Baby took a long nap, so I had time to run and properly cut the trace with my dremel and install the ground lift resistor on the right channel. It is now dead quiet, where the left channel still has the hum. It makes total sense that it would still be a 120hz hum, as the input signal was getting mixed up in the PS circuit by sharing the ground. Any ripple in the PS was just getting amplified as it makes its way into the input. I'll rest easy once I've done both channels, but I think we're going to get there. Thanks so much for all of the helpful replies. Learned a lot and ended up with an even better amp.
Ground Lift Resistor BEFORE - AFTER
Here is a simple test, to see if the ground lift resistor will help - with NO NEED TO REMOVE THE PCB.
It involves unsoldering of two wires, which is usually possible from the top of the PCB; hence, there is no need to remove the PCB. The ground wire and the speaker return (negative) wire are then soldered together, the ground lift resistor is then inerted (soldered), and its other end goes to the common on the PCB.
If it works -> you can remove the PCB's and cut the trace, and then insert the resistor "properly" (permanently), on the underside. The last photo shows how to do this.
Now, we have all 3 photos in one place.
EDIT: and NO, I did not unsolder the 2 black wires, just to create the second photo. I used Photoshop (which also took some time...)
Here is a simple test, to see if the ground lift resistor will help - with NO NEED TO REMOVE THE PCB.
It involves unsoldering of two wires, which is usually possible from the top of the PCB; hence, there is no need to remove the PCB. The ground wire and the speaker return (negative) wire are then soldered together, the ground lift resistor is then inerted (soldered), and its other end goes to the common on the PCB.
If it works -> you can remove the PCB's and cut the trace, and then insert the resistor "properly" (permanently), on the underside. The last photo shows how to do this.
Now, we have all 3 photos in one place.
EDIT: and NO, I did not unsolder the 2 black wires, just to create the second photo. I used Photoshop (which also took some time...)
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So your story mirrors mine from my A40 build years ago. I was frustrated with chasing around the chassis trying to find the hum source. I kept reading the ground lift scheme from the A40 instructions and didn't implement correctly and counted it out. After growing tired I spent alot of time trying to understand from schematics what I actually needed to do. When I got it all connected correctly the difference was alot of hum to some hum to absolutely dead quite! It was a Godsend for me. I had a feeling you kind of went through the same scenario on this ground connection. It works like magic!
I haven't tried the ground lift resistor, but might.
I've rerouted the mains underneath so they're sandwiched between the outer bottom panel and the floor; I've shortened the ground from the mains to close to the IEC terminal (star ground is just next to the IEC plug). I added a couple leads to bind together both rails of the PSU (ground). Twisted all my wires. And twisted around the transformer (vertically mounted) so that there's less slack in the secondary lines (twisted).
Result? I do think the hum is less, but still there. Less. What I notice is that when the RCA inputs are left open, I have a louder hum, and it gets louder when I touch the input lines. So it seems to have something to do with that? But I'm pretty sure I have all my grounds in the right place. I'd love to get this to rock quiet if at all possible. If I can get that done, I'll stop obsessing and call this done. Move onto another ;-)
I've rerouted the mains underneath so they're sandwiched between the outer bottom panel and the floor; I've shortened the ground from the mains to close to the IEC terminal (star ground is just next to the IEC plug). I added a couple leads to bind together both rails of the PSU (ground). Twisted all my wires. And twisted around the transformer (vertically mounted) so that there's less slack in the secondary lines (twisted).
Result? I do think the hum is less, but still there. Less. What I notice is that when the RCA inputs are left open, I have a louder hum, and it gets louder when I touch the input lines. So it seems to have something to do with that? But I'm pretty sure I have all my grounds in the right place. I'd love to get this to rock quiet if at all possible. If I can get that done, I'll stop obsessing and call this done. Move onto another ;-)
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