June 6th, 2018, 5:05 pm
June 6th, 2018, 5:18 pm
June 7th, 2018, 7:57 am
tomp wrote:This is a follow on to the noise problem with laptop power supplies and DACS. This is not meant to be an all encompassing solution but you may find it interesting as I went down the road with both switched and linear supplies.
I had noticed a significant amount of noise when using my HP Envy laptop with the standard switched mode supply that was supplied with the PC. When I lifted the AC supply, the noise immediately went away and then returned upon plugging it back in. I was using a DAC I got off ebay that uses an XMOS XU208 USB receiver andAK4490 DAC chip set. I got the noise when I used it with an external DC supply and also if I disconnected the supply and ran off internal USB power. The DAC can run either way. I also got it when using a DAC with an ES9018 chip set.
Next I decided to build an interface to the laptop and a linear lab power supply that is adjustable to 30 VDC and can supply 20 A. It has a very clean output. The interesting thing is that the HP laptops use a three wire DC supply. One lead is the positive 19.5V power source, one is the negative power return, and the third is a signaling lead that tells the PC which model of supply is being used so that an underpowered adapter with the same plug will not allow the laptop to use that supply. My supply is a 90 watt model and the center pin of the plug is connected to the positive supply through a 220K resistor. Apparently the lower powered supplies have a higher value of series resistor. Internally that lead is probably connected to a divider. If the voltage drop on that signaling lead is too low because of the higher impedance of the smaller supply the laptop will not use it. This is just background if anyone is planning to build an adapter for HP laptops. I'm including the schematic of adapter I build. Note that there is a reversed diode across the inputs after a fuse. If you plug the leads in backwards on the external supply, the diode will clamp the output and blow the fuse protecting the laptop against damage.
When I tested the linear supply I was anxious to see if the noise went away. Oh s__t! It was still there. So much for my theory that the noise was from the switching nature of the power supply. Whether I used the switcher or linear supply, the noise was the same. As soon as I unplugged either supply and ran off battery the noise went away.
One thing I did notice when building my interface to the linear supply was that the switcher had a three wire cord and the earth ground pin on the plug was connected to the return lead on the DC output. On my interface I provided a separate line to the ground terminal on the lab supply. For grins while the system was running with all the noise, I lifted the ground line from the lab supply and viola, the noise went away. As soon as I connected the earth ground of the supply to the adapter the noise returned.
Then I plugged the switcher back into the PC and the noise came back. I got a cheater (ground lift adapter) and put it on the end of the switcher supply and plugged it back in. Oh joy, the noise was again absent. So it seems that the safety connection on either the switcher or the linear supply caused the problem between the PC and the amp which also has an earth ground. Since lifting a safety earth ground is never a good idea the solution is probably to use a noise filter on the switcher or all the equipment. If you simply lift the ground connection and then get an AC short in the power supply it will find a path to ground either through you or other equipment to ground neither of which is a good solution. I have a Corcom noise filter and will be trying that when I get time. Stay tuned.
June 7th, 2018, 8:10 am
June 7th, 2018, 8:55 am
Pelliott321 wrote:Does it makes ice to measure voltage between neutral and ground?
Can one see noise with a scope between neutral and ground
June 7th, 2018, 9:04 am
June 7th, 2018, 9:33 am
June 7th, 2018, 10:22 am
June 7th, 2018, 10:57 am
June 7th, 2018, 11:06 am