Electronic – Sizing an RC snubber


My fridge is sending bad clacks to my loudspeakers when it turns on/off, I fear for the tweeters. I'd add a snubber/suppressor to the compressor but my notions about RC filters from school are 15 years old and I have no idea how to dimension it.

It's an old european fridge, so mains are 220V 50Hz, and the compressor mentions two other values In=0.8 and Icc=6.3, which I suppose are nominal and startup current. Do I need to compute the motor inductance? What condensator/resistor values to pick? Is an RC snubber even the correct fix?

How to estimate the RC values for snubbing an A/C motor?

Trying another approach. I've found a some usual R/C values for suppressors. For instance this one is 47ohm / 0.1µF, giving 33kHz of cutoff frequency… but the datasheet says up to 62Hz? Why is there 3 orders of magnitude difference ?

EDIT: in the end I added a capacitor to the fridge and that resolved the problems. That was quite long ago so I don't remember the exact value, but if anyone wants it just bump me…

Best Answer

The fridge may be dumping noise on your power line, but the real problem is the amplifier driving your speakers. Either the amp itself has bad line filtering, or your setup allows noise to get into its audio inputs.

Disconnect the audio input from the amp and see if the fridge can still cause clicks in the speakers. If so, then the amp has a crappy power supply circuit and needs help in the form of a line filter.

If the noise goes away when the input to the amp has been disconnected, then the noise is already on the audio signal by the time it gets to the amp. There are two likely causes to this, bad shielding and bad grounding. If you are using regular off the shelf shielded audio cable and it is properly connected at each end, then shielding is probably not the cause. The most likely cause is then a ground loop. Is the amp plugged into the same outlet as whatever equipment is producing the line level audio signals going into it? If not, then you have a ground loop. The fridge is probably causing a significant ground bounce when it turns on and off, and this is getting into your audio signal due to the different ground points. Plug all the audio equipment into the same outlet strip plugged into a single outlet in the house. Adding a line filter to that wouldn't be a bad idea.

You can try to reduce the noise the fridge is making, but that's just one noise source of many that the audio system needs to be immune to anyway. Crap on the power line happens, regularly. The audio system should be set up to deal with it properly.