Yeah, tried different cables, everything sounded fine in the front, I have a router box set up that allows you to switch pedals from the front to the loop by stomping a button. I tried with and without this router box as well, just in case.
Even just running a short 1ft patch cable from the effects send to effects return, with nothing even plugged into the front, made the Archon Classic buzz loudly. I had read about the effects loop being noisy on some of the older Archons and Tremonti amps, but it seemed as though they had worked out a fix for that.
Sometimes the problem is as simple as a ground loop. It can be somewhat difficult to track them down, but there are lots of ways to do so.
The first is to remove the pedals from the signal chain one by one and see if only one or two pedals are causing noise.
Another way is to plug a cable between the amp's send and return jacks; sometimes that tells you if the noise is internal to the amp or external in the pedal power supply or pedals.
Powering pedals with a good power supply that isolates each pedal from the other pedals reduces noise, but if the problem is the grounding of what powers the pedals, that won't work very well. Still, daisy chained power supplies like the old One Spot will cause significant noise issues.
If you have a box that safely lifts the grounds to the effects, lifting the power supply ground will often tell you if there's a loop. There are boxes that do this. If you have to use a 3-to-2 ground plug adapter, you can use it TEMPORARILY to lift the ground and check if there's a ground loop, but once you track down the source of the ground loop, you want to fix what's causing the grounding issue and make sure everything is properly grounded. For safety reasons don't leave the plug adapter in place!
Sometimes making sure the pedalboard and amp are on the same power outlet eliminates certain kinds of noise.
Some pedals need their input and output levels adjusted to eliminate noise. This is particularly true of digital pedals, where hiss and noise can become very noticeable once amplified by an effects loop.
I was getting lots of noise from my H9s at first, until - doh! - I realized I hadn't set the input and output levels of the pedals when I got them. Once I did that, no noise. Took me a day of wasted effort to wake up to that one!
Different amps and effects loops use different grounding schemes. So they can vary quite a bit. I have a couple of the older PRS CAD amps, and they have a very good grounding scheme that's hefty. Doug Sewell is a good amp designer, and the early Archons had a problem that was supposed to be fixed. There are lots of folks over at the PRS Forum with Archon experience, and I don't recall any complaints about the loops on the new ones, but regardless, insights in the amp section there might be helpful.
Finally, sometimes it really is the amp! In the case of PRS, they have a great warranty and take care of their customers.
