Ahhh. okay.
Just for chuckles and grins ... Measure the AC mains with your DVM on DC volts. A variac shouldn't hum or growl unless your current approaches or exceeds the maximum rating. Not the old ones anyway. I've seen some new ones that are rated for intermittent duty, smaller cores. That means they are rated much lower continuous. We used to buy them for their continuous ratings.
Just for chuckles and grins ... Measure the AC mains with your DVM on DC volts. A variac shouldn't hum or growl unless your current approaches or exceeds the maximum rating. Not the old ones anyway. I've seen some new ones that are rated for intermittent duty, smaller cores. That means they are rated much lower continuous. We used to buy them for their continuous ratings.
It hums while idling also. Not loud but noticable and it does not change with a load. It's brand new. Uniteck STG-1000W. Probably chinese, build quality is good but nevertheless it hums...
Again a link to good guy in Australia : https://sound-au.com/articles/xfmr-dc.htm
There he explains all in details including safety percussion, so no need to repeat, you can search here on same subject too. Just be aware that DC in mains might appear and disappear , but if this is the root cause, hum will do the same.
I know this guy, very good information, just read it. I understand now that a small DC voltage can create saturation, mainly in bigger torrodial trannies and cause problems. Indeed I have experienced it once in a while, this intermittent temporary hum from a transformer and I understand now how that is possible. Good read.
Hum in my variac is not intermittent, so I doubt DC is the cause -only if DC voltage would be permanent. I suspect that the variac transformer itself has some loose laminations or whatever that buzzes. I will not build a DC blocker, by the way, but instead I am more leaning towards using my off-grid system strictly for audio again, as stated in previous posts. I have to install some additional wiring across the home but that is no hassle. The inverter has it's own safety features and circuit breaker as well as earthing, so it is safe anyway. I went this route before and it sounded a little better as well and should be able to listen for >10 hours until the batteries have to be recharged (solar power or charger via mains) and during daytime when the sun shines even longer.
Then you are fully isolated from household mains and no voltage swings anymore and no humming variac... and possibly even saving the earth (grin).
Hum in my variac is not intermittent, so I doubt DC is the cause -only if DC voltage would be permanent. I suspect that the variac transformer itself has some loose laminations or whatever that buzzes. I will not build a DC blocker, by the way, but instead I am more leaning towards using my off-grid system strictly for audio again, as stated in previous posts. I have to install some additional wiring across the home but that is no hassle. The inverter has it's own safety features and circuit breaker as well as earthing, so it is safe anyway. I went this route before and it sounded a little better as well and should be able to listen for >10 hours until the batteries have to be recharged (solar power or charger via mains) and during daytime when the sun shines even longer.
Then you are fully isolated from household mains and no voltage swings anymore and no humming variac... and possibly even saving the earth (grin).
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Sounds like a plan.
You can consider extra HF filter after inverter, built in ones are just good to meet code. Inverter will eliminate overvoltage and DC issues, but brings in switching noise...
You can consider extra HF filter after inverter, built in ones are just good to meet code. Inverter will eliminate overvoltage and DC issues, but brings in switching noise...
Last update. I finished re-installing my solar battery system. I have two inverters, one with 600W continuous (800W short term) pure sine inverter to be used with the fridge and other low wattage kitchen appliances, and one 1200W continuous (1600W short term) pure sine inverter, dedicated to my audio and computer hardware. These are Victron inverters and pretty noise free (did not measure yet, but I don't hear any noise on my >110dB sensitivity horns even with my ear stuck in the horn). The inverters have a feature that the user can configure the output voltage, so I set it to 220V (default is 230V) for the audio gear. So far so good, sound is a little better, more relaxed and my older gear has no more overvoltage to deal with. And independent from the net, in case there is a short or longer power outage, like we experienced here recently. And the variac goes back to where it belongs: to the bench.