For sure the problem is in the preamp. I use for now my old GAS Thaedra with no problems. The ML make the noise in both single ended and xlr outputs with or without anything connected in the inputs. It makes this noise only in the right channel even if you put the selector in MONO.
Is the hiss level dependent or independent on the volume setting.
That would narrow down the problem to before or after the volume circuitry.
Hans
It is independent from the volume setting but it gets louder from the right balance setting
That's useful info.
Everything before and including the pot can be discarded as a cause and so is the 0dB inverting amp behind the 21dB main amp, since the problem is there for both SE and XLR output.
Hiss increases when turning the balance pot which confirms that the 21dB main amp is producing this hiss.
It would be great when you could make a simultaneous two channel .wav file of the output and the +17V supply and one .wav with the -17V supply showing what happens to the supply during this hiss.
Hans
Everything before and including the pot can be discarded as a cause and so is the 0dB inverting amp behind the 21dB main amp, since the problem is there for both SE and XLR output.
Hiss increases when turning the balance pot which confirms that the 21dB main amp is producing this hiss.
It would be great when you could make a simultaneous two channel .wav file of the output and the +17V supply and one .wav with the -17V supply showing what happens to the supply during this hiss.
Hans
The weird thing also with this noise is that it starts about 20-30 minutes after I open the preamp. Then it can make this noise for 15 minutes to 30 minutes and then disappear, and after that it can start again in sudden times. That’s why it’s not easy finding the issue!
have you checked the tightness on the ground of the volume potentiometer and balance 1 balance 2?
I've had this symptom before on some amps
I've had this symptom before on some amps
Since the volume setting had no effect on hiss and the balance acts as meant to, I see no reason how any of both could cause this effect.
I have extracted a .wav file from your Youtube file and tried to analyse it.
What you hear is wide band noise going up and down in level and not some oscillation that I feared.
But the level of the noise is much too high all the time also the moments you almost can’t hear it.
For that reason the most likely cause seems a cap either in the power supply or in the main amp.
As a first step the power supply should be exluded from the list as the possible cause.
This means: disconnecting the power supply going to this disfunctioning channel and reconnecting it in parallel to the other channels supply.
Then both channels will have the same supply, meaning that when the hiss is gone, the power supply is to blame but when it persists the cause is further narrowed down to the 21dB main amp.
Hans
I hope to be a cap or some cold solder ! One last is that this noise doesn’t effect the overall quality of the sound of this channel. It’s like a background noise only and don’t give distorted sound
The reason why I mention it is that the cases I encountered were similar, no variation in volume of hiss.
These are silly things that I have encountered in my career and which no one thinks about and moreover, it costs nothing to check.
A small update. The preamp goes to a really nice engineer that change some of the transistors except the VFET that there are no longer available. The preamp came back with unfortunately the same noise issues. So probably the problem is in the right channel VFET… does anybody got any inputs about these VFET? Because probably after all these the preamp will go to eBay for sale as is. Regards!
Excuse me, what VFet are you referring to, or do you mean the dual JFet at the input.
There are excellent replacements for this dual Fet.
Hans
Well I’m not an engineer but I know some basic things and I think I have some logic.
The preamp does this noise at summer after 30 minutes and at winter after an hour or so. What kind of component inside get hot after that much hour? Transistors work from the start and get temperature very soon, the capacitors inside are almost all new , maybe a resistor with not that much load that gets hot after some hour and is out of spec? Although if this resistor doesn’t have much load what’s the reason to fail? I’m really confused with this machine
The preamp does this noise at summer after 30 minutes and at winter after an hour or so. What kind of component inside get hot after that much hour? Transistors work from the start and get temperature very soon, the capacitors inside are almost all new , maybe a resistor with not that much load that gets hot after some hour and is out of spec? Although if this resistor doesn’t have much load what’s the reason to fail? I’m really confused with this machine
I don't know what part of the world you live in and if you can get access to it somehow, but for my part, I quite often use a thermal camera to see things that the eyes or fingers cannot see.
A small-big update. My preamp few months back got fixed from a really good guy and technician. It found that it was a faulty transistor in a later stage. Because I use the Mark Levinson as a second preamp, cause I really like my GAS Thaedra and its live sound, I found a second issue with the mark Levinson. The left channel (yes the one with the problem) after the preamp warm up a little , got around 2db less gain than the right channel. This issue left when I switch to mono so both channel got equal gain. I have to add something here, when I was searching the early issue with the noise I check also the power supply for cold solders, so I did some extra soldering in some places. Unfortunately after plugging the power supply (thankfully without the preamp on it) one of the toroidal transformers got really hot. I took the whole preamp to a technician that fix the problem i did with the soldering telling me that I was lucky that the transformer didn’t burned. The later technician check the voltage outputs and say that everything is ok. So now got only the issue with the imbalanced output in stereo mode …