Ground Loop problem

[jleaman]

Can any one suggest me some ideas / techniques for making some ground loops disappear. I have a stereo mon-block that a friend has built. It is a class a Mini a’ Both channels share a single CRC psu with a 300Va transformer. I have tried every thing I could possibly think of. When I connect a audio source via RCA it has a hum a low hum possible a ac ground.

I have connected my ipod to see if it was the source but it was not because I have tried many sources. IF I unplug one channel left/right input the hum goes away and it is dead quiet. I have tested the ac ripple on the dc psu and there is ripple there but my mini as that I have have the same amount of ac ripple on it. I seriously can’t think of any thing to do with this thing any more.. I have access to a scope. Can any one suggest any ideas ? I can post pics if needed.

Things I have tried

Replaced the input rca jacks
Twisted cables for rca and speaker terminal
Put a copper piece of metal around the transformer
Put a thermister on the ac input ground

 

[Mad_K]

Post that pic, also it might be a good idea to draw a gound map

 

[jleaman]

Here is a picture of the psu board.

It is setup as CRC with 3.oOhm resistors @ watt each.

An externally hosted image should be here but it was not working when we last tested it.

 

[jleaman]

Here is a pic of the amp inside. I have the rca's mounted directly to the amplifier like how i had mine.. I still get the ground loop problem.

 

[jleaman]

Here is the amplifier board. It only has 2 fets on the output stage.

 

[jleaman]

And here is a link to my crappy drawing to how it is all connected.


http://jleaman.ath.cx/aleph/testing-psusm.jpg

 

[Andypairo]

In my (former) Aleph30 all the gnds (input and output) are taken from a star point at the power supply. This star point is connected with a single wire to the gnd of the boards.

Cheers

Andrea

 

[jleaman]

So, All the input and speaker grounds go to the same point ? Right where the dc is connected on the ps going to the main amplifier boards ?

 

[Andypairo]

That's indeed the idea.

If you want to use a shielded wire for inputs just connect the shield to one side only.

Cheers

Andrea

 

[Mad_K]

Are you using R0? What value?

 

[jleaman]

Are you using R0? What value?

R0? huh

 

[khaho]

And here is a link to my crappy drawing to how it is all connected.


http://jleaman.ath.cx/aleph/testing-psusm.jpg

Is -IN really not connected at all ? It should be connected in SE too.

 

[Mad_K]

R0? huh

-IN should be connected to SG, but that's not creating a ground loop.
R0 should be 3R3@3W (according to A30 sch) or jumpered.

 

[jleaman]

-IN should be connected to SG, but that's not creating a ground loop.
R0 should be 3R3@3W (according to A30 sch) or jumpered.

I don't understand. and my other mini a's not in this chassi work 100% with out any hum only this one in a chassis.

 

[jleaman]

Ok i did what you suggested MAD_K but there is still hum. It sounds like it is more lighter but less deeper now. Now to test and see if i should be shielding the interconnect cable now from the pcb to the rca jack..

 

[Mad_K]

So what value did you use for R0?. Also, do you use a 10R/5A thermistor from GND (on psu board) to the chassis/Protective Earth? Both of these components are used to supress ground loops.

 

[jleaman]

So what value did you use for R0?. Also, do you use a 10R/5A thermistor from GND (on psu board) to the chassis/Protective Earth? Both of these components are used to supress ground loops.

R0 is = 3.0Ohm @ 3 watt

and i have a thermister on the ac side of the ac plug but not on the gnd i could try one there but i doubt it will do any thing. I have tried removing the ac gnd from the circuit and it does nothing.

 

[mpmarino]

What if you tried a 'series' ground. Take your ground from the psu to one amp board, and just ground the other amp board at the first amp board...see below. It worked for me on my mini a. Actually, it's the only thing that worked for me.

 

[jleaman]

What if you tried a 'series' ground. Take your ground from the psu to one amp board, and just ground the other amp board at the first amp board...see below. It worked for me on my mini a. Actually, it's the only thing that worked for me.

Where do you want the speakers Ground connected to ?

 

[Mad_K]

I would have tried to connect SG (RCA GND) to the other side of R0 (near Z4).