Gab wrote:
Anybody ?
Ok. Let's walk through this logically. The noise has to be coming from somewhere. So let's review what we know.
1. Noise is only present on one channel (left).
2. The ground lift switch does make some difference.
3. Swapping tubes makes no difference.
4. Noise is independent of the volume setting.
5. Noise is independent of input or output loading.
Number 1 indicates that there is a difference electrically between the left and right channels. There are several possibilities: grounding differences, cold solder joints, bad components, improper power connection. Number 2 indicates that it is likely not coupled noise from the environment or connected equipment. This can be verified by swapping left and right inputs and outputs and making sure the problem stays in the same channel of the preamp. Number 3 eliminates a bad tube. Number 4 says that it's not high side coupling of the input, but a ground problem is still possible. Number 5 helps confirm 4.
Now your input wiring is VERY complex. There are lots of prospective ground paths. It would be helpful if you could show us as as built schematic including the ground paths. I can't tell from the pictures how you wired the inputs.
Another thing to consider is that you have 4 independent noise sources in your amplifier (i,e, LEDs). Since you are likely switching these in and out with the same switch you're using for input selection, there is possibility for problems in this area. If you could draw out in detail the entire input switching mechanism, that would help. It would also hep to see the exact power supply diagram including how you are supplying the LEDs. It is important that their high frequency noise is kept out or the audio circuits.
From the picture I can't tell how the LEDS are wired into the circuit. However, it look like they are fed after the first inductor. This may be a problem as the inductor is required to choke out the high frequency noise (which may be modulated with the 120Hz ripple. You really need to take a look at the noise on the B+ with your oscilloscope and see if you can identify the primary frequency.
I await your reply.