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Old 20th November 2007, 10:53 PM   #1
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Question a resistor to ground?

Hi guys! I saw some post that some people put a resistor before chassis ground.is it for stopping any kind of noise?what is it for?.I have a Gainclone LM3875 and I did not used any resistors to ground.everything is working perfect.should I put a resistor or leave it like it is?.I just was worried about it,maybe I forgot something important here.thank you.
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Old 20th November 2007, 11:38 PM   #2
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I've seen people put a resistor and sometimes a parallel capacitor between circuit and chassis ground. IMO, it's bad practice. I'm not even sure it meets safety regs. I think the idea is to reduce currents and noise in the event of a ground loop, but not sure. FWIW, I've never done it, and never had a problem.
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Old 21st November 2007, 01:39 AM   #3
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It is ok to use a R and C when connecting the signal ground to the chassis ground. However, the chassis should be connected to the earth with a thick wire. One option is to loop a wire around a ferrite core like Mark did with his LM3875 chip amp / gainclone. He reports good results. You can salvage a ferrite core from a usb cable.
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Old 21st November 2007, 02:09 AM   #4
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Not sure it's a big issue, but the way I read safety regs, you can't have a potentially ungrounded metallic part that could conceivably contact an unsafe voltage inside the chassis. Admittedly the whole chain of events is low probability, but let's say the little ground resistor gets blown open. I've seen that happen. Now, let's say some supply flaw puts line voltage on the circuit ground. Even though the chassis is grounded, you now have line voltage accessible by the user. I.e., the shielding has been breached. Of course if you use non-insulated RCA connectors right on the chassis there's no problem, but IMO that's never a good idea for low hum/noise devices. On a power amp, there's the speaker return line to worry about. For commercial design work, you have to think this way!
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Old 21st November 2007, 03:26 AM   #5
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Thumbs up If it is not broken...

well,so far I do not have any problems with my Gainclone LM3875.it does not have any ham at all even at the maximum volume.so I will leave it alone.I just had this question,because I have seen many people putting resistors and caps.but if it is not broken ,do not fix it right?.thank you.
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Old 21st November 2007, 04:06 PM   #6
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It's a very primitive loop breaker I guess.

See http://sound.westhost.com/earthing.htm
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Old 22nd November 2007, 11:58 AM   #7
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This sounds like you are refering to a basic disconnecting network, it is something Andrew T has put quite a lot of informaiton on here about. I fitted one to my chip amp and it made a massive improvement to the mains noise I was getting from my fridge, etc.

Please see here:

Yet another 3886 gain clone!
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Old 23rd November 2007, 01:38 AM   #8
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Talking ooh it is from the board!

OOH! so the ground that has the resistor is from the ground of the boards not the main AC input(going to the main fuse).I thought it was from the Main AC input.so,mine does not have anything just the plain wire connected to ground.I do not hear a hum at all.so,my question is:
Should I put the resistor and diode(like post)?
or should I leave like it is (bare wire to ground).
I did a test and and I do not hear ham at all, even at hight volume.
so.should I leave it like it is or putting this resistor and diode etc.will improve sound?.
thank you.
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Old 23rd November 2007, 09:36 AM   #9
AndrewT is offline AndrewT  Scotland
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Default Re: ooh it is from the board!

Quote:
Originally posted by lanchile07
OOH! so the ground that has the resistor is from the ground of the boards not the main AC input(going to the main fuse).I thought it was from the Main AC input.so,mine does not have anything just the plain wire connected to ground.I do not hear a hum at all.so,my question is:
Should I put the resistor and diode(like post)?
or should I leave like it is (bare wire to ground).
I did a test and and I do not hear ham at all, even at hight volume.
so.should I leave it like it is or putting this resistor and diode etc.will improve sound?.
Confusion = Danger.

The chassis must be directly connected to mains Safety Earth - permanently.

This connection is best welded to the chassis, but most will bolt it on. Never remove this connection, except to inspect for lack of corrosion under the fixing. A dab of petroleum jelly obviates the need to ever dismantle the Safety Earth.

All exposed conductive parts must be permanently connected to the Safety Earth. This includes the input & output connectors if the metal parts can be touched and/or probed by inquisitive kids. This connection of the audio side to the Safety Earth must not require dismantling of the chassis to Safety Earth connection. It can be an extra nut on top or a separate connection elsewhere on the chassis.
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regards Andrew T.
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Old 23rd November 2007, 09:51 AM   #10
Nordic is offline Nordic  South Africa
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If you do not have hum, and case is connected to ground, then leave it alone.

Lol Andrew, I'd love to see you grounding my amp... two sides are heatsinks, other pannels are wood... only put together with 4 screws front and back...
To get to anything you need to dissaemble thewhole amp... well you can take off one heatsink at a time... so at least half of the amp...

To further complicate things it is a dual monoblock, with seperate transformers... in the end to cure my ground loop, I had to connect the RCA ground tab to both power ground star and input- on the amp itself... took months to figure it out, becasue it is so counter intuitive... now all is quiet, finally bringing with it the big dissapointment of not knowing what to next....
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