With the ST70 not hooked up to anything, you can only tell it is on by a very slight hum only audible in front of the speakers. I am hoping I can get it this quiet with the sp8 hooked up as well.
With the SP8 hooked up, everything on but with the volume off, there is a decent amount of hum noticeable from my listening chair on the phono side, and on the other selections it is less. I expect the phono to be the noisiest position, but I hope I can cut this down some more.
Things I think may be the problem:
I have the grounds hooked up with a buss bar between each RCA, and both wire going to the output have the grounds attached at both the buss bar and on the PCB. Each input wire has a wire attached to the buss bar and wrapped around the input wire but cut off at the selector switch. the output from the balance control is connected to the balance control ground on one side, and the PCB on both sides. There are also ground wires going from the volume control ground to the PCB ground, and a wire off of this ground to the chassis. The chassis is not connected to the ground at the power jack, so the 3rd leg of the power cable is floating. The phono grounds are both connected at the PCB, and the phono ground is connected to the chassis.
I cut the ground from the power cable because this reduced hum and a buzz from the connection to my TV.
The hum level drops slightly hen I touch the SP8 Chassis.
My shielded phono input wires are about 8" long, as I built this in a larger case.
When I touch my ST70, the hum is also slightly reduced.
My st 70 has the original 2 prong power cable.
This hum I am hearing from the SP8 is not affected by the volume setting, which means to me it is occurring after the phono stage and probably related to the output wiring, the wiring of the volume and balance control, and the power cable grounding.
Is the circuit ground supposed to have continuity with the chassis?
I am also thinking maybe the st70 needs a more modern power cable and needs to be grounded. Maybe I am also getting some noise from the original st70 input jacks..