Thread: Power grid interference: What does it look like measured with different tools?

If you're interested to see what power grid interference looks like, take a look at the videos below, or a comprehensive article about it here: F4SX Audio Power Filter - RDacoustic.cz (explains in detail what I did here and includes background info + why is it important in audio). A few months ago, I posted measurements of power grid interference with and without a power filter using a spectrum analyzer. Other measuring techniques include EMI line meters and oscilloscopes.


Measurements using an EMI meter:





Using an oscilloscope:





A closer look:





And using a spectrum analyzer:





This is Europe. I would be interested to see what it looks like on other continents if anybody has any sample measurements or experience.

The only place these trivial AC line irregularities have any impact is at the audio output of your components.
An well designed component will be able to deal with much larger irregularities.
If fact if you were to examine the AC power in to your amp with a current probe, you would see that the amp converts those pretty sine waves in to almost square waves.

Originally Posted by Speedskater

An well designed component will be able to deal with much larger irregularities.

A filter suppresses high frequency interference that comes from the power grid. The vast majority of audio components rely only on a simple transformer in the source section. However, due to its parasitic internal capacitance, high frequency interference gets from the source part to the signal. Most electronics then address this with a strong feedback loop, which diminishes the detail we were looking for in music in the first place.

Originally Posted by Speedskater

If fact if you were to examine the AC power in to your amp with a current probe, you would see that the amp converts those pretty sine waves in to almost square waves.

Absolutely, but this happens at 50–60 Hz, 100–120Hz if you want: I don't know what this has to do with high frequency interference that is the subject of our article. Speaking of amplifiers and your "square waves"... the area of interest here is at the edges where interference occurs at the switching diodes. This interference, which is in the DC part already, is addressed by hardly any manufacturers. If only, among others, because in the DC part, the solution would have to be large and expensive.

We go for details in music, so we use high sensitivity drivers that are capable of reproducing those details. With low-sensitivity speakers, or when someone uses equalization, for example, such details obviously won't be audible. Whether discarded by strong feedback in the amplifier or anything else, doing this kind of thing will loose sense. But there's no doubt it makes sense in our audio chain...

Power line noise and interference are best examined at the audio output of the connected components.
Circuit design engineers know of the probability of power line noise & interference and design the power supply and the audio circuit accordingly.

Originally Posted by Speedskater

Power line noise and interference are best examined at the audio output of the connected components.
Circuit design engineers know of the probability of power line noise & interference and design the power supply and the audio circuit accordingly.

Correct. And a signficant proportion of the "power line noise" in our audio systems comes from the full-wave bridge rectifier power supplies in...the components themselves. Many folks forget it's AC...alternating current. The current goes both ways, into the component and out of the component.

Originally Posted by DavidPiska

If you're interested to see what power grid interference looks like, take a look at the videos below, or a comprehensive article about it here: F4SX Audio Power Filter - RDacoustic.cz (explains in detail what I did here and includes background info + why is it important in audio). A few months ago, I posted measurements of power grid interference with and without a power filter using a spectrum analyzer. Other measuring techniques include EMI line meters and oscilloscopes.


Measurements using an EMI meter:



Using an oscilloscope:




A closer look:




And using a spectrum analyzer:




This is Europe. I would be interested to see what it looks like on other continents if anybody has any sample measurements or experience.

To get more feedback on measurements and experience, you might want to post your results in a forum that is objective-oriented such as ASR.

If you had a current probe (unfortunately very pricey) rather than a voltage probe, and examined at the AC inlet of each of your hi-fi components, you would see that the above before traces are very reasonable.