Douglas Self - NE5532 Line Input - KiCad Layout

Piotr

Well-known member
Joined
Jan 23, 2005
Messages
137
Location
Chambery, France
Hi to all,

I'm learning Kicad and trying to find more time for DIY...

So here is a Douglas Self line input circuit from "Small Signal Audio Design" that I've laid out on a PCB.

Curious to know what mistakes/bad practices you will see, I'm sure there's a lot, but I'm learning !!! Hopefully...
One question is that I left out the second half of the 5532, since it's not used, is it at all ok not to connect it to anything???

Thanks for your input !

Piotr.
 

Attachments

  • NE5532 Line Input by Douglas Self.pdf
    47.6 KB · Views: 42

Piotr

Well-known member
Joined
Jan 23, 2005
Messages
137
Location
Chambery, France
This is the top copper layer, signal and ground plane.
 

Attachments

  • Capture d’écran 2019-09-09 à 18.15.22.png
    Capture d’écran 2019-09-09 à 18.15.22.png
    42.1 KB · Views: 43

Piotr

Well-known member
Joined
Jan 23, 2005
Messages
137
Location
Chambery, France
This is the bottom layer, GND trace.
 

Attachments

  • Capture d’écran 2019-09-09 à 18.15.39.png
    Capture d’écran 2019-09-09 à 18.15.39.png
    33.5 KB · Views: 30

ruffrecords

Well-known member
Joined
Nov 10, 2006
Messages
13,180
Location
Norfolk - UK
The obvious mistake is that pin 1 of the XLR does not go to analogue 0V (or gnd as it seems to be labelled in your schematic).

Cheers

Ian
 

Piotr

Well-known member
Joined
Jan 23, 2005
Messages
137
Location
Chambery, France
Hi Ian,

thanks for having a look !

I'm not sure I get it, the blue copper being GND (analog 0V), pin 1 of the XLR is connected to it and to the red ground plane on the top layer.

Or am I completely dumb ?

Thanks,

Piotr.
 

squarewave

Well-known member
Joined
Dec 22, 2015
Messages
2,342
Location
New Jersey, USA
Your circuit is fine but there are some finer points that should be addressed.

First, the 100pF and 100K on the XLR input are for shunting RF interference to the chassis ground. So technically your XLR pin 1 should be connected to the chassis through a wire that is as short as possible (the length of that distance defines the frequency of RF that can be emitted into the enclosure). This is why some XLR connectors have a metal spike connected to pin 1 in the screw hole that stabs the metal chassis it's bolted to. Then, the 100pF and 100K should also be connected to that chassis ground. The chassis ground is then connected to the power supply at the power supply filter caps. Meaning the RF will be directed away from the analog circuit, through the chassis and then finally connect to the "star" ground point a the PS. If your chassis is not metal, this procedure is less important and possibly useless.

Second, you should have a "drain resistor" on the outside of the output (unless you know it's ALWAYS going to be connected to something that has a path to ground). This is because electrolytic capacitors, and expecially large ones, have some leakage. So if you don't have a path to ground, a significant voltage can build up on the downstream side. If there was some sort of switch downstream, it could easily cause loud popping noises as you switch it.
 

Winston OBoogie

Well-known member
Joined
Jun 3, 2004
Messages
2,168
Location
UK.
Piotr said:
Hi Ian,

thanks for having a look !

I'm not sure I get it, the blue copper being GND (analog 0V), pin 1 of the XLR is connected to it and to the red ground plane on the top layer.

Or am I completely dumb ?

Thanks,

Piotr.

Squarewave and Ian addressed the RF and pin 1 stuff.
Regarding connection of 0V to chassis, this usually happens back at the supply and only happens once.

P.S.  If you wanted, I think that The Signal Transfer Company used to sell a pcb of this circuit.  Good that you're getting to grips with your own layout though so  :)
 

ruffrecords

Well-known member
Joined
Nov 10, 2006
Messages
13,180
Location
Norfolk - UK
Piotr said:
Hi Ian,

thanks for having a look !

I'm not sure I get it, the blue copper being GND (analog 0V), pin 1 of the XLR is connected to it and to the red ground plane on the top layer.

Or am I completely dumb ?

Thanks,

Piotr.

Sorry, it was my poor explanation. You have pin 1 of the XLR connected to analogue 0V. This is incorrect and is known as the pin 1 problem:

http://pin1problem.com/

Pin 1 is not a signal carrying pin. It is a screen and should be connected to chassis.

Cheers

Ian
 

Piotr

Well-known member
Joined
Jan 23, 2005
Messages
137
Location
Chambery, France
Hi to all of you and thanks for sharing your thoughts !

So I've read some material linked earlier and some Rane documents to help me clarify the correct way to go and avoid the pin 1 problem.

As a sidenote I've also clarified the way Kicad handles and names the different "grounds" as in (https://electronics.stackexchange.com/questions/392911/kicad-5-what-is-the-significance-of-the-various-gnd-symbols).

GNDPWR is chassis ground.
GND is signal ground (or common or 0V if I read you correctly)
EARTH is mains ground.

So the correct schematic (omitting the power supply and drain resistor) would be:
 

Attachments

  • Capture d’écran 2019-09-10 à 14.29.12.png
    Capture d’écran 2019-09-10 à 14.29.12.png
    32.4 KB · Views: 38

squarewave

Well-known member
Joined
Dec 22, 2015
Messages
2,342
Location
New Jersey, USA
Looks good. Although I don't know why kicad calls the chassis ground GNDPWR since it has nothing to do with "power". That symbol should just be called "CHASSIS" IMO.

Also, one correction regarding my previous post. I said the 100K were part of the RF shunt. That is not true. They are just drain resistors like I described regarding the electrolytic on the output. Although they're not just for leaky 47uF since the source could be adding DC for some unknown reason (albeit unlikely which is to say you could probably skip them entirely). But 100K is large relative to the impedance of pins 2 and 3 so it doesn't really matter too much what ground they're connected to. Technically they should probably be on signal GND since they're connected to signal nets but I actually can't think of a reason why it would really matter.
 

abbey road d enfer

Well-known member
Moderator
Joined
Jan 22, 2008
Messages
11,944
Location
Marcelland
squarewave said:
Your circuit is fine but there are some finer points that should be addressed.

First, the 100pF and 100K on the XLR input are for shunting RF interference to the chassis ground. So technically your XLR pin 1 should be connected to the chassis through a wire that is as short as possible (the length of that distance defines the frequency of RF that can be emitted into the enclosure). This is why some XLR connectors have a metal spike connected to pin 1 in the screw hole that stabs the metal chassis it's bolted to. Then, the 100pF and 100K should also be connected to that chassis ground.
Actually, the 100k resistors could be terminated to almost any "ground", since they are here only to dump the capacitors leakage current. No significant current passes through them.

The chassis ground is then connected to the power supply at the power supply filter caps.
We already had this discussion. It's the PSU's 0V reference that must be grounded to chassis, so the "ground" that is distributed to the active circuits is clean, as referenced to the chassis. Now the 0V at the filter caps is dirty but it just doesn't matter, as long as nothing is connected to it.
 

Rocinante

Well-known member
Joined
Jan 17, 2014
Messages
1,125
Location
Minnehopeless
I use Ian's grounding 101 in all my designs and have had very few issues since I studied it. That link is a useful tool in itself.
 

Latest posts

Top