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Old 25th February 2007, 03:51 PM   #1
Niila is offline Niila  Finland
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Default LM3886 humming problem

Hello!

Suprise, i have a grounding problem. Im using BrianGt's LM3886 boards . So far, i have only assembled one channel of two monoblocks. The humming is not really a problem when listening music, but i want to know why it does that.

I use 5-lead xlr connectors to connect blocks to my PS.
Powergrounds go to the PS of cource, and channel grounds goes to the star point..you know what i mean Mains ground is also connected to the same point. Im usin plastic enclosures in both monoblocks and PS.

Signal ground from the rca connector goes through the pot to the pcb.

Small heatsink that i have inside the enclosure, is not grounded, as it isnt conncted to the circuit anyhow. Any way i did try to ground it, but i didnt make any difference.

For some reason, the humming sound amplifies a little bit when i touch the heatsink. The hum attenuates when i touch rca ground The chip is insulated, so there should be no problem here?

I will post pictures later

edit: oh, and im using 4ohm speaker for testing..i know 8 would be better as i use +-35V rails but...
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Old 25th February 2007, 08:19 PM   #2
AndrewT is offline AndrewT  Scotland
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Hi,
Quote:
Mains ground is also connected to the same point
How does the mains ground (safety earth) get to the audio ground?
Quote:
I use 5-lead xlr connectors to connect blocks to my PS
How have you connected these pins?
Quote:
Powergrounds go to the PS of cource, and channel grounds goes to the star point
Do these connect anywhere?
Quote:
plastic enclosures in both monoblocks and PS.
The only risks to a user are from the RCA or the speaker terminal becoming live and the risk of sending a live feed back down an interconnect to an ancilliary. Do you have any other metal or conductive parts that come through the enclosures?

Do ensure that the mains fuse will blow if any of these become live.
Quote:
im using 4ohm speaker for testing..i know 8 would be better as i use +-35V
bad decision.
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Old 26th February 2007, 04:40 PM   #3
Niila is offline Niila  Finland
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Hello and thanks for answering.

All grounds (audio and mains) except the "power grounds" goes to the same star point. This includes channel grounds from the monoblocks.

Xlr pins are connected in fololowing order, from eleven o'clock to one o'clock: -PG,-V,CG,+V,+PG. As only DC voltages are carried in the leads, no humming should be produced here.

No live should get to the amp. Theres always the risk. Potentiometer is half outside, but i use a plastic knob. Check out the pics in the gallery.

I now have 8ohm speakers. It didnt make any difference.
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Old 1st March 2007, 07:20 AM   #4
AndrewT is offline AndrewT  Scotland
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Quote:
Xlr pins are connected ....... -PG,-V,CG,+V,+PG. As only DC voltages are carried in the leads, no humming should be produced here.
What is CG connected to inside the mains PSU box? Is CG=0v?
What is CG connected to inside the amplifier box? Is there a connection from CG to TWO inputs leads?
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Old 1st March 2007, 12:52 PM   #5
Niila is offline Niila  Finland
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CG (Channel ground) is the main ground from the pcb. Input signal gound, output ground and both power grounds are connected to this point on the pcb. CG is the only ground from the amplifier which goes to the star point. So ground loop is not possible here. Mains ground also goes to this point.
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Old 1st March 2007, 08:34 PM   #6
AndrewT is offline AndrewT  Scotland
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Quote:
CG (Channel ground) is the main ground from the pcb. Input signal gound, output ground and both power grounds are connected to this point on the pcb. CG is the only ground from the amplifier which goes to the star point
This says it is very likely you will end up with an earth loop that causes hum and possibly buzz as well.
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Old 1st March 2007, 09:19 PM   #7
Niila is offline Niila  Finland
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Quote:
Originally posted by AndrewT
This says it is very likely you will end up with an earth loop that causes hum and possibly buzz as well.

How come, if the CG is the ONLY ground lead connected to the amp? Power grounds are NOT connected to the CG on the PSU end, so its not really a loop. Rather Y or U.
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Old 1st March 2007, 09:29 PM   #8
AndrewT is offline AndrewT  Scotland
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Quote:
Mains ground also goes to this point
what does this mean?
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Old 2nd March 2007, 01:52 PM   #9
Niila is offline Niila  Finland
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Quote:
Originally posted by AndrewT
what does this mean?
That mains ground or safety ground, (from the wall) is connected to the star-point.


Groundings
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Old 2nd March 2007, 02:03 PM   #10
AndrewT is offline AndrewT  Scotland
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Hi,
I think you are confirming that the audio ground is connected to the safety ground which in turn is connected to the house earth ring/star back to distribution board.

You have also confirmed that audio ground is connected to the input terminal ground/return.

If you fit two or more inputs into this system you are guaranteed to create an earth loop.

If you connect just one input and that source is also connected to earth then again you have created an earth loop.

What you are describing is very likely to incorporate an earth loop, no matter how many times you say it can't. You have to do something to break the loop/s.

That linked layout will create a hum loop. It also omits the 0v from the PSU and omits the safety earth. That alone creates a hum loop. Then connect a two channel source to the two input RCAs and you have three hum loops. I do not recommend that you follow that layout.
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