First post, by darry
I have an Akai DPS12 that I use as a mixer and a retro PC (PC, monitor, OSSC, HDMI scaler) with multiple sound cards (AWE64 Audiotrix 3D/XG and Gravis Ultrasound) as well 3 MIDI modules (MT-32 , SC-88 and MU500 ) and a component input switcher .
Setup :
- Everything is plugged into the same power bar (MIDI modules, DPS12, PC, monitor, OSSC, HDMI scaler, monitor, component input switcher)
- There is an Ethernet cable connected between the PC and and Ethernet switch (switch is on the same circuit, but different outlet) .
- The DPS12's optical S/PDIF output is plugged into an AVR (hardly ever used).
- Other than b) there are no connections to other AC powered devices .
- The AWE64 and Audiotrix 3D/XG are each connected directly to a dedicated stereo input on the DPS12
- The MT-32, SC-88, MU500 and Gravis Ultrasound are connected to the component switcher, which in turn is connected to a dedicated stereo input on the DPS12 .
- line outputs on all cards and modules and gain on DPs12 are roughly adjusted to similar volume levels, but below clipping level .
- All midi modules are fed from the AWE64's midi port using a joystick MIDI adapter from the 90s and a MIDISPORT 2x4 in passive mode with one out looped-back to input to act as three output MIDI thru box .
Observations/issues :
1) When I isolate either the AWE64 or Audiotrix 3D/XG on the DPS12 mixer and set a high volume on the DPS12's headphone out , I get hum
2) When I isolate the input used by the component switcher on the DPS12 mixer, select the Gravis Ultrasound and set a high volume on the DPS12's headphone out , I get hum .
3) When I isolate the input used by the component switcher on the DPS12 mixer, select the MT-32 and set a high volume on the DPS12's headphone out , I get no discernible hum .
4) When I isolate the input used by the component switcher on the DPS12 mixer, select the SC-88 and set a high volume on the DPS12's headphone out , I get ALMOST no discernible hum .
I found 2 Nexxtech branded ground-loop isolators in my stash of parts and installed them on AWE64 and Audiotrix 3D/XG outputs, which completely eliminated the hum on those devices .
I intend to get another ground-loop isolator if that is what is required to fix remaining issues 2 and 4 .
My 2 questions are :
a) Why is there an apparent ground loop between devices on the same ground .?
b) Is there a better way to fix this than with isolation transformers ?
EDIT: Disconnecting the MIDI adapter cable from the AWE64 or the Ethernet cable from the NIC changes nothing to the hum .
EDIT2 : The impulsive idiot that I am ordered 3 of these Rolls HE18 https://www.bhphotovideo.com/c/product/380761 … 8_Buzz_Off.html