D11 help!! squealing / grounding issues? with thru to amp

[photoresistor]

hello! looking for some help, ive managed to put together an n72, D11, and T15 but now my D11 module is now acting funny!

i was using it to record a DI of an electric guitar and then send the signal back out to a guitar amp to simultaneously record the guitar with a microphone on a cab.

heres the problem:

when i have the volume/gain up enough on the amp and the volume on the guitar either fully up or mostly/fully down, an awful squeal is heard through the guitar cab. sometimes i get humming as well and one time (the chasis might have been turned off) i heard the radio when the tip of the guitar cable was touching the input (squeal will happen when the tip is even just touching the outside of the jack). i suspect i have some grounding issues or something, but i am pretty new to projects like this so i could use some help fixing my problem... where should i be looking??

the D11 is setup with a jumper "bridging" J1 and J2 horizontally (so a guitar is plugged into input 1 and then another guitar cable is inserted into the other input and plugged into an amp).
ive tried the alternate jumper settings for J3, J4, J5, and J6 to no avail. the initial voltage checks across R15/R32 are fine.

my solder joints seem alright and i dont see anywhere where there is a short or anything. by itself, the DI works fine, its just when the amp is plugged into the other input on the D11 that its not working as it should.

ive checked my chasis ground as well and it seems fine. the weirdest part of this is i used it fine for awhile then it just sort of started happening randomly (though thinking now i may have initially had the gain low enough on the amp so to have not noticed the problem... if that or the volume is low enough it seems to work ok).

help??? this is my first time doing a project of this scope and im still learning so sorry if ive left out important information or if ive been unclear.

thanks!

[tpryan]

Does the squealing occur with only the D11 and guitar amp connected?

Try plugging the preamp chassis and the amp into the same outlet strip. Any change?

Have you tried different / shorter guitar cables?

[photoresistor]

yeah the squealing only happens when the the guitar amp is hooked up to the second input of the D11... it works perfectly fine (and sounds awesome btw!) when im just going to straight in (and not thru to an amp).

i will try those ideas and report back! cables were somewhat long (20ft) but they are good quality canare ones (not sure if that really matters or not).