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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
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.
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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.
mechans
3,048 posts
 
I did look at Rodger's (RAM) tube offerings. While expensive he does offer some rare NOS tubes that he tests the hell out of to select truly quiet ones. It might be worth the investment. I had thought that Upscale was doing good work but have only bought 1 pair of Mullard 12AX7s a decade ago and used them as an output stage of a CD player. They worked great in that capacity but that is hardly a true test.
I have a lot tubes as rolling turned into collecting. There is a big difference between noise generated by the tube's normal function and microphonics. Testing for microphonics is sort of useless in my book, because the sound very very rarely feedsback into the tube to cause a problem. So, unless you are tapping the tubes with a pencil while playing for some strange reason, microphonic tubes have little impact on the sound.
Noisy tubes are different. They make a noisy base signal that gets picked up and amplified. In a phono stage or other very high gain situation, this is obviously a big problem. Sometime the tube has both a noisy output and a distorted output but I don't want to write a novel here. The most common noise is from a near dead tube or one in the process of dying and it makes that tell tale hiss, which makes this case frustrating. Upscale should and I expect would have tested the tube enough to find hiss. Could they have missed it ?? I don't know. But substituting any other compatible tubes should have fixed the problem...
grannyring's avatar
grannyring OP
6,530 posts
 
It is doing far better now with 12au7 tubes from Tube Depot......
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