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 Post subject: Re: Line noise filters
PostPosted: July 7th, 2020, 9:33 am 
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Posts: 257
SoundMods wrote:
Grover Gardner wrote:
Walt, it may ineed be that, but with the inputs shorted, where's the loop? I've suggested cheater plugs which he will try, but I suspect the loft room of the condo where the stereo is is on a circuit with some dimmers. This wasn't a problem until the hot weather hit.

Although I hate to suggest it -- it might be an issue with the power-supply filter capacitors. Aluminum electrolytic caps can be very reliable, yet sneaky. Default failure can be an internal short (very nasty) or an "open" condition where the measured capacitance drops like a stone. Why an open condition? I don't know, but I have experienced it especially if it cannot work with the amount of ripple current it has to deal with. I found that power supply capacitor ratings do not stop at size, WVDC, and temperature. I found that a major rating that goes unnoticed is ripple-current capability. If undersized that way even 105-degree C rated caps can get into trouble. It can start with a relatively benign hum at first --then all bets are off.


That's possible, Walt, but *both* amps exhibiting the same level of of hum, which reportedly diminished when the ceiling fans were turned off? The amps are only six months old (I built them) and I've used the same Cornell-Dubilier caps in my own version of the same amps for over a year now without any problems, and for years before thatin other projects. But it's a possibility. I'd like to explore either line noise or ground issues before I have him send them back for a rehaul.


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 Post subject: Re: Line noise filters
PostPosted: July 7th, 2020, 10:12 am 
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Maybe he can run an extension cord to an outlet on a different circuit to see if it persists?


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 Post subject: Re: Line noise filters
PostPosted: July 7th, 2020, 10:21 am 
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Location: Parkville, Maryland
Grover Gardner wrote:
SoundMods wrote:
Grover Gardner wrote:
Walt, it may ineed be that, but with the inputs shorted, where's the loop? I've suggested cheater plugs which he will try, but I suspect the loft room of the condo where the stereo is is on a circuit with some dimmers. This wasn't a problem until the hot weather hit.

Although I hate to suggest it -- it might be an issue with the power-supply filter capacitors. Aluminum electrolytic caps can be very reliable, yet sneaky. Default failure can be an internal short (very nasty) or an "open" condition where the measured capacitance drops like a stone. Why an open condition? I don't know, but I have experienced it especially if it cannot work with the amount of ripple current it has to deal with. I found that power supply capacitor ratings do not stop at size, WVDC, and temperature. I found that a major rating that goes unnoticed is ripple-current capability. If undersized that way even 105-degree C rated caps can get into trouble. It can start with a relatively benign hum at first --then all bets are off.


That's possible, Walt, but *both* amps exhibiting the same level of of hum, which reportedly diminished when the ceiling fans were turned off? The amps are only six months old (I built them) and I've used the same Cornell-Dubilier caps in my own version of the same amps for over a year now without any problems, and for years before thatin other projects. But it's a possibility. I'd like to explore either line noise or ground issues before I have him send them back for a rehaul.

OUCH! Let the hum chasing begin! It's definitely not my favorite indoor sport.

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 Post subject: Re: Line noise filters
PostPosted: July 7th, 2020, 10:50 am 
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Finding noise problems is one of the most difficult things you can do. As far as power supplies, a good supply should essentially allow the audio circuit to not see the power line. If you have enough filtration and HF noise suppression the PS should act like a very low pass filter to the line.


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 Post subject: Re: Line noise filters
PostPosted: July 8th, 2020, 11:00 am 
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Jim G wrote:
Maybe he can run an extension cord to an outlet on a different circuit to see if it persists?


He's going to try that and report back.


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 Post subject: Re: Line noise filters
PostPosted: July 8th, 2020, 11:24 am 
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Location: Parkville, Maryland
Grover Gardner wrote:
Jim G wrote:
Maybe he can run an extension cord to an outlet on a different circuit to see if it persists?


He's going to try that and report back.


A point of information -- a different outlet is not a different circuit -- it is typically only a different circuit-breaker protected feed off of the same circuit.

Now if he can try to see what receptacles are on which 240-volt pole they are on (the left side of the panel-board vs. the right side) that may help. At least if one amp. is on one pole and the other on the other that puts the amps at opposite ends of the service-entrance transformer. A speculation at best -- but what the hell -- it may be worth a try.


Attachments:
Residential Power Distribution.jpg
Residential Power Distribution.jpg [ 27.52 KiB | Viewed 347 times ]

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 Post subject: Re: Line noise filters
PostPosted: July 8th, 2020, 11:27 am 
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Residential breaker panels don't put the two phases of the 240V line on opposite sides, they put them on alternating slots in the panel so that two adjacent slots can be used with a two-pole breaker for 240V loads....

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 Post subject: Re: Line noise filters
PostPosted: July 8th, 2020, 2:14 pm 
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Roscoe Primrose wrote:
Residential breaker panels don't put the two phases of the 240V line on opposite sides, they put them on alternating slots in the panel so that two adjacent slots can be used with a two-pole breaker for 240V loads....

That's right -- but we're talking about single-pole that's derived from one pole referenced to neutral. 2-pole breakers for 240-volt service will connect to both poles. Granted the single-pole protected feeds alternate as you drop down the panel from top to bottom - one pole to the other, but one can choose (with a bit of effort) one feed on one side and one feed on the other side and hope that the alternate receptacles are near enough each other for the trial.


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panelboard layout.jpg
panelboard layout.jpg [ 10.97 KiB | Viewed 343 times ]

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 Post subject: Re: Line noise filters
PostPosted: July 8th, 2020, 2:29 pm 
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Joined: January 15th, 2015, 7:19 am
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Location: Baltimore MD
what works for me is unplug everything. nothing connected together nothing plugged into the mains.
Start with the power amps hook up to speakers, plug it in, turn on listen for hum. If there is hum then short the inputs. If the hum goes away then there is a problem in the amps.
next hook up preamp and go through the same procedure.
add your sources one at a time, go through the same procedure and so on.
you will soon find the culprit. The the mains on that piece and hopefully that will fix the problem


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