[Ian Thompson-Bell]

When building tube circuits on a chassis it is relatively easy to create
a star ground where necessary. On a PCB is is probably easier to create
a ground plane. What are the relative pros and cons of each?

Cheers

ian

[Eeyore]

Ian Thompson-Bell wrote:

When building tube circuits on a chassis it is relatively easy to create
a star ground where necessary. On a PCB is is probably easier to create
a ground plane. What are the relative pros and cons of each?

A ground plane doesn't do the same thing at all.

The whole point of stars is to force currents to flow in specific paths.
Notably to avoid power supply related currents or output stage currents
flowing in the ground wires of sensitive stages where the resulting IR
voltage will get amplified and degrade the signal.

You can easily make a star (or multiple stars) on a pcb.

Graham

[Ian Thompson-Bell]

Eeyore wrote:

Ian Thompson-Bell wrote:

When building tube circuits on a chassis it is relatively easy to create
a star ground where necessary. On a PCB is is probably easier to create
a ground plane. What are the relative pros and cons of each?

A ground plane doesn't do the same thing at all.


I didn't for a moment think it did.

The whole point of stars is to force currents to flow in specific paths.
Notably to avoid power supply related currents or output stage currents
flowing in the ground wires of sensitive stages where the resulting IR
voltage will get amplified and degrade the signal.


OK, that I understand. AIUI that implies the local ground of a preamp
stage would be starred with the power stage ground, at the PSU itself so
that power currents do not flow in the preamp ground line.

What then would be the purpose of starring the preamp local ground? as I
have seen in many preamp designs.

Cheers

Ian

[Eeyore]

Ian Thompson-Bell wrote:

Eeyore wrote:
Ian Thompson-Bell wrote:

When building tube circuits on a chassis it is relatively easy to create
a star ground where necessary. On a PCB is is probably easier to create
a ground plane. What are the relative pros and cons of each?

A ground plane doesn't do the same thing at all.


I didn't for a moment think it did.

The whole point of stars is to force currents to flow in specific paths.
Notably to avoid power supply related currents or output stage currents
flowing in the ground wires of sensitive stages where the resulting IR
voltage will get amplified and degrade the signal.

OK, that I understand. AIUI that implies the local ground of a preamp
stage would be starred with the power stage ground, at the PSU itself so
that power currents do not flow in the preamp ground line.

What then would be the purpose of starring the preamp local ground? as I
have seen in many preamp designs.

It's simply not practical take every wire / trace back to the PSU star point,
so you create another one.

Multiple stars work very nicely. OTOH not *everything* needs starring, just
those bits that carry significant current.

Graham

[Ian Thompson-Bell]

Eeyore wrote:

Ian Thompson-Bell wrote:

Eeyore wrote:
Ian Thompson-Bell wrote:

When building tube circuits on a chassis it is relatively easy to create
a star ground where necessary. On a PCB is is probably easier to create
a ground plane. What are the relative pros and cons of each?
A ground plane doesn't do the same thing at all.

I didn't for a moment think it did.

The whole point of stars is to force currents to flow in specific paths.
Notably to avoid power supply related currents or output stage currents
flowing in the ground wires of sensitive stages where the resulting IR
voltage will get amplified and degrade the signal.
OK, that I understand. AIUI that implies the local ground of a preamp
stage would be starred with the power stage ground, at the PSU itself so
that power currents do not flow in the preamp ground line.

What then would be the purpose of starring the preamp local ground? as I
have seen in many preamp designs.

It's simply not practical take every wire / trace back to the PSU star point,
so you create another one.

Multiple stars work very nicely. OTOH not *everything* needs starring, just
those bits that carry significant current.


So would you say then that there is little point in starring in a preamp?

Ian

[Eeyore]

Ian Thompson-Bell wrote:

Eeyore wrote:
Ian Thompson-Bell wrote:
Eeyore wrote:
Ian Thompson-Bell wrote:

When building tube circuits on a chassis it is relatively easy to create
a star ground where necessary. On a PCB is is probably easier to create
a ground plane. What are the relative pros and cons of each?
A ground plane doesn't do the same thing at all.

