Turn off the amp first. Connect both positive and negative input to the ground. Turn on the amp, and see you can still hear the hum. If you're worried, disconnect the speaker, and check AC and DC voltage of the speaker output.
Here are some pictures. Hopefully I did something really obvious and easy to correct. 🙂
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Would you recommend that this be used instead of the .0033uf line cap in the standard Pass power supply or in addition to that cap? Just curious on your findings.
I think it's a matter of personal preference. I personally would choose one or the other but not both. The final option of course, is neither.
So, I could accomplish this by connecting an alligator clip to the positive on each input and connecting the other end of the alligator clip to ground?
Same with the negative of each input?
Your negative inputs should be already connected to the ground for unbalanced connection, anyway.
Yes, I did install the jumper on the amp boards between the negative out and ground.
So with that said, can I just attach alligator clips across the positive and negative of each input?
Looking at the pictures, I don't see a ground lift in the power supply ground to chassis ground. Use either a thermistor as in the FW supplies or bridge/diodes.
Another thing to do is always twist wires together with their returns. For instance I see that your positive and negative supply wires are twisted together but the ground return wire is free. The ground wire should be twisted together with the positive and negative wires from power supply to boards.
Likewise I see that the speaker ground wire is not twisted with the signal wire for its entirety. Twist the two wires together all the way to the speaker protection board and then twist together to the speaker jacks.
Another item is the location of the ground connection to the PS board. It is best to join the positive and negative supply grounds and take the ground wire from the board at exactly the midway point between the negative and positive grounds.
Once all that is done, try rotating the power transformer. Usually there is one position where the noise caused by radiation from the transformer is minimized.
A good read on minimizing amplifier hum and noise is by diyAudio member Bonsai:
http://hifisonix.com/wordpress/wp-content/uploads/2019/02/Ground-Loops.pdf
It is rather long but you can skip to the end where he summarizes and shows practical information/tips.
Just to verify, even if I am using the soft start board, I will still want to install a thermistor inline between the PSU board ground and the STR ground?
Are you referring to the V+, V- and GRND from each amp board to the PSU board? I don't have any of those twisted together.
Ok. There is maybe an inch on one end and a half an inch on the other just before these wires meet either the board or the jacks. I can certainly add a few more twists at each end if you think it will make a difference.
This I don't follow. There are two banks where you can connect ground to the PSU. One is on the V+ board and one is on the V- board. With that said, I did install several jumpers to connect the two boards.
This is something I haven't tried. Is there a certain orientation that tends to work best?
Or hum is from your preamp (or source equipment). Check your preamp's line output AC voltage (hum voltage) with your voltmeter without connecting AJ, while no music playing. If you see considerable amount of hum, transformer or cutting a ground loop would not help.