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tokyofan · Posted: Mon May 24, 2010 9:02 am Post subject: Mic tests and hum

A friend stopped by yesterday to audition a few new MXL mics and just about every one of them had a distinct hum when run though my regular signal chain: mic --> Speck 5.0 --> RME AD-2--> Mac-Mini (via Digital in/out). Odd as my Neumann U87ai (in the same chain) exhibits NO hum. We then tried his mics through a couple USB preamps plugged directly into the Mac-Mini and no hum! So, it appears there's a ground-looping issue somewhere in my chain that MXL's are picking up on (but not the Neumann). Any ideas?
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Gp · Guest

Check cables one at a time....replacing them as you go. You may end up needing a ground loop isolator...it may just be your power.

Frank F · Posted: Mon May 24, 2010 10:28 am Post subject:

or... the direction of the un-shielded cables as related to power cables. Move the cables around and as GP stated, replace them one at a time with a new or alternate cable.

Frank F
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Mike Sommer · Posted: Mon May 24, 2010 10:56 am Post subject:

More than likely it is not a Ground-Loop problem on such a short chain and on modern equipment.

The hum is probably caused by a lack of ground contact.

What microphones were making the hum? (Hopefully your friend left them for you to test)

First try pressing the “Output Signal Path” (O.S.P.) switch next to the XLR output on the Speck.

Next as Gp suggested check your XLR cables, since there should only be two, just get two new quality cables. Look for Quad 4 cables.

If you are using 1/4" phone jacks at any point make sure you are using TRS connectors like this:

Let us know what you find out.
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tokyofan · Posted: Mon May 24, 2010 5:00 pm Post subject: re:

The mics in question were the MXL 3000, 67G, V63M, and 909.

I tried two different XLR cables: a Canare and a Blue Kiwi Quad....both had hum.

It might be the Speck...we tried his mics through two USB preamps AND the RME AD-2...and no noise.

From the Speck I've got a Monster XLR Male-1/4" TRS Male inserted into the RME's left Analog IN port.

I also have a Samson S-monitor plugged into the Speck's Insert Send.

What does the O.S.P. button do exactly and should I have that engaged all the time?

I no longer have the mics here but will try out the suggestions next time we test gear.Thx!
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Mike Sommer · Posted: Mon May 24, 2010 9:53 pm Post subject:

From Spec:
"When going for the absolute minimal signal path, the “Output Signal Path” (O.S.P.) switch will bypass all non-essential circuitry to the XLR pre out."
http://www.speck.com/pdf/mp50signal.pdf
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Acoustics are counter-intuitive. If one thing is certain about acoustics, it is that if anything seems obvious it is probably wrong.