Grounding Issues

Bonce · 7th June 2006, 04:00 PM

Hi Im sure this has been exhausted before, but I have searched and havent been able to find any topics on this.

Anbyway I'll explain my setup. I've had my chip amp sat on a piece of MDF board for a little while and decided to put it into an aluminium box. Im not using a star grounding system, as I didnt on the MDF board and never had any problems. Im using the PCB's from Chipamp.com, and am using one trafo and rectifier PCB for two amplifier boards. The trafo (0-25, 0-25 rectifies to +-36v) has two seperate windings so Im using 8 diodes and have the V+, PG+, V- and PG - connections going from the rectifier to each Amp board. Also each input and ouput ground is connected to the PCB at the correct place. The only electrical connection to my alumunium chassis is earth from the mains socket.

The problem I have is that there is a voltage from any of the grounds on the PCB to the chassis which is earthed. The strange thing is that it isnt 36v, but 12v. This is fine when im using a source which isnt referenced to earth (laptop etc) but If i try to use my PC for a source the soundcard's ground is earthed and so I get 12v running from my amp back to earth through the PCB, making the cable very hot!

I know you may just say "do a star ground" but I dont see how this would solve the problem. If someone could explain what Im doing wrong It would be much appreciated!

joensd · 7th June 2006, 04:19 PM

Hi,
you can read up on the subject of earthing here,
http://sound.westhost.com/earthing.htm
Although you´ll find 1000x as much here in the forums,
it´s just not so nicely summed up.

Quote:

but If i try to use my PC for a source the soundcard's ground is earthed and so I get 12v running from my amp back to earth through the PCB, making the cable very hot!

But the 12V as you say are only on the chassis and earth from the main socket. So how can that cause any current flow and particularly go through your PCB?

greets

Bonce · 7th June 2006, 04:45 PM

Well my RCA grounds are 12v DC above the earth in the mains, whereas the grounds on the output of my soundcard are at 0v with reference to earth, so there is 12v between the inputs to my amp and the ouputs of the soundcard, and when i connect them up with a phono cable there is 12v with about 0.1 ohm of resistance in the cable = bad

I just tried another amp I have, and its RCA connectors are at ground, just like my PC.

Thanks for the link though, I'll have a read!

Mad_K · 7th June 2006, 09:00 PM

Try the grounding arrangement described here:

LM3875 circuit ready to solder?

Bonce · 8th June 2006, 10:25 PM

I looked at that site, but I cant see how exactly that will help me. As far as I know I dont have a ground loop problem. In fact maybe I should change the title of this thread.

Any other suggestions before I rip this amp apart and put it back on the piece of MDF?

poobah · 8th June 2006, 11:11 PM

What is the voltage from V+ to PG?

What is the voltage from V- to PG?

What is the voltage from PG to EARTH? both AC and DC?

What is the voltage from NEUTRAL to EARTH? both AC ans DC?

cpemma · 8th June 2006, 11:50 PM

If you've got +12V on the amp's signal ground you have a fault in the amp circuit. The input cap should block any DC from the chip side.

preiter · 8th June 2006, 11:59 PM

Is the ground from the mains connected to the ground coming from the power supply and the ground from the amplifier boards?

poobah · 9th June 2006, 02:27 AM

I think the DC he is seeing, is actually alot of AC... probably due to asymmetry in the AC input and the capacitance fron pri to sec, OR the phasing (polarity) of the secondaries.

It's probably nothing... but the 12 volts DC to earth is cause for question.

Bonce · 9th June 2006, 10:02 AM

Hello again. I've just made a bit of a strange discovery. The box the amplifier is in is very small so I have to take it apart somewhat in an attempt to get the voltages you asked for. Well I found that when the amp was apart the voltage from earth to PG/SG/OG was gone completely, and the amp is now working fine.

After reassembling the case I was expecting the problem to return, but it didnt. Looking at how tightly everything is crammed in Im thinking that maybe the top of one of the rectifying diodes could have been touching the bare metal on the top of a capacitor. Would this have caused problems?

Anyway, I've decided that when I get time, I will probbaly redo the whole grounding as suggested in the other thread and perhaps use the whole chassis as a star ground that is also earthed. The one thing Im worried about now is the offset reappearing in use and destroying something.

