Hum from my gain clone when using a preamp

[mac-attack]

Well, I never implemented any of the colored connections seen in my sketch to begin with, I added them to the sketch only as possibilities to be discussed.

Right now I have implemented the black and yellow lines only, connecting the two CHG connectors (chassi-gnd) on the two amp boards. This seems to work, my humming sound is gone. Perhaps someone can elaborate on why?

I did do some other smaller adjustments to the wiring while I had the amp apart, shortened a few wires and removed the pot I don't need anymore. Not sure, but I suppose the CHG-connection was the key to removing the hum. I have not tried to remove the CHG-connection after the other adjustments, don't really like to take the amp apart again ...

M.A.

The current wiring:

[jimbo1968]

I think I get it now.

Without the chassis ground connection you have a star ground for each channel and they are connected by a loop back through the power supply boards. Whereas they apparently want a nice direct connection or no connection (dual mono)

[digi01]

mac-attack,

remove one rectifier.just use one as combo.

connect 2 CHG points to the chassis ground.

[AndrewT]

Hi Mac,
I disagree with Digi, keep the two rectifiers. It helps prevent channel interaction and reduces load on the diodes.

I think his reference to chassis ground is an oversight, he's forgotten you have a non conducting case.

But it does occur to me that the two CHG connections should, maybe, not be connected together.
How about a resistor between them? and a parallel switch to short out the resistor if it cures a hum problem with some obscure pre-amp/source input connections.
Or not connected at all, did you try this already?

[mac-attack]

Originally posted by AndrewT
Hi Mac,
I disagree with Digi, keep the two rectifiers. It helps prevent channel interaction and reduces load on the diodes.

I think his reference to chassis ground is an oversight, he's forgotten you have a non conducting case.

But it does occur to me that the two CHG connections should, maybe, not be connected together.
How about a resistor between them? and a parallel switch to short out the resistor if it cures a hum problem with some obscure pre-amp/source input connections.
Or not connected at all, did you try this already?

Hi,

I get the humming type sound when there is no connection between the two CHG-connectors on the amp boards.

Can you elaborate on why the CHG:s should not be connected? If eg channel interaction could be negatively effected, please let me know. Right now though, I have no other solution to rid myself of the humming sound than connecting the two CHG:s.

What value for a resistor are we talking about if I were to try that option?

Can there be anything obscure about my setup? It seems fairly straight forward and normal to me. No high-end setup though by any means, but it delivers nicely:
- Marantz DV4500
- Rotel 980 preamp
- Mezzo Proteus speakers (DIY from a Tony Gee design)

M.A.

[AndrewT]

Hi,
I cannot explain why the CHGs need to be connected but you have tried it and it works. You have also proved that it does not work without the connection.

What is CHG connected to on the PCB? It is neither signal ground nor power ground nor speaker return since these already exist.

If you want to experiment further then try a 10r resistor between the CHGs. No guarantees that it will improve things.

[jimbo1968]

Andrew,

mac-attack has no problem with the CHGs connected.

For me, connecting the CHGs reduced the hum greatly.

CHG is connected to all the other grounds on the amp pcb.

[AndrewT]

Hi,
is there a public knowledge schematic that shows the grounding of the SKA GBxx0?

I am NOT asking you to divulge trade secrets. Only that which Greg has already authorised for release.

Links or posting please.

[digi01]

another way is just connect 2 signal gnd to the chassis,isolated 2 CHG and 2 output G to the chassis.

wish it helps.
can you take a inner photo of the layout?

[AndrewT]

Hi,
there is no conductive chassis. He has used wood.

Would you like to repost some advice? and/or enlightenment?