[sdiy] Seperate Grounds

[harry]

Hi Mark...

I'm posting your additional questions to the list as well... I hope you
don't mind but there are others who will want to hear the answers as
well...

Mark Melvin wrote:

> Harry-
>
> OK, wow.  First of all, thanks for the time you put
> into your response.  I really appreciate it!
>
> Now, the fog is still a little thick but I can see the
> lighthouse, at least.  So I'll try to ask the right
> questions to help clear it up.  (...remember those
> kids in school that always said "I don't get it" and
> that's all they would say...?!)
>
> > Power supplies are like water pipes... the plus and
> > minus 15V are the
> > feeds.. water mains so to speak. The ground is the
> > drain pipe.
> >
>
> ...
>
> > So you keep them separate... and tie the pipes
> > together only
> > at one spot... right near the final sewer.  If the
> > toilet backs up...
> > then NO WAY can it get into the pool...
> >
>
> THIS I understand... and GOOD analogy!  Nuthin works
> like a GOOD analogy.
>
> > "Signal Ground"...
> >
> > Thats the "drinking water pipe"... its where you let
> > the modules
> > tie together for audio signals... but not high
> > current, noisy grounds...
>
> OK, the analogy is working, but I'm not sure
> SPECIFICALLY how to do this.  Exactly what can I
> connect to "Signal Ground?"

Signal Ground (to me) is the ground of all audio inputs and
outputs. It MIGHT include some CV grounds as well... unless they are
"noisy"...

> Am I just designating
> some of the modules as Noisy and others as not Noisy?

If they are noisy or not depends on design. Don Tillman made an
excellent
post on his technique for keeping module noise to a minimum. He does not

ALLOW his modules to be noisy... by design.

Noise is caused by things that draw a lot of current and change
rapidly... like
an LED flashing on and off... that might be 10-20 mA.  If it is going at
LFO rate...
it might rock the grounds (or power supplies) enough to make the VCO
modulate
even with no patch cord installed... a bad thing.  He designs so that
the circuit
ALWAYS draws that 10-20mA... it does not change dynamically.
Therefore... the
flow in the pipe is constant and does not disturb anyone else...

>
> OR should I instead be looking at specific points on
> each board... some to ground as clean and some to
> ground as noisy?

Yes thats right.  If I'm doing (lets say) a module with a preamp and
some LED indicators... I run separate traces back to the point where the

power enters the board... and there are DECOUPLING capacitors there...
these will minimize the interaction.  If this is not enough, there are
more agressive
ways to isolate.  You'll probably not have troubles in most cases just
using
careful layout / wiring techniques.

> In either case I'm still not sure
> what's clean and what's noisy.

Clean is someting that always consumes a steady flow of current and is
not
prone to making interference that will disturb other modules. Also clean
might
be for something that is PRONE to picking up other circuits
disturbances.
A preamp or maybe VCF... VCA ... might be clean modules

Noisy is things that draw varying currents... and Env Generator will
usually draw
a lot of current during attack and decay and almost nothing during
sustain. Some
VCO's bang their power supplies pretty hard... these could be noisy.
Fast moving
signals (like square waves) tend to be noisy. LED indicators are a
notorious noise
source.

> >
> > > "Reference Ground"
> >
> > A water level point that should never move... even
> > if you flush the toilet the water here stays
> > undisturbed... like a mirror surface on a pond...
> > no ripples.  This is a point where you get important
> > voltages from... usually
> > to set things like your expo converters, VCO
> > reference points... Its the
> > real zero volts in your system (wired properly)
>
> OK, OK.  Again, the analogy is crystal clear but I
> remain confused.  If I have two seperate ground paths
> on each board (one clean "Signal Ground" and one noisy
> "Dirty Ground") then where does the Reference
> originate?  Is this a THIRD and completely seperate
> path the goes all the way back to power supply?

