Hum, lots of hum.

[Pepsifx357]

This just recently happened. My SCA DI and 2 N72 models have been working perfectly for about 2 months now.

Yesterday I was going to play guitar through the DI and N72 and all i heard was this horrible hum and poor signal coming through. I tried different cables and completely removed all connections from the patchbay. I've unplugged everything inside the chassis and tried to narrow it down. It seems to be some grounding problem but it does it with any combination of the modules plugged in. So I'm thinking maybe the power supply? When I have a cable attached to the N72 output I can thump the cable and hear it. Also, you can hear the clicks of the switch when it is being moved as well as knocking on the chassis and hearing that.

Any help would be greatly appreciated.

Thanks,
Ben

[tpryan]

Are the 3 LEDs on the PS03 lighting up?
You hear hum from both N72s or just one?

[Pepsifx357]

Yes, all three leds light up and any combination of the modules hooked up produces this noise.

[tpryan]

If the chassis is in a rack, remove it. Disconnect everything from the outputs except a single powered monitor or headphone amplifier. Do you still get hum?

[Pepsifx357]

Here is the last thing I did. I took the rack out, opened it up, removed the top, took out all of the modules and while they were not touching the chassis, connected each module to the power supply one by one with one mic hooked up to the N72 and the output going directly into my MOTU interface. Still got the hum no matter what I did. I'll try to take a headphone amp out of the rack. Just to let you know, the rack rails are wood and I've made painstakingly sure that none of the other racks touch eachother. But I don't have a guarantee on that so that will be my next objective. When the SCA chassis was in the rack However it was above in order: SCA>MOTU MIDI>MOTU HD192>Computer>Yamaha Power amp, and below in order: SCA>Lexicon MX400>CD Recorder>Furman Power Condish.

[tpryan]

What kind of cable are you using to connect to the MOTU? If it's an XLR to TRS adapter, you must make sure that both pin 2 and pin 3 of the XLR actually go somewhere. Pin 3 can't float.

[Pepsifx357]

It's the HD 192; No worries about the pinout. I'm hoping that this is just the HD 192 touching something funny. hopefully...

[Pepsifx357]

Ok, so I took my headphone amp and the pre amps to a different room and power outlet all together and hooked them up. Sadly I experienced the hum again. And without touching anything else other than the switch knob I heard the distinct sound like when you touch the end of a guitar cable with your finger. I tapped the preamp and could hear my tapping on it. And, again, could hear the physical clicks of the switch as I turned the gain up. Almost as if it was acting like a guitar pickup. So I've done about everything I can. As far as I know. This was without a microphone hooked up, just the output to the headphone amp. Also I took the pre amp out of the chassis and set it on a cardboard box for this test.

Any Ideas, things I could try?

Thank you for your help. Still love the pre. Just confused as to what could have possibly gone wrong. I haven't used the pre in a month.

Ben

[tpryan]

You must test with the module installed in the chassis with the front panel attached and the locknuts snug on the bushings. You also need to terminate the input, either with a 150 ohm resistor or an actual low impedance source.

[Pepsifx357]

Will do....I don't think I have a resistor so I'll just use an SM-57 for the job.

[Pepsifx357]

Hey Tim,

I did what you said by putting one N72 in the box with the front, plugged up a few mics, and viola! No hum. Very wierd. What could have caused what I was hearing? I'm not out of the woods yet though. I still have to put the other N72 in there and the DI and in my rack without that horrible hum.

Thank You very much!
Ben