troubleshooting sid hum

Testing/Troubleshooting
1

I’ve done a forum search on this issue and have tried shorting the audio in pins, no effect.  I’m also fairly sure my connection to ground is good (is there a way to test this for sure?)  Here are the symptoms:

When connected to headphones I can barely hear the hum, but it is there.

When I connect my sid to my little crate guitar amp I get the hum but in this scenario it renders the sid pretty much unplayable.

When I have just the audio out signal (not ground) connected to the tip of the 1/4" audio connector the hum amplitude scales with the amp volume (volume 0 = no hum).

When I connect ground to the upper portion of the 1/4" connector the hum is constant and does not change when I turn the volume up or down.

When both audio and ground are connected to the audio jack the hum is present in the same way as the above ground-only example.

Im guessing this is an issue with ground but I’m not sure how to go about testing the connection.  Any ideas?

2

How do you power your box? Is the transformator inside the box? Bring it out then.

How did you interconnect the audio signals? Did you use “normal” wires to get them out of the box or to the connectors? Try shielded ones. The shielding connected to ground.

What is the housing made of? Wood? Plastic? In best case, it should be some metal (e.g. aluminium) connected to ground.

There *will* be some hum staying (that´s some illness of *all* SIDs - but the SNR should be well over 60-80 dB or even more).

Greets & good luck.