I'm not sure I understand why a DMX or AES/EBU cable shielding would be any different than a Microphone cable. Both are Sheilded with a twisted pair. That the cable made a difference is what you found, but I don't know why..
My HD500 to DT50 (112) has alot of high frequency hissing but that is preset dependant and I suspect it is the HD500 noise, not the DT. The DT on similar amp configurations is near dead quiet (using the DT preamp vs the HD). This is with the guitar cable removed as well..
One possibility is that the digital audio (and data) flowing from the HD to the DT is getting corrupted by cable reflections (though a MIC cable is perhaps over 100 ohm but less than 600 ohm). The corruption might 'sound' like hiss or noise (and as you stated, not being 60/120Hz AC hum).
From 'the web':
AES/EBU is a 6.1MHz clocked (3.0 Mbps) data stream (clock is embedded in the data.. manchester like) and is specified for a 110 ohm balanced cable.
General microphone cables are: Analog differential (baseband and common mode DC), 600 ohm source and termination (but the cable is NOT specifically specified for differential impedance).
It is likely that the low capacitance balanced cables may be much higher differential impedance than 600 ohms but would not be much over 1k ohm (why? since it is very difficult to build impedance with close spaced conductors.... think of old 'twin lead TV cable... that was 300 ohm).
Net result is that for a run of MIC cable, the ringing bounce of the mismatch at 6MHz clocked data may cause errors in the data bits and perhaps even frame blocks (if AES/EBU spec format is used in the DT, and I suspect it is, if not just for convienience). This might explain why we are getting this noise...
I might purchase an AES/EBU cable and see if that makes any difference with my setup... (or DMX...which I believe is the same as an AES/EBU spec).
Thanks good responses. My DMX is cable also meets the AES/EBU standard. In summary DMX or AES/EBU is designed to carry data and is shielded more thouroughly where as Microphone is designed for analog phase reversal noise canceling of an analog signal (higher guage and not shielded as well). I'm sure in a lot of cases the mic cable is sufficient. In my case any mic cable picks up noise (in the form of digital artifacting) from my environment when used in this system and my DMX do not. I have a lot of computer equiptment in my studio and a 25' run to cross with the cable, so the shielding on mic cable has proven to be insufficient. link to cable I used http://www.sweetwater.com/store/detail/EBU025. Not sure if all DMX are AES3 compliant, so check the package.
My experiment with the AES/EBU cable (purchased from BestTronics) yielded no change in noise. I'm sure the AES/EBU has better digital signal performance but the noise I'm getting must just be the HD500.
It is at a level that I can't imagine recording with this configuration...At 'playing' volume it would not be perhaps noticable but in a studio environment, it is too noisy (and the DT is quiet on its own, as is the HD500 dirrect into the DAW via the USB connection. I believe I also disconnected the USB to see if that was injecting noise into the HD500-DT50 combo.... no difference..