Mic preamp Hum - my Multimeter Common probe kills it !?

Hi Guys,

I have a small mic preamp (That 1510 based) on the bench at the moment. It makes a small hum so I decided to have a look at it. The moment I touch the ground path with my digital multimeter's common probe, the hum goes away! 

My multimeter is a Fluke 87. What's connected to the Common probe inside a multimeter?

Thanks

elskardio said:

Hi Guys,

I have a small mic preamp (That 1510 based) on the bench at the moment. It makes a small hum so I decided to have a look at it. The moment I touch the ground path with my digital multimeter's common probe, the hum goes away! 

My multimeter is a Fluke 87. What's connected to the Common probe inside a multimeter?

Click to expand...

Nothing but the reference ("ground") of teh circuit.
I guess your preamp is totally floating; you should earth it.

That sort of thing is a very common problem when building circuits on the bench. You really need to think about grounding from the beginning. For a mic pre that means having a really solid ground wire with pin 1 of your XLR in connected to it and then to your power supply ground. Then either the power supply ground needs a sturdy connection to earth or, if it's floating like an SMPS, then the ground of the device needs a study ground path to something which is presumably whatever it is you're listening to the output with. But the fact that the noise goes away from touching a hand-held meter means that the noise is electromagnetic / electrostatic induced. There is electromagnetic noise all around us. The most common is mains noise from wires in the walls or extension cords nearby. The more current in those wires the stronger the noise. Those magnetic fields will induce currents in high impedance circuitry. If those currents make it into a high gain amplifier like a mic pre, they will be amplified and heard as hum. Because the noise goes away from a device not connected to ground (your hand-held meter), that means that the circuit does not have a good path to ground and that capacitance of your body touching the meter touching the circuit, is enough to AC couple the noise to ground. Meaning the entire circuit is floating as Abbey said. The solution is to run a wire from your power supply filter cap ground to an earth point (normally a bolt in a chassis that is connected to the earth wire from mains) OR make sure that the ground of the output is connected properly to the ground of the device that it's plugged into and that it has a proper connection to earth. These days it's very common for everything you're connected to to be running SMPS and be "floating" so you might have to really think about what has a proper connection to earth. So my preference is to have a proper earth connection from the mic pre supply to earth ground. For many devices this is not technically required to achive good performance. You could have an SMPS powered device without a proper earth ground (aka "floating") and then a balanced or even just an impedance balanced output and that is enough to eliminate ground related hum. But for a mic pre or a guitar input where the input is connected to a device that needs a good path to earth like a guitar or a mic body, you really should have a solid path from the supply of your device to earth ground (using said chassis bolt). But for now try just connecting an alligator clip jumper from the ground of your device to something that you know is earth grounded. That should make the noise go away. But it is very common to have multiple independent ground problems if you're not thinking about proper grounding from the beginning.

There are classic stories about expensive government/military rack gear shipped with scope probes still attached because that was the only way it worked.

JR

JohnRoberts said:

There are classic stories about expensive government/military rack gear shipped with scope probes still attached because that was the only way it worked.

JR

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Ah, the wonders of low-bid and/or crony contracting never cease!