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Old 19th October 2005, 03:57 AM   #1131
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Ah Ok, I did not understand (or retain in my memory ) That the relays were not triping, it does indeed sound like a power issue with your source selector.

Well time to go read a book, until tomorrow...

Cheers!
Russ
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Old 19th October 2005, 11:13 AM   #1132
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Question Loud hum with open inputs

Well, Im not sure if I should call it a problem, but for better understanding: With open inputs my REV C ( to be seen here: http://www.diyaudio.com/forums/showt...874#post744874 ) hums like hell. With shorted input there is no noise, except of a very faint trafo hum. So it is no real problem as with an input device connected there is no noise at all.

But what is the reason ? Never had such a behavior before ...
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Old 19th October 2005, 11:57 AM   #1133
mikelm is offline mikelm  England
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Default Re: Loud hum with open inputs

Quote:
Originally posted by SmellOfPoo
With shorted input there is no noise, except of a very faint trafo hum. So it is no real problem as with an input device connected there is no noise at all.

But what is the reason ? Never had such a behavior before ...

High i/p impedance, quite high gain and few unshielded parts is my guess

mike
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Old 19th October 2005, 12:32 PM   #1134
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Default Re: Loud hum with open inputs

Hi SOP,

It looks like you have your shielded cable attached to your case which is attached to mains GND. As a test try leaving mains GND unconnected and see if the hum goes away.

Even with open inputs I have never experienced this.
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Old 19th October 2005, 01:17 PM   #1135
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Lightbulb Re: Re: Loud hum with open inputs

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Originally posted by Russ White
. As a test try leaving mains GND unconnected and see if the hum goes away.
Hi Russ,

Ill try that as soon as Im home. Thanks!

Gerd
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Old 19th October 2005, 01:35 PM   #1136
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I find when I have open inputs, and no music playing, it's me that's doing the humming... as I try to get everything hooked up
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Old 19th October 2005, 03:44 PM   #1137
Nordic is offline Nordic  South Africa
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Well I bumped the wires a bit hard last night, and activated the smoke emitter, the poor resistor connected to the earth line is absolute toast. Replaced it but getting a little hum now.

Could the capacitors have packed up in the process? Or do you think the chip is damaged? Still plays fine... just the hum.
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Old 19th October 2005, 03:53 PM   #1138
troystg is offline troystg  United States
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Default Grnd loops

A few simple rules I follow.

1. Isolate the signal grd from pwr or chassis grnd. All 3 are different.
2. tie the left and right grnds together @ the input. They should be tied together @ the source output too.
3. Cross your fingers, stand on your left foot and chant a prayer to the guru gods of ground that you didn't miss anything during construction that may cause a ground loop.
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Old 19th October 2005, 04:01 PM   #1139
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Quote:
the poor resistor connected to the earth line is absolute toast
Which one? And what exactly shorted out?
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Old 19th October 2005, 04:58 PM   #1140
Nordic is offline Nordic  South Africa
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heck I should have opened a new thread, don't want to hijack... got caught up in the moment

Its the standard IGC from decibel dungeon, the resistor sits in parallel with a 100nf cap between the PGS and the earth... It basicaly connects the PGS to the chassis, which in turn is connected to the mains' earth.

I was doing some testing work with everything open so I don't know what exactly shorted, the moment I saw smoke I ripped the powercable out (benefit of banana plugs)...

No scorching on the veroboard (don't laugh), only the resistor is totaly burned out (brown), and it looks like some brown marks on the outside of the one 1000ufd cap (which is adjacent on the board), I think this is just a smoke stain though.

Was quite surprised that it still works, as the first one I build completely blew up after a small shortcircuit, with some brown stuff actualy comming out of a crack in the chip...
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