Measuring earth currents - tips?

[primalsea]

Is there any best way to measure earth currents with a scope where the act of measuring does not effect what is being measured?

I want to track down the last traces of noise in my phono stage and suspect that it is a mix of mainsbourne EMI pick up and localised ground loops, the first stage of the amp seems very sensitive to these.

I wondering if I will end up chasing my tail where the measuring effects what is being measured.

 

[JonSnell Electronic]

There should only be an earth current under a fault condition.

 

[primalsea]

Yep, could have phrased that better. I mean measuring very low level ground loops that might exist and be causing noise in the output of the amplifier.

 

[ubergeeknz]

I think you'll have a very hard time, as you've perhaps figured out the meter itself will dramatically affect the situation.

Post pictures of your layout and wiring scheme, and I'm sure someone here can help you identify your problem.

 

[Speedskater]

If you are doing voltage measurements then use a filament or power transformer as an isolation transformer connected to the O-scope probe.

 

[Vovk Z]

I think you'll have a very hard time, as you've perhaps figured out the meter itself will dramatically affect the situation.

- if we use battery powered multimeter then you are right - there isn't a big problem. But if we start to use mains-connected devices (oscillograph or else) - then the problem arises at full height.
For example: my Tektronix 2465A have some portion of bad old film X2 caps in power supply (I had changed only one yet, it was several years ago). So when I connect the osc probe to the device under test (audio power amplifier for example) and look at on-line spectrogram - I clearly see additional mains harmonics (they were much lower several years ago, so I see that some more X2 caps died).
The solution may be, for example, to use battery powered oscillograph - modern digital devices give us such a possibility.

 

[trobbins]

Are you focusing on pre-amplifier circuitry, or the wiring and pickup outside the pre-amplifier, or a main amplifier ?

If it relates to quite low noise and hum levels (considering the phono input application), then I'd suggest using a soundcard and software spectrum analyser setup, which can provide a noise floor well below any record. In that same vein, you won't be able to lower the record noise floor, so reducing circuit based noise has a diminishing return on effort.

 

[DF96]

Don't measure ground loops. Get rid of them.