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.
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.
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?
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?
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.
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?
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 !
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 .
Mechans, you are right about NOS's and perhaps Upscale's tubes are still noisy? I am going to try another set which the builder says are the quietest in the preamp. ECC802s from JJ.
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?
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.
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.
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.
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.
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...
Hi Bill, I am way late here but I was busy in May and never saw this post. An easy way to check out the gain issue would be to try something as simple as Rothwell in-line attenuators which cut the gain by 10dB using a divider network. I used to have both the rca and XLR varieties but I am not sure whether I still have the rca version. If so, I would be happy to loan them to you. I have also made my own (XLR version only) using better resistors. I believe Rothwell uses SMD resistors. They sound clean and don't muck things up although IMO they can reduce dynamics a touch. Anyway, if these reduce your tube hiss then you know it is a gain issue and you can then decide whether you want to try and tackle it internally.
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