This is how the input looks taken from the attached service manual provided by BSST and even though the signal path is the same between the 980 and 985, the 985 protection circuit is different with the NJM4558.
I also tested keeping the cables attached to the amp and connecting ground at the open end of the cables and the amp stayed silent so I am starting to think its not a groundloop issue.
I also tested keeping the cables attached to the amp and connecting ground at the open end of the cables and the amp stayed silent so I am starting to think its not a groundloop issue.
Try repeating the experiment with source end of cables shorted but not connected to preamp; I presume hum will be absent. Then touch shield end to RCA shell of preamp source—- any hum?
So I connected the left signal-grounds directly to the right signal-grounds and the noise was there whatever I did with the cables.
The PCB ground layout is a bit strange as one of the signal/pre-stage power supply caps is connected directly to power-ground while the other is not.
I decided to cut this pcb trace and separate signal ground from power ground with a 2.2 ohm resistor on all channels and the amp is now audible silent however there are still residues of the problem as you can see on the FFT below.
Anyone know what the source of the problem is when this change sort of fixed it?
Also, any guidance on how high I can go with the ground separating resistor? (assuming that will will improve the noise floor further)
The PCB ground layout is a bit strange as one of the signal/pre-stage power supply caps is connected directly to power-ground while the other is not.
I decided to cut this pcb trace and separate signal ground from power ground with a 2.2 ohm resistor on all channels and the amp is now audible silent however there are still residues of the problem as you can see on the FFT below.
Anyone know what the source of the problem is when this change sort of fixed it?
Also, any guidance on how high I can go with the ground separating resistor? (assuming that will will improve the noise floor further)
Bob Cordell seems to use 4.7 Ohm in some of his designs. Look for DC drops across these resistors. If there's only a few mV or less, I think it would be safe to experiment with larger values.
P.S. Using your post 22 schematic as an example, instinct tells me there needs to be a low noise path (devoid of hum currents) between C104 ground and the corresponding input jack ground.
P.S. Using your post 22 schematic as an example, instinct tells me there needs to be a low noise path (devoid of hum currents) between C104 ground and the corresponding input jack ground.
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Agreed assuming you mean C404 and C504 or the ground point for the feedback loop? After getting a solid powerground that was my next goto adding a direct and solid connection to signal ground.