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Old 19th February 2011, 01:41 PM   #1
diyAudio Member
Join Date: Nov 2010
Default Help with Ground and Layout of Preamp+Amp Combo

Hello all, here is my proposed layout for my Tube pre + SS dual mono amp hybrid

Imageshack - layout1o.jpg
(sorry for the link but I get a broken image when I use insert pic)

Each PCB has single grounding point.

Not pictured:

AC input paths to power switch and Xformers are omitted for simplicity

Isolated RCA in (+)(-) -> Volume Pot (+)(-) -> Preamp In (-)(+)

Only (+) outs from Pre would go into Ampx2
Only (+) outs from Ampx2 would go to Binding Post (+)

Binding Post (-) : Directly to star ground

This is my interpretation of a star grounding system. I'm putting it up here because I don't know if it is right! Thank You
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Old 20th February 2011, 03:10 PM   #2
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Old 21st February 2011, 11:31 AM   #3
DF96 is offline DF96  England
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Join Date: May 2007
You may have ground loops, difficult to tell. You are unfortunately having to rely on whoever designed the PCBs. Are your input signals balanced or unbalanced? If balanced you may be OK. If unbalanced then you have grounded the input twice: once to the preamp input, and again to the star point - remove the input ground from the star point.

When you say that each PCB has a single ground point, do you really mean that? They ought to have an input ground and an output ground. These will be connected on the PCB, but should not be connected together externally as that would create a loop.
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Old 21st February 2011, 11:53 PM   #4
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Thank you for your reply!

The inputs are unbalanced. You are right there are multiple points on the board possible to connect GND, what I really meant was it is possible and I plan to ground at only one point for each PCB.

I see the possible ground loop now. If I grounded the preamp to the (-) input only and removed the preamp GND from the star ground, that will work fine. Part of me is nervous using the (-) RCA as the GND for the preamp, and the star for the amps, but that should work fine I'd think?
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Old 23rd February 2011, 11:39 AM   #5
DF96 is offline DF96  England
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The rule to follow is that wherever possible grounds should track the signal route, and no loops (so from any grounded point there should be one and only one route to the star point). Also, power grounds and signal grounds should be kept separate. Some PCB designs can make this difficult.

That probably means your incoming signals should be grounded at the input to the preamp (no separate connection to the star). The preamp should be connected to the star somewhere near its output. Having two separate preamps can create a loop via the input cable screens, but this may not be a problem provided both cables run closely in parallel.
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Old 23rd February 2011, 12:02 PM   #6
marce is offline marce  United Kingdom
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Join Date: Jun 2007
Location: Blackburn, Lancs
All current flows in a loop, draw a schematic out and trace the loops in pretty colours, you'll then start getting a visual picture of what you are trying to achieve.
At the end of the day the GND is an equipotential reference plane as well as providing a path for return currents, so one ground is best, (that should set the cat amongst the pigeons) if required the return currents controlled by how the ground is laid out.
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