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> Ground Plane
Tubenirvana
Posted: November 20, 2014 05:44 pm
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Hi,

new member here.
I am relatively new to pcb design, I build custom tube audio gear, and have not used pcb's much, but decided that for some of my gear a pcb is a much better way to go.

I am wondering, if using a large ground plane on the top as well as the bottom of the pcb is going to cause any problems, such as possible capacitance or even ground loop type of thing?
I am designing a pcb for a tube preamp, so want to make sure to eliminate any possible 'noise' by using a ground plane/s.
Thank for any input.
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CWB
Posted: November 21, 2014 01:38 pm
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pcbs lend themselves to good engineering practices .
for most audio frequency circuits , a pcb will cause no more hassle than hand (PTP) wiring .
the same basic rules apply :
keep the input away from the output
decouple , decouple , decouple

high gain circuits (think magnetic pickup pre-amp) can give fits ... sometimes the use of "all grounds" connecting back to a central point will eliminate loops .

good ground planes can help eliminate problems , bad ones can cause problems .
there is another recent post pertaining to your question .


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Tubenirvana
Posted: November 21, 2014 03:17 pm
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QUOTE (CWB @ November 21, 2014 05:38 am)
pcbs lend themselves to good engineering practices .
for most audio frequency circuits , a pcb will cause no more hassle than hand (PTP) wiring .
the same basic rules apply :
keep the input away from the output
decouple , decouple , decouple

high gain circuits (think magnetic pickup pre-amp) can give fits ... sometimes the use of "all grounds" connecting back to a central point will eliminate loops .

good ground planes can help eliminate problems , bad ones can cause problems .
there is another recent post pertaining to your question .

Thank you for your reply. Yes, I of course use 'star' grounding on all the chassis wiring, this is a proven way for grounding tube amp wiring etc..
The way that I have designed my pcb's is that I am using a ground plane over the whole surface, generally on the top/component side, and any components that need to be connected to ground, I use a 'thermal pad' connected to the ground plane.
The ground of the pcb is then connected to the 'star' ground of the chassis. This has been working fine, my preamp, especially phono preamps, are as quiet as a church mouse.
My 'concern' is about adding a ground plane on both sides of the pcb, interconnected through via's, that this may in some ways create some kind of capacitance effect.

This post has been edited by Tubenirvana on November 21, 2014 03:18 pm
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Sch3mat1c
Posted: November 21, 2014 04:47 pm
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With impedances so high, the advantage is actually quite small. You get the most benefit from grounded metal shields around the circuit; usually the chassis. If this can't be connected to signal ground (due to loops from safety ground), you may want to design a second floating shield inside.

The PCB itself can somewhat serve this role, but the disadvantage is a few pF extra capacitance, which will reduce your bandwidth. For audio, probably not enough to matter, but you'll want to keep this in mind especially around feedback paths and precision filters.

The additional capacitance due to stitched grounds on both sides (forming coplanar waveguide type traces) is much less than that due to grounding at all (forming microstrip type traces), so if you're already doing it, it won't hurt much to go the full length.

Tim


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kl27x
Posted: November 21, 2014 08:28 pm
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No, not a problem. Double ground planes also make the pcb design easier to layout.
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CWB
Posted: November 22, 2014 01:41 pm
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as tim mentioned about the dimensions of the stitched side-to-side vias and areas ...
they can form resonant circuits or cause problems at rf frequencies (at suhf/uW think stripline techniques) .

for audio work though , the capacitive reactance will be low enough to be of little concern .
inductive reactance of traces/areas will likewise be negligible in effect .

this is not to say that one can ignore "good housekeeping" practices .
in theory , any circuit with gain at or over unity can be made to whistle or suffer from deleterious effects accidentally , given enough bad engineering .

what tube(s) are you using ?


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Tubenirvana
Posted: November 24, 2014 03:04 am
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QUOTE (CWB @ November 22, 2014 05:41 am)
as tim mentioned about the dimensions of the stitched side-to-side vias and areas ...
they can form resonant circuits or cause problems at rf frequencies (at suhf/uW think stripline techniques) .

for audio work though , the capacitive reactance will be low enough to be of little concern .
inductive reactance of traces/areas will likewise be negligible in effect .

this is not to say that one can ignore "good housekeeping" practices .
in theory , any circuit with gain at or over unity can be made to whistle or suffer from deleterious effects accidentally , given enough bad engineering .

what tube(s) are you using ?

thanks for all the very informative replies.
For the preamps, I use a number of different model tubes, 12AY7, 12AX7, 12AZ7, 12AU7, 6SN7, 6SL7 and derivatives of those, just depends on the model and type of preamp.
For power tubes, which are mostly wired point to point, I use EL84, KT88 and KT120's.
I do use pcb's even for some power tubes, again it all depends on the model of the power amp.
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