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Hum/buzz when equipment is not plugged in Dynamics Processors (HW)
Old 26th November 2016
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chrismeraz's Avatar

Hum/buzz when equipment is not plugged in

I hear hum or buzz in a return from a hardware eq, even when the eq is powered down and unplugged (no 240V input, no audio input)!

I verified it's not the cables or my interface, and all my equipment has a common ground.

How can a unit cause noise when it's not even powered up?
Old 26th November 2016
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chrismeraz's Avatar


I powered down the unit and removed the power cable. When I short across XLR pins 1 and 3 with a screwdriver, the noise disappears.

I am not an electrical engineer. I will not try this while the unit has power. But does this narrow down the problem?
Old 26th November 2016
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chrismeraz's Avatar

I actually hear this noise coming from all my outboard equipment! But it sounds different in each case. Only one channel of my ELOP makes noise. The Langevin preamp is dead quiet. I'm sorry to be so vague, but this is something I have no experience with.
Old 27th November 2016
Registered User
Anything with a transformer input is likely to hum until you connect a load to it. That's normal and expected ... an open ended transformer coil is essentially a little radio receiver for hum ...
Old 27th November 2016
Registered User
For diagnosing hum I sometimes use a single coil guitar pickup and a battery amp, so I can sniff around the room looking for sources of hum. It seemed a bit spooky when I first did this, because I found that non-powered things such as mic stands, keyboard stands etc emanated hum! But I now realise that the EMI fields are very strong in all rooms, unless you build a Faraday Cage, and this is mainly coming from the AC power grid - nothing you can do about it. But steel is about 1000x more permeable to magnetic flux than air - so steel object have this effect of intensifying the hum, and this cause current to flow in the steel and it emanates it's own field. This is exactly what grounding is all about - steel cases are great for diverting this EMI away from your signal. But in some cases, it can cause hum ... sheilding and grounding is a massively complex subject and lots of ways to screw this up, between different equipment and cable makers - the more you can understand how this works the easier it is to diagnose.
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