Sorry for delay on results. Better late than never I suppose..

So building the caps into the J10 connected cable (by bridging power and gnd pins/wires) seemed to have little if any effect. However putting it directly across the diode bridge worked pretty well and cut out a lot of hum. I went with a 4100uF cap, since 8200uF seemed to have no added benefit, but was noticeably better than 100uF and even 1000uF.

I also wired in a switch in series with the J10 cable to simply break the power pin, and JB welded it to a cutout on the back panel. It's pretty dumb but works to completely kill DSP+associated noise (both hum and the slight digital "air" noise). Just make sure the amp is powered off when toggling this one or you get a great pop.

Thanks for all the help! I think I have the best of all reasonably possible worlds now with this amp.

Copy on leaving the resistor alone.

To be clear the sudden hum was the loose J10. The hum now is back in it's "normal" state, however since I noticed this thing can really whisper with the dsp disconnected, trying to see if I can improve on the original "with dsp" circuit with some more filtering.

Roly, thank you for your in depth reply! It helped me learn a bit more about filter circuits.

I suspect adding a higher F cap will provide much better results in getting the 120hz out, but I will also give the higher R and 0.1uF a go. Just waiting on digi-key now.. I even picked up an 8200uF cap in addition to the 100uF for fun, since it was less than 2 bucks

For the interested, a bit of backstory I think leads to an explanation at why I think this "crate dsp hum" problem is so prevalent on the internet:

In my case I did not notice hum when I bought the amp many years ago. Then one day it got a REALLY LOUD hum, out of the blue. After many hours googling I found a solution was to disconnect the dsp and it was dead quiet.

Fast forward a few years to last week when I decide to take a look at it again and see if I can salvage the dsp use. More relentless googling leads me to the answer, in a 2011 forum post (last: http://music-electronics-forum.com/t13806/). The problem is the crappy pc power connector (J10). The J10 on the dsp is mounted horizontal, so after hours of play the connector rattles loose and causes a huge hum. Crimping the connector or better yet soldering it removed the LOUD hum.

So if the LOUD hum is fixed why am I posting this? Well because the dsp still adds hum (I'd say 20% of the LOUD hum) over no dsp, but since I never knew the crate could be ninja silent with the dsp disconnected, I didn't notice the problem when I first used it. My ears have probably gotten more picky as well.

So in final, if txfloods 'over filter' cap does indeed decrease this 20% hum by 2/3 I will indeed be a happy hacker! Of course I'm still adding the true bypass switch because I'm persnickety

Roly, I am also seeing the 470 ohm.

How exactly would this 'bypass filter' work? Wouldn't the 0.1uF cap just roll off the very high frequencies?  The hum is pretty low, I'd guess around 240hz.

Thank you for the quick reply! Exciting to think some headway may be made on this. I am new to this forum and indeed just getting into electronics hacking!

Please bear with my noobness if I am overlooking where the pdf is.. Could you please refer to the pdf with the 47ohm R and bypass cap? They do not appear in this thread, and the closest I can find elsewhere is http://elektrotanya.com/crate_gx_120,_gx_212_sch.pdf/download.html but that does not appear to have j10 dsp connection.

I assume you are referring to R107 being increased 10 > 47 ohms? Is the bypass cap an additional 0.1uf cap put in parallel to R107? (I assume you are not referring to C63 as this is in my schematic/amp as well).

Sorry if it is bad form to revive a year old thread, but all the info is here and my findings directly build on the previous posts.

My GTX-65 has the same terrible "hum with dsp attached" problem as txflood and many others. I attempted perform the "switch J10's pin 1 to ground" as pondered in this thread, and while some high freq noise was cut (from dsp I assume) the main hum was still there.

I have since found that simply disconnecting J10's pin 4 (the power pin) removes the hum completely (as well as dsp air noise).

My current plan is to add a switch to break/join pin 4, however this creates a loud pop when disconnecting, which I suppose is not good. I do not mind only using this switch when the amp is off, but perhaps this gives some people in here some insight into the nature of the problem/a better fix?

Reference schematic: http://music-electronics-forum.com/attachments/6109d1251661835-195sch_2.pdf

Edit: This may also explain why over filtering at C64 also reduces the hum significantly, perhaps the dsp board is not well-behaved at its power input?