"tony.r" wrote in message
...
Star Grounds

What a lot of fun and games I have had with locating a squeal problem in
my
project amp. I have done everything.

1/ the microphonic test tapping the preamp tubes with an eraser to see if
you hear a reaction from your speaker.
2/ Replaced a bunch of cable with shielded cable.
3/ Grounded the centre spigots on my preamp
But when I separated the input jack ground out of the star I got a big
result.
I grounded to input jack separately and the problem has been located.
Star Ground or Bus ? The star is to difficult to locate problems from.
The problem is still their but at least now I know where it is.

http://www.ncable.com.au/~tony.r/

tony.r


Hi Tony.
The "pure"star system never worked for me - the slightest miscalculation
will set up ground loops that you'll never be able to track down - to many
variables. The easiest grond system (for me, at least), is a modified star,
or star-bus-starbusstar. Most comercial gear uses that, but a *really*
clear example of it is a Fender amp. Look at one, and see the way the
filter caps are grounded to the brass plate behind the pots, and the general
grounding layout. It's simplicity itself, but people kind'a underapreciate
it, or not notice it at all. I've tried pure star, with the sam e results as
you - spent hours on end chasing groundloops, and then... just junked the
layout. Oh, house wiring is a great source of easily-worked bus wire -
just tin the whole wire...
'luck,
-dim

Hi tony.r,

Star Ground or Bus ? The star is to difficult
to locate problems from.

I don't think so. If star ground is _consequently_
used, it is a good as any other _consequently_
used grounding strategy, like plane ground.

Problems only arise when one mixes several strategies.

BTW, a bus ground is just a stretched star ground :-)
with the exception, that sometimes it isn't as
easy to predict in which direction the electrons
flow between the taps to the bus ...

Personally, I prefer star grounds with sub-stars:
Each channel has its own star (or 2, for the driver
and the power stages, at convenient points, where the
low level signals are fed in), and each of this
stars is connected to a central ground star at the big
PSU filter caps with _one_ wire (so no star-to-star
conections, execept to the central star).

It is good practice to give low-level high-gain
stages their own sub-stars, with ground connections
as short as possible to the star placed at strategic
position near by.

All ground stars are insulated from the chassis,
and there is only _one_ connection from the central
star to the chassis plane via a 10 ohms / 10 watts
resistor (tnx to Patrick Turner for that tip), and
the chassis plane being connected to mains earth wire.

Regarding inputs and volume controls, I just follow
Fred Nachbaurs "patch cable approach" with great
results.

Except for the missing chassis shielding effect
I get even very hum-free breadboard level circuits
this way :-)

I think with a consequent star ground strategy you
simply can't go wrong and I don't mind that this
approach will need some feet of black insulated
wire even for medium comlexity projects, since this
wire is cheap when you use 1 mm^2 solid copper wire
as used in home mains installations.

Tom

--
A consultant is a man who knows 40 ways to make love,
but doesn't know any women.

Hi Shiva,

The "pure"star system never worked for me - the
slightest miscalculation will set up ground loops

No, it won't, if you really keep it "pure" :-)

to many variables.

No, quite contrarily, consequent star ground
has not many "variables" except for maybe
pacing the substars where they make sense, but it
likely has some more wires than a clever mixed
scheme or plane ground. Essentially, pure star
ground (with substars) is nearly a no-brainer.

The easiest grond system (for me, at least), is a
modified star, or star-bus-starbusstar.

I would only recommend cleverly "mixed" grouding
schemes if one really knows what one is doing (i.e,
one is clever and/or has patience to try out
by trial and error), or for highly commercial
projects, where some inches less or more copper
wire per amp sum up to kilometers by thousands
of built amps.

Tom

--
Anyone who considers protocol unimportant has
never dealt with a cat. - R. Heinlein