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 Post subject: Line noise filters
PostPosted: July 6th, 2020, 8:42 pm 
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I followed only a bit of the discussion on isolation transformers, but I have a specific question: My friend Jim lives in a small condo in Rehoboth Beach, and lately has noticed that the Williamson amps I built for him have a slight hum, very slight but it bothers me that he has to live with it. I'm pretty sure it's not the amps themselves because they were dead quiet when I sent them, and he only mentioned it recently. But we narrowed it down to the amps with the inputs shorted. Bias and balance settings are fine. I had him turn off a ceiling fan in the living room, and he thinks the noise diminished. So my thought is that, now that the warm weather has set in and they are now using several ceilings on dimmers in the condo, he's getting some line noise. What sort of device is best to limit or remove this? I don't think he wants to spend a fortune on an audiophile-type power conditioner. Is there something more basic and easily obtainable that would remove this line noise?


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 Post subject: Re: Line noise filters
PostPosted: July 6th, 2020, 8:44 pm 
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https://www.mouser.com/manufacturer/corcom/

Roscoe

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 Post subject: Re: Line noise filters
PostPosted: July 6th, 2020, 9:01 pm 
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Thanks for that, but this would have to be something external he can plug the amps into.


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 Post subject: Re: Line noise filters
PostPosted: July 6th, 2020, 9:47 pm 
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Does it need to be off the shelf? Put a corcom in a box with a duplex receptacle...

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 Post subject: Re: Line noise filters
PostPosted: July 6th, 2020, 10:12 pm 
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Location: Parkville, Maryland
Grover Gardner wrote:
I followed only a bit of the discussion on isolation transformers, but I have a specific question: My friend Jim lives in a small condo in Rehoboth Beach, and lately has noticed that the Williamson amps I built for him have a slight hum, very slight but it bothers me that he has to live with it. I'm pretty sure it's not the amps themselves because they were dead quiet when I sent them, and he only mentioned it recently. But we narrowed it down to the amps with the inputs shorted. Bias and balance settings are fine. I had him turn off a ceiling fan in the living room, and he thinks the noise diminished. So my thought is that, now that the warm weather has set in and they are now using several ceilings on dimmers in the condo, he's getting some line noise. What sort of device is best to limit or remove this? I don't think he wants to spend a fortune on an audiophile-type power conditioner. Is there something more basic and easily obtainable that would remove this line noise?

It sounds like (no pun intended) a ground loop. Maybe connecting the amps or the pre-amp to the AC using a "cheater" to delete the third pin ground bond. That has worked for me. Of course with phono playback hum chasing gets to be a chore beyond cheater plugs.

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 Post subject: Re: Line noise filters
PostPosted: July 6th, 2020, 11:24 pm 
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Roscoe Primrose wrote:
Does it need to be off the shelf? Put a corcom in a box with a duplex receptacle...

Roscoe


No it needs to be off the shelf. A medical-grade islaotion device, maybe?


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 Post subject: Re: Line noise filters
PostPosted: July 6th, 2020, 11:25 pm 
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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.


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 Post subject: Re: Line noise filters
PostPosted: July 7th, 2020, 6:55 am 
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If it is a 120 Hz hum it is probably not dimmer noise. That is generally higher frequency hash from current spikes as the triac turns on. The chokes they use in common dimmers to suppress the spikes are pitiful at best.


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 Post subject: Re: Line noise filters
PostPosted: July 7th, 2020, 6:58 am 
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tomp wrote:
If it is a 120 Hz hum it is probably not dimmer noise. That is generally higher frequency hash from current spikes as the triac turns on. The chokes they use in common dimmers to suppress the spikes are pitiful at best.


The problem is I'm not there to hear it, I have to rely on his description. If it *is* noise from his ceiling fans, is there something that will isolate the amps from it?


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 Post subject: Re: Line noise filters
PostPosted: July 7th, 2020, 8:45 am 
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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.

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