I have just experienced my first issue with dc offset on the utility system. This is due to the threshold new to me amplifier with a toroidal transformer. I have been gone from home for some time and upon returning powered up the system to get a music fix. The threshold made a rather aggressive growl. I turn the unit off thinking the tranny had given up. After a quick inspection I noticed that the transformer was rubber mounted and concluded that MAYBE tightening the mounting bolts might cure the noise. No. So next checked the rectifier bridge and main caps, nothing wrong. To the internet and the discovery of dc offset problems could cause hum on transformer, especially toroidal ones.
I measure the offset around the house and found universal 1.9vdc offset. Wow, is this normal? I cycled through all the breakers in the panel with no change in the offset. The cause of the offset did not appear to be anything in my house. With a .5 ohm primary dc resistance and a 1.9v dc source, well you can see the problem. Not sure what has changed as I have been running this amp before without this issue, ok last time I use this amp was 3 months ago.
There are units made to address this problem, but what could have change to create this problem? No new houses/business in the area, or changes in my house?
I measure the offset around the house and found universal 1.9vdc offset. Wow, is this normal? I cycled through all the breakers in the panel with no change in the offset. The cause of the offset did not appear to be anything in my house. With a .5 ohm primary dc resistance and a 1.9v dc source, well you can see the problem. Not sure what has changed as I have been running this amp before without this issue, ok last time I use this amp was 3 months ago.
There are units made to address this problem, but what could have change to create this problem? No new houses/business in the area, or changes in my house?
It could be a number of things causing an imbalance in the distribution network. Easily remedied though Mains DC and Transformers
Thanks for the reply. Just roughing out the requirements it appear like 2 ea, 10000uf 63v caps and 4 ea hight current diodes might suffice. I understand placing electrolytics neg. to neg. pass ac signals like a non polarized cap, I am a little reluctant to use this type of connection in a load bearing circuit. I understand that the voltage across the caps remains small, but most recommendation for caps to be used on any mains should be x2 types with self healing features.
I may monitor the dc offset for some time to determine if this is just a temporary situation. I have live here for 20 year and never had this happen. Of course none of my previous equipment had a toroidal transformer.
I may monitor the dc offset for some time to determine if this is just a temporary situation. I have live here for 20 year and never had this happen. Of course none of my previous equipment had a toroidal transformer.
I could check with the utility after the holiday weekend. I did google for a possible state or national standard for dc offset but didn't have much luck. Most of the sites that had any information that I could find indicated level in the mv range, nothing like 1.9v(using a good DVM).
It's fairly easy to figure out whether it's the utility or your house that's the source, if you know your way around an electrical panel and are willing to open all your breakers. Also, I think that Rod Elliott page is out of date (2008), there are probably a lot more appliances putting DC on the mains with CFL and LED lighting.
Line ac voltage at the time of the dc measurement was 120vac. I did check the dc offset this morning, still 1.9vdc. I will check the offset on a different utility transformer in the neighborhood.
Check with another meter. I'm not convinced all DC meters read correctly with very large AC voltage on the probes.
Also try with say a 0.05uFd 400V cap in series with the meter, to block DC. Be Careful!! The exposd cap legs are deadly, and the cap may hold stray charge for days.
Also try with say a 0.05uFd 400V cap in series with the meter, to block DC. Be Careful!! The exposd cap legs are deadly, and the cap may hold stray charge for days.
All measurements are with a BK 2709B. Using a .022uf cap, the offset remains at 1.9vdc.
I have used the threshold here 3 months ago with no issue. The area is a resort with little occupancy most of the time, and there is no new construction close by. I checked the Denon AVR system no issue. The treshold has a distinct 60hz buzz not a hum. There is no noise or buzz/hum on the output/speakers. The transformer is encapsulated in an epoxy like substance. Doesn't seem likely to have a loose winding.
I have used the threshold here 3 months ago with no issue. The area is a resort with little occupancy most of the time, and there is no new construction close by. I checked the Denon AVR system no issue. The treshold has a distinct 60hz buzz not a hum. There is no noise or buzz/hum on the output/speakers. The transformer is encapsulated in an epoxy like substance. Doesn't seem likely to have a loose winding.
I have checked another house not on the same utility transformer and there is the same 1.9vdc offset. In my absents of three months, something has changed to cause enough offset to affect the toroidal transformer or the transformer has a problem. Any Ideas?
What do mean the offset remains with the cap? Are you measuring with and without the cap in series? If you are reading 1.9VDC with a cap in series then your meter is wrong.
I see your point. If the offset measures the same with and without the cap, what does this show us? How can I accurately measure the dc offset? Does this confirm a bad transformer?
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Maybe you could describe the problem more? You are hearing mechanical buzzing, or a hum coming through the audio? The amp did it ever since you plugged it in, or developed it over time?
The threshold operates as in the past. There is no issue with the output.
I have checked the main PS rails, both +47v and -47v seem fine. I believe it started suddenly upon my return from being absent for 3 months. I guess I would discribe the sound as a hard edged hum(a "arnn" sound to it not a uum). The sound is distinct and loud enough to have not been missed in the past.
I have checked the main PS rails, both +47v and -47v seem fine. I believe it started suddenly upon my return from being absent for 3 months. I guess I would discribe the sound as a hard edged hum(a "arnn" sound to it not a uum). The sound is distinct and loud enough to have not been missed in the past.
The "arnn" noise is constant. Starts up 1 sec after turn on. I would describe it as a overload transformer hard driving sound.
Just turned the system on and play a disc turned up loud. Sounds great, no issue with the sound. It must be something of course, but what? will try this subject on pass forum, maybe someone has had this problem.
It may sound daft, but easy and worth trying, is reversing the transformer primary. Depending on how the Earth is bonded it just may make a difference
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