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Alex, are you able to comment on the cause of the faint high-pitched noise that some of us have experienced? I know you've said that, in some cases, it was due to faulty tubes. But where the issue was not caused by tubes, have you been able to discover the source of the issue and what the solution is?
I'm on vacation and away from my LF for a few weeks, so I haven't had enough time to see whether the noise appears with both my Siemens and the stock tubes--so far, I've only heard the noise with the Siemens. The noise generally went away when I unplugged my headphones and plugged them back in (to the same output), at least temporarily.
Thank you and thanks to dB for helping me try to find a solution to this problem. I really love my LF, aside from this issue and maybe one other minor complaint: The volume is too high with my LCD-3 and PS Audio PWD DAC set anywhere at 60% or higher of its output. This may be primarily due to my DAC's output being too hot, but I'm rarely able to set the LF's volume at higher than 7 or 8:00 if I'm using the #1 headphone port and have my DAC's output set to 60 or higher (which is what most users of the DAC seem to recommend).
Is there a way to add a gain reducer without sacrificing resolution? I'm able to get a wider volume range with the #2 output, but I slightly prefer the sound of #1 with my LCD-3s.
Yes, I can tell you what I know at this point. This low level, but audible noise seems to be in a small number of LFs. I had never heard it during burn in because it usually happens many hours after that. I use AD2000s to listen to the amps before shipping. They are very sensitive headphones and I can hear everything, including the basic noise floor of the LF from jack #1.
Unfortunately for Morbid Toaster (who lives nearby) his LF began to show this noise too. He let me have the amp for a while and I was able to hear, for the first time, the noise signature. I was also able to probe around a bit and do some experimentation.
The first thing is, this is a very difficult problem to pin down because it is so intermittent. I had the amp on for days, listening repeatedly. Sometimes it was there, sometimes not. When I heard it, sometimes it would disappear as soon as I started probing with the scope or a DVM. Sometimes I could get it to stop by gently rapping the tubes with my fingernail. Sometimes not. It doesn't happen with all tubes, just a small number of them. But, since it stops when I knock the tubes it is definitely an internal mechanical issue. Possible to stop with tube dampers.
So, what I know is that some small number of tubes exhibit this problem. I also believe that it is not solely related to the tube, but is in some interaction with the problem tubes and the circuit. I don't know what it is about the problem tubes that causes this interaction. And, on the face of it, the circuit is behaving properly (and measures ok both with DVM and scope).
Because the problem is intermittent (sometimes not showing up for days) and because it often disappears when I start to probe around, it is very hard to identify the exact source or cause. Thus, the only thing to do is to try some reasonable steps and then to listen for a long time.
I have purchased some low noise JJs for Morbid Toaster's amp (tested by the seller). I've also added tube damping rings (two per tube) to his amp and returned it to him. He is kind enough to run the amp normally to see if the noise returns. This is the first step. I'll know more after Morbid either says it's back or if it goes for a week or two without the problem.
Thanks for your patience on this.
There is not an easy way to change the gain of the LF. Initially, I thought about giving you several NFB options which could be used to set the gain. But SQ is really affected by the amount of NFB. The LF has low open loop gain so it has low NFB. Adding more NFB to reduce the gain would probably change the sound signature. So I opted for a single gain. Do you know what the output of the DAC is? The gain of the LF is about 8.5 and its maximum peak output voltage is about 22V. This means that about 2.5V peak or about 1.8V RMS will saturate the amp.
