Hello everyone,
I’m new to Kicad (version 6.0.5-0) and I made an audio preamp PCB with OPA. The board will be soldered with only thru-hole components. The voltage supply here is +/- 18v.
A friend advised me to work in four-layers mode and here’s how I distributed them:
Layer 1 (top) = audio signal
Layer 2 = audio ground
Layer 3 = V+
Layer 4 (bottom) = V-
I set all the pads to F.Cu, B.Cu and connected layers and asked for thermal relief which I set to 0.5 (spoke) and 0.5 (gap).
Now for maximum RF protection I add a filled zone on layer 2 (2nd from top) which is audio GND. Then I read in Kicad doc. that a filled zone can’t be directly connected to any layer and I noticed that in the concerned layer (2) the pads & tracks are “isolated” from the filled zone. So I set one of these pads (R99) as connected to all cooper layers where I will connect the PSU ground to the audio ground.
My questions are
Is it a good way to proceed ?
can someone have a look at how I arranged the different layers ?
Then another point : since I will connect wires to the board I add some pads chosen in the connector wires menu but some of them refuse to be paired to the net I’d like : more precisely, the net to which I wish to associate them does not appear in the possible choices of the drop-down menu so I added some resistor’s footprints where I will able to solder my connected wires instead of putting a resistor…
Then a last point : I did not find a dedicated footprint for a TSSOP14 adapter in my footprints library so I drew one in the FP editor (U1 on the schematic) > maybe it’s because my Kicad version is not recent…
Sorry I don’t have comments on your layout, but is there a reason you are using the ancient 6.0.5 version? In your place I would start with 9.0.2 (your project is unlikely to hit the edge case bugs currently being sorted out), or 8.0.9. You don’t have to start again, KiCad can convert your project.
Many people have forgotten how the menus are laid out, or whether a certain capability was in v6, so you will get better help with a recent version.
Thank you for your advice but for some personal reasons my macOs is old (Mojave 10.14.6) and does not support Kicad 9 so I"m in trouble with getting some help I guess
No, not true, even in such an ancient KiCad version, you can simple edit the zone properties, and then set a net name for the zone. And then after a redraw with b, the zone will connect to all pads with that same net name.
Thank you, you’re right. I did connected the filled zone to some pads but why does it refuse to connect to pad already connected by a track ? In the scrolling menu not all the available pads are shown…
No, I mean that the idea of an “audio ground” and a “power supply ground” is outmoded.
Just use a single ground plane.
You can, of course, use multiple ground planes, but make sure they are well stitched together.
I have only one ground plane in my schematic.
All my grounds (PSU & audio) are then connected to PEN/Earth thru a anti // pair of diodes at one point only (star ground). Of course PEN/Earth is tied to chassis…
So :
channel 1 audio ground > one separate wire connected to star ground point
channel 2 audio ground > one separate wire connected to star ground point
+/-18v PSU ground > one separate wire connected to star ground point
+48v phantom PSU > one separate wire connected to star ground point
Then star ground connected to PEN/Earth thru a pair of anti // diodes (or a 10Ω + 100nF X2 in //)
Yeah, audio gear isn’t very demanding (low frequencies).
But, it’s still a deprecated design practice.
Although - it’s always necessary to evaluate what is the best design for a particular circuit.
I have asked to see schematic because I had a problem to understand what you are speaking about GNDs.
You have separate net for GND and audio GND.
From first post I understood that at layer2 you will have audio GND.
What is the role of audio GND when audio circuits (I understand your OpAmps are audio circuits) have their references connected to GND and not audio GND.
I have never designed audio circuits (except long time ago when I were beginning to be interested in electronic) so it is very possible that I do not know something that is basic knowledge in audio electronics.
For me if you use separate GNDs from any reason it should be clear from schematic which part of schematic lies on which GND and then each part should have under it the zone of proper GND net.
To form separate GND regions in KiCad except using 0Rs you can use net-ties.
Indeed your question allowed me to realize that I was wrong, either in my analysis or in my diagram.
In my diagram the audio GND and the PSU GND (capacitors among others) are connected by pin 10 of the OPA1679 (third pin at the bottom right) which is the inverting input of one of the 4 channels and by the negative input of the audio signal, so I have no reason to separate these GNDs.
I have the GND on layer 2 and I place a filled zone to “shield” against possible interference. And then I connect the GND of each audio board and of the PSU +/-18v as well as the GND of the +48v for the phantom power supply to the star ground (which is connected to PEN & chassis thru a diode network).
Thank you !
Add on : OPAs don’t need/use GND but a volatge potential difference when powered by “+” and “-” voltages (they only need GND when powered by mono voltage, like +36v & GND). In my schematic the OPA which needs GND it’s for audio ref.
