My dac also has a volume that I keep 12 in order use the preamp's volume in the 9-1 dial range. Like to to able to,turn my dac all the way up and take that Attenuator out of the system. Simply too,much gain when I do that.
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