mic/pre combination hums

Keith Sylvester · 18th November 2011

Hi all. My first post in this forum. I've learned quite a bit from lurking here - thanks to everyone who has contributed to my education. I have a little problem maybe someone can help me with:

I have a JoeMeek VC2 that I use mainly for vocal tracking with a U87 and acoustic guitar tracking with a KM84. Both combinations are nice and quiet. A few days ago I put a Shure SM137 on that pre and I get what sounds like a 60hZ hum. I switched XLR cables, I switched lines from live to control room, I switched patch cables that go out of the VC2 and I switched to a second SM137 - hum is still there. I even swapped the tube out of the pre. Hum is still there. If I put the U87, KM 84 or any other mic in the VC2, it's quiet. If I put the SM137 into my 003 rack pre, it's quiet. All connections are balanced.

So why is the combination of VC2 and SM 137 "hummy"

bogosort · 18th November 2011

Hi Keith, that's an interesting issue. The first thing I would do is figure out what kind of noise we're dealing with -- is it 60 Hz hum, 120 Hz buzz, or something else? To determine this, insert a spectral analyzer plug-in to view the frequency of the noise.

Generally speaking, hum is magnetically induced (e.g., from a power transformer to a signal transformer) or caused by ground problems. Buzz can come from AC ripple on a rectified DC voltage. But since only the SM137-VC2 combination is causing noise, I'm going to guess that your SM137s are acting like antenna and the VC2 has the pin 1 problem.

My conjecture: the SM137 is picking up RF and transferring it to the shield of the balanced cable, which is what it's supposed to do. If, at the preamp end, the shield on the XLR (pin 1) is connected to signal ground in the VC2, then the RF has a path to the preamp's circuit and will modulate its ground. This can couple noise into the signal a variety of ways.

Your other mics may not be sensitive to this particular RF and thus not cause a problem. And if the 003 (properly) connects XLR pin 1 to the chassis, the RF on the SM137s never makes it to the circuit.

Again, this is pure conjecture and I'm no expert. There are methods to determine if something has the pin 1 problem (google jensen hummer pin 1, or you can inspect the device). You could also try ruling out RF by covering your SM137 with lots of tin foil and using the shortest cable you have. Whatever the case may be, I think the first step is to determine the nature of the problem. Hopefully this gives you some ideas. Good luck.

18th November 2011	#1
Keith Sylvester Gear Head Joined: Sep 2011 Location: Eastern Panhandle of WV Posts: 35 Thread Starter	mic/pre combination hums Hi all. My first post in this forum. I've learned quite a bit from lurking here - thanks to everyone who has contributed to my education. I have a little problem maybe someone can help me with: I have a JoeMeek VC2 that I use mainly for vocal tracking with a U87 and acoustic guitar tracking with a KM84. Both combinations are nice and quiet. A few days ago I put a Shure SM137 on that pre and I get what sounds like a 60hZ hum. I switched XLR cables, I switched lines from live to control room, I switched patch cables that go out of the VC2 and I switched to a second SM137 - hum is still there. I even swapped the tube out of the pre. Hum is still there. If I put the U87, KM 84 or any other mic in the VC2, it's quiet. If I put the SM137 into my 003 rack pre, it's quiet. All connections are balanced. So why is the combination of VC2 and SM 137 "hummy"

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18th November 2011	#2
bogosort Gear nut Joined: Jan 2010 Location: Miami, FL Posts: 75	Hi Keith, that's an interesting issue. The first thing I would do is figure out what kind of noise we're dealing with -- is it 60 Hz hum, 120 Hz buzz, or something else? To determine this, insert a spectral analyzer plug-in to view the frequency of the noise. Generally speaking, hum is magnetically induced (e.g., from a power transformer to a signal transformer) or caused by ground problems. Buzz can come from AC ripple on a rectified DC voltage. But since only the SM137-VC2 combination is causing noise, I'm going to guess that your SM137s are acting like antenna and the VC2 has the pin 1 problem. My conjecture: the SM137 is picking up RF and transferring it to the shield of the balanced cable, which is what it's supposed to do. If, at the preamp end, the shield on the XLR (pin 1) is connected to signal ground in the VC2, then the RF has a path to the preamp's circuit and will modulate its ground. This can couple noise into the signal a variety of ways. Your other mics may not be sensitive to this particular RF and thus not cause a problem. And if the 003 (properly) connects XLR pin 1 to the chassis, the RF on the SM137s never makes it to the circuit. Again, this is pure conjecture and I'm no expert. There are methods to determine if something has the pin 1 problem (google jensen hummer pin 1, or you can inspect the device). You could also try ruling out RF by covering your SM137 with lots of tin foil and using the shortest cable you have. Whatever the case may be, I think the first step is to determine the nature of the problem. Hopefully this gives you some ideas. Good luck.