I didn't for a moment think it did.

The whole point of stars is to force currents to flow in specific paths.
Notably to avoid power supply related currents or output stage currents
flowing in the ground wires of sensitive stages where the resulting IR
voltage will get amplified and degrade the signal.
OK, that I understand. AIUI that implies the local ground of a preamp
stage would be starred with the power stage ground, at the PSU itself so
that power currents do not flow in the preamp ground line.

What then would be the purpose of starring the preamp local ground? as I
have seen in many preamp designs.

It's simply not practical take every wire / trace back to the PSU star point,
so you create another one.

Multiple stars work very nicely. OTOH not *everything* needs starring, just
those bits that carry significant current.


So would you say then that there is little point in starring in a preamp?

You mean a piece of equipment that's purely a preamp only ?

You still need if for the power supply to constrain ripple currents for sure.

Graham

[Ian Thompson-Bell]

Eeyore wrote:

Ian Thompson-Bell wrote:

snip


So would you say then that there is little point in starring in a preamp?

You mean a piece of equipment that's purely a preamp only ?

Correct


You still need if for the power supply to constrain ripple currents for sure.


So, supposing I have my preamp on a PCB with just its local decoupling
and the PSU (transformer, bridge and reservoir cap) some distance away
(to reduce hum pick up in the preamp inputs), then basically starring is
just the ground of the reservoir cap and the ground of the transformer
and a ground wire from the preamp stared at the PSU?

Cheers

Ian

[Tim Shoppa]

On Dec 5, 5:13 am, Ian Thompson-Bell wrote:
Eeyore wrote:

Ian Thompson-Bell wrote:

Eeyore wrote:
Ian Thompson-Bell wrote:

When building tube circuits on a chassis it is relatively easy to create
a star ground where necessary. On a PCB is is probably easier to create
a ground plane. What are the relative pros and cons of each?
A ground plane doesn't do the same thing at all.

I didn't for a moment think it did.

The whole point of stars is to force currents to flow in specific paths.
Notably to avoid power supply related currents or output stage currents
flowing in the ground wires of sensitive stages where the resulting IR
voltage will get amplified and degrade the signal.
OK, that I understand. AIUI that implies the local ground of a preamp
stage would be starred with the power stage ground, at the PSU itself so
that power currents do not flow in the preamp ground line.

What then would be the purpose of starring the preamp local ground? as I
have seen in many preamp designs.

It's simply not practical take every wire / trace back to the PSU star point,
so you create another one.

Multiple stars work very nicely. OTOH not *everything* needs starring, just
those bits that carry significant current.

So would you say then that there is little point in starring in a preamp?

You could still have several amps of filament current, and if it's a
capacitor-input filter and low-impedance rectifier the current
charging the capacitors might come in little bursts of several amps.
But doing the power supply wiring right is still trivial - just make
sure that signal grounds don't share a wire with the (presumably
short!) high current wire. You'd have to be pretty stupid to wire a
power supply up the wrong way, and I'm guessing you wouldn't have done
it anyway :-).

All that said... for preamp levels at high impedances it is
drastically important to lay out the chassis and wiring such that
there aren't big loops with magnetic fields through them. For point-to-
point wiring, you avoid this by twisting wires and their ground
returns together; when laying out the PC board, just make sure the
return runs parallel. I've seen a few layouts when they criss-cross
every so often with vias to emulate twisted pair but it's probably
unnecessary.

Note that "starring the ground" is possibly at odds with "run the
return parallel to the wire" for some layouts. If so, it's probably a
bad layout! Look at the way old point-to-point metal chassis were laid
out, especially with respect to filament, power supply, and high-
impedance wiring, emulate this on a PCB, and you're probably in the
right ballpark.

Tim.