Anyway, some pics:

7th June 2006, 04:00 PM	#1
Bonce diyAudio Member Join Date: Dec 2005 Location: Newbury/York, England	Grounding Issues Hi Im sure this has been exhausted before, but I have searched and havent been able to find any topics on this. Anbyway I'll explain my setup. I've had my chip amp sat on a piece of MDF board for a little while and decided to put it into an aluminium box. Im not using a star grounding system, as I didnt on the MDF board and never had any problems. Im using the PCB's from Chipamp.com, and am using one trafo and rectifier PCB for two amplifier boards. The trafo (0-25, 0-25 rectifies to +-36v) has two seperate windings so Im using 8 diodes and have the V+, PG+, V- and PG - connections going from the rectifier to each Amp board. Also each input and ouput ground is connected to the PCB at the correct place. The only electrical connection to my alumunium chassis is earth from the mains socket. The problem I have is that there is a voltage from any of the grounds on the PCB to the chassis which is earthed. The strange thing is that it isnt 36v, but 12v. This is fine when im using a source which isnt referenced to earth (laptop etc) but If i try to use my PC for a source the soundcard's ground is earthed and so I get 12v running from my amp back to earth through the PCB, making the cable very hot! I know you may just say "do a star ground" but I dont see how this would solve the problem. If someone could explain what Im doing wrong It would be much appreciated!

7th June 2006, 09:00 PM	#4
Mad_K diyAudio Member Join Date: Sep 2002 Location: Norway	Try the grounding arrangement described here: LM3875 circuit ready to solder? __________________ Mads K

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7th June 2006, 04:45 PM	#3
Bonce diyAudio Member Join Date: Dec 2005 Location: Newbury/York, England	Well my RCA grounds are 12v DC above the earth in the mains, whereas the grounds on the output of my soundcard are at 0v with reference to earth, so there is 12v between the inputs to my amp and the ouputs of the soundcard, and when i connect them up with a phono cable there is 12v with about 0.1 ohm of resistance in the cable = bad I just tried another amp I have, and its RCA connectors are at ground, just like my PC. Thanks for the link though, I'll have a read!

8th June 2006, 10:25 PM	#5
Bonce diyAudio Member Join Date: Dec 2005 Location: Newbury/York, England	I looked at that site, but I cant see how exactly that will help me. As far as I know I dont have a ground loop problem. In fact maybe I should change the title of this thread. Any other suggestions before I rip this amp apart and put it back on the piece of MDF?

8th June 2006, 11:11 PM	#6
poobah diyAudio Member Join Date: Nov 2005	What is the voltage from V+ to PG? What is the voltage from V- to PG? What is the voltage from PG to EARTH? both AC and DC? What is the voltage from NEUTRAL to EARTH? both AC ans DC?

8th June 2006, 11:50 PM	#7
cpemma diyAudio Member Join Date: Jul 2003 Location: S Yorkshire OK	If you've got +12V on the amp's signal ground you have a fault in the amp circuit. The input cap should block any DC from the chip side.

8th June 2006, 11:59 PM	#8
preiter diyAudio Member Join Date: May 2006	Is the ground from the mains connected to the ground coming from the power supply and the ground from the amplifier boards?

9th June 2006, 02:27 AM	#9
poobah diyAudio Member Join Date: Nov 2005	I think the DC he is seeing, is actually alot of AC... probably due to asymmetry in the AC input and the capacitance fron pri to sec, OR the phasing (polarity) of the secondaries. It's probably nothing... but the 12 volts DC to earth is cause for question.

9th June 2006, 10:02 AM	#10
Bonce diyAudio Member Join Date: Dec 2005 Location: Newbury/York, England	Hello again. I've just made a bit of a strange discovery. The box the amplifier is in is very small so I have to take it apart somewhat in an attempt to get the voltages you asked for. Well I found that when the amp was apart the voltage from earth to PG/SG/OG was gone completely, and the amp is now working fine. After reassembling the case I was expecting the problem to return, but it didnt. Looking at how tightly everything is crammed in Im thinking that maybe the top of one of the rectifying diodes could have been touching the bare metal on the top of a capacitor. Would this have caused problems? Anyway, I've decided that when I get time, I will probbaly redo the whole grounding as suggested in the other thread and perhaps use the whole chassis as a star ground that is also earthed. The one thing Im worried about now is the offset reappearing in use and destroying something. Anyway, some pics:

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