[sdiy] keeping signals seperate..avoiding noise

Seb Francis seb at burnit.co.uk
Sat Nov 15 23:08:53 CET 2008

Justin Owen wrote:
> One pin on each of the volume pots on board 3 has to go to Ground (of the 78XX) - so should it
> a) go via a separate lead from its own board to the 78XX/power supply board
> b) go to same point on the motherboard where Ground from the power supply is connected, or...
> c) in a small, single-supply circuit - it's not going to make much difference.

Well, it's probably not going to make much difference.  But I would wire 
the ground from the pots to somewhere in the circuit that the other 
terminal from the pots are connected to.  The idea being that you want 
the ground reference that the amp circuit uses to be the same as the 
ground reference connected to the pots.

But I really wouldn't worry too much - I very much doubt you would 
notice any difference if you just connected the pot grounds direct to 
the power supply (if this happened to be more convenient).

Where power routing makes a difference is when you have fast switching 
current transients.  Consider this example ...

You have:
(1) A power board
(2) A board with a square wave LFO with an LED indicator
(3) A VCA board

Now if you chain the power so board (1) goes to (2) goes to (3), when 
the LED switches on (or off), you get a sudden change in current 
consumption of board (2) and consequently the power voltage at that 
board glitches slightly because of the resistance and particularly the 
inductance of the wires, etc.  So if you are running board (3) from the 
power on board (2) then this spike may get coupled into the VCA as an 
audible click.

Makes sense?


P.S. Many things can create fast current transients - an LED driven by a 
square wave is just an (extreme) example.

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