DC on AC line
 

This is a good question for Dan. I have 650mv dc on the AC line. I checked this because things just didn't sound right. Dan how much dc is acceptable ?

I use a Burmester 948 power conditioner, which removes DC on the line.

When I forget to engage the DC circuit in the 948, such as after a reboot of the system, it takes me about 30 seconds to realise something is wrong. I have around 250mV on the AC line. 650mV seems very high.

Tom.......First, how are you coming up with the 650mv measurement? Are you measuring with a DMM or an oscilloscope? Which wires are you measuring, i.e. hot to neutral, hot to ground, or neutral to ground?

It is not unusual to have some level of DC offset on AC lines in your home. This is often caused by variable speed AC motors like those found on a hair dryer and by switch mode power supplies in electronics and computers that create DC by chopping half the AC sine wave to make a pulsing DC output. This causes the other half of the sine wave, also a pulsing DC, to be back fed onto the neutral which generates heat in the cores on transformers. Under perfect conditions the neutral tap on any transformer should have zero AC or DC when referenced to ground.

650 millivolts (0.65 VDC) is not a serious problem on a AC line but ideally zero DC would be the best. You can probably reduce the DC offset on your home power by turning off (unplugging) anything with a switch mode power supply when not in use.

Dan, I thought the issue was coming from switching power supplies and linear PSU was the way to go.

Georges.......Not all linear power supplies are created equal but you are correct that the largest contributors to DC offset on AC power lines are switch mode power supplies.

It is easy to overlook just how many switch mode power supplies are in the typical home. Nearly all wall wart charges that make DC for cellphones, calculators, laptop computers, tablet computers, iPods, rechargeable flashlight batteries, security cameras, LCD displays, desktop computer speakers, routers, and other devices can all contribute to the DC offset issue on a home's AC lines, not to mention switch mode power supplies in desktop computers and power bricks that produce DC power for audio devices.

Dan I'm using a Fluke Model 87 (true rms) DVM and measuring hot to neutral. My audio line is direct to the box with nothing else on the circuit. However there are several devices in the house which have switching power supplies. :yes: