I'm just in the process of finishing to put together an original hybrid design (MOSFET AB output stage). Unfortunately I've run into a bit of an issue.
I first built the sand output stage and wired it up directly to the RCAs to test for background noise. It was dead silent in both channels. Next I built the valve PS, the input stage and added the pot. This is when the trouble began. The first thing that I noticed was a high pitched grounding noise whenever I got close to the to the case, even though everything is grounded (pot is a blue velvet grounded through one of the screws). This noise increases as the volume increases, up to about 4/5 of the turn, then subsides. This sounds like simple grounding and I'm going to go back in and create 3 star grounds (1 for the valve PS, 1 for the valve input, and one for the Sand bits).
Any advice for actually creating these star grounds? The issue I always run into when trying to implement a star ground is all the wires springing off whenever I try to solder one more to the node.
The other noise that I heard was a low frequency (<200Hz) 'whopper whopper' noise that doesn't vary with volume, I also tried bypassing the input stage and this noise didn't diminish meaning that it must be coupling from the valve power supplies to the SS stage. A bit of experimentation soon revealed that the noise was coming from the valve heaters of all places. The heaters are running from DC, (originally an SMPS, which I replaced when I heard this noise with a power resistor which drops the 18V rail to 12V). How can this be putting serious noise into the other stages? It's the same DC as used for the other rails just passed through a resistor! The two heater wires are tightly twisted. Any ideas? Anything? I'm at a real loss here.
One more question that shouldn't be to hard to answer is about wiring the pot. I'm using shielded wiring from the RCAs to the pot. Do I take the ground from the RCA to the pot and then to a star ground, or the RCA to a star ground and a star ground to the ground of the pot? And at which end do I connect the shield?
Any responses to any of these questions would be greatly appreciated.