Reference Ground (or power supply references) are usually part
of the clean power system.  For instance... the level that a VCO uses
when it begins a cycle muct not vary... or the frequency will change.
Tying this point to some LED on the same trace would be a big mistake...

everytime the LED lights... the frequency shifts. You've seen some posts

where people talked about using on-card regulation or separate
references
for this purpose. This is the kind of thing that you will pick up when
you do
it wrong... see the problem... and fix it.  If you don't notice any
problem...
you don't have a problem....  ;^)

> > > and
> > > "Dirty Ground"
> >
> > This is the shit pipe... you run your nasty drains
> > through here... Some
> > VCO cores
> > are looking at a few microamps... ONE LED could be
> > 20 milliamps...
> > thousands
> > of times larger. So you run a separate pipe for this
> > to go back to the
> > ocean without
> > disturbing anything else...
> >
>
> Right, OK.  So what exactly qualifies as "dirty?"
> There are no LEDs in my synth.

Dirty signals screw up the clean ones. If you hear the LFO modulating
your signal even when its supposed to be off... THAT'S dirty...

> > Separate wires... that connect only at the OCEAN
> > (the large filter caps
> > in your
> > system... at the power supply).
>
> OK... does this point where they all meet ALSO connect
> to the Chassis?  Or is it only the Earth ground from
> the AC that goes to the Chassis?

I've done both.  Usually I connect the chassis to earth (mains) ground
at
exactly one place. Often I try to NOT connect earth ground to the
chassis
at all. This is dangerous and should not be tried without a full
understanding
of the safety hazard. Better is to use a "wall wart" power supply that
keeps the
mains voltage in a safe double insulated plastic box... Then there IS no
"mains"
ground on your chassis... only the ground from the power supply. This
should
touch the boz at exactly one place.

The problems arise if the grounds have different voltages on them. If
they were all
at the same potential it would not matter how you connect them....

>
>
> > I tie the power supplies from each module directly
> > to the power
> > supply... if necessary
> > I use a small PCB with connectors (Blacet makes a
> > thing like this...) or
> > Terminal
> > strips.  This is my "dirty power".
>
> Sadly, this confused me again.  Are you talking about
> the +/-V, or the Dirty Ground, or both?

OOPS meant to say "Bulk Power"...

> I understand
> that you're NOT talking about a daisy-chain.  That
> much is clear.

right.

> I guess it's the term "Dirty Power"
> that confused me.  Do I have "Clean Power" as well?

Yes... clean and dirty power. The envelope generator AND the VCF both
run from the same supply... but with separate wiring to a central
location...
a distribution bus or terminal strip. Usually you want a way to remove
separate
modules without disturbing others. A lot of people are using a 4 pin
.156" header
for this connection... Blacet, MOTM, EFM... all use this. So do I
(now....).

>
>
> > > 2. Regarding the pots on the front panel, most all
> > of
> > > them have one terminal that must connect to
> > ground.
> > > What's the best way to approach that?  Tie ALL of
> > the
> > > grounds on the whole panel together?
> >
> > If it is on ONE panel... I tie them all together in
> > a way I can easily
> > separate later... plan ahead.  I allow this ground,
> > the power supply
> > ground,
> > and the chassis ground to touch at only one point.
> > This makes everything
> >
> > VERY quiet.
> >
>
> Good, ok.  I can do that.  Again though, does the
> "Earth Ground" from the 117V also meet at this point?
> I'm assuming "yes."

I'd rather not have earth ground involved.  I'd rather isolate the
supply
elaswhere so that only low voltages are involved. The earth ground has
LOTS of problems. Wanna talk about Dirty Ground ??? Try your household
"third wire"... with the refrigerator AND the washing machine running...
and the TV...
and the air conditioner... HELP !!!

There are inevitably going to be several possible grounds in an audio
system...
there are grounds between the synth and the mixer, power amp, etc.  So
if you tie all the chassis to earth ground, and through the audio
cables... you can have the noise from the electrical system breaking
into the audio path.  DO NOT just use those little
ground lift plugs... you might get a shocking experience.  If you are
careful in the synth design... there will be only ONE ground
(ultimately) that you will be dealing with.

H^) harry

>
>
> > > Or seperate them
> > > by module and return the ground to that module?
> >
> > That would work OK too... if you are careful how you
> > run the power to
> > each module.
>
> No, I don't want to do that.  That's more work!  I
> just thought that might be a better way to do it.
>
> > Hey good luck. write again with any questions !!!
> >
> > H^) harry
>
> I hope my questions are clear.  If not, let me know.
> Thanks A LOT for your help.
>
> -Mark
>
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