Annoying 50 hz hum

[fallonp]

I'm getting a very annoying ground loop -esque 50 HZ Hum in my Project Studio when I turn up the input Trim on my pre-amp mic input.

I am using:
Cubase 4.52 on an XP PC
RME Hammerfall DSP AES-32 soundcard
Focusrite ISA 220 pre-amp
AKG Solidtube mic (which uses it's own Power supply unit)
My troubleshooting so far has established the following facts -

1. All of my AC Power supply plugs go through the same multi plug.
2. I've changed all the power supply leads to rule out dodging wiring - this had no effect on the hum.
3. When I press the ground lift on the AKG Solidtube PSU, it has NO Effect on the Hum
4. When I use a HOSA Groundlift on all my XLR connections, it also has NO effect on the hum.
BUT -
5. When I touch the PC chassis or the Solidtube PSU or the ISA 220, the hum reduces by approx 10-15 db
6. When I use my AKG C1000 Phantom power mic instead of the Solidtube, the Hum stops completely.
7. On occasion (normally after I have unplugged everything then plugged it back), the hum disappears, but always returns after I restart my system.

Since the ground lifts seem to have no effect, I've concluded that this isn't a 'ground loop' as such, but is instead caused by some kind of 'chassis grounding leakage'?!? since the hum can be fixed by touching the equipment chassis's i.e. grounding it through myself.

I'm considering a balanced power supply -
http://www.airlinktransformers.com/b...r-supplies.asp

But would rather save the money because my cashflow is currently extremely tight.

I've read about star-grounding - I tried connecting a wire from a screw on the back of my PC to the mains Earth (mimicing me touching the PC) but this had no effect on the hum.

Any advice on where to go from here would be greatly appreciated!

Cheers,

Paul
---------------------------
www.fallonmusic.com
Indie Power Pop with Pianos from South East London

[unsolved]

A silly question: Is everytning connected to a grounded wall socket??

[fallonp]

Yes, I've tested the wall socket and multi-plug using a Mains Socket Tester which confirms that the Ground is Ok

[HK26147]

When I touch the PC chassis or the Solidtube PSU or the ISA 220, the hum reduces by approx 10-15 db

That to my experience says a difference in ground potential.
I'll assume you have used a meter to check all wires and connectors for integrity.
Make sure no power grounds are lifted anywhere!
Here is an experiment: Use a strand of hookup wire and touch between the metal chassis of the "affected units" and other chassis to hear if there is a reduction.
Old turnable veterans know that the many tables had a ground wire that was to be attached ( usually to a preamp/amp chassis ).

[Nigel Goodwin]

Are ANY of the units actually grounded?, as touching the chassis reduces the hum this leads me to believe that you no ground connection at all.

[unsolved]

Originally posted by HK26147

Old turnable veterans know that the many tables had a ground wire that was to be attached ( usually to a preamp/amp chassis ).

Ouch!! that means i'm old....and veteran... and i'm not 40 yet.... Sure, the newer the thing is, the faster it gets old... (i think i will put that line to my signature :-)

anyway:
It was when the solitube was connected he had the hum... I haven't been working with them, can it be something wrong with the psu for it? Have you tried it in another mic preamp??

[Conrad Hoffman]

I'm not familiar with the equipment, but if you touch ground and the hum is reduced, you're the source of the hum! It's proximity to something that's incompletely shielded. Look for some low level mic, wire, circuitry or screen that's not grounded. AFAIK, simple guitar pickups are prone to this, and it's inherent in the design, thus "hum buckers" were born. BTW, if the 50Hz really bothers you, just move to the USA and it will be completely gone. Unfortunately it will be replaced by even more bothersome 60Hz hum!

[jfofnj]

I have built 2- f2b clones, but need to know how the 3 outputs are wired. one is for ch. a, one for ch. b. The third combines both channels and cancels the other two out when nothing is plugged in a or b. What resistors are used and im guessing you use a grounding lug jack. any ideas?

[fallonp]

FYI - Following tresting, I've concluded that my mic PSU has a fault which I will have to get repaired.

Thanks for all postings