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Old 25th June 2004, 02:05 PM  
MWP is offline MWP  Australia
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Default Grounding scheme

Hi all,

After some big problems with ground loops with my big bridged gainclone amp, im redoing all the internal wiring.

Im now trying to work out the best way to re-wire it all.
The amp uses 8 amp PCBs (two LM3886s on each), each board with the 3 power supply pins, 2 signal in (signal, gnd) and the speaker out pins.

Do i connect the power earth to the chassis and signal/amp-power ground?

Do i ground the RCA connectors individually back to the amp boards, or connect all the ground points at the connectors and run them back to the supply caps ground?

Etc, etc.

Thanks for any help...
 
Old 27th June 2004, 05:50 PM  
moamps is offline moamps  Croatia
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one example
(sgnd and pgnd connected together only on starground)

Regards
Milan
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Old 27th June 2004, 06:16 PM  
MWP is offline MWP  Australia
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Thanks for that.

I rewired the amp to pretty much that plan yesterday.
The old wiring setup was quite bad.

No more problems with ground loops, but now i have big problems with transformer induced humm (ARGH!!!).
 
Old 28th June 2004, 05:42 PM  
dhenryp is online now dhenryp  United States
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Is your XFormer making mechanical noise itself or do you think it is inducing noise into your circuits magnetically?
 
Old 28th June 2004, 06:10 PM  
MWP is offline MWP  Australia
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Well im not sure now...

Ive moved the trafo out of the 19" 2U rackmount box, and the problem is still there.

Its leaving me with only one more possibilty... supply ripple.
The weird thing is that this Amp never had a problem with noise until about 1 week ago.
Surely the 4 10,000uF caps smoothing the power after the bridge-rect cant degrade that quickly.

Hmmmmmm
 
Old 28th June 2004, 07:48 PM  
dhenryp is online now dhenryp  United States
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If you hear the sound coming out of the speaker then your transformer is not vibrating mechanically.

Are you hearing a 50-60 hz hum (line frequency) or 100-120 hz sound (capacitor charging) or other higher frequency noise (maybe rf or other hash on you mains)? Do all of your channels have the problem or some subset?

If what you are saying is that you've done the star grounding and still have hum somewhere in you amp, I think it is more likely that you still have a gound loop problem rather than a transformer problem. Check to see if it is a problem with the source you have connected. Try feeding it with a baterry powered CD player or something else that does not connect to mains power or ground. If the hum goes away, then your problem is a loop between your amp and the mains connected input device.

If you still have a hum with a battery input device check to make sure your line level input is shielded and/or physically separated from mains voltage wires and away from diode bridges if possible. Make sure all you connections to power and grounds are solid.

Make sure your connection to power are as far away, physically from the diode bridges as possible. I.E. pick up your V+ and V- from the filter capacitor that is farthest from the bridge, not at the bridge. High current charging current between the bridge and capacitors can cause 100-120 hz noise.

Is your transformer a Toroid or a square El? If you have tried your transformer from a foot or more away and you have tried various physical orientations you can be pretty sure you are not getting hum coupled magnetically.
 
Old 28th June 2004, 08:27 PM  
Franz G is offline Franz G  Switzerland
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Default My way

First: connect the gnd from the input-connectors to the signal ground.

Connect the amps PGND to the PSU GND, the same way you did with the SGND.

If this does not help, try another radical solution:

Remove the PSU from the ground and connect it directly to every amp-board's gnd, exactly between the bypass c's, every cable exactly the same length.

Make a THICK rail between all amp boards gnd. In the middle of this rail, you have now your new central gnd. Here you can connect the signal ground, the enclosure and eventually the speakers.

This is my prefered solution for multichannel amps.

Franz
 
Old 29th June 2004, 07:07 AM  
MWP is offline MWP  Australia
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Quote:
Are you hearing a 50-60 hz hum (line frequency) or 100-120 hz sound (capacitor charging) or other higher frequency noise (maybe rf or other hash on you mains)? Do all of your channels have the problem or some subset?
All channels have the hum.
Im not sure if its 50Hz or 100.

Quote:
If what you are saying is that you've done the star grounding and still have hum somewhere in you amp, I think it is more likely that you still have a gound loop problem rather than a transformer problem. Check to see if it is a problem with the source you have connected. Try feeding it with a baterry powered CD player or something else that does not connect to mains power or ground. If the hum goes away, then your problem is a loop between your amp and the mains connected input device.
Its not a ground loop (well to external sources anyway).
I can ground the input to the channels and itll still hum.

Quote:
Make sure your connection to power are as far away, physically from the diode bridges as possible. I.E. pick up your V+ and V- from the filter capacitor that is farthest from the bridge, not at the bridge. High current charging current between the bridge and capacitors can cause 100-120 hz noise.
I am doing this already.

Quote:
Is your transformer a Toroid or a square El? If you have tried your transformer from a foot or more away and you have tried various physical orientations you can be pretty sure you are not getting hum coupled magnetically.
Torid.
And i have moved it away an dthe problem still continues.

ARGH... i need to buy myself a CRO.
 
Old 29th June 2004, 08:37 AM  
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Is the enclosure connected to the circuit ground anywhere?

Are the RCA grounds insulated from the enclosure? Speaker grounds too?

Where is the safety earth connected? Chassis? Circuit?

If it is a ground loop problem there must be a physical loop somewhere.
 
Old 29th June 2004, 11:22 AM  
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To MVP : If you will look at Per - Anders home pages, you will see there too my amp PA - 03, where you can to take inspiration about correct grounding. This amp is quite silent, no noise, no hum. It is with LM 4780, but it is the same as with LM 3886 .
 

 

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